TY - JOUR
AU1 - Green, David
AU2 - Heyburn, Ross
AU3 - Keeble, Jessica
AU4 - Nippress, Alexandra
AU5 - Nippress, Stuart
AU6 - Peacock, Sheila
AU7 - Young, John
AB - Abstract David Green, Ross Heyburn, Jessica Keeble, Alexandra Nippress, Stuart Nippress, Sheila Peacock and John Young review the history of the UK's seismological monitoring of underground nuclear testing For 60 years, AWE Blacknest in Berkshire, UK, has been home to several generations of scientists and engineers who have helped develop seismology into a robust verification technology for monitoring underground nuclear testing. This field is referred to as “forensic seismology”, for experts may be expected to provide seismological evidence to answer questions regarding suspected nuclear-test-ban treaty violations: Where was a seismic source located? Can we distinguish between signals generated by explosions and earthquakes? If the source was an explosion, how big was it? This brief history details the UK contribution to this global effort. 1 Open in new tabDownload slide Blacknest, located on the Berkshire/Hampshire border, UK, became the home of the AWRE forensic seismology team in 1961. 1 Open in new tabDownload slide Blacknest, located on the Berkshire/Hampshire border, UK, became the home of the AWRE forensic seismology team in 1961. In 1958, tri-lateral negotiations between the US, UK and Soviet Union began on a Comprehensive Nuclear-Test-Ban Treaty (CTBT), seeking to ban all nuclear weapons testing. In the 13 years since the first US test, the rate of testing had accelerated (figure 2), and there was increasing global concern regarding both the risks of a nuclear arms race and the danger posed by radioactive fallout from atmospheric testing. The treaty negotiations, held in Geneva, were preceded by an international meeting to discuss how compliance with a treaty could be verified: the Geneva Conference of Experts. Although monitoring explosions in the atmosphere, underwater and in space was deemed feasible, the detection and identification of underground explosions proved contentious. Would contained underground explosions (those that vent no radioactive material into the atmosphere) generate recordable, and interpretable, ground motions? The UK delegation technical lead, Sir William Penney, who had contributed to the Manhattan Project and was now director of the Atomic Weapons Research Establishment (AWRE – the “R” was dropped in the 1990s and the organization became AWE), expressed serious concern about the state of the art in seismology, and is attributed as describing it as “a stone-age science”. 2 Open in new tabDownload slide Nuclear testing and selected nuclear-test-ban verification activities. The upper panel shows the number of tests over time, and the timespans of testing for individual states (numbers in parentheses indicate number of tests per state, as of January 2021, from Bergkvist & Ferm 2000). The lower panel shows a number of the verification activities that have influenced the research and work undertaken at Blacknest. LTBT, TTBT and CTBT refer to the Limited, Threshold and Comprehensive Nuclear-Test-Ban Treaties, respectively. 2 Open in new tabDownload slide Nuclear testing and selected nuclear-test-ban verification activities. The upper panel shows the number of tests over time, and the timespans of testing for individual states (numbers in parentheses indicate number of tests per state, as of January 2021, from Bergkvist & Ferm 2000). The lower panel shows a number of the verification activities that have influenced the research and work undertaken at Blacknest. LTBT, TTBT and CTBT refer to the Limited, Threshold and Comprehensive Nuclear-Test-Ban Treaties, respectively. Explosion vs earthquake A lack of data hampered the debate on seismic generation from underground nuclear testing, with estimates of the detection capability at a given distance being revised as more seismograms became available from contained US underground tests. The difficulties in distinguishing explosions from earthquakes were also becoming apparent. Further complications arose when the US delegation tabled a paper on seismic decoupling of underground explosions: muffling the seismic signals by detonation within a large air-filled cavity. The US predicted that seismic signal amplitudes could be reduced by factors of at least 100. This led to concerns that decoupling, although a huge engineering challenge, might allow states to test large nuclear devices and evade seismic detection. Penney realized that the UK was well placed to make its first significant contribution to the debate. He contacted AWRE's ground shock division, based at Foulness, Essex, to enquire whether cavity decoupling of explosions could be verified. Eric Carpenter and his scientific assistant Peter Marshall (who would both become Blacknest luminaries) responded promptly and conducted fully coupled and decoupled two-ounce chemical explosions, recording the resulting ground velocity from each. The results, showing decoupling factors of between 10 and 20, were sent to Geneva where they were tabled by the UK delegation. The presentation made a significant impact, and Carpenter led further decoupling trials in quarries and mines across the UK (Project Orpheus). The success of these early experiments persuaded the then prime minister, Harold Macmillan, to fund a UK technical programme to support test-ban-treaty negotiations. Although considerably smaller than the equivalent US programme (Vela Uniform), the UK forensic seismology programme was up and running. Ieuan Maddock, AWRE's field test director, named the programme Tabor Pluto (tabor: a drum; Pluto: the lord of the underworld). Forensic seismology at Blacknest In 1960, the UK government approved £3–4m over three years to the UK Atomic Energy Authority and Ministry of Defence to set up a seismological research facility with Commonwealth participation. Initially located at AWRE