CHARACTERIZATION OF MINING EXPLOSIONS AT REGIONAL DISTANCES: IMPLICATIONS WITH THE INTERNATIONAL MONITORING SYSTEM

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1 CHARACTERIZATION OF MINING EXPLOSIONS AT REGIONAL DISTANCES: IMPLICATIONS WITH THE INTERNATIONAL MONITORING SYSTEM Brian W. Stump, 1 Michael A. H. Hedlin, 2 D. Craig Pearson, 3 and Vindell Hsu 4 Received 19 October 1998; revised 17 January 2002; accepted 4 October 2002; published 31 December [1] The International Monitoring System (IMS) being constructed in support of the Comprehensive Nuclear- Test-Ban Treaty introduces new opportunities to nuclear test monitoring by providing open access to global data from seismic, hydroacoustic, infrasonic, and radionuclide sensors. These sensors will detect myriad natural and manmade events and can be used to identify those that have explosive characteristics and therefore might be clandestine nuclear tests. Detection and identification of seismic events must be conducted at a lower magnitude threshold (m b 3.5 and lower) than has been previously considered. Concomitant with the lower monitoring threshold will be an increased number of events that must be scrutinized. This collection will be largely composed of regional observations in which the seismic waves have traversed complex geological structures. High-fidelity regional, geophysical models will be needed to support accurate location and source identification. Source identification will not be limited to the separation of single-fired nuclear explosions from earthquakes as in previous testing treaties. The lowermagnitude threshold and increased reliance on regional observations assures that mining explosions will be detected by the monitoring system. It is important that the signals from mining explosions are properly identified to avoid false alarms of the monitoring system. Cooperation with the mining industry, including deployment of close-in instrumentation and extensive documentation of the explosions, provides critical information for interpreting the performance of regional discriminants. Linkage of these observations to appropriate physical models of the blasting process is also enhanced through this cooperative research effort. A number of discriminants for characterizing mining explosions have been identified including P to L g ratios at high and low frequencies, surface wave to high-frequency body wave amplitudes, R g at short distances, high- and low-frequency time-independent spectral modulations, signal correlation, and temporal clustering. Variable performance of individual discriminants results from such factors as mine-specific blasting practices and the complexity of the regional wave propagation. This variability attests to the need for a suite of region-specific discriminants. Regional calibration with modest-size, single-fired contained chemical explosions can provide a basis for developing region-specific procedures. Broadband data provide the basis for the most robust set of discriminants. The inclusion of infrasonic data as part of the IMS introduces a potential for the combined use of seismic and infrasonic data for the identification of nearsurface explosions. The generation and, to a greater extent, the propagation of mining explosion infrasonic signals is not well understood, but empirical data attest to its future utility. Evidence for the accidental, nearsimultaneous detonation of a large amount of explosives during standard delay-fired explosions is presented. Events such as these have single-fired characteristics and may prove to be problematic in discrimination analysis. INDEX TERMS: 7219 Seismology: Nuclear explosion seismology; 7205 Seismology: Continental crust (1242); 7294 Seismology: Instruments and techniques; KEYWORDS: regional, monitoring, seismology, detection, location, characterization Citation: Stump, B. W., M. A. H. Hedlin, D. C. Pearson, and V. Hsu, Characterization of mining explosions at regional distances: Implications with the International Monitoring System, Rev. Geophys., 40(4), 1011, doi:1998rg000048, Department of Geological Sciences, Southern Methodist University, Dallas, Texas, USA. 2 Institute of Geophysics and Planetary Physics, University of California, San Diego, California, USA. 3 Los Alamos National Laboratory, Los Alamos, New Mexico, USA. 4 Air Force Technical Applications Center, Patrick Air Force Base, Florida, USA. 1. PROBLEM STATEMENT 1.1. Monitoring Goals [2] After many years of international discussion and debate the Comprehensive Nuclear-Test-Ban Treaty(CTBT) was tabled for signature at the United Nations in September of It is quite different from previous nuclear testing treaties, such as the Threshold Test Ban Copyright 2002 by the American Geophysical Union. Reviews of Geophysics, 40, 4 / December /02/1998RG , doi: /1998rg000048

