TY - JOUR
AU - Dionísio,, A
AB - Abstract Geophysical methodologies have been implemented, tested and validated as diagnostic and /or monitoring tools in artworks or historical monuments. They are non-destructive and can give an image of internal structure of investigated medium. This paper is a review about the main geophysical techniques applied to the study of cultural built heritage (excluding the archaeology field). A brief description of the used methodologies is presented, the main investigations done in this field are showed, the method or methods most appropriate to answer each problem (moisture detection, characterization of the materials, study of the structural continuity of the material, assessment of intervention’s effectiveness) are indicated and the main advances and gaps and future developments are also pointed out. geophysical methods, non-invasive, diagnosis, conservation, cultural built heritage 1. Introduction Monitoring of historical monuments is an issue of great interest and of upmost importance, for their effective preservation and restoration. Detailed knowledge of its internal structure is the key to their conservation and/or restoration. In the case of buildings, such structure is composed of different types of stones, bricks, with wooden or iron elements inserted into walls and cavities as ties, etc. (Valle and Zanzi 1998). Moreover, the wall thickness, the recognition of detachments and cracks is crucial to verify the stability of buildings. In the case of the study of cultural built heritage, the major problem is related to the impossibility of touching and taking samples in order to be studied in the laboratory or even in-situ. The assessment of decay’s degree can be performed by various methods and with different levels of accuracy. In general, it can be carried out by qualitative and quantitative methods. Although visual assessment and tactile methods are very important and useful tool they are not enough for further characterization of the materials used, its pathologies as well as for the development and implementation of conservation and restoration measures. One serious drawback is that the results depend on the observer’s experience. Therefore, non-invasive techniques are considered as the most appropriate to evaluate the internal structure of historic buildings and assess the quality of the materials that have historical or artistic value. The development of the technologies associated to applied geophysics, namely, the improvement of performance and information resolution of sensors and devices and the increasing availability of user-friendly data/image analysis, and processing software and routines, have led to an increasing interest in the use of geophysical methods for the study and conservation of cultural heritage. They are non-invasive, can be used in situ and are characterized by a resolution power that can be defined between ‘microscopic’ and ‘macroscopic’ investigations (Cosentino et al2011) ranging from a few centimeters to a few decimeters, i.e. useful for artifacts having dimensions from a few cubic decimeters to a few cubic meters. Therefore, they can be used in small-sized targets as artifacts, columns, pillars, pottery, sculptures, museum objects, and so forth. The geophysical methods most commonly used in the field of cultural built heritage are ground penetrating radar (GPR), seismic (sonic and ultrasonic techniques) and electrical methods (resistivity and self-potential). Other geophysical techniques such as microgravimetry and magnetometry can also be applied in cultural heritage, namely, in archaeology (the topic goes beyond the scope of this paper), but they are more specific and therefore less used (e.g. Abad et al2007, Trinks et al2010, Panisova et al2013). Passive seismic is a new technique, consisting in seismic ambient noise measurements, which can be used for stratigraphic investigation of cultural heritage sites subsoil (Castellaro et al2008, Spizzichino et al2013). Another method that could be used in cultural built heritage is the nuclear magnetic resonance (NMR) but is rather recent (e.g. Capitani et al2012). Generally, the decision about the use of a particular method depends on several factors, including: spatial and physical characteristics of the cultural targets and their surroundings, aim of the investigation (research, conservation), issues to be addressed (e.g. decay assessment, moisture), depth of investigation and resolution. Depending on the type of investigation and since decay takes many different forms, the integration of different geophysical methods is, in some cases, recommended (Binda et al2003b, Binda et al2011). The integration increases the reliability of the non-invasive indirect examinations. 2. Brief description of main geophysical techniques applied to cultural built heritage 2.1. Ground penetrating radar Ground penetrating radar (GPR) is a high-resolution technique that allows obtaining an image of subsurface structures using electromagnetic waves in the frequency band of 10–2600 MHz. Therefore, signals of relatively short wavelength can be generated and radiated into the ground to detect anomalous variations in the dielectric properties of the geological material. GPR is commonly applied for geological, engineering, environmental, archaeological investigations and cultural built heritage. It can be used to study geological contacts between different superficial materials and contacts between superficial materials and bedrock (e.g. Dallimore and Davis 1987). Applications to environmental and civil engineering problems include permafrost, groundwater and overburden delineation, and detection of voids, fractures, seepage, and soil and groundwater contamination (e.g. Benson 1995, Stevens et al1995, Longoni et al2012). The method is also used for non-destructive testing of stone quality in quarries (Lualdi and Zanzi 2004), and in pre-excavation archaeological investigations (e.g. Conyers and Leckebusch 2010). In the field of cultural built heritage, GPR is increasingly used in the assessment of the preservation state of artworks and monuments (e.g. Pérez-Gracia et al2009, Binda et al2010, Cosentino et al2011). A GPR system essentially measures energy reflected or scattered in targets; the amplitudes are recorded as a function of travel time. The system comprises a signal generator, transmitting and receiving antennae, and a receiver that may or may not have recording facilities or hardcopy graphical output. Some advanced systems have an onboard computer that facilitates data processing both while acquiring data in the field, and post-recording. There are three modes of deployment of radar systems (e.g. Reynolds 1997): (i) reflection profiling of common offset mode; (ii) wide-angle reflection and refraction (WARR) or common-midpoint (CMP) sounding, and (iii) transillumination or radar tomography. In the reflection profile one or more radar antennae are moved over the ground surface simultaneously, with the measured travel times to radar reflectors being displayed on the vertical axis while the distance the antenna has travelled is shown on the horizontal axis. In the WARR antenna configuration the transmitter is kept at a fixed location and the receiver is towed away at increasing offsets. An alternative is the CMP sounding. In this case, both the transmitter and receiver are moved away from each other so that the midpoint between them stays at a fixed location. The point of reflection on each subsurface reflector does not change, and thus areal consistency at depth is not a requirement. In the transillumination mode of deployment the transmitter and receiver are on opposite sides of the medium under investigation. The radar antennae can be located down boreholes and the radar signals are then propagated from one, through the medium in between, to the other. The transillumination mode is also common in non-destructive testing (NDT) investigations of man-made structures (Reynolds 1997), particularly using very high frequency and hence small antennae (e.g. 900 MHz centre frequency). The technique, has the main advantage of allowing high resolution diagnosis (from centimeters to tens centimeters), also thanks to the available advanced processing techniques. Examples include testing concrete columns and masonry pillars (e.g. Masini et al2010). The depth of penetration of a radar pulse depends largely of the electrical conductivity of the investigated material and the frequency of the antenna used. In general, the range of penetration will decrease with increasing conductivity. Moreover for a given material, lower antenna frequencies increase penetration depth but resolution is decreased. Therefore, a compromise must be established between the penetration depth and resolution. 2.2. Seismic methods (sonic and ultrasonic technique) Seismic investigations uses the fact that elastic waves (also called seismic waves) travel with different velocities in different rocks. By generating seismic waves and measuring the time required for the waves to travel from the sources to the receptors, it is possible to determine the velocity distribution and from this, the nature of the subsurface layers. The seismic methods can provide elements of interest to detect the thickness and position of the weathering layer, physical properties of the different materials, including mechanical characteristics and the state of cracking, fractures and other discontinuous elements (e.g. Cosentino et al2009, Capizzi et al2013). The ultrasound technique can be used in the field of cultural heritage in order to evaluate the degradation of the element (locating areas of decay and structural weakness hidden within, assessing the extent of decay visible on the surface, and measuring the depth and extent of fractures). In addition this non-destructive technique can be carried out in situ and in laboratory samples as well. In studies of cultural built heritage, sonic or ultrasonic transducers are used to transmit a pulse through a material in question. Two probes are used, one which emits and the other which receives ultrasonic pulses, transducer and receiver, respectively. From the measured travel time of pulse through the material and knowledge of the distance between two probes, the velocity can be calculated. Depending on the position of transducer and receiver, S and P-waves velocities can be measured. The ultrasonic velocities measured in a material mainly depend on its water content (degree of saturation), density (composition) and state of preservation. Commonly, the surveys are carried out with a 2D array, for instance, along longitudinal or transverse sections of columns or thin sheets, walls, etc. Sometimes many 2D tomographic profiles are arranged to construct a 3D model. The full 3D sonic or ultrasonic tomographies are especially devoted to the internal study of artifacts (e.g. Cosentino et al2011). 