Aldermaston, in March 1961 the team moved two miles west to Blacknest (figure 1). By the 1950s, this former hunting lodge (built in 1901) was being used as a hostel for AWRE apprentices. The move was motivated by access restrictions at the main AWRE site. To develop nuclear test monitoring expertise, the team would need to work closely with UK and foreign seismologists; Blacknest allowed such collaboration. But why was AWRE chosen to host the team? Firstly, AWRE already had expertise in other methods of detection, and secondly, a body of instrumentation experts and their resources were unoccupied as a result of a global nuclear test moratorium (lasting from 1958 until 1961, although the French conducted their first test in 1960). What is perhaps surprising is that while these experts were skilled in electronics, optics, nuclear measurements and communication theory, they had no previous experience of seismology; this was to shape their approach to nuclear test monitoring. Brought in to lead the research programme and provide observational seismology experience was Hal Thirlaway, a Durham geology graduate (figure 3). After gaining experience at radar school, Thirlaway had volunteered for overseas service during the second world war and had been posted to Pakistan, where in the 1950s he helped UNESCO establish a geophysical observatory. The personnel that Thirlaway welcomed to Blacknest in the early 1960s were primarily from two areas: the ground shock division from Foulness (including Carpenter and Marshall), and weapons-testing instrumentation experts led by Frank Whiteway. Carpenter and colleagues rapidly developed methods of computing seismograms of the first arriving phases (P-waves) using explosion source and Earth models. They also derived relationships to estimate explosive yield from seismic wave amplitudes (or seismic magnitude). Incidentally, it was Carpenter's wife Kathleen who suggested the term forensic seismology. Meanwhile, Whiteway and his team of scientists and engineers were designing and building advanced seismological recording arrays. These provided (and continue to provide) some of the highest quality seismic data available, giving AWRE seismologists a significant advantage when attempting to understand explosively generated seismic signals. 3 Open in new tabDownload slide Open in new tabDownload slide Science leaders at Blacknest. (Top) Hal Thirlaway (left) who led the group from 1961 to 1982, with Peter Marshall, whose involvement in test-ban-treaty negotiations led him to be awarded an OBE and CMG. The photo was taken on the occasion of Peter's MSc conferment at the University of Durham in 1969. (Bottom) Alan Douglas, who led the group between 1982 and 2001, at his desk studying a set of seismograms. 3 Open in new tabDownload slide Open in new tabDownload slide Science leaders at Blacknest. (Top) Hal Thirlaway (left) who led the group from 1961 to 1982, with Peter Marshall, whose involvement in test-ban-treaty negotiations led him to be awarded an OBE and CMG. The photo was taken on the occasion of Peter's MSc conferment at the University of Durham in 1969. (Bottom) Alan Douglas, who led the group between 1982 and 2001, at his desk studying a set of seismograms. Seismometer array development A seismometer array is a collection of sensors, distributed over an area. The design aim is to record coherent seismic signals at each sensor, while noise with shorter (apparent) wavelengths is incoherent across the array. By combining the recordings, the noise is suppressed while leaving the coherent signal unchanged. To improve the signal-to-noise ratio, the 1958 Geneva Conference of Experts recommended arrays with spatial extents of 3 km. Whiteway's team, with their experience of radar, realized that the arrays could be designed better. By increasing the array aperture, signal-to-noise improvements could be made while additional, and highly valuable, information about the speed and direction of the propagating seismic wavefronts could be estimated. Applying time-shifts to recordings at individual sensors causes signals arriving from a chosen direction to align. These can then be summed coherently (delay-and-sum processing). Steering the array in this manner is a filtering operation and allows signals from a particular location to be enhanced. In the late 1950s, many seismic recordings were still made on paper charts and relied on smoked paper, pen-and-ink or photographic recording methods. AWRE electronic engineers already had expertise in recording signals that could be replayed for electronic processing. Therefore, the team was able to design equipment that recorded array seismograms onto analogue magnetic tape. These tapes had a maximum of 24 channels; once channels had been used to store time codes and error compensation values, approximately 20 remained for sensor data (thus impacting the number of seismometers deployed within an array). Array processing techniques and jargon were borrowed from radar: “channel” for the recording at a single seismometer; “aperture” for the spatial extent of the array; “beamforming” for the process of combining individual channels to enhance the signal, just as aerials radiate and receive beams of electromagnetic radiation. Teleseismic observations By 1961, AWRE scientists were planning to build large seismic arrays that would require the application of time delays. Several different prototype array configurations were constructed at Porton Down on Salisbury Plain, chosen for being a quiet region (figure 4). To test the array, the Royal Navy fired a number of depth charges at varying locations in the English Channel. Recordings from this experiment, named Operational Seagull, confirmed the delay-and-sum beamforming concept worked. 