TY - JOUR
AU1 - Wang,, Baoshan
AU2 - Yang,, Wei
AU3 - Yuan,, Songyong
AU4 - Guo,, Shijun
AU5 - Ge,, Hongkui
AU6 - Xu,, Ping
AU7 - Chen,, Yong
AB - Abstract A large volume airgun array is effective in generating seismic waves, which is extensively used in large volume bodies of water such as oceans, lakes and reservoirs. So far, the application of large volume airguns is subject to the distribution of large volume bodies of water. This paper reports an attempt to utilize large volume airguns in a small body of water as a seismic source for seismotectonic studies. We carried out a field experiment in Mapaoquan pond, Fangshan district, Beijing, during the period 25–30 May 2009. Bolt LL1500 airguns, each with volumes of 2000 in3, the largest commercial airguns available today, were used in this experiment. We tested the excitation of the airgun array with one or two guns. The airgun array was placed 7–11 m below the water's surface. The near- and far-field seismic motions induced by the airgun source were recorded by a 100 km long seismic profile composed of 16 portable seismometers and a 100 m long strong motion seismograph profile, respectively. The following conclusions can be drawn from this experiment. First, it is feasible to excite large volume airguns in a small volume body of water. Second, seismic signals from a single shot of one airgun can be recognized at the offset up to 15 km. Taking advantage of high source repeatability, we stacked records from 128 shots to enhance the signal-to-noise ratio, and direct P-waves can be easily identified at the offset ∼50 km in stacked records. Third, no detectable damage to fish or near-field constructions was caused by the airgun shots. Those results suggest that large volume airguns excited in small bodies of water can be used as a routinely operated seismic source for mid-scale (tens of kilometres) subsurface explorations and monitoring under various running conditions. large volume airgun, small volume water body, active monitoring 1 Introduction Imaging a subsurface structure and its temporal evolution has long been the main task for seismologists (Chen et al2008). With the densification of seismic networks (e.g. Zheng et al (2009)), we obtained ever more clear images of the Earth's interior. In contrast to the situation of structural studies, there is still much to be sought about our understandings on the subtle temporal variations of subsurface structures. A little progress in monitoring subsurface changes has been made by using passive sources such as repeating earthquake (e.g. Song and Richards (1996)) and ambient seismic noise (e.g. Sens-Schönfelder and Wegler (2006)). However, as passive sources are unevenly distributed both spatially and temporally, the corresponding resolutions are limited. Monitoring subsurface changes with an active source is an alternative method. The seismic source is critical to active source monitoring. An ideal source should be highly repeatable, less damaging to near source environments, and efficient in seismic energy radiation. There are three commonly used seismic sources—explosives, vibrators and airguns. In onshore explorations, chemical dynamite is the most widely used seismic source. However, the use of dynamite on land has been gradually restricted because of its negative environmental effects. To use a vibrator, one needs specified and uniform receivers. Airguns or an airgun array source have the advantages of being highly repeatable, environmentally friendly and low in maintenance costs. They have also been widely used in exploring oceanic and paralic structures (e.g. Okaya et al (2002) and Qiu et al (2007)). The use of large volume airguns has been extended for on-land seismotectonic studies recently (Chen et al2007, 2008). Two airgun experiments were carried out in Shangguan Lake reservoir, Zunhua, Hebei province, in 2006 and 2007 (Chen et al2007, Lin et al2010). Successful excitation of large volume airgun arrays in a reservoir provided a new seismic source for land exploration (Chen et al2007, 2008). Results from those two experiments suggest that large volume airguns are an environmentally friendly, highly repeatable seismic source with high seismic energy radiation efficiency (Chen et al2007, 2008, Lin et al2008) that can be used for crust structure exploration (Lin et al2008), as well as continuous subsurface monitoring. Shangguan Lake is a medium-sized reservoir. The water surface is several km in dimension, which is much larger than the size of the air bubble (∼tens of metres) generated by the airgun source. In a manmade hole with depth 1.65 m and diameter 1.75 m, Reasenberg and Aki (1974) used a 40 in3 (1 in3 = 0.016 liter) airgun as a seismic source to monitor subsurface velocity changes in the early 1970s. The authors claimed that the velocity changes induced by solid earth tides were observed from seismic records at a geophone 200 m away from the source. The seismic signals recorded by the geophone with offset 750 m were not strong enough to obtain any reliable velocity change measurements. Leary and Malin (1982) extended the measurements to a baseline 18 km long by exciting a larger volume airgun (1000 in3) in a reservoir with dimensions of several hundred metres. However, to detect velocity