2.3. Electrical resistivity imaging The electrical resistivity imaging (ERI) technique is considered a relatively new geophysical method and of considerable current interest owing to its application to a wide range of geophysical problems (Dahlin 2001). As a non-invasive and inexpensive tool, this technique has been widely used for geological (e.g. Schwindt and Kneisel 2009), environmental (e.g. Bentley and Gharibi 2004), engineering (Almeida et al2001), archaeological (Papadopoulos et al2006, Tsokas et al2007, Compare et al2009) and recently, in studies of stone cultural heritage (e.g. Martinho et al2012). The method allows obtaining images of subsurface geological structures from electrical measurements made at the surface or inside boreholes. Electric imaging combines sounding and profiling, so it can give information about the resistivity distribution as a function of both depth and horizontal distance (Diamanti et al2005). Resistivity images are created by inverting hundreds to thousands of individual resistivity measurements (e.g. Loke and Barker 1996) to produce a graphical representation of the subsurface resistivity. Material’s resistivity is related to various parameters such as the mineral and fluid content, porosity and degree of water saturation. The resistivity measurements are made by injecting current into the ground through the two current electrodes and measuring the resulting voltage difference at two potential electrodes. An enormous number of electrode dispositions can be used in electrical resistivity imaging. Several commonly used array types are Wenner, Schlumberger, gradient, pole-dipole and dipole-dipole (Sharma 1997). Data in two-dimensional and three-dimensional forms can be obtained. The two-dimensional electrical resistivity image (ERI) can be achieved from the collection of the data along a profile, with continuously increasing inner-electrode spacing (pseudo-section) or as a series of successive vertical electrical soundings along a line. In practice, a number of electrodes with a given spacing between them are inserted into the ground along a line and various measurements are obtained for different electrode spacing. The result is an image of the subsurface based on the resistivity changes in the vertical and horizontal direction along the survey line (2D model). Nowadays the most common practice to obtain the three-dimensional resistivity variation of the subsurface is from parallel two-dimensional lines whose data are interpreted with two-dimensional inversion algorithms and the results combined to generate a quasi-three-dimensional image. Due to the cost and time involved, three-dimensional surveying is not yet routinely employed for near-surface applications. In the full three-dimensional surveys the pole-pole configuration is commonly used and the electrodes are normally arranged in a square or rectangular grid with the same unit electrode spacing in the x and y direction (Loke and Barker 1996). If the 3D survey is carried out with pole-dipole or dipole-dipole array, the distance between the lines can be larger about two or three times the inline unit electrode spacing (Dobrin and Savit 1988). Ideally, the data must be collected from a set of survey lines with measurements in the x-direction, followed by another series of lines in the y-direction. The use of measurements in two perpendicular directions helps to reduce any directional bias in the data. 2.4. Self-potential The SP method is passive, i.e. differences in natural ground potentials are measured between any two points on the ground surface. Self-potentials are generated by a number of natural sources, although the exact physical processes by which some are caused remain still unclear. The common factor among the various processes thought to be responsible for self-potentials is groundwater. Four main source mechanisms are known (Sharma 1997): (i) electrofiltration potential, (ii) thermoelectric potential, (iii) electrochemical potential and (iv) mineralization potential. SP signals in the field of cultural heritage can be related to redox reactions at the interfaces between pore-occluding mineral particles and interstitial humidity within stones and mortars (Cammarano et al2000). An important source of SP signals can also be the unsettling of the electric double layer across the walls of capillaries, cracks and fissures due to a naturally or artificially impressed movement of the electrolytic fluid saturating the voids in stone and muddy materials (Bogoslovsky and Ogilvy 1973). In recent years the SP method has found increasing use in geothermal, environmental and engineering applications to help locate and delineate sources associated with the movement of thermal fluids and groundwater. In the field of cultural built heritage, the SP method can be used, mainly, for determination of moisture content (Cammarano et al2000, Martinho et al2014). The measurement of self-potentials is performed by using two non-polarisable porous-pot electrodes connected to a precision multimeter with an input impedance greater than 108 ohms and capable of measuring to at least 1 mV (Telford et al1990). Two different electrode configurations can be used, namely the potential gradient and the potential amplitude method. The potential gradient method uses two electrodes, at a fixed separation, between which the potential difference measured is divided by the electrode separation to give a potential gradient (mV/V). The point to which this observation applies is the midpoint between the two electrodes. The potential amplitude method uses a stationary electrode fixed at a base station and measuring the potential difference (in mV) between it and the second one which is moved along the traverse. 3. The main geophysical investigations in cultural built heritage 3.1. Ground penetrating radar Geophysical methods have been used to archaeological research since 1950, but only recently have been applied to monuments and cultural artifacts. They are non-invasive and allow obtaining a mapping of different physical parameters of the geological materials. In the last decade GPR has been used to conduct many geophysical surveys on monumental heritage (Colla and Maierhofer 2000, Ranalli et al2004, Binda et al2005, Pieraccini et al2005, Leucci et al2007). One of the major targets of its application has been the historic buildings during remodeling/restoration processes (e.g. Binda et al2003b, Ranalli et al2004, Pérez-Gracia et al2009). The design of the structural rehabilitation of the historical and heritage buildings is a complex work because of the impacts of adaptive reuses that these buildings have been submitted, most of the times, along their history. Therefore, any repair or remodeling work needs the knowledge of the materials (old and new), construction techniques, as well as a careful analysis of their conservation level. The GPR provides an unique non-invasive means for retrieving information about thickness of the walls and of the degradation level, with particular reference to void, cracks and decay of the building materials (Ranalli et al2004), and localization and size of reinforcement bars (Pérez-Gracia et al2009, Masini et al2010). The interpretation of GPR data is one of the major challenges of this technique and depends on the quality of the data obtained. Therefore, ways of acquiring and the processing techniques used are crucial for a good interpretation. As can be seen below, different published studies in this field during the last decade show some of the techniques used for acquisition and processing in order to obtain a better interpretation. Ranalli et al (2004) used the GPR in the project of structural monitoring and restoration of the facade of the Collemaggio Basilica, a medieval church of 13th century, located in L’Aquila (central Italy), for evaluating the state of conservation of the facade, identifying the thickness of walls, internal masonry structure and location of detachments or cracks. The investigation was made with 600 MHz and 1600 MHz antennas to identify wall thickness and to detect the internal features of the masonry and the possible detachment of its ashlar facing from the rubble core, respectively. The processing of the data involved the application of two filters: the first for removing the effect of distortion due to the air-ground interface between the GPR antenna and ground (in this case instead of ground, the wall); the second for removing background noise in the vertical and horizontal directions. The study allowed to detect local or diffuse zones of ashlar facing instability including more degraded zones (from the high-frequency GPR antenna). Moreover it was showed, for the first time, a fairly variable wall thickness (from the medium-frequency GPR antenna). Pieraccini et al (2005) developed a high-frequency large-bandwidth synthetic aperture penetrating radar intended for the inspection of masonry walls. The system was conceived and designed for non-contact operation, in particular for allowing walls covered by paintings to be inspected avoiding damages to the surface artworks. This type system (non-contact) required some additional processing in data, in order to take into account that the electromagnetic signal propagates through two different media, from air to wall. The system was applied to an intrawall investigation in the Palazzo Vecchio in Florence in order to detect hollow spaces or embedded discontinuities in historic painted walls of the ‘Hall of 500’, where fragments of the famous fresco the ‘Battle of Anghiari’ by Leonardo da Vinci could be hidden. The radar signal was a Continuous-Wave Step-Frequency (CW-SF) waveform, sampling a 4 GHz bandwidth at 10 GHz center frequency, which provided relatively high resolution images of the investigated structures. The survey was performed in the eastern and western walls and allowed identify a discontinuity on the eastern wall that can be associated to a cavity able to preserve some fragments of the lost fresco. However, the authors concluded that only a destructive introspection can provide a definitive answer. Binda et al (2005) applied the georadar on the detection of the structural problems of the Bell Tower of Cremona (Italy). Three main investigations were made: (i) to find and define the extent of the detachment of the thin external leaf of the load bearing wall; (ii) to understand the morphology of an arch where the inspection of a scaffolding hole was suggesting that the external masonry leaf hides an arch with a shape different from the shape shown by the external brick work, and (iii) to detect the hidden structure of the concrete frame supporting the bells. The GPR surveys were performed with high frequency antennas (1.5 and 1.6 GHz). On the first investigation the data were acquired along many parallel profiles so that a 3D reconstruction of the external part of the wall was possible; on the second and third, orthogonal profiles were made. On the second and third investigations, the datasets were processed with 3D software; in the first, the radar profiles were pre-processed in 2D mode and after assembled in two 3D volumes, which were also submitted to a 3D processing