2 2-2 STUMP ET AL.: REGIONAL SEISMOGRAMS FROM MINES 40, 4 / REVIEWS OF GEOPHYSICS Figure 1. Body wave magnitude versus explosive yield [after Murphy, 1996]. The line thickness represents the effect of variability in propagation paths expected for stable and tectonic regions (0.4 m b ). Well-coupled materials are saturated, low-porosity media similar to those found at the former Semipalitinsk Test Site. Poorly coupled materials are dry, porous media similar to those found at the Nevada Test Site. Cavity decoupling magnitudes are estimated. Treaty (TTBT), in that it includes an extensive international monitoring system and concomitant interest in monitoring to a much smaller magnitude. The TTBT focused attention on seismic events of relatively large magnitude (m b 6[Adushkin, 1996]) near the testing threshold of 150 kt. These large events were well observed at teleseismic distances. The primary problems for the TTBT were the establishment of magnitude-yield relations and the discrimination of earthquake and other sources from nuclear explosion sources [Office of Technology Assessment, 1988]. [3] The establishment of the International Monitoring System (IMS) provides a new opportunity to study natural and man-made events of small magnitude using a global network of seismic, infrasonic, hydroacoustic, and radionuclide sensors. Relationships between seismic magnitude and yield for nuclear explosions detonated in different materials and under a variety of conditions are summarized in Figure 1. The magnitude threshold one wishes to monitor depends on the material as well as the size of the explosion. Khalturin et al. [1998] studied the magnitudes of mining explosions throughout the world and report magnitudes reaching the mid fours for mining explosions, with most events producing smaller magnitudes. Observational data illustrate that mining explosions seldom exceed magnitude 4, while mine collapses approach magnitude 5 [Heuze and Stump, 1999]. This magnitude range overlaps with that expected for small nuclear explosions (Figure 1). Comparison of the observed magnitudes with that expected for a fully coupled single-fired explosion of the same size led Khalturin et al. [1998] to conclude that mining explosion magnitudes are depressed by magnitude units (for most events magnitude units) compared with contained, fully coupled, single-fired explosions. [4] Increased reliance on regional observations is dictated by this need to detect and identify small events. There remain unanswered questions associated with the detection, location, and identification of small events using regional seismic data. The National Research Council [1997] has stated that research in many areas of seismology is needed to reach monitoring goals. Seismic research recommendations proposed by the National Research Council include the following: (1) improve characterization and modeling of regional seismic wave propagation effects; (2) improve capabilities to detect, locate, and identify small events using sparsely distributed seismic arrays; (3) perform theoretical and observational investigations of the full range of seismic sources; and (4) develop high-resolution velocity models for regions of monitoring concern. Complementary recommendations were made for problems associated with infrasonic, hydroacoustic, and radionuclide monitoring as well Detection [5] Identification of mining explosions first depends on detection of a signal by the IMS [Claussen, 1996; Wüster et al., 2000]. Small signals with modest signal to noise ratios ( 5) make detection difficult. Claussen [1996] has estimated seismic detection thresholds for a fully deployed, 50 primary station IMS. The estimates are based upon achieving a 99% probability of three or more P (compressional wave) detections. In Eurasia the expected detection threshold is magnitude 3.75, whereas in other areas it is as low as magnitude These detection capabilities rely on the fact that a number of the IMS stations consist of arrays of closely spaced (several kilometers) seismometers that provide the opportunity for improving signal detection through beam forming [Kværna and Ringdal, 1996]. The density, type, and quality of observing stations likewise affect detection and classification of later arriving seismic phases (i.e., S (shear wave), L g (trapped, higher-mode surface waves), and R (Rayleigh-type surface waves)). Once detections at multiple stations, possibly with multiple phases, are achieved, the individual phases are associated with one another to form a single event Location [6] Typically, arrival time data can be determined from regional seismic phases to within 0.1 s where compressional (P) velocities range from 5 to 8 km/s Theoretical expectations for location errors would be on the order of 1 2 km. However, variations in the crust and upper mantle velocity structure result in much larger location errors. On the basis of CTBT requirements for an on-site inspection a location error ellipse with a maximum total area of 1000 km 2 (17.8 km radius) has been established. Reduction in location errors, including

3 40, 4 / REVIEWS OF GEOPHYSICS STUMP ET AL.: REGIONAL SEISMOGRAMS FROM MINES 2-3 Figure 2. Distribution of primary (dark) and auxiliary (light) seismic stations in the International Monitoring System (IMS). Array sites are represented as circles, while three-component stations are triangles. systematic bias introduced by the three-dimensional velocity structure of the Earth, can be obtained by deploying dense networks of seismic observing stations and calibration of the effects of the three-dimensional velocity structure using known, documented sources including mining explosions Identification [7] Once events are formed and located with adequate accuracy and precision, they must be identified as to the type of seismic source. The difficulty in distinguishing a contained, single-fired (simultaneously detonated) chemical explosion from a nuclear explosion was illustrated by the Non-Proliferation Experiment, in which a large, contained chemical explosion was detonated in an area of the Nevada Test Site where previous nuclear tests had been conducted [Denny, 1994]. The observed seismic data provided no means of classifying the single-fired chemical explosion as being distinguishable from a nuclear explosion. This observation motivates the need to develop discriminants for mining explosions based on the fact that the vast majority are delay-fired, that is, detonated as a sequence of many, relatively small explosions distributed in space and time [Langefors and Kihlström, 1978; Borg et al., 1987]. [8] This paper focuses on the characteristics of seismic waves from mining explosions that provide for the robust identification of these events. Outstanding research issues will also be identified. The complementary nature of low-frequency atmospheric waves (infrasound) in the Hz band will also be reviewed. Utilization of infrasound signals for event identification is not as developed as seismic techniques but offers promise for sources near the solid Earth-atmosphere boundary [Bedard and Georges, 2000]. 2. INTERNATIONAL MONITORING SYSTEM [9] Four monitoring technologies compose the IMS. Two of these, seismic and infrasound, are of primary interest in the study of land-based, man-made events, especially mining explosions. Figure 2 illustrates the planned worldwide distribution of seismic stations. This network includes 50 primary stations that continuously transmit data and an additional 120 auxiliary stations from which data can be requested. [10] The average station spacing of the primary stations on the continents is on the order of 2000 km (Figure 2). The primary and auxiliary seismic station configuration is derived from the need to detect, locate, and characterize small events. Given such a station configuration and the inability of small-magnitude events to propagate seismic phases with amplitudes greater than typical Earth noise beyond 2000 km, many of the signals will be detected by only a few of the primary and auxiliary seismic stations of the IMS. Propagation effects at ranges 2000 km (regional distances) are strongly influenced by crust and upper mantle structure. The heterogeneity of such paths motivates the need to calibrate path effects globally [Jenkins and Sereno, 2001]. [11] Figure 3 quantifies the interplay between the number of observing stations and the distance at which seismic observations will be made for the IMS. The percent of landmass within range of N primary and auxiliary stations (N 1 5) is plotted for ranges out to 20. IMS seismic station coverage assures that over 70% of Earth s landmass is within 10 of one station. If the sources are energetic enough to propagate to 15, then 50% of the landmass has three IMS observations. This result suggests that multiple regional