changes associated with large earthquakes occurring at the depth of 10 km or deeper, one needs a baseline of tens of kilometres and therefore sources of higher energy. Active seismic sources other than the airgun may cause near source damage and thus affect velocity measurement (e.g. Yamaoka et al (2001)). Utilizing a large volume airgun source in a large volume body of water can provide seismic energy large enough for the monitoring of velocity changes deeper than 10 km, but it is limited by the distribution of large volume bodies of water. Excitation of a large volume airgun in a small volume body of water may provide a seismic source with high seismic energy and fewer limitations. A field experiment using large volume airguns in a small body of water as the seismic source and its preliminary results are reported in this paper. This paper is arranged as follows. The experiment site, experimental setup and steps are described in section 2; near-field effects of the airgun source are depicted in section 3; far-field seismic signals from the airgun source are given in section 4; and section 5 presents the discussion and conclusions. 2 Field experiment To test the possibility of generating seismic waves by exciting large volume airguns in a small volume body of water, we carried out a field experiment in Mapaoquan, Fangshan district, Beijing, during the period 25–30 May 2009. Mapaoquan (figure 1) is a manmade pond built around a natural spring in the 1970s. The water surface is in the shape of an ellipse with long and short axes measuring 37 m and 25 m, respectively. The water depth at the airgun firing point was measured to be 14.5 m. Figure 1 Open in new tabDownload slide Map shows the experiment site and station distribution. The Mapaoquan pond is shown as an ellipse, and the airgun source (red star) was fired at the southwest part of the pond; three groups of hydrophones (hollow triangles) were deployed down the pond wall slope. Three periods of strong motion measurements were carried out. Stations for the first strong motion measurement are shown as black stars, and the later two measurements were carried out along a 100 m profile south-west (dashed line), with an average inter-station distance of 10 m. The nearest permanent seismic station NKY is located 950 m northeast of the airgun source. For clarity, the NKY station is not marked to scale. The map on the bottom right shows the airgun source (red star); seismic stations (solid triangles) that form a 100 km long portable seismic profile and permanent stations (inversed hollow triangles) nearby. Figure 1 Open in new tabDownload slide Map shows the experiment site and station distribution. The Mapaoquan pond is shown as an ellipse, and the airgun source (red star) was fired at the southwest part of the pond; three groups of hydrophones (hollow triangles) were deployed down the pond wall slope. Three periods of strong motion measurements were carried out. Stations for the first strong motion measurement are shown as black stars, and the later two measurements were carried out along a 100 m profile south-west (dashed line), with an average inter-station distance of 10 m. The nearest permanent seismic station NKY is located 950 m northeast of the airgun source. For clarity, the NKY station is not marked to scale. The map on the bottom right shows the airgun source (red star); seismic stations (solid triangles) that form a 100 km long portable seismic profile and permanent stations (inversed hollow triangles) nearby. To test the excitation of an airgun array with different numbers of airguns, we built a steel frame as the airgun holder (figure 2). This frame can be used to hold one to three guns. The installation of one gun is explained in figure 2(a). Two guns should be fixed on the two sides to keep the frame in balance. The airgun holder is suspended horizontally by a 20 ton crane. Using a crane, it was more feasible to adjust the depth of submergence than the float scheme (figure 1 in Chen et al (2007)). The airgun seismic source system is similar to that described by Chen et al (2007). It is composed of an air source (electrical compressor), an air buffer (air bottles), a control centre (including controllers for compressed air and electric circuits), airguns and cables. For portability of the seismic source system, the air compressor and air bottles employed in this experiment are smaller than our previous experiments (Chen et al2007). We used a Longlife model 1500LL airgun produced by Bolt Co., each with a capacity of 2000 in3 in this experiment. Figure 2 Open in new tabDownload slide (a) A specially designed frame for an airgun holder; solid lines indicates the outline of the steel frame. Airguns were tightened to the frame with a wire rope (gray lines). The airgun is shown as a tank in the middle of the frame. Numbers 1–5 indicate the air chamber for the storage of high pressure air, the vent for releasing high pressure air, the solenoid valves control the airgun firing, electric cable and air hose, respectively. (b) The submerging and uplifting of the airgun is operated by a 20 ton crane rather than a float as in our previous experiment (Chen et al2007). The frame is horizontally submerged below the water surface. Dumper ropes were tied