procedure. The authors concluded that the use of the method in the detection of the defects, voids, inclusions and flaws, can be very successful when supported by a deep knowledge of the construction. The GPR was also used by Leucci et al (2007) in the structural monitoring of the columns inside the crypt of the ‘Cattedrale di Otranto’ (Lecce, Italy) presenting clear fractures and signals of deterioration. The survey was carried out with a georadar System 2 (SIR 2) and a 1000 MHz (centre frequency antennas) in a continuous mode. Two possible strategies for acquisition and data processing are discussed given the round shape of the columns: (1) a circular array of transmitting and receiving GPR sensors placed all around the columns; (2) check the consistence and similarity of the diagnostic results achieved by different measurement and data processing techniques. The authors have chosen the second approach that allowed them to exploit simpler measurement set-ups and data processing algorithms, thus obtaining results in a much cheaper and faster way. They used both a standard GPR processing (in a time domain) and a linear inverse scattering algorithm (in a frequency domain) in order to detect and achieve information on the damaged zones inside the columns. In this research, the linear inverse scattering approach was the most suited for the detection of local fractures compared to the classical time-domain processing. The GPR was also the technique chosen by Pérez-Gracia et al (2009) for studying a S. XV building (Marques de Llió palace, in Barcelona) that was remodeled several times. The data acquisition was performed with a SIR 3000 system equipped with a 1.6 MHz nominal centre frequency antenna. A 13 ns time window and 512 samples per trace were applied. Five points gain function was applied to the data to compensate the attenuation due to the geometrical spreading. Kirchhoff migration was applied in order to obtain accurate images of the metallic beams and to differentiate them from the other reflectors like arches and vaults. The final interpretation was carried out using invasive information (invasive drillings were performed in several points). The survey allowed determining the specific zones affected by the last remodeling done, the typology of the constructive elements and pieces, detecting some of the bars inside the beams, and detecting some oldest structural changes and the adopted solutions. A monitoring of the different constructive elements, typical of historical buildings (a wall, a masonry pillar and a marble column) was made by Masini et al (2010) with aim to detect cracks, characterize the masonry and imaging of metallic reinforcement bars. Two studies, one pillar and a masonry wall, were performed in the Cathedral of Matera (in Basilicata, Italy), and a third, involving the study of the columns, was made on the church of San Giovanni al Sepolcro (in Brindisi, Italy). In the pillar, the GPR prospecting was performed with a SIR3000 equipped with a 1000 MHz antenna whose band was about extended from 800 to 1600 MHz while on a masonry wall the SIR3000 system was equipped with an antenna at 1500 MHz. Data processing was related to the quality of the data obtained. In the pillar, the acquisition data was performed with a stacking on 64 acquisitions for each position. Aiming to confirm the discontinuities in a quite evident way, the authors calculated the arithmetic average of all the radar traces. The interpretation of the data was performed with the help of information derived by some core test performed in 1989 and allowed to know the internal structure of the pillar (interfaces between different materials and material discontinuities or cracks). Already on the altar masonry wall (Cathedral of Matera) due to the good quality of the data, the processing consisted only of the zero timing and a 2D average filtering. In columns, both scans along the columns and scan all around the columns were performed. The data were processed with a linear inverse scattering algorithm. Only results of the two columns were presented. The GPR results showed that one of the columns seems quite well preserved and does not show any particular internal anomaly while in the other it was possible to identify two reinforcement bars and a discontinuity probably ascribable to the residual trace of some restored fracture. With this study, the authors concluded that the obtained information from GPR survey can be relevant to stability issues, but also just to achieve a better knowledge of the history of the monument and its current internal state, to be accounted for in any further possible restoration intervention. Figure 1, shows the results of two GPR profiles (28, 29) performed in room 5 on the 1st floor of the Marques de Llió palace (Barcelona) (Pérez-Gracia et al2009), is an example of the GPR ability to study the constructive elements and pieces of historical buildings. Figure 1. Open in new tabDownload slide GPR profiles 28 and 29 in room 5. The main structure in this part is formed by great arches and columns. Over these arches, the floor is supported by cockloft slight wand. Reflections on the corners of these boxes are clearly observed in profile 28. (After Pérez-Gracia et al2009). Figure 1. Open in new tabDownload slide GPR profiles 28 and 29 in room 5. The main structure in this part is formed by great arches and columns. Over these arches, the floor is supported by cockloft slight wand. Reflections on the corners of these boxes are clearly observed in profile 28. (After Pérez-Gracia et al2009). Moisture is the main cause of most deterioration processes of materials, acting directly as a solvent or indirectly by carrying salts or favoring the growth of biodeterioration agents. Therefore, Cataldo et al (2005), in the analysis of the deterioration of the Crypt of the “Cattedrale di Otranto (Italy) used the GPR to confirm the possible rise of water from the pavement. The survey included non-destructive, different biological and physical (microclimatic and GPR) techniques because the Crypt was showing various forms of the decay stone, including mildew, efflorescence and moulds. The high values of relative humidity through the whole year suggested that an important rising of water from the pavement was possible. Then, to assess the presence of this phenomenon, the authors performed a GPR survey. A 500 MHz (central frequency) antenna was used and the processing steps involved horizontal scaling, background removal and Kirchhoff migration. The GPR survey allowed to conclude that degree of moisture in the subsoil of the Crypt was relatively high and that played an important role in the degrade processes observed on the stone columns (marble, granite and breccias) and on the frescos inside the Crypt. A similar study was performed by Cataldo et al (2013) in the Crypt of the Duomo of Lecce (Italy) affected by salt damage on the masonry and columns. The GPR surveys were carried out with a 270 MHz antenna aiming to localize potential sources of moisture, involved in the damage. The authors concluded that the microclimatic conditions and the presence of sources of moisture in the subsoil of the Crypt enhanced salt crystallization. 3.2. Integrated GPR and sonic tomography The above mentioned studies show that the GPR can be used for the purpose of solving specific diagnostic and /or monitoring problems regarding historical buildings studies. Despite the radar equipment that is available for the inspections, still arise some problems. The dimension and shape of the antennas is still a problem especially on rough surfaces and small artifacts like columns, pottery, sculptures, etc. In these cases, the GPR does not allow up to now a resolution similar to that of ultrasonic and ERT tomographies (Cosentino et al2011). Therefore, GPR surveys are generally integrated with other analytical or non-destructive techniques (e.g. ultrasonic tomography), substantially improving the obtained results for successful assessment and diagnosis of the structural problems associated with cultural built heritage. One of these cases was performed by Cosentino et al (2011) that used different methodologies (GPR and ultrasonic tomography) to investigate the physical consistency (the internal extension of all the visible fractures and to search for the hidden ones) of a marble slab of the II-III century AD of the archaeological Museum of Rome. The radar tomography obtained with a 2 GHz bipolar antenna, showed a strong correlation with observable cracks in the slab and with ultrasonic tomography. Figure 2 shows an example of the result of the integrated application of the GPR and ultrasonic tomographies on a marble slab (Cosentino et al2011). A similar investigation was carried out by Sambuelli et al (2011) on the base of the sculpture of the Pharaoh with god Amun (piece of the ‘Museo delle Antichitá Egizie’ of Turin) in order to estimate the persistence of the visible fractures, to look for unknown ones and to provide information about the overall mechanical strength of the base (limestone prism). The GPR was used to detect and locate hidden fractures and give information on their persistence while the ultrasonic tomography was used to evaluate the average mechanical properties distribution within the object. The GPR measurements were acquired with two different antennas (1500 and 1000 MHz). The measurements on the base of the sculpture were performed with the 1500 MHz antenna while the electromagnetic wave velocity in the limestone was evaluated with the 1500 and 1000 MHz antennas. The authors concluded that although the full reconstruction of the spatial arrangement of a network of fractures (such as those present in the sculpture) to be nearly impossible, indications on critical weaknesses can be drawn using at least these two techniques. Integrated GPR and sonic tomography were also used by Leucci et al (2011) on a diagnostics survey to characterize the deterioration level of the pillars (cracks, inhomogeneities in the inner structure of masonry building elements and quality) of the cathedral of Tricarico (Southern Italy). The results were compared with those obtained by direct inspection from one of the investigated pillars (coring the pillar and examining both the core and the hole) and showed their capabilities to detect the inner features of building elements. Sonic and radar tests were also applied on remaining walls and piers of the Cathedral of Noto (Binda et al2003a) whose right aisle and most of the dome collapsed in 1996. The aim was to verify the state of damage and the possibility of conservation of the walls and piers in view of the reconstruction of the damaged part of the Cathedral. The GPR survey was performed with 500 MHz antenna after tests carried out with the 900 MHz antenna that shown that this did not provide enough penetration to investigate all the pier sections because of the pier dimensions (height near 6 m and horizontal section approximately 3  ×  2 m) and the high moisture content. The radar test allowed the control of the lack of continuity of the stone leaf in the high part of the pier (first strong reflection) but not in the lower part due to high moisture content (attenuated reflection). The sonic tests allowed detecting the variation in the materials of the external layers of the piers and walls and evaluate the state of masonry. These tests also allowed evaluation of the effectiveness of grout injection; they allowed control the distribution of the grout in the masonry. Figure 2. Open in new tabDownload slide Ultrasonic (a) and GPR (b) tomographies superimposed to slab image. (After Cosentino et al2011). Figure 2. Open in new tabDownload slide Ultrasonic (a) and GPR (b) tomographies superimposed to slab image. (After Cosentino et al2011). 