4 2-4 STUMP ET AL.: REGIONAL SEISMOGRAMS FROM MINES 40, 4 / REVIEWS OF GEOPHYSICS Figure 3. Coverage by IMS seismic stations. Coverage of the Earth s landmass permitted by the current and proposed IMS primary and secondary seismic networks is indicated by the solid curves. Single-station coverage (N 1) is nearly complete within 10 of the source. If multiple recordings of an event are required for adequate source characterization (N 1), most of the observations will be made at mid to far regional or teleseismic range. Coverage of three prominent mining regions is indicated by symbols. Krasnogorsky, the main Kuzbass mine in Russia, at 53.6 N, 87.8 E (diamonds); Powder River Basin in Wyoming at 43.7 N, W (triangles); and Kursk, Russia, at 51.8 N, 36.5 E (circles). These mining regions will be monitored somewhat more closely than the average. For example, three stations are within 8 of the Kuzbass mine. Just 8% of the Earth s landmass has better coverage. The dashed curve is coverage given by the IMS networks (current and proposed) and existing stations in the Global Seismic Network (GSN). Many stations in the GSN that are not already in the IMS are located on oceanic islands, and so the improvement is limited. signals from some mining regions may be possible. The IMS coverage for three well-known mining regions is included in Figure 3: Powder River Basin of Wyoming (triangles); Kuzbass of Russia (diamonds); and Kursk of Russia (circles). There are four IMS seismic stations within 14 of the Powder River Basin. All three regions have one station within 5. These results illustrate that regional distance seismic waveforms will play a key role in characterizing small mining explosions. [12] The infrasound network will consist of 60 stations distributed globally, some colocated with primary or auxiliary seismic stations (Figure 4). It s more sparse sampling of the globe suggests that the typical distances the waves will travel to these stations will be greater than for the seismic waves. The combined use of seismic and infrasound data for the identification of near-surface mining explosions is discussed in section 4.7. In such a case it is useful to have the complimentary seismic and infrasound sensors colocated. [13] If an event is not readily identifiable using data from the IMS exclusively, two additional provisions are included in the Treaty. The first is consultation and clarification, a process by which countries can ask one another for information that might resolve a questionable event. The second is confidence-building measures (CBM) which are cooperative actions that can be taken by nations to improve the performance of the monitoring system (i.e., sharing of information and conducting calibration experiments). Included within CBM is a voluntary provision for the exchange of information on large mining explosions. This information could be used for calibration of regional travel times as well as supporting event identification efforts. If these provisions fail to resolve all questions, then there are provisions to request an on-site inspection. Figure 4. Distribution of infrasonic stations in the IMS.