4 Open in new tabDownload slide The configuration of seismic arrays designed by, and deployed with the assistance of, Blacknest staff. The VLA (Very Large Array, New Mexico, US) radio telescope array is shown for comparative scale. EKA (Scotland), YKA (Canada), WRA (Australia) and GBA (India) became known collectively as the Commonwealth arrays, and provide excellent global coverage (see figure 7). 4 Open in new tabDownload slide The configuration of seismic arrays designed by, and deployed with the assistance of, Blacknest staff. The VLA (Very Large Array, New Mexico, US) radio telescope array is shown for comparative scale. EKA (Scotland), YKA (Canada), WRA (Australia) and GBA (India) became known collectively as the Commonwealth arrays, and provide excellent global coverage (see figure 7). Following the success of Seagull, AWRE collaborated with the US Advanced Research Projects Agency (ARPA) to construct an experimental array (PMW) at Pole Mountain, Wyoming. The site was located about 1000 km from Gnome, a planned 3.1 kiloton (kt) underground explosion in New Mexico; the low anticipated P-wave amplitudes at this distance would be a challenge for signal detection. The array design was L-shaped to allow simple analogue computer processing, with each arm of the array being 9 km long. This was considered a near-optimum design for the predicted signal wavelengths. Signals observed at PMW from Gnome confirmed that arrays could successfully suppress noise and enhance coherent seismic signals. In addition, the observed Gnome signals lasted tens of seconds, with numerous crustal seismic phases identifiable. These seismograms were described as being “complex”. However, the most important results from PMW were not from Gnome, but from observations of the February 1962 USSR nuclear detonation in East Kazakhstan (at 9900 km) and the May 1962 French test in the Sahara (at 9800 km). These signals were large, coherent and relatively simple. The amplitude of these signals at teleseismic distances (3000–10 000 km, or 30–90 arc degrees) were about the same as those observed at 500–1500 km distance. This led Thirlaway to propose an Earth model where short-range signals are complex because they propagate in the laterally heterogeneous crust and upper mantle. In contrast, propagation paths to teleseismic distances mostly traverse the relatively uniform lower mantle, leading to simple seismic arrivals (figure 5). Thirlaway named the 30–90° range the “source window”, with the implication that if a station could not be established within 500 km of a test site, an array within this window would likely record signals. Additionally, the simpler signals observed might provide a clearer picture of the seismic source. With these results, global monitoring of underground nuclear explosions seemed significantly more feasible. 5 Open in new tabDownload slide Seismograms recorded at three AWRE arrays located at teleseismic distances (EKA: 3540 km, YKA: 9048 km, PMW: 9841 km) from the 18 March 1963 French nuclear test in the Sahara. 5 Open in new tabDownload slide Seismograms recorded at three AWRE arrays located at teleseismic distances (EKA: 3540 km, YKA: 9048 km, PMW: 9841 km) from the 18 March 1963 French nuclear test in the Sahara. With the compelling observations from PMW, the UK began to shift its efforts to recording signals at long range. In March 1962, a group led by Sir Solly Zuckerman (chief scientific advisor to the MoD) flew to Washington to try to persuade US experts to concentrate their research on teleseismic observations. The US were reluctant, as they believed it would be impossible to process all the data collected by these large arrays. Whiteway and his group disagreed and built an analogue–digital hybrid computer, but this was quickly superseded by faster digital minicomputers. Becoming seismologists AWRE constructed and opened a second L-shaped array (EKA) at Eskdalemuir, southern Scotland, in May 1962. Similar to PMW, EKA's 10 km aperture was optimized for regional signals, but the array has proved invaluable for observing teleseismic arrivals. After the earlier success of Operation Seagull, the Navy detonated a series of depth charges on either side of southern Scotland: Operation MacSeagull. The recorded signals allowed the crustal structure beneath the array to be determined. As Carpenter noted, the AWRE team were becoming seismologists. Global coverage was desired and working with Commonwealth partners was key to achieving this. In collaboration with local seismological experts, the UK team deployed the YKA array at Yellowknife, Canada, in November 1962, and two arrays in 1965: GBA at Gauribidanur, India, and WRA at Warramunga, Australia. The shift to recording at teleseismic distances required larger aperture arrays; teleseismic signals arrive more steeply at an array than regional signals and therefore have a longer apparent wavelength. YKA, GBA and WRA all have apertures of 25 km. Including EKA, the four Commonwealth arrays have provided AWE with a near-complete library of seismograms from the entire period of underground testing. With array designs being finalized, others at Blacknest focused on developing sensor technology. In the early 1960s, AWRE designed short-period and long-period seismometers, but when commercially available sensors became available this first phase of instrumentation research ceased. Yet understanding the sensor responses of available seismometers was important, as it allowed the Blacknest team to interpret seismic magnitude estimates from other institutions. In particular, the Soviets had pioneered broadband seismometer recordings, and AWRE adapted commercial seismometers to simulate the response of the Soviet Kirnos sensors. A number of these modified seismometers were installed in specially constructed vaults at Blacknest, and showed that measuring across a broad frequency passband provided a clearer picture of the source. Yet early broadband sensors were difficult to deploy as they were large and heavy. Therefore, throughout the 1970s the Blacknest team collaborated with Reading University to