to the airgun for safety. Figure 2 Open in new tabDownload slide (a) A specially designed frame for an airgun holder; solid lines indicates the outline of the steel frame. Airguns were tightened to the frame with a wire rope (gray lines). The airgun is shown as a tank in the middle of the frame. Numbers 1–5 indicate the air chamber for the storage of high pressure air, the vent for releasing high pressure air, the solenoid valves control the airgun firing, electric cable and air hose, respectively. (b) The submerging and uplifting of the airgun is operated by a 20 ton crane rather than a float as in our previous experiment (Chen et al2007). The frame is horizontally submerged below the water surface. Dumper ropes were tied to the airgun for safety. There are several factors that may affect the seismic energy output of an airgun source (e.g. Caldwell and Dragoset (2000)). Among them, the water depth, total volume of the airgun array, depth of submergence (tow) and working pressure are four primary factors. The contributions of these factors affecting airgun sources fired in large volumes of water are well studied. In this experiment, we tested how these factors may affect the airgun output when fired in a small volume body of water by adjusting the last three factors. The change in airgun volume was implemented by changing the number of guns. In this paper, we focus on the preliminary results from the experiments under the same working conditions with depth of submergence 11 m and working pressure 15 MPa (∼2175 psi). The seismic signals radiated from the airgun source were recorded by a 100 km long seismic profile (figure 1). This profile composed of 16 portable seismic stations. Each station consisted of one Guralp 40T seismometer and one Reftek 130B data logger. The portable seismic profile, combined with permanent stations adjacent to the profile, constituted a new profile with an inter-station distance of ∼5 km. Three stages of ground motion measurements in the vicinity of the airgun source were also carried out in the experiment (figure 1). During the period of the one-gun experiment, strong motions were measured diffusely (solid stars in figure 1) and linearly (dashed line in figure 1), separately. Only the linear distribution of ground motion was measured during the two-gun experiment. ETNA-type accelerometers produced by KINEMATRIX were used for near-field ground motion measurements in this experiment. To reduce the disturbance of local human activities and recording noise, we only fired the airgun source during the night, usually 8 pm to 4 am During the period 25–29 May 2009, we conducted one-gun experiments and fired 142 shots in total. Among them, 128 shots were fired under the same running conditions of 11 m depth of submergence and 15 MPa air pressure. On 30 May 2009, 30 shots were fired with a two-gun array. According to the small air compressor used in this experiment and violent water vibration caused by an airgun shot, we set the shot interval to no shorter than 2 min to allow the water to regain equilibrium. 3 Effects of the airgun shot on local environment and fresh water fish Because environmental protection organizations have concerns that airgun signals may harm marine mammals, there are more and more limitations for the use of large volume airguns in oceanic explorations (e.g. Caldwell 2002, Gausland 2003, Dalton 2008) despite the airgun source being highly repeatable and easy to maintain. In our previous reservoir airgun experiment (Chen et al2007), we did not find any observable damage of the airgun source on fresh water fish (Chen et al2008). To investigate the effect of a large volume airgun source excited in a small volume body of water on fresh water fish, we placed a netted bag containing several kinds of fish 5–20 m away from the firing airgun source. We did not find any visible harm to the fish after several airgun shots (figure 3). Figure 3 Open in new tabDownload slide A netted bag of fish was placed 5 m away from the airgun source. The fish were examined after several shots, and no detectable harm to the fish was observed. Figure 3 Open in new tabDownload slide A netted bag of fish was placed 5 m away from the airgun source. The fish were examined after several shots, and no detectable harm to the fish was observed. We also investigated the effects of the airgun source on near-field constructions. We monitored widths of pre-existing cracks in the pond wall continuously, and no obvious change was recorded during the entire period of the experiment. Since the airgun is a type of pure compression source, less shear deformation was applied on near-field constructions by the airgun source. Moreover, the airgun source is fired tens of metres under the water surface, under the confining pressure of an overlaying water layer, where a large portion of energy is converted into elastic seismic energy rather than causing local damage. These observations on the other hand indicate that the airgun source is less harmful to the local environment than other seismic sources such as a chemical explosion. Near-field ground motions induced by airgun sources can be quantitatively described by our strong motion measurements (figure 1). Ground accelerations along the strong motion