3.3. Sonic and ultrasonic technique Despite the integrated prospecting be the most recommended for a good structural diagnosis of historical monuments, ultrasonic measurements, only, have been used both in-situ and also in laboratory to characterize building materials, to measure the extent of decay (below the surface) and also to control conservation procedures/evaluate the performance of materials that have been treated with a consolidant. Many articles using ultrasonic techniques are consequently presented in conference publications. The testing of stone with ultrasonic wave measurements started early in the history of stone conservation. The testing was developed in the 1950s by the conservation scientist Mamillan in Paris. Today it is one of the most used NDT methods in stone conservation. In this paper a special attention will be given to the application of ultrasonic technique in-situ for evaluation of building materials degradation degree and extent. Galan et al (1992) used ultrasonic pulsed on marbles columns of the Court of the Lions at the Alhambra (Granada, Spain) and using this methodology (direct method) it was possible to group them into five types depending on the range velocity values. Moreover the ultrasonic indirect method was also used to obtain the form and thickness of the degradation crusts that were observed on the surface layer of the columns. These authors concluded that ultrasonic measurements were in agreement with the macroscopic assessment. Montoto et al (1996) have used ultrasonic tomography to investigate the internal deterioration of Axeito megaliths (NE, Spain). The technique was useful for determining the position of internal fissures but was less reliable at assessing the condition of stone immediately below the surface. Galan et al (2000) also used ultrasound velocity to evaluate stone decay at the Columbus monument in Huelva (SW Spain). The indirect measurements taken in the monument do not revealed any relationship between the variation of ultrasound transmission and the different orientations and/or heights. Siegesmund et al (2000) used ultrasonic tomography on a column at Marmopalais Postdam (Germany) and the results were cross checked by detailed mapping of macroscopically visible structures. Moreover laboratory velocity data and their anisotropy were the basic input for modeling the synthetic tomogram and concluded that anisotropy significantly influences the tomogram and must be taken into consideration when interpreting velocity data, together with the rock composition and rock fabric. Recheis et al (2000) used ultrasonic measurements on two marble portals of the Sloss Tirol to obtain information on weathering state and correlated differences in the ultrasonic values with differences in the outdoor exposure conditions that the portals were submitted to. Cardarelli and Nardis (2001) successfully applied the seismic method (seismic refraction and seismic transmission tomography) to investigate three marble columns of the Pronaos of the Antonino and Faustina temple (AD 141) in Rome, Italy. The correlation between the methods enabled highlights the characteristics of the marble (anisotropy, depth of the weathering, fractures and small cracks). Fais et al (2002) used combination of ultrasonic and seismic techniques and were able to detect shallow altered areas within the masonry and to define the thickness of the alteration of palazzo Regio within the fortifications of the ‘castello’ in the historical city of Cagliari (Italy). Zezza (2002) used ultrasonic methods, with homeosurface measurements, to control weathering degree of the stone support of the paints in Altamira cave (Spain). It was thus possible to determine number and thickness of the decayed layers and to correlate those results with variations of environmental microclimatic conditions inside the cave. Fort and Alvarez de Buergo (2012) used ultrasonic measurements for detection of fissures inside the ashlars non visible at the stone surface occurring at architrave of the entablature of the 4 façades of the Madrid’s Royal Palace (Spain). Ultrasonic 2D tomography was carried out along the left arm of the Venus statue (Cosentino et al2009), a marble statue of human height. The results showed low velocity zones, typical of the degradation of the marble. Pamplona et al (2010) studied 25 Carrara marble sculptures with ultrasonic pulse velocity testing to sustain a conservation plan. Measured values were converted into damage classes, which were mapped on the front and back side views of each sculpture. This diagnosis study allowed to quantify weathering and gave input to the conservation team, who can establish sustained priorities for the conservation plan. Faella et al (2012) used an integrated approach, incorporating sonic and ultrasonic pulse velocity measurements, to evaluate the masonry structures of Church of the Nativity in Bethlehem (Palestine). Pulse velocity testing, surface penetrating radar testing and infrared thermography allowed qualifying the masonry and to locate and quantify degraded or deteriorated areas. Bromblet et al (2012) revisited the columns of the church Saint Trophime of Arles (France) that were first studied thorough ultrasonic investigation in 1993 in order to provide an update of their condition and to get information on the evolution of their degradation over 16 years. The analysis of the results allowed to classify the columns into damage classes and to compare ultrasonic velocities between 1993 and 2009. Over 16 years the degradation has significantly changed for many columns distributed in different parts of the cloister. This work showed how the ultrasonic velocity measurements and its ability to quantify physico-mechanical conditions for natural stone, allow for a long-term, reliable monitoring of change, degradation or consolidation by conservation intervention. Pamplona and Simon (2012) performed an ultrasonic investigation (direct transmission method and ultrasonic computer tomography) on outdoor marble sculptures located in Munich (Germany). Through this non-destructive technique was possible to quantify physico-mechanical conditions of natural stones applied, and also reliable monitoring of change, degradation or consolidation by past conservation interventions. Ruedrich et al (2013) used ultrasonic wave velocities together with rock strength measurements to assess the stability of marble statuaries of the Schlossbrücke (Berlin, Germany). The strength properties together with the internal stress calculations revealed that some mechanical critical parts of the statues should be replaced. Capizzi et al (2013) used high resolution 3D ultrasonic tomography to study the structural continuity of the marble of the bust of Eleonora d’Aragona (by F. Laurana, 1468). The resulting velocities model showed that marble is sufficiently homogeneous and is well-preserved with exception of the lower front portion of the trunk at the breasts where the model shows low values. The result of this study is presented as an example in figure 3. Figure 3. Open in new tabDownload slide Frontal 3D image with transparency of the investigated volume (right) showing an area with low velocity, corresponding to the support point of the bust. (After Capizzi et al2013). Figure 3. Open in new tabDownload slide Frontal 3D image with transparency of the investigated volume (right) showing an area with low velocity, corresponding to the support point of the bust. (After Capizzi et al2013). Ultrasonic testing has also been found useful for addressing the evaluation of stone treatments. The impregnation depth is a key factor for the assessment of consolidation treatment efficiency. Ultrasonic testing has been used as an non-destructive tool, for monitoring penetration depth (Villegas et al1994, Simon and Lind 1999, Favaro et al2006, Favaro et al2007, López-Arce et al2010, El-Gohary 2013, Moropoulou et al2013). 3.4. Electrical methods (electric resistivity tomography and self-potential) The above mentioned works show that the GPR and sonic /ultrasonic methods are perhaps the most known and commonly used non-invasive techniques to map the internal geometry and degradation state of cultural heritage, including historical buildings, and check the restoration quality when consolidation treatments are being employed. Nevertheless, the employment of the GPR becomes useless when materials are saturated with water, mainly salt-water, as the penetration depth of the electromagnetic waves is dramatically reduced due to the high electrical conductivity. The ultrasonic tomography requires that opposite sides are accessible to allow efficient and cost-effective mapping of velocity variation. When only one surface is accessible, the ultrasonic tomography employment becomes very cumbersome, since holes have to be drilled to host both acoustic sources and receivers. In this case the method becomes invasive. Moreover, these methods cannot be used when the evaluation is to be done underwater. Another method that also allows to evaluate the state of degradation of historical buildings and cultural artifacts, and that can be used under the condition referred is the resistivity method in tomographic modality (Electric Resistivity Tomography, ERT). This method is perhaps the most suitable technique for the exploration of these structures and for determining spatial moisture distribution in stone or masonry monuments, since neither the quality of the collected data, nor the depth of investigation and resolution of the estimated models of resistivity distribution are reduced by the highly conductive environment. Furthermore, it is only needed an accessible side. For some time the resistivity measurements have been used to monitor the degradation state of historic buildings walls. In 1993, Haelterman et al (1993) initiated a research program aiming to evaluate the capability of the electrical resistivity method to analyse of the internal structure of the masonry and the changes induced by grout injection on works consolidation of historic masonry. As the technique cannot be used without adaptations, they started studying the influence of the geometrical boundaries, electrode configuration and moisture content in the measurements. They concluded that the method could become a good tool for the