5 40, 4 / REVIEWS OF GEOPHYSICS STUMP ET AL.: REGIONAL SEISMOGRAMS FROM MINES 2-5 Figure 5. Mining events and earthquakes that are part of the routine U.S. mining event catalog published by the U.S. Geological Survey (available at 3. EVENT TYPES, NUMBERS, AND LOCATIONS [14] Mining explosions are used throughout the world in the recovery of energy and mineral resources. In assessing the types of mining activities and the numbers of mine-related events that might be observed at regional distances, Richards et al. [1992] conducted a preliminary survey of blasting activity in the United States. The U.S. Department of Energy in cooperation with the U.S. mining industry assessed the effect mining operations in the United States would have on regional monitoring [Heuze and Stump, 1999]. The study concluded that the largest mine blasts and ground failures will be detected by the IMS and that the resulting signals must be identified as distinct from those generated by a small nuclear explosion. [15] Regional earthquake bulletins often discard mining explosions since their focus is on tectonic activity, and so the quantification of the numbers of mining explosions that generate regional seismic signals has been difficult to estimate [Richards et al., 1992]. The United States Geological Survey (USGS) began in May 1997 to identify mine-related events for the 48 conterminous states in an attempt to quantify the numbers of events that are observed seismically. The resulting catalog, Routine United States Mining Seismicity (available at includes routine explosions and planned roof collapses at mines and quarries. Events with magnitude 2.5 or greater in the western United States and 3.0 or greater in the eastern United States are included. Figure 5 taken from this catalog shows the locations of the mining events in this catalog with comparison to earthquake locations. The monthly bulletins from the USGS indicate that there are several U.S. mine-related events per day. Similar patterns exist for other regions of the world [Leith, 1994]. [16] These routine mining event locations delineate a number of regions in the United States: surface coal mines in the Powder River Basin of NE Wyoming; surface copper mines in SE Arizona and SW New Mexico; surface iron mines in upper Minnesota; surface and underground coal mines in West Virginia; and surface coal mines in NE Arizona and NW New Mexico. The map in Figure 5 illustrates that mining events may be isolated in a number of local regions Surface and Underground Mining Explosions [17] There are many different types of explosions used by the mining industry (Table 1) [Borg et al., 1987; Langefors and Kihlström, 1978; Persson et al., 1994; Dowding, 1985]. Mining explosions that utilize the largest amount of explosives per blast are those associated with surface coal mining and open pit metal mining. For surface coal mining, explosions are detonated in the overburden material that lies on top of the coal. These

6 2-6 STUMP ET AL.: REGIONAL SEISMOGRAMS FROM MINES 40, 4 / REVIEWS OF GEOPHYSICS Table 1. Types of U.S. Mining Operations and Their Typical Use of Explosives Type of Operation Explosives/Hole, kg Total Explosives, t Regional Signal Likelihood Surface coal and above likely Open pit and above likely Underground to 500 likely to possible Oil shale to 150 possible Dredging to 130 possible Quarrying to 130 possible Construction to 45 unlikely Reclamation 1 50 to 22 unlikely Specialty insignificant unlikely explosions are designed to cast the overburden into the mine pit, exposing the coal for recovery. Open pit metal mining also uses relatively large explosions designed to fracture the rock for extraction. Underground operations use explosives, but, because of the need to maintain the integrity of underground openings the explosions are most often small relative to surface operations [Hamrin, 1986]. Rock quarries comprise the largest number of surface mines but typically utilize smaller amounts of explosives [Borg et al., 1987]. [18] The ability of each of these types of mining explosions to generate regional seismic signals that can be detected and located by the monitoring system depends on a number of conditions. The efficiency of coupling explosive energy into seismic waves is fundamental to regional detection. The more poorly coupled an explosion is the less likely it will be detectable. Poor coupling is often due to a lack of confinement of the explosive, compressible materials in contact with the explosives, or the detonation sequence of individual explosive charges in a delay-fired mining blast. Khalturin et al. [1998] have documented that mining explosions are decoupled by magnitude units. Larger, more impulsive events are of most interest since they produce signals of the amplitude and character observed from underground nuclear explosions Rock Bursts and Collapses [19] A number of other types of events associated with mining will be detected by the IMS. The most important event types are listed in Table 2, including a qualitative estimate of the likelihood that signals will be detected at regional seismic stations. Underground failures in room and pillar mines [Zipf, 1996; Pechmann et al., 1995], rock Table 2. Mining Operations and Underground Failures Type of Collapse Room and pillar mine failure Rock bursts Coal bumps First caves in long-wall coal mining Block caving Shrinkage stopping Regional Signal Likelihood likely likely likely possible possible to unlikely possible to unlikely bursts [Young, 1993; Wong and McGarr, 1990; Blake, 1984], and coal bumps can all produce seismic signals that exhibit characteristics similar to those of nuclear explosions. These event types have been detected, analyzed, and in some cases modeled at regional distances [Yang et al., 1998; Taylor, 1994; Bennett et al., 1994, 1995, 1996; Walter, 1995; Pechmann et al., 1995; Gibowicz, 1990, 1993]. Seismic characterization tools similar to those used for mining explosions are under development although they are not explicitly addressed in this review. 4. EVENT CHARACTERIZATION TECHNIQUES [20] While significant mining activity occurs worldwide, blasting practices are tailored to local needs and are thus highly variable. Development of a strong physical basis for identification of mining blasts provides the foundation for implementation of characterization tools that can take this variability into account. It is important to link regional signal characteristics directly to documented blasting practices and phenomena. [21] Since the early 1960s [Devine, 1962; Devine and Duvall, 1963; Pollack, 1963; Frantti, 1963], research has been conducted to develop techniques for characterizing ground motion from mining explosions. These early studies were focused on the reduction of ground motion by delay-firing, a technique in which individual boreholes in an explosive array are fired at different time delays [Borg et al., 1987]. The purpose of these delays is to maximize the fragmentation and, possibly, the movement of rock and dirt while minimizing ground motion [Dowding, 1985]. [22] Successful event identification employs analysis techniques utilizing regional seismic data that identify characteristics for mining explosions that are distinct from those produced by contained, single-fired explosions and earthquakes. Special consideration is given to techniques that separate single-fired, contained explosions from delay-fired explosions typical of mining operations [Baumgardt and Ziegler, 1988; Smith, 1989; Hedlin et al., 1989; Chapman et al., 1992]. Procedures must minimize false alarms, the misidentification of a mining