profile (dashed line in figure 1) and their amplitude spectra are shown in figure 4. Airgun-induced ground motion at the nearest (29 m) macroseismograph is quite complicated, which may result from near-source noises of supporting instruments. Records at distances of 41–89 m appear to be simpler, which may be due to the decaying of operational noise. At distances further than 90 m, the wave trains become complicated again, which is likely to be caused by the emergence of surface waves and local scattering. Near-field recordings are observed to have dominating frequencies between 10 and 25 Hz (figure 4(b)). Note that multiplications of 5 Hz are clearly shown in all records, which is supposed to be the response of air bubble oscillation (Tang et al2009, Lin et al2010). Figure 4 Open in new tabDownload slide Ground accelerations (a) and corresponding amplitude spectra (b) at various distances recorded by strong motion profiles while one single gun was fired under the running condition of 11 m depth of submergence and 15 MPa working pressure. Figure 4 Open in new tabDownload slide Ground accelerations (a) and corresponding amplitude spectra (b) at various distances recorded by strong motion profiles while one single gun was fired under the running condition of 11 m depth of submergence and 15 MPa working pressure. The comparison of peak ground accelerations (PGA) induced by one- and two-gun excitations is shown in figure 5. The maximum PGA is the vertical component of acceleration induced by the two-gun shot at the nearest station, which can reach up to 0.5 g. The airgun-induced PGA decays approximately in an exponential way. The two-gun-induced PGA reduces to 0.04 g (∼0.4 m s−2) at a distance of 90 m (figure 5). The PGA induced by the one-gun shot at a distance of 99 m is about 0.02 g (∼0.2 m s−2), which is about half of that induced by a two-gun excitation but much larger than the PGA (∼0.038 m s−2) recorded at 100 m from a single gun (1500 in3) shot in a reservoir (see table 1 in Chen et al2007). In this experiment, the body of water is much smaller than that in our previous experiments (Chen et al2007), and vibration propagates much longer in solid media, which may result in larger observed ground shake at a similar distance in the near field. Figure 5 Open in new tabDownload slide Peak ground acceleration induced by one- and two-gun shots shown as a function of distance. Figure 5 Open in new tabDownload slide Peak ground acceleration induced by one- and two-gun shots shown as a function of distance. 4 Seismic signals radiated from the airgun source As described in the last section, the near-field ground motion induced by a pond-excited airgun source is much larger than that by an airgun excited in a reservoir, which is likely to result in less energy radiated to far field, i.e., lower far-field movements, by a pond-excited airgun than a reservoir-excited airgun. Seismic signals from an airgun excited in the pond recorded by two permanent stations Niukouyu (NKY) and Zhoukoudian (ZKD) are shown in figure 6; these two stations are located 950 m and 3.5 km away from the source, respectively. The peak ground velocity registered on the NKY station is about 15 µm s−1, and the peak ground velocity decreases to ∼1 µm s−1 at ZKD. Similar to the near-field acceleration records (figure 4(b)), the amplitude spectra at NKY show obvious multiplications of 5 Hz corresponding to air bubble oscillations. Due to scattering and nonlinear attenuation (Aki and Richards 2002, Chen et al2007, Wang et al2001), high frequency signals decay faster with distance, and lower frequency signals can propagate to longer distances. Thus, signals at frequencies higher than 15 Hz attenuate dramatically and low-frequency (2–12 Hz) signals dominate the waveform at ZKD. In addition, low-frequency (lower than 15 Hz) signals fall in the optimal frequency range for current permanent seismic stations, which provides a wonderful opportunity for us to utilize the existing and densified permanent seismic networks (e.g. Zheng et al (2009)) to detect the airgun signal. Figure 6 Open in new tabDownload slide Vertical component seismograms (a) and corresponding amplitude spectra (b) from the one-gun shot recorded at permanent stations ZKD and NKY. Figure 6 Open in new tabDownload slide Vertical component seismograms (a) and corresponding amplitude spectra (b) from the one-gun shot recorded at permanent stations ZKD and NKY. Seismic signals radiated from one-gun shots recorded by the seismic profile and local permanent seismic stations (figure 1) are shown as recording sections in figure 7. Surface wave signals from a single one-gun shot can be easily recognized to a distance of 15 km (ellipse in figure 7(a)). Similar to the reservoir-excited airgun source (e.g. Lin et al (2008)), the pond-excited airgun source was also found to be highly repeatable. The similarity of source functions makes it possible to enhance the signal-to-noise ratio (SNR) by stacking seismograms from individual shots. After being stacked 128 times, the SNR was enhanced remarkably, and direct P-waves can be identified up to ∼50 km (figure 7(b)) corresponding to a penetrating depth of 10–20 km. Figure 7 Open in new tabDownload slide (a) Seismic record section from a single one-gun