design and control of the injection technique. Similarly, Van Gemert et al (1996) and Venderickx and Gemert (2000) developed computer programs where the effect of the geometrical boundaries was incorporated, so that the resistivity maps were a picture of the real anomalies in the structure. They used these data processing techniques to visualize the internal structure of a thin wall surrounding the park of the Castle of Arenberg in Heverlee (Belgium) and the walls of the building ‘Duke’s Mills (13th Century) at Aarschot (Belgium). Schueremans et al (2003) used resistivity measurements for monitoring crack development and sealing done by cement grout injection on an experimental wall. Resistivity measurements were done in the sound, cracked and repaired situation. Relative difference mapping of the resistivity values showed a clear picture of the internal situation in the masonry, showing the applicability of the method in this field. Moisture is the main cause of most deterioration processes, acting directly as a solvent or indirectly by carrying salts or favoring the growth of biodeterioration agents. Knowledge of moisture content, distribution and temporal variations in porous materials is critical to better understanding material decay in cultural heritage sites and objects. As the moisture content is one of the factors that influence the resistivity, Sass and Viles (2006) successfully used the 2D electrical resistivity method to investigate the moisture content in the walls at two abbey ruins. With aim to investigate the degradation state of the brick walls along a canal located in Venice (Italy), an electrical resistivity tomography (ERT) test was performed above and below m.s.l. along a portion of a canal’s wall (Abu-Zeid and Santarato 2006). From the results was possible to recognize the degradation state where the visual inspection was impossible, which was useful for the restoration project. Similar work was performed by Abu-Zeid et al (2010) on selected wall portions of the historical church of Montepetriolo, Perugia, Central Italy. 3D resistivity distribution models were obtained before and after grouting in order to assess the possibility of achieving a quantitative estimate of the injected mortar volumes to increase the mechanical resistance of the studied wall portions. In the most damaged marble column of the Crypt of the Cathedral of Otranto, Leucci et al (2007) used ERT to study the presence of fractures within the column. As mentioned above, the columns of the crypt were also investigated with GPR. The authors concluded that although both techniques are in good agreement, 2D electrical resistivity tomography seems to be less resolute than GPR because the results show a longitudinal fracture larger than its real dimension. However, the ERT results show clearly the wet zones of the column, coincident with the visible phenomena of biodeterioration, decomposition and loosening of the tight fabric of the mineral. Viles et al (2008), Sass and Viles (2010a, 2010b) again used 2D resistivity to provide information regarding moisture content in historical masonry walls in Oxford, England. Their work has proved electrical resistivity imaging (ERI) to be a non-destructive, inexpensive and effective tool for assessing moisture in stone monuments. The results gave preliminary confirmation of a simple model of catastrophic decay and illustrated the complexity of moisture regimes in historic walls. Lehmann and Krüger (2011) used impedance measurements to monitor the capillary transport of brine in sandstone samples. The experiment was carried out in laboratory aiming to study the effects of water and brine flow on the electrical impedance. The salts mainly influenced the impedance during the drying phase in the rate of increase of the impedance. Full 3D electrical resistivity imaging was used by Martinho et al (2012) to obtain the moisture distribution patterns in a stone cultural heritage (three magnificient limestone bas-reliefs with scenes from the Passion of Christ, created by the sculptor Nicolau Chanterenne). Since resistivity readings are heavily influenced by the presence of soluble salts, the presence and spatial distribution of salts in the panels were also identified and classified. The results showed the value of the full 3D ERI technique for the study of the moisture distribution in stone cultural heritage monuments, even when salts are present. The moisture distribution patterns pointed to the existence of more than one source of moisture, which was consistent with data on water-soluble salts that suggested multiple saline sources contributing to the salt weathering on the panels analyzed. The lower resistivity values observed in the panels also indicated severe decay. As an example of the resolution of the method in defining patterns of moisture distribution, presents in figure 4 the resistivity tomogram obtained in one of the panels. Another method that has not been very often used in cultural heritage but that also enables to detect the presence of water in a porous media, is the self-potential (SP) method. Cammarano et al (2000) carried out successfully a detailed microgeophysical surveys with the SP, resistivity and radar methods on the Aksum obelisk (Rome, Italy) for assessment of its state of conservation (SP and resistivity) and to try to individuate the exact position of the metallic pivots (radar) that were used to reassemble the five different blocks forming the obelisk. Martinho et al (2014) used the SP and ultrasonic methods (refraction and transmission) combined with IR thermography and identification of decay products and quantification of soluble salts in a fifteen century artistic tomb made of a porous limestone that presents severe decay phenomena. The SP results against temperature and salts distribution allowed to detect the areas of the tomb where the moisture content is higher (low temperatures and negative SP values) and the mechanism of salts diffusion (the salts are diffused from zones where the concentration is higher for the zones where the concentration is lower resulting in potential diffusion). The ultrasonic results allowed relating the strength of the stone with the moisture content (least strength where the moisture content is higher). Figure 5 shows the results of the integrated application of SP and ultrasonic techniques in that tomb. Figure 4. Open in new tabDownload slide (a) Descent from the Cross panel; (b) Resistivity tomogram obtained in Descent from the Cross panel. (After Dionisio et al2011). Figure 4. Open in new tabDownload slide (a) Descent from the Cross panel; (b) Resistivity tomogram obtained in Descent from the Cross panel. (After Dionisio et al2011). Figure 5. Open in new tabDownload slide Distribution of SP, Vs and Vp values and location of Versus seismic refraction profiles on all faces of the tomb. (a) View of northern and eastern faces; (b) View of southern and western faces. (After Mendes et al2014). Figure 5. Open in new tabDownload slide Distribution of SP, Vs and Vp values and location of Versus seismic refraction profiles on all faces of the tomb. (a) View of northern and eastern faces; (b) View of southern and western faces. (After Mendes et al2014). 4. Targets The application of non-invasive surface geophysical techniques in the investigation of degradation state of cultural heritage is an expanding field. As described above some geophysical methods are being widely applied not only to map the internal geometry and degradation state of cultural artifacts and historical buildings but also to check the restoration/conservation quality when consolidation treatments is being employed. Table 1 lists some of the most important cultural heritage problems/questions and classifies the applicability of geophysical techniques in three categories: applicable (the technique is appropriate for the problem investigation), limited applicability (the application of the technique is limited by the dimension, geometry and roughness of the artifacts) and not applicable (the technique is not appropriate for the problem investigation). From table 1 one can see, depending on the target involved and geometry and dimension of the elements considered, one or more geophysical methods can be considered as evaluation tools. These geophysical methods provide either qualitative or quantitative information on materials, deterioration patterns (the visible consequences of the impact of environment factors on the objects) and in specific cases, the decay factors (extrinsic factors, e.g. moisture content, that contribute to degradation of the materials). Table 1. Important targets of cultural heritage investigations by geophysical techniques. . Ancient monuments masonry walls, masonry pillars, stone columns . Cultural artifacts pottery, statues, bas-reliefs, friezes, mural painting . . . Preservation state . . . . . Methods . Thickness of the walls . Location of voids or cracks . Decay of materials . Location and size of reinforcement bars . Moisture content . Evaluation of repair interventions . Location of detachments, voids or cracks . Decay of building materials . Moisture content . Evaluation of consolidation treatments . Ground penetrating radar (GPR) + + – + + + ⚪ – ⚪ ⚪ Seismic methods (sonic and ultrasonic tomography) – + + – – + + + – + Electrical resistivity tomography (ERT) + ⚪ + – + + ⚪ ⚪ ⚪ ⚪ Self-potential (SP) – – – – + – – – + – . Ancient monuments masonry walls, masonry pillars, stone columns . Cultural artifacts pottery, statues, bas-reliefs, friezes, mural painting . . . Preservation state . . . . . Methods . Thickness of the walls . Location of voids or cracks . Decay of materials . Location and size of reinforcement bars . Moisture content . Evaluation of repair interventions . Location of detachments, voids or cracks . Decay of building materials . Moisture content . Evaluation of consolidation treatments . Ground penetrating radar (GPR) + + – + + + ⚪ – ⚪ ⚪ Seismic methods (sonic and ultrasonic tomography) – + + – – + + + – + Electrical resistivity tomography (ERT) + ⚪ + – + + ⚪ ⚪ ⚪ ⚪ Self-potential (SP) – – – – + – – – + – + applicable: the technique is appropriate for the problem investigation. limited applicability: the application of the technique is limited by the dimension, geometry and roughness of the artifacts. not applicable: the technique is not appropriate for the problem investigation. Open in new tab Table 1. Important targets of cultural heritage investigations by geophysical techniques. . Ancient monuments masonry walls, masonry pillars, stone columns . Cultural artifacts pottery, statues, bas-reliefs, friezes, mural painting . . . Preservation state . . . . . Methods . Thickness of the walls . Location of voids or cracks . Decay of materials . Location and size of reinforcement bars . Moisture content . Evaluation of repair interventions . Location of detachments, voids or cracks . Decay of building materials . Moisture content . Evaluation of consolidation treatments . Ground penetrating radar (GPR) + + – + + + ⚪ – ⚪ ⚪ Seismic methods (sonic and ultrasonic tomography) – + + – – + + + – + Electrical resistivity tomography (ERT) + ⚪ + – + + ⚪ ⚪ ⚪ ⚪ Self-potential (SP) – – – – + – – – + – . Ancient monuments masonry walls, masonry pillars, stone columns . Cultural artifacts pottery, statues, bas-reliefs, friezes, mural painting . . . Preservation state . . . . . Methods . Thickness of the walls . Location of voids or cracks . Decay of materials . Location and size of reinforcement bars . Moisture content . Evaluation of repair interventions . Location of detachments, voids or cracks . Decay of building materials . Moisture content . Evaluation of consolidation treatments . Ground penetrating radar (GPR) + + – + + + ⚪ – ⚪ ⚪ Seismic methods (sonic and ultrasonic tomography) – + + – – + + + – + Electrical resistivity tomography (ERT) + ⚪ + – + + ⚪ ⚪ ⚪ ⚪ Self-potential (SP) – – – – + – – – + – + applicable: the technique is appropriate for the problem investigation. limited applicability: the application of the technique is limited by the dimension, geometry and roughness of the artifacts. not applicable: the technique is not appropriate for the problem investigation. Open in new tab In what concerns moisture evaluation the most prominent methods are those based on the electrical properties of the materials, i.e. Self potential (SP) and the electrical resistivity tomography (ERT). The former together with Ground Penetration Radar are useful tools in the estimation of wall thickness as well as for location of detachments, voids or cracks. Ground penetrating radar (GPR), Seismic methods (sonic and ultrasonic tomography) and Electrical resistivity tomography (ERT) are important diagnostic tools when decay consists of a loss of cohesion within the applied material, or the development of detached layers, blisters, or internal voids. Regarding cultural artifacts, the limited applicability of the georadar in locating voids and cracks is due to presence of irregular surfaces on majority of this type of objects, and the lack of suitable antennas to operate in such surfaces. Regarding ERT, limitation is related mainly with the limited dimensions and geometry of the artifacts (influences of geometrical boundaries). 5. Major advances and significant gaps The geophysics applied to the study of cultural heritage, also known as microgeophysics, involves the adaptation of several methodologies derived from geophysics. The main adaptation lies in the more or less miniaturization of geophysical instruments, especially transducers (both transmitters and receivers), so that they can be used in small volumes of material. There are some important differences regarding the methodology to be used, i.e. potential fields (mainly the electric fields) or wave fields, either mechanical or electromagnetic. In what concerns transducers, different types were tried with success namely, the self-adhesive medical electrodes (e.g. Viles et al2008) and non-polarizable Cu/CuSO4 miniaturized electrodes (e.g. Martinho et al2012) or medical Ag/AgCl (e.g. Bavusi et al2012), that are non-invasive, for using electrical fields. As can be seen by the mentioned above works, the use of the electromagnetic waves fields (georadar) has led to the development, by the geophysical companies, of the small size antennas that operate at higher frequencies. These companies have tried to establish a compromise between resolution and penetration depth. Higher frequencies (increasing the resolution) imply smaller penetration depth; on the contrary, if lower frequencies are used to obtain a higher penetration depth, diffracted waves can be used to study small targets but their analysis is complicated and represents an emerging filed of microgeophysics (Cosentino et al2009). Regarding data acquisition techniques, great advances have been recorded with different types of research to be implemented, often derived from others fields of study, for instance, the tomographic techniques (2D and 3D), which are derived from medical diagnostics. These methodologies, that can be used with both potential and wave fields, allow a more detailed resolution of the investigated structures but require the acquisition of a large amount of data, so multi-channel instruments are generally required. Multi- channel systems already exist but in some cases, as for example in the sonic and ultrasonic tomography, the transducers are not suitable for very small scale studies, such as in laboratory studies involving small sample sizes. Inversion software that allows the 2D and 3D complete inversion procedure for interpretation data already exists, although commercial software modules for 3D radar analysis are not flexible enough to import data affected by some irregularities (geometry of targets, inhomogeneity of targets), which means that in some cases are the researchers that prepare software for the acquisition of specific geometries. In the case of the GPR, which the quality data is one of the most important aspects for a good interpretation, different ways of acquisition data and processing techniques have been tried. In some cases, a comparison was made between the results obtained from different processing techniques. However, there is still the need to improve the resolution of the surveys in order to obtain more accurate information especially in surveys of very small scale (about columns, statues, etc.). The size and shape of the antennas is still a problem in the study of rough surfaces or small artifacts. Compared to methods of GPR and ultrasonic tomography, the electrical methods are still little used and the developed investigations were made, mainly, to assess the response of the methods in the determination of moisture distribution patterns and their source. However, the works performed, for example, by Van Gemert et al (1996), Abu-Zeid and Santarato (2006) and Abu-Zeid et al (2009) show that the resistivity method may also be used to monitor the degradation state of building walls and the changes induced by the grouting. Therefore, further research should be done in this type of targets in order to optimize the acquisition and interpretation data. Nuclear magnetic resonance (NMR) is another geophysical method that can be used to determine the moisture content of the artifacts (the method only works if some water is present). However, this methodology is still under study, at least as far as the previously mentioned purpose is concerned, because suitable instruments have not been created by geophysical companies. 6. Future directions The geophysical techniques applied to study of cultural heritage must be refined in order to obtain investigation tools that can be applied and interpreted as best as possible. Future developments should include the creation of appropriate instruments and software for certain types of studies. Multi-channel GPR systems should be developed in order to obtain three-dimensional images with high precision. State-of-the-art processing software can also improve the raw data obtained and give GPR a 3D imaging capability. This is crucial in the field of cultural built heritage conservation, since structural information is often very limited and the historic materials vary greatly having so, different properties (dielectric constant, etc.) that in general are largely unknown. The use of GPR requires a compromise between the resolution and penetration depth. In the case of using low frequencies (higher penetration depth), diffracted waves can be used to study small targets. However, the analysis of diffracted waves is complicated and, therefore, it also represents an emerging field of geophysics applied to cultural built heritage conservation. Antennas that allow for obtaining data on rough surfaces and small objects should also be created. Transducers of reduced dimensions for performing sonic or ultrasonic tomography must be developed, either for laboratory studies (very small size laboratory samples) or case studies (small dimension artifacts and/or very irregular surfaces). The laboratory investigations are necessary to predict the behavior of materials in specific situations and to assess the accuracy of the method. The study of cultural artifacts of small size requires a resolution increasingly higher of the techniques. The NMR technique must be studied and suitable instruments of NMR must be created for testing the method in assessing the moisture content. Another important aspect is that new instrumentation must be as cheap as possible either in what concerns construction and also operational costs (reducing the time required to conduct research) so that the application of geophysical methods can be expanded. The performed analysis shows that electrical methods (ERT and SP) are the less used in studies of cultural heritage despite the good results obtained mainly, in the determination of patterns moisture distribution and their sources. As future development, should be considered the use the ERT technique for locating detachments, voids and cracks, analysis of the internal structure of the materials (masonry or stone) and changes induced by the injection of consolidation materials. Given the resistivity contrasts, is expected a good resolution of the technique in this type of the targets. The results already obtained with SP method are an incentive to the use of the method, especially in the monitoring of moisture content. References Abad R , García G , Abad R , Blanco R , Conesa M , Marco B , Lladró C . , 2007 Non-destructive assessment of a buried rainwater cistern at the Carthusian Monastery Vall de Crist (Spain, 14th century) derived by microgravimetric 2D modeling , J. Cultur. Heritage , vol. 8 (pg. 197 - 201 ) 10.1016/j.culher.2006.10.009 Google Scholar Crossref Search ADS WorldCat Crossref Abu-Zeid N , Santarato G . , 2006 Non-invasive imaging of ancient foundations status in venice (italy) using the electrical resistivity tomography technique. heritage, weathering and conservation – fort , Alvarez de Buergo Gomez-Heras , Vasquez-Calvo . London Taylor and Francis Group ISBN 0-415-41272-2 Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Abu Zeid N , Balducci M , Bartocci F , Regni R , Santarato G . , 2010 Indirect estimation of injected mortar volume in historical walls using the electrical resistivity tomography , J. Cultur. Heritage , vol. 11 (pg. 220 - 227 ) 10.1016/j.culher.2009.07.001 Google Scholar Crossref Search ADS WorldCat Crossref Almeida F , Carminé P , Gonçalves L , Daniel L . , 2001 Odd-Even Pole- Pole spread electrode commutation in resistivity - 2D cross borehole and 3D surface imaging (Porto/Portugal underground tunnelling case studies) Proceedings –Extended Abstracts - 7th Meeting of the Environmental and Engineering Geophysical Society (European Section) Birmingham England EGSI01 (pg. 50 - 51 ) Bavusi M , Loperte A , Soldovieri F . , 2012 A low cost ERT prototype in the Cultural Heritage monitoring , Geophys. Res. Abstracts 14 EGU2012-10064 OpenURL Placeholder Text WorldCat Benson K . , 1995 Applications of ground penetrating radar in assessing