7 40, 4 / REVIEWS OF GEOPHYSICS STUMP ET AL.: REGIONAL SEISMOGRAMS FROM MINES 2-7 explosion as a contained, single-fired explosion, possibly nuclear. [23] For purposes of illustration some event characterization techniques reviewed in this paper are documented with observational data from a set of regional experiments based upon large-scale coal cast blasting and contained, single-fired calibration explosions conducted in the Powder River Basin of NE Wyoming [Pearson et al., 1995; Hedlin et al., 2002]. These blasts are used since they were well documented as a result of cooperation with the mine. This ground truth information is critical to the physical understanding of the resulting discriminants. Regional observations were obtained from portable broadband stations at km and the IMS primary array, PDAR, at 360 km from the mine. [24] We now turn to the central focus of this review, a discussion of the collection of tools used for identifying mining explosions P/L g at High and Low Frequency [25] One of the most robust regional discriminants for separating earthquakes from explosions has been the high-frequency ratio of P to L g energy in comparison to low-frequency ratios for the same phases [Dysart and Pulli, 1990; Baumgardt and Young, 1990; Kim et al., 1993; Walter et al, 1995; Hartse et al., 1997]. Explosions and earthquakes show a small P to L g ratio at intermediate frequencies ( 1 Hz), while explosions show a large ratio at the higher frequencies, separating them from the earthquake population. The physical process that underlies this discriminant is the preferential excitation by explosions of P energy relative to S energy. Regionspecific phase excitation and propagation must be understood for proper interpretation of the discriminants [Xie and Patton, 1999]. A number of researchers have suggested methodologies for correcting regional waveforms for propagation path effects prior to application of the discriminants [Kennett, 1993; Taylor and Hartse, 1998; Phillips, 1999; Rogers et al., 1999; Schultz et al., 1998]. In new regions, modest-size explosions have provided calibration [Gitterman and Shapira, 2001]. [26] Figure 6 compares seismograms from a singlefired contained explosion (circle to right of waveforms) and delay-fired mining explosions (rectangles to right of waveforms) detonated in the same mine in the Powder River Basin of Wyoming and observed at the IMS array, PDAR. The data illustrate the robust excitation of highfrequency P energy (8 16 Hz) relative to the low-frequency L g (1 2 Hz) for both the contained, single-fired calibration explosion and delay-fired mining explosion. [27] At intermediate frequencies (1 2 Hz) the L g amplitude dominates over the P waves for all events. These ratios are similar to those successfully applied to separate nuclear explosions from earthquake populations [Hartse et al., 1997]. In this example it is not possible to distinguish the contained, single-fired explosion from the delay-fired mining explosion using a single discriminant and thus the requirement for additional discrimination tools. Wüster [1993] found in a study of chemical explosions and earthquakes in central Europe that no single discriminant separated the earthquake and explosion population as well. [28] Su et al. [1991] successfully used high-frequency regional coda waves for discrimination of earthquakes and quarry blasts. Differences for the quarry blasts and earthquakes were attributed to the source functions for the two different source types. [29] A third comparison between the single-fired and cast blasts is made at very low frequency ( Hz) in Figure 6. The mining explosions show clear surface wave arrivals, while the single-fired explosion has no signal above the background noise (note differences in amplitude scales for the individual plots). The mining explosions can be used as a single-fired explosion surrogate for P to L g ratios at intermediate and high frequencies. Utilization of very low frequencies in combination with the high frequencies may allow separation of the single-fired explosions from the large cast blasts Surface Wave to Body Wave Amplitudes [30] Teleseismic monitoring, developed for the TTBT, benefited from the fact that events to be characterized produced large-amplitude waveforms providing teleseismic body and surface waves with large signal to noise ratios. Under that regime the difference between surface wave magnitude (M S ) and body wave magnitude (m b ) was found to be a robust discriminant of nuclear explosions from earthquakes. Recent work [Barker et al., 1993; Bonner et al., 1996; Anandakrishnan et al., 1997; Yang, 1998] has investigated long-period surface wave and short-period body wave excitation at regional distances for both typical mining explosions and single-fired contained explosions (Figure 7). These studies demonstrate that the large cast blasts associated with overburden removal in surface coal mines produce long-period waves that are not observed from single-fired contained explosions. The utilization of broadband data provides a mechanism for separating some types of mining explosions from single-fired contained explosions. The long duration of many cast blasts (several seconds) and the mass of the material that is cast into the open pit of the mine accounts for this earthquake-like characteristic (enriched surface wave energy) from these mining explosions [Hedlin et al., 2002; Bonner and Orrey, 2001]. [31] R g can also be used to identify shallow events, including mining explosions, if observations are available at relatively short distances ( 200 km) [Wüster, 1993; Kafka and Dollin, 1985; Kafka and Reiter, 1987; Woods et al., 1989]. Bonner et al. [1996] have used R g waves to quantify quarry blasts at short distances and short periods. This phase is not an important discriminant at greater ranges since lateral variations in the near-surface material properties preclude its propagation to very great ranges ( 200 km).