miniaturize broadband seismometers. Cansun Güralp, who went on to found the seismometer manufacturer Güralp Systems Ltd, was one of the PhD students who successfully developed a vertical-component broadband sensor. The focus on wideband recordings led to the development of the first UK broadband seismometer network, UKNET, which Blacknest continues to operate and maintain. Techniques for the digital processing of the large amount of seismic data generated at the arrays were developed by a specialist computing group. Programs were mostly written in FORTRAN IV, a modern language in the early 1960s. Much of the programming was at the cutting edge. For example, the group leader T L van Raalte had to formulate an internal standard for flow diagrams before a universal standard was available. While the team had various minicomputers (including an early DEC PDP-11 that is now in the UK National Museum of Computing's collection), it relied on the mainframe computer on the Aldermaston site for major calculations. For security reasons there was no direct connection to Blacknest. Programs had to be punched on to cards, which were collected twice a day to be fed into the computer, and piles of printout returned. Errors were costly in terms of time; the team learnt to think and type carefully. Seismic source characterization With arrays and computing facilities in place, the team began to focus on interpreting the signals recorded. Experts at the 1958 Geneva meeting realized some earthquakes would be difficult to distinguish from explosions, causing problems for monitoring. Early identification techniques such as observing the direction of ground motion of the first arriving P-wave, and measuring seismogram complexity, had limitations. For P-waves, observation of a downward first ground motion is sufficient to identify a source as an earthquake, as theoretically contained underground explosions should generate compression in all directions. In practice it was often difficult to determine the first motion unambiguously, particularly for small events. Furthermore, although early teleseismic recordings of underground explosions exhibited simple waveforms when compared to earthquakes, some explosions (e.g. large underground explosions at the Soviet test site on Novaya Zemlya) were observed to generate complex seismograms. The group at Blacknest therefore needed to develop robust criteria for seismic source discrimination. Following the 1965 Long Shot nuclear explosion in the Aleutian Islands, one of the few US nuclear explosions fired as a geophysical experiment, seismologists at Blacknest recognized that differences in the relative sizes of body and surface wave phases observed from earthquakes and explosions might provide an effective method for distinguishing between the source types (the phase names describe their propagation path: either through the body of the Earth or along the surface). This approach, known as the mb: MS discriminant (figure 6), has become one of the most widely used techniques for seismic source discrimination. Application of the method was complicated by the variable effects of distance and propagation on body wave magnitudes (mb) and particularly surface wave magnitudes (MS). To overcome this, Marshall and a Canadian colleague, Peter Basham, revised the Rayleigh (surface) wave magnitude formula allowing observations at any distance and over any path to be used. 6 Open in new tabDownload slide Body wave and surface wave magnitudes for selected recent underground nuclear test explosions (red stars), and Eurasian earthquakes from the Reviewed Event Bulletin (REB) produced by the International Data Centre (blue dots). The IDC experimental screening line is the revised relationship proposed by Selby et al. (2012). (Courtesy of N Selby) 6 Open in new tabDownload slide Body wave and surface wave magnitudes for selected recent underground nuclear test explosions (red stars), and Eurasian earthquakes from the Reviewed Event Bulletin (REB) produced by the International Data Centre (blue dots). The IDC experimental screening line is the revised relationship proposed by Selby et al. (2012). (Courtesy of N Selby) Early source-discrimination criteria were based upon empirical evidence so understanding of when and why they worked, and when they might fail, was limited. To address this, Carpenter developed a simplified model of a P-wave seismogram consisting of a source, elastic and non-elastic propagation effects in the Earth, and the response of the recording instrument. Alan Douglas (figure 3), who joined the team in 1965 after working as a government prospector in the Australian outback, developed Carpenter's early modelling work into a co-operative project with John Hudson at the Department of Applied Mathematics and Theoretical Physics at Cambridge University. Theory developed at Cambridge was used to compute model seismograms for explosion and earthquake sources and helped understand why the recently developed mb: MS discriminant worked, and crucially which types of earthquake focal mechanisms might look explosion-like based on this discriminant. Douglas would later lead the team at Blacknest, and literally “wrote the book” on forensic seismology (Douglas 2013). Modelling work also suggested that simplicity rather than complexity could be a criterion for recognizing suspicious disturbances; simple seismograms from earthquakes are only likely to be observed at a limited range of stations. Bob Pearce extended a method that used relative amplitudes of P-waves and surface reflected phases to determine earthquake focal mechanisms, which could also be used to rule out the possibility that a source was an earthquake and identify it as suspicious. As well as being applicable to explosion monitoring, the waveform modelling and relative amplitude methods developed at Blacknest have been used extensively for estimating source parameters (e.g. depth, focal mechanism) of