some geological hazards: examples of groundwater contamination, faults, cavities , J. Appl. Geophys , vol. 33 (pg. 177 - 193 ) 10.1016/0926-9851(95)90040-3 Google Scholar Crossref Search ADS WorldCat Crossref Bentley L R , Gharibi M . , 2004 Two-and three-dimensional electrical resistivity imaging at a heterogeneous remediation site , Geophysics , vol. 69 (pg. 674 - 680 ) 10.1190/1.1759453 Google Scholar Crossref Search ADS WorldCat Crossref Binda L , Saisi A , Tiraboschi C , Valle S , Colla C , Forde M . , 2003 Application of sonic and radar tests on the piers and walls of the cathedral of noto , Constr. Build. Mater. , vol. 17 (pg. 613 - 627 ) 10.1016/S0950-0618(03)00056-4 Google Scholar Crossref Search ADS WorldCat Crossref Binda L , Saisi A , Zanzi L . , 2003b Sonic tomography and flat jack tests as complementary investigation procedures for the stone pillars of the temple of S.Nicolo’ L’Arena (Italy) , NDT&E Int , vol. 36 (pg. 215 - 227 ) 10.1016/S0963-8695(02)00066-X Google Scholar Crossref Search ADS WorldCat Crossref Binda L , Zanzi L , Lualdi M , Condoleo P . , 2005 The use of georadar to assess damage to a masonry bell tower in cremona , Italy NDT&E Int. , vol. 38 (pg. 171 - 179 ) Google Scholar Crossref Search ADS WorldCat Binda L , Cardani G , Zanzi L . , 2010 Nondestructive testing evaluation of drying process in flooded full-scale masonry walls asce , J. Perform. Constr. Fac. , vol. 24 (pg. 473 - 483 ) 10.1061/(ASCE)CF.1943-5509.0000097 Google Scholar Crossref Search ADS WorldCat Crossref Binda L , Lualdi M , Saisi A , Zanzi L . , 2011 Radar investigation as a complementary tool for the diagnosis of historic masonry buildings , Int. J. Mater. Struct. Integrity , vol. 5 (pg. 1 - 25 ) Google Scholar Crossref Search ADS WorldCat Bogoslovsky V V , Ogilvy A A . , 1973 Deformation of natural electric field near drainage structures , Geophys. Prospect , vol. 21 (pg. 716 - 723 ) 10.1111/j.1365-2478.1973.tb00053.x Google Scholar Crossref Search ADS WorldCat Crossref Bromblet P , Vergès-Belmin V , Simon S . , 2012 Ultrasonic velocity measurements for the long-term monitoring of the degradation of marble columns in the cloister of the church of saint-trophime in arles (France) Proc. of 12th Int. Congress on the Deterioration and Conservation of Stone New York USA Columbia University (in press) Cammarano F , Di Fiore B , Mauriello P , Patella D . , 2000 Examples of application of electrical tomographies and radar profiling to cultural heritage , Annali di Geofisica , vol. 43 (pg. 309 - 324 ) OpenURL Placeholder Text WorldCat Capitani D , Di Tullio V , Proietti N . , 2012 Nuclear magnetic resonance to characterize and monitor cultural heritage , Prog. Nucl. Mag. Res. Sp. , vol. 64 (pg. 29 - 69 ) 10.1016/j.pnmrs.2011.11.001 Google Scholar Crossref Search ADS WorldCat Crossref Capizzi P , Cosentino P L , Schiavone S . , 2013 Some tests of 3D ultrasonic traveltime tomography on the Eleonora d’Aragona statue (F Laurana, 1468) , J. Appl. Geophys. , vol. 91 (pg. 14 - 20 ) 10.1016/j.jappgeo.2013.01.012 Google Scholar Crossref Search ADS WorldCat Crossref Cardarelli E , Nardis R . , 2001 Seismic refraction, isotropic anisotropic seismic tomography on an ancient monument (Antonino and Faustina temple AD 141) , Geophys. Prospect , vol. 49 (pg. 228 - 240 ) 10.1046/j.1365-2478.2001.00251.x Google Scholar Crossref Search ADS WorldCat Crossref Castellaro S , Imposa S , Barone F , Chiavetta F , Gresta S , Mulargia F . , 2008 Georadar and passive seismic survey in the roman amphitheatre of catania (Sicily) , J. Cultur. Heritage , vol. 9 (pg. 357 - 366 ) 10.1016/j.culher.2008.03.004 Google Scholar Crossref Search ADS WorldCat Crossref Cataldo R , D’Agostino D , Leucci G . , 2013 Microclimatic and Ground-Penetrating Radar surveys for damage diagnosis , The case of the Crypt of the Duomo of Lecce (Italy) Science and Technology for the Conservation of Cultural Heritage London, UK Taylor & Francis Group ISBN: 978-1-138-00009-4 (pg. 51 - 54 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Cataldo R , Donno A , Nunzio G , Leucci G , Nuzzo L , Siviero S . , 2005 Integrated methods for analysis of deterioration of cultural heritage: the crypt of cattedrale di otranto , J. Cultur. Heritage , vol. 6 (pg. 29 - 38 ) 10.1016/j.culher.2004.05.004 Google Scholar Crossref Search ADS WorldCat Crossref Colla C , Maierhofer C . , 2000 Investigation of historic masonry via radar reflection and tomography In Eighth Int. Conf. on Ground Penetrating Radar Australia Gold Coast (pg. 893 - 898 ) Compare V , Cozzolino M , Mauriello P , Patella D . , 2009 Three-dimensional resistivity probability tomography at the prehistoric site of Grotta Reali Molise, Italy , Archaeol. Prospect. , vol. 16 (pg. 53 - 63 ) 10.1002/arp.347 Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Crossref Conyers L , Leckebusch J . , 2010 Geophysical archaeology research agendas for the future: Some ground-penetrating radar examples , Archaeol. Prospect. , vol. 17 (pg. 117 - 123 ) OpenURL Placeholder Text WorldCat Cosentino P L , Capizzi P , Fiandaca G , Martorana R , Messina P . , 2009 Advances in micro geophysics for engineering and cultural heritage , J. Earth. Sci. , vol. 20 (pg. 626 - 639 ) 10.1007/s12583-009-0052-x Google Scholar Crossref Search ADS WorldCat Crossref Cosentino P L , Capizzi P , Martorana R , Messina P , Schiavone S . , 2011 From geophysics to microgeophysics for engineering and cultural heritage , Int. J. Geophys. , vol. 2011 Article ID 428412 pg. 8 Google Scholar Crossref Search ADS WorldCat Dahlin T . , 2001 The development of electrical imaging techniques , Comput. Geosci. , vol. 27 (pg. 1019 - 1029 ) 10.1016/S0098-3004(00)00160-6 Google Scholar Crossref Search ADS WorldCat Crossref Dallimore R , Davis L . , 1987 Ground probing radar investigations of massive ground ice and near-surface geology in continuous permafrost in , Geol. Surv. Can. paper , vol. 87 1A (pg. 913 - 918 ) OpenURL Placeholder Text WorldCat Diamanti N , Tsokas G , Tsourlos P , Vafidis A . , 2005 Integrated interpretation of geophysical data in the archaeological site of europos (northern Greece) , Archaeol. Prospect. , vol. 12 (pg. 79 - 91 ) 10.1002/arp.249 Google Scholar Crossref Search ADS WorldCat Crossref Dionisio A , Alegria F , Martinho E , Grangeia C , Almeida F . , 2011 3-D electrical resistivity imaging as a smart monitoring method for identification of moisture sources in stone cultural heritage Proc. of the European Workshop on cultural heritage preservation September 26–28 Berlin Germany (pg. 218 - 223 ) Dobrin M B , Savit C H . , 1988 , Introduction to Geophysical Prospecting McGraw-Hill International Editions Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC El-Gohary M A . , 2013 Evaluation of treated and un-treated Nubia Sandstone using ultrasonic as a non-destructive technique , J. Archaeol. Sci. , vol. 40 (pg. 2190 - 2195 ) 10.1016/j.jas.2012.12.023 Google Scholar Crossref Search ADS WorldCat Crossref Faella G , Frunzio G , Guadagnuolo M , Donadio A , Ferri L . , 2012 The Church of the Nativity in Bethlehem: Non-destructive tests for the structural knowledge , J. Cultur. Heritage , vol. 13 (pg. e27 - e41 ) 10.1016/j.culher.2012.10.014 Google Scholar Crossref Search ADS WorldCat Crossref Fais S , Ligas P , Palomba M , Tocco R . , 2002 Evaluation of preservation state of monumental buildings by ND acoustic techniques and mineralogical Studies Proc. 5th Int. Symp. Protection and Conservation of the Cultural Heritage of the Mediterranean Cities (pg. 307 - 314 ) Favaro M , Mendichi R , Ossola F , Russo U , Simon S , Tomasin P , Vigato P A . , 2006 Evaluation of polymers for conservation treatments of outdoor exposed stone monuments, Part I: Photo-oxidative weathering , Polym. Degrad. Stabil. , vol. 91 (pg. 3083 - 3096 ) 10.1016/j.polymdegradstab.2006.08.012 Google Scholar Crossref Search ADS WorldCat Crossref Favaro M , Mendichi R , Ossola F , Simon S , Tomasin P , Vigato P A . , 2007 Evaluation of polymers for conservation treatments of outdoor exposed stone monuments, part ii: photo-oxidative and salt-induced weathering of acrylicsilicone mixtures , Polym. Degrad. Stabil. , vol. 92 (pg. 335 - 351 ) 10.1016/j.polymdegradstab.2006.12.008 Google Scholar Crossref Search ADS WorldCat Crossref Fort R , Alvarez de Buergo M . , 2012 Stone decay assessment of the Madrid’s Royal Palace (Spain) by means of ultrasound and magnetometric prospection , Geophys. Res. Abstracts 14 EGU2012-1791 OpenURL Placeholder Text WorldCat Galan E , Carretero M I , Bernabe J M , Fernandez-Caliani J C , Requena A . , 2000 The Columbus monument at Huelva (sw Spain): preliminary survey on stone decay Proc. of the 9th Int. Congress on Deterioration and Conservation of Stone Amsterdam Elsevier , vol. 2 (pg. 715 - 720 ) Galan E , Guerrero M A , Vazquez M A , Maert F , Zezza F . , 1992 Marble weathering: relation between ultrasonic data and macroscopic observations. The case of the columns of the court of the lions at the alhambra in granada, spain”; In La conservation des monuments dans le basin méditerranéen: Actes du 2ème Symposium international = The Conservation of Monuments in the Mediterranean Basin: Proc. of the 2nd Int. Symposium Decrouez D , Chamay J , Zezza F . (pg. 193 - 207 ) Geneva Ville de Genève, Muséum d’histoire naturelle, Musée d’art et d’histoire (pg. 193 - 207 ) Haelterman K , Lambrechts A , Janssens H , Van Gemert D . , 1993 Geo-electrical survey of masonry , Mater. Struct. , vol. 26 (pg. 495 - 499 ) 10.1007/BF02472809 Google Scholar Crossref Search ADS WorldCat Crossref Lehmann F , Krüger M . , 2011 Wireless impedance measurements to monitor moisture and salt migration in natural stone Cultural Heritage Preservation, Proceedings of the European Workshop on Cultural Heritage Preservation Germany Fraunhofer IRB Verlag ISBN: 978-3-8167-8560-6 (pg. 224 - 231 ) Leucci G , Cataldo R , De Nunzio G . , 2007 Assessment of fractures in some columns inside the crypt of the Cattedrale di Otranto using integrated geophysical methods , J. Archaeol. Sci. , vol. 34 (pg. 222 - 232 ) 10.1016/j.jas.2006.04.012 Google Scholar Crossref Search ADS WorldCat Crossref Leucci G , Masini N , Persico R , Soldovieri F . , 2011 GPR and sonic tomography for structural restoration: the case of the cathedral of Tricarico , J. Geophys. Eng. , vol. 8 (pg. S76 - S92 ) 10.1088/1742-2132/8/3/S08 Google Scholar Crossref Search ADS WorldCat Crossref Leucci G , Persico R , Soldovieri F . , 2007 Detection of fractures from GPR data: the case history of the cathedral of otranto , J. Geophys. Eng. , vol. 4 (pg. 452 - 461 ) 10.1088/1742-2132/4/4/011 Google Scholar Crossref Search ADS WorldCat Crossref Loke M H , Barker R D . , 1996 Pratical techniques for 3D resistivity surveys and data inversion , Geophys Prospect. , vol. 44 (pg. 499 - 423 ) 10.1111/j.1365-2478.1996.tb00162.x Google Scholar Crossref Search ADS WorldCat Crossref Longoni L , Arosio D , Scaioni M , Papini M , Zanzi L , Roncella R , Brambilla D . , 2012 Surface and subsurface non-invasive investigations to improve the characterization of