shot registered on portable seismic stations (figure 1); (b) stacked waveforms of 128 individual one-gun shots recorded by portable (thick line) and permanent (thin line) seismic stations. All shots were fired at the same depth of submergence (11 m) and working pressure (15 MPa). The horizontal axis is the travel time reduced with a velocity of 6 km s−1. Figure 7 Open in new tabDownload slide (a) Seismic record section from a single one-gun shot registered on portable seismic stations (figure 1); (b) stacked waveforms of 128 individual one-gun shots recorded by portable (thick line) and permanent (thin line) seismic stations. All shots were fired at the same depth of submergence (11 m) and working pressure (15 MPa). The horizontal axis is the travel time reduced with a velocity of 6 km s−1. 5 Discussion and conclusions The total energy released by an airgun source can be estimated by using the following equation (e.g. Ronen (2002) and Chen et al (2007)): 1 where E, V and P are released energy, total volume of the airgun source and working pressure, respectively. Po is the environmental pressure (∼1 atm = 0.101 MPa). In this experiment, we have V ∼ 2000 in3 and P ∼ 15 MPa, and the total energy released by the one-gun shot is estimated to be 15 MPa × 32.8 l × ln(15 MPa/0.101 MPa) = 2.46 MJ, which is equivalent to the energy released by 1 kg chemical dynamite (e.g. Rao et al (2007) and Chen et al (2007)). Dynamite charges of several kilograms are frequently used on land for shallow structure (several hundred metres to kilometres) explorations. At the same time, seismic signals from the airgun source with equivalent energy can propagate to longer distances (e.g. 15 km in this experiment). This may suggest that the airgun source is more efficient in converting energy into elastic seismic motion than chemical explosions. Near source plastic deformation is the most likely cause of low energy conversion efficiency and low repeatability for sources such as dynamite. In addition, differences in frequency content of different sources may also affect the propagation of seismic signals (Chen et al2007). The peak ground velocity at 3.5 km (ZKD) induced by a one-gun shot is ∼1 µm s−1 (figure 6), while in our 2007 reservoir experiment, the one-gun (2000 in3) shot produced a direct P phase with vertical ground velocity ∼2 µm s−1 at the station ZUH (Zunhua) with an epicentral distance of 7 km. Taking into account only geometric spreading, we estimate the ground velocity of the pond-excited airgun at the epicentral distance of 7 km as 1 µm/s/(7 km/3.5 km) = 0.5 µm s−1, which is about one-fourth of that from a reservoir excitation. These two experiments shared similar working conditions (water depth ∼15 m, depth of submergence 11 m, working pressure 15 MPa and airgun volume 2000 in3), and the different efficiencies in radiating seismic waves are mostly likely a result of the differences in the shape of the bodies of water. The shape of the body of water may affect the airgun excitation in two ways: (1) a funnel-type pond wall can reduce the effective bottom area and reflect some seismic energy upward, which results in stronger near-field motion as shown in section 3, and (2) bubble pressure released by the air bubble rising from the airgun is also an important part of the seismic signal from the large volume airgun (Lin et al2010). The small volume pond prohibits the bubble from rapid releasing, resulting in longer bubble oscillations and lower bubble pressure. Despite the airgun source excited in a small pond being less efficient than a reservoir-excited airgun source, the highly repeatable airgun source provides a possible way to enhance the signal SNR and thus exploration ability by stacking signals from individual shots. And it is supposed, according to our results, that due to the high repeatability and low near-field damage, the pond-excited airgun source can be used as an ideal source for mid-scale (tens of kilometres) subsurface exploration and monitoring (Leary and Malin 1982, Leary et al1979, Reasenberg and Aki 1974). Acknowledgments Thanks to the graduate students who participated in the field work. Thanks to the earthquake administration of Fangshan district for their help during the experiment. This work was funded by the Scientific Research Institutes' Basic R & D Operations Special Fund of the institute of geophysics, China Earthquake Administration (grant no DQJB07A01), National Natural Science Foundation of China (grant no 40874095) and National Public Benefit Research Foundation on Earthquake (grant no 200808002). Waveform data of permanent stations are provided by the Data Management Centre of the China National Seismic Network at Institute of Geophysics, China, Earthquake Administration. Critical comments from two anonymous reviewers significantly improved the quality of this paper. 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TI - An experimental study on the excitation of large volume airguns in a small volume body of water
JF - Journal of Geophysics and Engineering
DO - 10.1088/1742-2132/7/4/005
DA - 2010-12-01
UR - https://www.deepdyve.com/lp/oxford-university-press/an-experimental-study-on-the-excitation-of-large-volume-airguns-in-a-Dby2kre6Lf
SP - 388
VL - 7
IS - 4
DP - DeepDyve
ER -