small-to-medium sized earthquakes, making an important contribution to numerous tectonic studies. Other methods developed at Blacknest, such as one to simultaneously estimate the epicentres of seismic events from the same region (Douglas 1967), have also been widely used in both forensic and earthquake seismology. As estimates of the size of nuclear test explosions are often required, relationships between the explosive yield and the expected seismic amplitudes (or magnitudes) were needed. After initially using scaling laws to extrapolate near-field ground motion recordings of NTS explosions to the far-field, Carpenter subsequently utilized his numerical model to predict a relationship between mb and yield. Following the 1974 Threshold Test Ban Treaty (TTBT) between the USA and USSR, which limited the yield of underground nuclear tests to 150 kt, interest in yield estimation increased. Explosions in the USSR were observed to have a higher mb than explosions of a similar size in the US. In what became known as the “yield wars”, US yield estimates of foreign nuclear tests were higher than those estimated at Blacknest, leading some to question the USSR's declaration of compliance with the TTBT. Collaborative analysis, by Marshall at Blacknest and US colleagues at Lawrence Livermore National Laboratory, identified that differences in upper mantle seismic attenuation beneath test sites contributed to the discrepancies in yield estimates. This allowed corrections to be made to the observed mb values and UK yield estimates were eventually accepted. Yield estimation using seismic magnitudes remains a challenge, as Douglas noted wryly in the BGA's first Bullerwell Lecture (in 1981): “There is no creature more deserving of pity than the seismologist who attempts to make sense of magnitude–yield observations … just when you think you can explain why all the observations do not fit a simple straight line, a new observation comes to light which requires another refinement.” International co-operation Seismology and nuclear-test-ban-treaty monitoring are, by their nature, global sciences. To achieve acceptable event location accuracy, and to improve the seismologist's confidence when undertaking source characterization studies, a station network that provides good azimuthal coverage around any event is desirable. This necessitates co-operation between states and the ability to share data in a timely manner. The near-real-time exchange of seismic data in standardized formats is now commonplace, for example through the use of open-access online repositories such as that managed by IRIS (the Incorporated Research Institutes for Seismology). The development of such systems took time and considerable effort; scientists and engineers at Blacknest played their part in bringing this about. One of the crucial vehicles for rapid data analysis and dissemination efforts was the Ad Hoc Group of Scientific Experts, established in 1976. This group (referred to as the GSE; Dahlman et al. 2020) was proposed by the Swedish delegation to the Conference of the Committee on Disarmament (CCD). The GSE secured a mandate to specify the characteristics of a global (seismic) monitoring system, including: the station network required; the data required to facilitate the detection, location and identification of seismic events; and the facilities needed for timely data exchange. In addition, the GSE would endeavour to estimate the detection and identification capabilities of such an international co-operative system. These goals were well aligned with the verification research programme undertaken at Blacknest, and the UK were keen to be involved. Thirlaway and Douglas both travelled to Geneva in early 1976 for informal discussions regarding the scope of the work to be undertaken, and UK scientists would participate in GSE activities for the next 20 years. Global bulletins The Blacknest team had built up experience in the sharing and dissemination of seismic data, in particular through the combination of reported event parameters from the Commonwealth arrays. By 1975, the team coordinating this effort, known as the BDAC (Blacknest Data Analysis Centre), was building digital bulletins of seismic onset times for event hypocentre determination. This involved using an IBM 370/195 computer located at the Rutherford Laboratories in Oxfordshire, connected to Blacknest via a dialled line. These bulletins were made available to US experts via ARPANET (the Advanced Research Projects Agency Network, widely regarded as the ancestor of the modern internet). John Young made a significant contribution to global bulletin production, culminating in the 1995 revision of the Flinn–Engdahl regionalization scheme: the standard for associating event epicentre information with general regions of the Earth. The international system proposed by the GSE in the late 1970s for the exchange and processing of seismological data was largely unproven, and required a significant scaling up of national efforts such as those undertaken by the BDAC. To address this, the GSE coordinated three international technical tests (GSETT1, 2 and 3); UK participation allowed the BDAC to improve the timeliness and efficiency of data reporting. For GSETT1 in 1984, all parameters were measured by hand. By GSETT2 in 1991, the BDAC had invested heavily in automating data transmission and processing, such that interactive analysis tasks were undertaken on a Sun 3/160 computer that supported data transmission to experimental international data centres via an X.25 packet-switched network. The automation meant that only five people were involved in the GSETT2 procedures, rather than the eight required for GSETT1. Crucially, GSETT2 was the first system-wide test that involved the transmission of waveform data (rather than processed parameters); over a six-week period the BDAC supplied just under 10 megabytes of data via the network. The era of global