a fractured rock mass , J. Geophys. Eng. , vol. 9 (pg. 461 - 472 ) 10.1088/1742-2132/9/5/461 Google Scholar Crossref Search ADS WorldCat Crossref López-Arce P , Gomez-Villalba L S , Pinho L , Fernández-Vallec M E , Álvarez de Buergo M , Fort R . , 2010 Influence of porosity and relative humidity on consolidation of dolostone with calcium hydroxide nanoparticles: Effectiveness assessment with non-destructive techniques , Mater. Charact. , vol. 61 (pg. 168 - 184 ) 10.1016/j.matchar.2009.11.007 Google Scholar Crossref Search ADS WorldCat Crossref Lualdi M , Zanzi L . , 2004 2D and 3D experiments to explore the potential benefit of GPR investigations in planning the mining activity of a limestone quarry Proc. 10th Int. Conf. on Ground Penetrating Radar GPR 2004 June 21–24 Delft (pg. 613 - 616 ) Martinho E , Alegria F , Dionisio A , Grangeia C , Almeida F . , 2012 3D-resistivity imaging and distribution of water soluble salts in Portuguese Renaissance stone bas-reliefs , Eng. Geol. (pg. 141 - 142 )(pg. 33 - 44 ) OpenURL Placeholder Text WorldCat Martinho E , Dionisio A , Almeida F , Mendes M , Grangeia C . , 2014 Integrated geophysical approach for stone decay diagnosis in cultural heritage , Constr. Build. Mater. , vol. 52 (pg. 345 - 352 ) 10.1016/j.conbuildmat.2013.11.047 Google Scholar Crossref Search ADS WorldCat Crossref Masini N , Persico R , Rizzo E . , 2010 Some examples of GPR prospecting for monitoring of the monumental heritage , J. Geophys. Eng. , vol. 7 (pg. 190 - 199 ) 10.1088/1742-2132/7/2/S05 Google Scholar Crossref Search ADS WorldCat Crossref Mendes M , Martinho E , Dionisio A , Almeida F , Grangeia C . , 2014 Diagnóstico do estado de conservação da pedra de uma obra de arte Portuguesa do século XV aplicando técnicas geofisicas não invasivas Resumos da 8ª Assembleia Luso Espanhola de Geodesia e Geofisica January 29–31 Évora Portugal pg. 40 Montoto M , Valdeón L , Esbert R M . , 1996 Non-destructive tests in stone conservation: tomography of the axeitos (la coruña, spain) megalith Proc. of the EC Workshop November 28–30 Santiago de Compostela Spain (pg. 281 - 287 ) Moropoulou A , Labropoulos K , Delegou E , Karoglou M , Bakolas A . , 2013 Non-destructive techniques as a tool for the protection of built cultural heritage , Constr. Build. Mater. , vol. 48 (pg. 1222 - 1239 ) 10.1016/j.conbuildmat.2013.03.044 Google Scholar Crossref Search ADS WorldCat Crossref Pamplona M , Ahmad A , Simon S , Abel E , Theissen A . , 2010 Ultrasonic pulse velocity- a tool for the condition assessment of outdoor marble sculptures Proc. 8th Int. Symposium on the Conservation of Monuments in the Mediterranean Basin Pamplona M , Simon S . , 2012 Long-term condition survey by ultrasonic velocity testing of outdoor marble sculptures Proc. of 12th Int. Congress on the Deterioration and Conservation of Stone New York USA Columbia University (in press) Panisova J , Frastia M , Wunderlich T , Pasteka R , Kusnirák D . , 2013 Microgravity and ground penetrating radar investigations of subsurface features at the St Catherine’s Monastery, Slovakia , Archaeol. Prospect. , vol. 20 (pg. 163 - 174 ) 10.1002/arp.1450 Google Scholar Crossref Search ADS WorldCat Crossref Papadopoulos N G , Tsourlos P , Tsokas G N , Sarris A . , 2006 Two-dimensional and three-dimensional resistivity imaging in archaeological site investigation , Archaol. Prospect. , vol. 13 (pg. 163 - 181 ) Google Scholar Crossref Search ADS WorldCat Pérez-Gracia V , Caselles O , Clapés J , Osorio R , Canas J A , Pujades L G . , 2009 Radar exploration applied to historical buildings: A case study of the Marques de Llió palace, in Barcelona (Spain) , Eng. Fail. Anal. , vol. 16 (pg. 1039 - 1050 ) 10.1016/j.engfailanal.2008.05.007 Google Scholar Crossref Search ADS WorldCat Crossref Pieraccini M , Mecatti D , Luzi G , Seracini M , Pinelli G , Atzeni C . , 2005 Non-contact intrawall penetrating radar for heritage survey: the search of the ‘Battle of Anghiari’ by Leonardo da Vinci , NDT&E Int. , vol. 38 (pg. 151 - 157 ) 10.1016/j.ndteint.2004.07.010 Google Scholar Crossref Search ADS WorldCat Crossref Ranalli D , Scozzafava M , Tallini M . , 2004 Ground penetrating radar investigations for restoration of historical building: the case study of Collemaggio Basilicata (L’aquila, Italy) , J. Cultur. Heritage , vol. 5 (pg. 91 - 99 ) 10.1016/j.culher.2003.05.001 Google Scholar Crossref Search ADS WorldCat Crossref Recheis A , Bidner T , Mirwald P . , 2000 Ultrasonic measurements on weathering alpine marble. A study on field exposed samples and on the medieval marble portals of schloss tirol/South Tyrol- Italy 9th, Int. Congress on Deterioration and Conservation of Stone Elsevier , vol. 2 (pg. 139 - 144 ) Reynolds J M . , 1997 , An Introduction to Applied and Environmental Geophysics England John Wiley & Sons Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Ruedrich J , Knell C , Enseleit J , Rieffel Y , Siegesmund S . 2013 Stability assessment of marble statuaries of the Schlossbrüke (Berlin, Germany) based on rock strength measurements and ultrasonic wave velocities , Environ. Earth Sci. , vol. 69 (pg. 1451 - 1469 ) Crossref Search ADS WorldCat Sambuelli L , Bohm G , Capizzi P , Cardarelli E , Cosentino P . , 2011 Comparison among GPR measurements and ultrasonic tomographies with different inversion algorithms. An application to the basement of an ancient Egyptian sculpture , J. Geophys. Eng. , vol. 8 (pg. 1 - 11 ) 10.1088/1742-2132/8/1/001 Google Scholar Crossref Search ADS WorldCat Crossref Sass O , Viles H A . , 2006 How wet are these walls? Testing a novel technique for measuring moisture in ruined walls , J. Cultur. Heritage , vol. 7 (pg. 257 - 263 ) 10.1016/j.culher.2006.08.001 Google Scholar Crossref Search ADS WorldCat Crossref Sass O , Viles H A . , 2010 Wetting and drying of masonry walls: 2-D resistivity monitoring of driving rain experiments on historic stonework in Oxford, UK , J. Appl. Geophys. , vol. 70 (pg. 72 - 83 ) 10.1016/j.jappgeo.2009.11.006 Google Scholar Crossref Search ADS WorldCat Crossref Sass O , Viles H A . , 2010b Two-dimensional resistivity surveys of the moisture content of historic limestone walls in Oxford, UK: implications for understanding catastrophic stone deterioration , Limestone in the Built Environment: Present-Day Challenges for the Preservation of the Past: Geological Society London Special Publications , vol. 331 (pg. 237 - 249 ) 10.1144/SP331.22 Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Crossref Schueremans L , Rickstal F , Venderickx K , Van Gemert D . , 2003 Evaluation of masonry consolidation by geo-electrical relative difference resistivity mapping , Mater. Struct. , vol. 36 (pg. 46 - 50 ) 10.1007/BF02481570 Google Scholar Crossref Search ADS WorldCat Crossref Schwindt D , Kneisel C . , 2009 Quasi-3D resistivity imaging – results from geophysical mapping and forward modeling , Geophys. Res. Abstracts 11 1156 OpenURL Placeholder Text WorldCat Sharma P V . , 1997 , Environmental and Engineering Geophysics Cambridge Cambridge University Press Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Siegemsund S , Pretzschner C , Ruedrich J , Lindner H , Weiss T , Richter I , Richter D , Woyde M . , 2000 Deterioration characteristics of columns from the Marmopalais Postdam (Germany) by Ultrasonic-tomography Proc. of the 9th, International Congress on Deterioration and Conservation of Stone Elsevier , vol. 2 (pg. 145 - 153 ) Simon S , Lind A M . , 1999 Decay of limestone blocks in the block fields of Karnak Temple (Egypt): Non-destructive damage analysis and control of consolidation treatments In 12th Triennial Meeting, ICOM Committee for Conservation 29 August–3 September Lyon France (pg. 743 - 749 ) Spizzichino D , Margottini C , Castellaro S , Mulargia F . , 2013 , Passive Seismic Survey for Cultural Heritage Landslide Risk Assessment Landslide Science and Practice . , vol. 6 Heidelberg, Berlin Springer-Verlag Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Stevens M , Lodha S , Holloway L , Soonawala M . , 1995 The application of ground penetrating radar for mapping fractures zones in plutonic rocks within the Whiteshell Research Area, Pinawa, Manitoba, Canada , J. Appl. Geophys , vol. 33 (pg. 125 - 141 ) 10.1016/0926-9851(95)90036-5 Google Scholar Crossref Search ADS WorldCat Crossref Telford W , Geldart L , Sheriff R . , 1990 , Applied Geophysics Cambridge Cambridge University Press Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Trinks I , Gansum T , Hinterleitner A . , 2010 Mapping iron-age graves in norway using magnetic and GPR prospection explore the project gallery , Antiquity , vol. 084 326 December , 2010 OpenURL Placeholder Text WorldCat Tsokas G N , Stampolidis A , Mertzanidis I . , 2007 Geophysical exploration in the church of Protaton in Karyes of Mount Athos (Holy Mountain) in Northern Greece , Archaeol. Prospect. , vol. 14 (pg. 75 - 86 ) 10.1002/arp.305 Google Scholar Crossref Search ADS WorldCat Crossref Valle S , Zanzi L . , 1998 Traveltime radar tomography for NDT on masonry and concrete structures , Eur. J. Environ. Eng. Geophys , vol. 2 (pg. 229 - 246 ) OpenURL Placeholder Text WorldCat Van Gemert D , Janssens H , Van Rickstal F . , 1996 Evaluation of electrical resistivity maps for ancient masonry , Mater. Struct. , vol. 29 (pg. 158 - 163 ) 10.1007/BF02486161 Google Scholar Crossref Search ADS WorldCat Crossref Venderickx K , Van Gemert D . , 2000 Geo-electrical survey of masonry for restoration projects , Internationale Zeitsschrift für Bauinstandsetzen und Baudenkmalpflege , vol. 6 Jahrgang Heft 2 (pg. 151 - 172 ) OpenURL Placeholder Text WorldCat Viles H , Sass O , Mol L . , 2008 2D resistivity surveys as a tool for investigating moisture in historic masonry walls Proc. of the Int. Workshop In Situ Monitoring of Monumental Surfaces (SMW08) (pg. P439 - p444 ) Tiano P , Pardini C . Villegas R , Vale J F , Miguel A B . , 1994 Evaluation of consolidants by means of ultrasonic and surface hardness measurements In Conservation of Monuments in the Mediterranean Basin: Stone Monuments, Methodologies for the Analysis of Weathering and Conservation Proc. of the 3rd Int. Symposium Venice Fassina V , Ott H , Zezza F . (pg. 919 - 923 ) Venice Soprintendenza ai beni artistici e storici di Venezia Zezza F . , 2002 Non-destructive technique for the assessment of the deterioration processes of prehistoric rock art in karstic caves: The paleolithic paintings of Altamira (Spain) , In Protection and Conservation of the Cultural Heritage of the Mediterranean Cities Galán Huertos E , Zezza Lisse F . Netherlands and Exton PA Balkema (pg. 377 - 388 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC © 2014 Sinopec Geophysical Research Institute
TI - Main geophysical techniques used for non-destructive evaluation in cultural built heritage: a review
JF - Journal of Geophysics and Engineering
DO - 10.1088/1742-2132/11/5/053001
DA - 2014-10-01
UR - https://www.deepdyve.com/lp/oxford-university-press/main-geophysical-techniques-used-for-non-destructive-evaluation-in-FQLlwp5RyC
VL - 11
IS - 5
DP - DeepDyve
ER -