8 2-8 STUMP ET AL.: REGIONAL SEISMOGRAMS FROM MINES 40, 4 / REVIEWS OF GEOPHYSICS Figure 6. Band-pass-filtered regional seismograms from a single-fired, contained explosion (circle to right of waveform) and two delay-fired mining explosions (rectangles to right of waveforms, size of rectangle proportional to size of explosion) at 360 km. The height of the vertical bar to the right of each waveform is proportional to the relative peak amplitude Time-Varying Spectral Estimates [32] Mining blasts conducted with a regular pattern of delays between individual explosions in the blast exhibit interference effects which result in a repeated frequency domain pattern of high spectral values transitioning to low values known as scalloping [Bell and Alexander, 1977; Baumgardt and Ziegler, 1988; Hedlin et al., 1989; Hedlin, 1998]. These scallops are not confined to the

9 40, 4 / REVIEWS OF GEOPHYSICS STUMP ET AL.: REGIONAL SEISMOGRAMS FROM MINES 2-9 Figure 7. Peak long-period amplitudes (10 20 s) plotted against peak high-frequency amplitudes (1 10 Hz) for mining cast blasts (hexagons, diamonds, crosses, and stars) and single-fired contained explosions (squares and triangles) detected at regional distances. All observations are made at the same range from the mine (200 km) but at different azimuths. Dotted lines are constant logarithmic long-period to short-period ratios. The sympathetic detonation explosion had a large, near-simultaneous detonation of a significant fraction of the total explosives. main arrivals but persist into the P and S coda and can be visualized using time-varying spectral estimates [Bell and Alexander, 1977; Hedlin et al., 1989]. The effectiveness of this identification procedure depends on the details of the actual blasting practice as well as the bandwidth of the observational data. Long delays produce interference effects beginning at low frequencies, while short delays manifest themselves at higher frequencies that are rapidly attenuated by wave propagation to regional distances. The utilization of high frequencies for identification has led to proposals for inclusion of seismic stations within active mining regions. [33] A comprehensive study of spectral banding in Wyoming [Carr and Garbin, 1998] used a regional data set of 118 known earthquakes and 176 known delay-fired explosions to test spectral banding for identifying mining explosions. Only 50% of the explosions are properly identified based on spectral banding criteria using 16 Hz data. When higher-bandwidth data were used (to 50 Hz), there was no improvement in identification. Delay times for cast blasts in the Powder River Basin of Wyoming can be complex and can produce irregular spectral interference effects. Spectra that are repetitive in the frequency domain can be characterized by the cepstrum (Fourier transform of the log of the spectrum). Numerous authors (including Baumgardt and Ziegler [1988]; Dysart and Pulli [1990], and Shumway [1996]) have successfully used the cepstrum to identify delayfired explosions. The cepstrum indirectly senses signal repetition that occurs at the source or is acquired during propagation by quantifying regularly spaced spectral modulations Low-Frequency Modulations [34] While time-independent spectral modulations due to millisecond intershot delays exist at high frequencies (generally above 10 Hz), recent work by Baumgardt and Ziegler [1988], Hedlin et al. [1990], Gitterman and van Eck [1993], and Hedlin [1997] has revealed significant spectral modulations below 10 Hz. An example, taken from observations of a Wyoming cast blast using an azimuthal network at 200 km, is shown in Figure 8. [35] These modulations are largely independent of recording component and time. It is clear that these features are not directly due to delay firing but are most likely the result of long inter-row delays and the extended size of the source in both space and time (source finiteness) and are not produced by instantaneous explosions (Figure 8). The low frequencies are not attenuated as severely as the high frequencies and thus may be

10 2-10 STUMP ET AL.: REGIONAL SEISMOGRAMS FROM MINES 40, 4 / REVIEWS OF GEOPHYSICS Figure 8. Detrended log average spectra taken from the azimuthal network deployed in Wyoming in 1996 and 1997 [Hedlin et al., 2002]. Shown in red (solid) are time-averaged spectra taken from a Wyoming cast shot. In blue (dashed) are spectra from a 7300 kg calibration shot (contained and single-fired) detonated in the same mine in Each curve represents the average of 46 spectral estimates taken from staggered 15 s windows that span 125 s of P and S wave coda. Each three-component station yields three curves. useful for discrimination at mid regional distances ( km) Correlation Analysis [36] Recognizing seismic signatures from a particular mine provides an additional tool for characterizing an event. Such a procedure relies on empirical observations from a single mining operation. Different blasts from an operation must provide repetitive signals for the procedure to be useful. Harris [1991] has illustrated this technique in a single mine, while Israelson [1991] and Riviere- Barbier and Grant [1993] have used the technique for identifying individual mines in a mining region. Without detailed information from cooperating mines it is not known if these correlation methods identify distinct mines, portions of mines, or various mining practices. [37] A set of single-fired contained explosions is used to illustrate the correlation between closely spaced explosions. Seven, single-fired explosions were detonated in the Black Thunder coal mine, six closely spaced ( 100 m) and the seventh located 3 km away (Figure 9). [38] The amount of explosive ranged from 2500 to