data sharing and collaboration was taking shape. The CTBT era By the early 1990s, political changes led to a series of unilateral nuclear test moratoria, starting with the USSR in 1991. This gave impetus to renewed calls for a CTBT, and in 1993 the Conference on Disarmament gave a mandate to the Ad Hoc Committee on a Nuclear Test Ban to begin negotiations. Early discussions showed that the declared nuclear weapons states would only accept a CTBT if an effective verification regime was provided; the experience gained through the activities of the GSE provided the confidence that such a regime was feasible. Marshall had the knowledge and experience required to provide technical support to the UK CTBT delegation, having been a seismological verification adviser to tripartite Test Ban Treaty negotiations in the late 1970s. He had been awarded an OBE in 1991 for his “long and expert role in seismology”. Marshall was soon helping to find a solution to the long-standing issue of effective verification, first as friend of the chair for non-seismic techniques, then as chair of the International Monitoring System (IMS) Expert Group. He was part of an international team, many of whom he knew well from GSE meetings and data exchanges. They helped outline the requirements for a global system (the IMS) comprising seismic, infrasound, hydroacoustic and radionuclide sensors. The CTBT opened for signature in September 1996 and prohibits all nuclear explosions anywhere on Earth, whether that be in the atmosphere, underwater or underground. Marshall was awarded a CMG, an honour for service in foreign affairs, in 2002. In the 25 years since the treaty opened for signature, Blacknest staff have continued to work alongside international colleagues to understand better the capabilities of the IMS, and where improvements might be made. The team also provides advice on waveform technologies to Ministry of Defence colleagues who are the UK National Authority to the Preparatory Commission (PrepCom) of the Treaty Organization (CTBTO). In particular, David Bowers, who has been the Blacknest science lead since 2001, currently chairs the Waveform Expert Group that advises the technical policymaking body reporting to the PrepCom on aspects of the progressive development of the IMS. This is part of a wider AWE contribution to CTBT verification that includes a radionuclide laboratory, where samples from IMS particulate and noble-gas sensors are analysed, and technical support to on-site inspection activities. Although not yet entered into force, the CTBT has ushered in an era of an almost global de facto moratorium on nuclear testing (only the DPRK has tested in the past 20 years). This poses a unique challenge to forensic seismology. During the development of the IMS there have been very few nuclear tests to act as calibration events for understanding the capability of the new global network. In addition, in the years prior to the CTBT most nuclear explosions took place at a small number of test sites for which propagation effects to long-running stations were well understood. An ongoing challenge in forensic seismology is to detect (and identify) low-yield, or evasively emplaced (e.g. decoupled) clandestine tests, potentially at new test sites for which source-to-station propagation effects are not well characterized. For smaller events the forensic seismologist has to distinguish an underground test from both the larger number of earthquakes that occur at these lower event magnitudes, and mining explosions or collapses. The team at Blacknest continues to address these challenges. An example is its contributions to the CTBTO event-screening expert group. For CTBT purposes, event screening is the process of removing all obvious earthquakes from a compiled global event bulletin, such that it is hoped the residual population contains any nuclear test. One criterion that continues to be used is the mb: MS discriminant. Following the 2006 and 2009 announced DPRK nuclear tests, it was recognized that these small events exhibited mb: MS ratios that fell close to the provisional screening line. Combining previous magnitude determinations made at Blacknest with new data, Selby et al. (2012) proposed a revised screening relationship (figure 6). This work has helped to increase confidence that the politically unpalatable scenario of a future underground nuclear test being screened out will not occur. Despite advances in event characterization, there will always be a subset of seismic disturbances that are difficult to identify as being either an earthquake or explosion. In the past 20 years, Bowers and Neil Selby have developed statistics-based detection algorithms to suppress spatially coherent noise in array seismograms. This has allowed better identification of depth phases (energy that has propagated upwards from a disturbance to the surface, and is then reflected back into the Earth's interior). A long enough time lag between the direct source-to-receiver arrival and a depth phase indicates that an event is too deep to be caused by a near-surface explosion. Ross Heyburn has worked to combine such observations with better understanding of surface-wave spectra and event focal mechanisms to estimate event depths (and the associated uncertainties). As seismic networks have expanded, regional recordings (up to 2000 km from the source) have been utilized to assist the identification of smaller magnitude events. In particular, it was possible to identify the 16 August 1997 event in the Kara Sea, near to the ex-USSR test site at Novaya Zemlya, and the 13 March 2003 event near the Chinese Test site at Lop Nur as most probably earthquakes. As Douglas noted, the main application of identification criteria up to now has been to “acquit the innocent”, demonstrating that seismic disturbances with some characteristics of unannounced tests were earthquakes. Alongside the seismometer network, the IMS contains two other waveform technologies: hydrophones deployed to measure signals from underwater explosions, and microbarometer arrays to measure low-frequency (<20 Hz) acoustic waves propagating in the atmosphere. The team at Blacknest conducts a modest research programme into their application for explosion monitoring purposes. This work has often helped the team to analyse unusual signals from a wide variety of sources. For example, Heyburn et al. (2020) used both hydroacoustic and seismic recordings to provide location estimates, and event characterization, for signals presumed to be associated with the loss of an Argentine Navy submarine, the ARA San Juan, in the South Atlantic Ocean. Data archive When investigating events of interest, a comparison to previously recorded waveforms from comparable sources is useful. Since the early 1960s, recordings from the four Commonwealth arrays were shipped on tape to Blacknest for analysis. As this archive contains signals from >800 presumed nuclear explosions, it is hugely important for our understanding of explosively generated seismic signals. Therefore, since 2007 significant effort has been expended to build a modern digital archive, by translating old digital tape content into modern formats, and digitizing more than 7000 analogue tapes. Data stored on these tapes as frequency modulations of a 270 Hz carrier would have taken 11 person-years to digitize with the original Blacknest equipment, but with modern equipment that digitized the carrier wave at high frequency and demodulated in software it was possible to process most of the tapes in a few years. The tapes had deteriorated in storage, absorbing moisture particularly at the edges where the crucial timecode channel was recorded. The moisture was driven off by baking the tapes in an industrial oven at 50°C for 3–10 days. The digitized data often lack metadata to identify stations, channels and times, and post-digitization quality control by skilled analysts was necessary. One benefit of the archive is finding arrivals that were not identified previously. Figure 7 shows signals from a 0.3 kt Soviet nuclear explosion detonated at the Degelen test site in present-day Kazakhstan. Information regarding this low-yield test was only published after the dissolution of the USSR, and remarkably teleseismic signals from the test were recorded at both EKA, Scotland, and YKA, Canada. Signals such as these provide forensic seismologists with useful data for assessing station performance. 7 Open in new tabDownload slide Signals from an announced 0.3 kt nuclear explosion at the USSR's Semipalatinsk Test Site (now Kazakhstan), fired at a depth of 152 m, recorded at (a) YKA, Canada, and (b) EKA, Scotland. Both time series are array beams, with the beam passband, backazimuth and slowness given below the seismogram. The blue lines above the traces indicate time windows for which an F-statistic signal detector indicates a signal with a signal-to-noise ratio ≥2 exists with ≥90% probability. Both stations are at teleseismic distances from the source (c). 7 Open in new tabDownload slide Signals from an announced 0.3 kt nuclear explosion at the USSR's Semipalatinsk Test Site (now Kazakhstan), fired at a depth of 152 m, recorded at (a) YKA, Canada, and (b) EKA, Scotland. Both time series are array beams, with the beam passband, backazimuth and slowness given below the seismogram. The blue lines above the traces indicate time windows for which an F-statistic signal detector indicates a signal with a signal-to-noise ratio ≥2 exists with ≥90% probability. Both stations are at teleseismic distances from the source (c). Looking to the future Advances in other fields of Earth observation will provide more information that can be combined with seismic data for test-ban-treaty verification. For example, the increased resolution of satellite imagery and interferometric synthetic-aperture radar (InSAR) have allowed ground surface deformation studies to be incorporated into event analyses. As the diversity, sensitivity and number of deployed sensors increases, so does data volume. To address this, event characterization by skilled analysts is being supplemented by robust automated processes capable of analysing low-magnitude events in noisy data. However, event assessments need to be underpinned by understanding of the physical processes that generate the observed seismograms. In December 1972, Thirlaway gave the RAS Harold Jeffreys Lecture, noting: “It is not too unlikely to suppose that one day geophysicists may be required to give evidence before international courts of some kind as to whether or not violations have occurred. The history of technical negotiations associated with the test-ban treaty … warns us that it is not a task to be undertaken lightly.” The requirement to effectively, and expertly, communicate event assessments (and the associated uncertainties) to state officials and the international treaty monitoring community continues to be the focus of the forensic seismology research programme. Over the past 60 years, the Blacknest team has cascaded knowledge of seismic wave generation, propagation and recording down through several generations of scientists and engineers. Douglas and Marshall, among others, were inspired by the early leadership of Carpenter, Whiteway and Thirlaway. Many of the current team then benefited from Douglas and Marshall's expertise and encouragement. Moreover, many students have passed through Blacknest, getting a thorough education in forensic seismology, before having rewarding careers in both academia and industry. Looking forward, the need for expertise to provide technically rigorous assessments of potential nuclear tests is likely to persist. Sustained international effort, including contributions from Blacknest, have ensured that seismology is no longer a “stone-age science”. Yet there is still work to be done to develop monitoring techniques to