11 40, 4 / REVIEWS OF GEOPHYSICS STUMP ET AL.: REGIONAL SEISMOGRAMS FROM MINES Figure 9. (left) Map of mine 1 to the left with stars at the locations of the seven, single-fired explosions. (right) Cross-correlation functions of the regional seismograms from the seven, single-fired explosions are plotted on the right. Seismogram from shot 5 is used to correlate with all other shots. 22,100 kg. The signal to noise ratios (SNR) at regional distance ranged from near the noise level to 2 orders of magnitude above the noise ( SNR). Cross correlations of the regional signal from event 5 (one of the closely spaced events) with itself and seismograms from the six other explosions are displayed in Figure 9. Results for the six closely spaced explosions provide evidence for good signal coherence despite a significant range of charge size and signal to noise ratios. This event assessment technique is even able to recover the matched signal when the signal to noise ratio is 1 or less in the time domain as in the case of shot 2 because of poor explosive performance. [39] The one explosion that is widely separated (number 7 in Figure 9) from the others produces a complex cross-correlation function. This result suggests that at high frequencies ( 1 Hz) the separation of the sources within a single mine can be important in terms of the observed regional seismograms. Correlation may still exist at lower frequencies and prove to be useful for characterizing an event. Figure 10 displays broadband regional observations at five stations at varying azimuths from three large cast blasts at the Powder River Basin coal mine. In this comparison the long periods ( 1 Hz) from these three explosions correlate visually in the time domain at all azimuths. [40] These results suggest that broadband data from explosions in active mining regions can be used to investigate the separation distances between sources where ground truth information is available. There are a num- Figure 10. Comparison of broadband regional seismograms from three different cast blasts at five different stations at varying azimuths from the Powder River Basin coal mine. Vertical surface waves shown are s. 2-11

12 2-12 STUMP ET AL.: REGIONAL SEISMOGRAMS FROM MINES 40, 4 / REVIEWS OF GEOPHYSICS Figure 11. Event time of day for 1179 suspected mining events from near Novosibirsk, Russia [Hedlin and Khalturin, 2001]. ber of other processes that may affect this interpretation, such as the temporal and spatial finiteness of the source Time-Space Clustering [41] Time and space clustering of mine-related seismic events are commonly observed [Israelson, 1991; Sorrells et al., 1997] in regional studies. Mining operations in new areas can be identified by investigating the clustering of events in time and space [Agnew, 1990]. Although these characteristics cannot be used alone for event identification, they can be supportive of a source interpretation (Figure 11). An event that has the characteristics of a mining explosion but occurs during the nighttime hours would be subject to increased scrutiny. Alternatively, an event that occurs at a known mine location might not be immediately eliminated from further analysis Acoustic and Seismic Signals [42] In the 1950s and 1960s, there was considerable research of the infrasonic energy produced by atmospheric nuclear tests [McKisic, 1996a, 1996b, 1997]. With the advent of the Limited Test Ban Treaty in 1963, testing in the United States and the Soviet Union moved underground, and seismic monitoring became the principal tool of treaty verification. Increasing interest in small and shallow events as a result of the CTBT along with the possibility of clandestine nuclear explosions conducted in the atmosphere has renewed interest in infrasound as a monitoring tool. Many natural or manmade events emit energy into the atmosphere and the solid Earth [Bedard and Georges, 2000]. Recently, Sorrells et al. [1997] have shown the utility of combining seismic and infrasound data to characterize mining explosions in northern Mexico, southeastern Arizona, and southwestern New Mexico. Using a colocated seismic and infrasound array near Lajitas, Texas, they documented associated seismic and infrasound signals from known mines. A seismoacoustic event recorded by an array in the Republic of Korea [Stump et al., 2000] is reproduced in Figure 12. The association of infrasound signals with seismic signals can provide not only additional constraints upon the source type, but combining seismic and infrasound data for purposes of location may provide some advantages. [43] Under the CTBT, accurate location and depth estimates remain paramount indicators of event type. Although strong, shallow earthquakes can produce a piston-like ground displacement and excite infrasonic waves in the atmosphere [Blanc, 1989], near-surface explosions are much more efficient sources of infrasound signals. A recent study [Calais et al., 1998] found infrasound ionospheric perturbations resulting from a small-magnitude mine blast (magnitude 3) that were comparable to those produced by the magnitude 6.7 Northridge earthquake. The relative strength of infrasound and seismic signals might provide an effective depth discriminant. The effect of regional propagation must be taken into account for both seismic and infrasound signals. Stratospheric winds have a significant effect on the acoustic wave amplitudes [Wallace and Hobbs, 1977]. Since these winds can change systematically with the seasons, the array sensitivity to mining explosions will vary with the time of the year [Hsu, 1999]. [44] Infrasound signals might further complement seismic data by providing a more precise back azimuth estimate and thus give a more precise event direction. In an example illustrated in Figure 13, a three-element, 100 m aperture portable infrasound array deployed 200 km from the Black Thunder coal mine recorded signals from a 19 July 1996 cast blast. A frequency-wavenumber (f-k) analysis of the onset energy produced a back azimuth estimate within 1.75 of the true mine azimuth. The back azimuth estimates remain stable for 6 s while the signal levels are strong. 5. PROBLEMATIC ISSUES [45] A number of practical issues must be considered in developing a robust set of discriminants for the smallto moderate-size signals generated by mining explosions. Since these events are relatively small (magnitude 4 and below, Figure 1) and the station density is sparse (Figures 2 and 3), it will often be the case that the signals will have small signal to noise ratios and limited bandwidth. The propagation paths are almost totally regional and