improve CTBT verification. Hopefully, this brief history can inspire future forensic seismologists by showing an example of what can be achieved through dedicated, patient, geophysical investigation. AUTHORS Open in new tabDownload slide Open in new tabDownload slide Open in new tabDownload slide Open in new tabDownload slide Open in new tabDownload slide Open in new tabDownload slide Open in new tabDownload slide Open in new tabDownload slide Open in new tabDownload slide Open in new tabDownload slide Open in new tabDownload slide Open in new tabDownload slide Open in new tabDownload slide Open in new tabDownload slide D N Green, R Heyburn, J Keeble, A Nippress, S E J Nippress, S Peacock and J B Young (pictured top to bottom)are all current members of the forensic seismology team at Blacknest. Researching this article has provided an excellent way for them to learn about the team's past achievements – except for John Young: he was remembering them REFERENCES Bergkvist N-O & Ferm R 2000 Nuclear Explosions 1945–1998 technical report FOA-R-00-01572-180-SE (Swedish Defence Research Establishment [FOA] and Stockholm International Peace Research Institute [SIPRI]) Dahlman O et al. 2020 Nonproliferation Review doi:10.1080/10736700.2020.1764717 Douglas A 1967 Nature 215 47 Douglas A 2013 Forensic Seismology and Nuclear Test Bans ( Cambridge University Press ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Heyburn R D et al. 2020 Geophys. J. Int. 223 289 Selby N D et al. 2012 Bull. Seism. Soc. Am. 102 88 FURTHER READING Atomic Weapons Research Establishment 1965 The Detection and Recognition of Underground Explosions technical report (UKAEA) Bates C C et al. 1982 Geophysics in the Affairs of Man: A Personalized History of Exploration Geophysics and its Allied Sciences of Seismology and Oceanography ( Elsevier ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Birtill J W & Whiteway F E 1965 The application of phased arrays to the analysis of seismic body waves Phil. Trans. R. Soc. Lond. A 258 421 Google Scholar OpenURL Placeholder Text WorldCat Bowers D 2015 Obituary: Alan Douglas (1936–2015) A&G 56 4.11 Google Scholar OpenURL Placeholder Text WorldCat Bowers D & Selby N D 2009 Forensic seismology and the Comprehensive Nuclear-Test-Ban Treaty Ann. Rev. Earth & Planet. Sci. 37 209 Google Scholar OpenURL Placeholder Text WorldCat Carpenter E W 1966 A quantitative evaluation of teleseismic explosion records Proc. R. Soc. Lond. A 290 396 Google Scholar OpenURL Placeholder Text WorldCat Douglas A 1981 Seismic source identification: a review of past and present research efforts , in Identification of Seismic Sources – Earthquake or Underground Explosion 1 ( Springer ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Douglas A 1982 Two problems in forensic seismology twenty years on Q. J. R. Astron. Soc. 23 26 Google Scholar OpenURL Placeholder Text WorldCat Douglas A 2001 The UK Broadband Seismology Network A&G 42 2.21 Google Scholar OpenURL Placeholder Text WorldCat Douglas A 2002 Seismometer arrays, their use in earthquake and test ban seismology , in International Handbook of Earthquake & Engineering Seismology 357 Google Scholar OpenURL Placeholder Text WorldCat Douglas A 2013 Obituary: Peter David Marshall (1937–2012) A&G 54 1.35 Google Scholar OpenURL Placeholder Text WorldCat Douglas A et al. 1972 A quantitative evaluation of seismic signals at teleseismic distances – III. Computed P and Rayleigh wave seismograms Geophys. J. R. Astron. Soc. 28 385 Google Scholar OpenURL Placeholder Text WorldCat Douglas A & Marshall P D 2010 Obituary: Henry (Hal) Ivison Shipley Thirlaway (1917–2009) A&G 51 1.36 Google Scholar OpenURL Placeholder Text WorldCat Fox B D et al. 2012 Shallow seismic source parameter determination using intermediate-period surface wave amplitude spectra Geophys. J. Int. 191 601 Google Scholar OpenURL Placeholder Text WorldCat Marshall P D & Basham P W 1972 Discrimination between earthquakes and underground explosions employing an improved MS scale Geophys. J. Int. 28 431 Google Scholar OpenURL Placeholder Text WorldCat Marshall P D et al. 1979 Magnitude corrections for attenuation in the upper mantle Geophys. J. R. Astron. Soc. 57 609 Google Scholar OpenURL Placeholder Text WorldCat Nippress S E J et al. 2017 The 2008 and 2012 Moosiyan earthquake sequences: rare insights into the role of strike slip and thrust faulting within the Simply Folded Belt (Iran) Bull. Seism. Soc. Am. 107 1625 Google Scholar OpenURL Placeholder Text WorldCat Peacock S et al. 2017 Joint maximum-likelihood magnitudes of presumed underground nuclear test explosions Geophys. J. Int. 210 621 Google Scholar OpenURL Placeholder Text WorldCat Pearce R G 1980 Fault plane solutions using relative amplitudes of P and surface reflections: further studies Geophys. J. R. Astron. Soc. 60 459 Google Scholar OpenURL Placeholder Text WorldCat Thirlaway H I S 1973 Forensic seismology Q. J. R. Astron. Soc. 14 297 Google Scholar OpenURL Placeholder Text WorldCat Walker J R 2010 British Nuclear Weapons and the Test Ban 1954–1973: Britain, the United States, Weapons Policies and Nuclear Testing: Tensions and Contradictions ( Ashgate Publishing ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Whiteway F E 1966 The use of arrays for earthquake seismology Proc. R. Soc. Lond. A 290 328 Google Scholar OpenURL Placeholder Text WorldCat Young J B B W et al. 1996 The Flinn-Engdahl Regionalisation Scheme: the 1995 Revision Phys. Earth Planet. Int. 96 223 Google Scholar OpenURL Placeholder Text WorldCat © British Crown Owned Copyright / AWE This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
TI - Sixty years of forensic seismology at AWE Blacknest
JF - Astronomy & Geophysics
DO - 10.1093/astrogeo/atab082
DA - 2021-08-01
UR - https://www.deepdyve.com/lp/oxford-university-press/sixty-years-of-forensic-seismology-at-awe-blacknest-LDMn2M4NSI
SP - 4.36
EP - 4.42
VL - 62
IS - 4
DP - DeepDyve
ER -