13 40, 4 / REVIEWS OF GEOPHYSICS STUMP ET AL.: REGIONAL SEISMOGRAMS FROM MINES 2-13 Figure 12. Associated seismic and infrasonic signals from an event in Korea [Stump et al., 2000]. thus can be quite heterogeneous. This effect can significantly modify the signals propagating along different paths and must be taken into account in the interpretation [Jenkins and Sereno, 2001]. There are many different types of blasting practices, which are employed by the mining industry, that affect the seismic waves. Finally, there is evidence of radical departures between design and actual blasts, even to the extent that there are accidental, simultaneous detonations of significant amounts of explosives during some delay-fired mining explosions [Martin et al., 1997] Small Events With Low Signal to Noise Ratio [46] It is important to quantify the size of the smallest mining explosions that will be observed at regional distances. The single-fired explosions introduced in Figure 8 are used here as an empirical data set for determining the minimum charge size detectable. The observations were made on a single component of the Pinedale array (IMS primary array, PDAR) 360 km from the sources. Figure 14 displays the seismograms from the seven, single-fired explosions with explosive weights ranging from 2500 to 22,700 kg. The first two 2500 kg shots (1 and 2) detonated and burned improperly at low order, producing a factor of 10 decrease in near-source and regional amplitudes. Low-order detonation means the chemical reaction did not go to completion, resulting in decreased energy release and thus smaller seismograms. Comparison of near-source and regional observations suggests that the equivalent yields of the first two events were 450 kg. Taking this effect into account, one can see the increase in amplitude versus yield for these single-fired explosions. Results suggest that, in this geology, a kg (1000 pound) charge detonated simultaneously and fully contained will be observed at or near the noise level at this near-regional distance ( 300 km) Limited Bandwidth [47] Spectral modulation from delay times between boreholes may be best observed at high frequencies, while modulations attributable to source finiteness are evident at lower frequencies. Waveform correlation methods become less effective at higher frequencies because of variations in propagation path effects. P n /L g ratios work well for separating explosions from earthquakes but only at the highest frequencies. As the seismic waves propagate, the high frequencies are attenuated, resulting in mixed performance of the different

14 2-14 STUMP ET AL.: REGIONAL SEISMOGRAMS FROM MINES 40, 4 / REVIEWS OF GEOPHYSICS Figure 13. Acoustic data (upper traces) from a three-element, 100 m aperture array deployed 200 km from the Black Thunder coal mine. Frequency wave number (f-k) estimates are shown in the upper right-hand corner, while back azimuth estimates and signal power are shown below. discriminants. The optimum collection of discriminants for the purposes of event identification will depend on the available station spacing. Limited bandwidth data will not necessarily result in poor discriminant performance especially if some of the tools that work at low frequencies are used to complement those using high frequencies Complex Regional Propagation [48] Implicit in the search for robust discriminants is the necessity that complex propagation path effects not mask source effects. Experience illustrates that propagation path effects can dominate, such as the numerous documented structures that block the propagation of L g. This blockage, if not taken into account, could result in identifying an earthquake as a probable explosion based upon P n or P g to L g ratios [McNamara and Walter, 2001; Baumgardt, 2001; Sandvol et al., 2001]. [49] Propagation complexities also hamper infrasonic source studies. As shown in Figure 13, in the first6s after the onset of the acoustic signal from a mining blast, there is an eastward bias in the back azimuth estimates. Although the bias at onset is very small, later bias ranges from 1 to 8 while the signal is strong but increases to 15 in the lull between arrivals. The error is likely due to atmosphere wind since, at the time of the shot, highaltitude (30 km) winds were strong ( 30 m/s) and directed almost due west from the source to the receiver. Wind will affect signal arrival direction and will have a significant impact on the strength of the refracted energy and the location at which the refraction first returns to the ground Effect of Blasting Practices [50] There are many different mining practices that utilize explosives to fracture or move rock and soil. These practices can have a strong impact on the result-

15 40, 4 / REVIEWS OF GEOPHYSICS STUMP ET AL.: REGIONAL SEISMOGRAMS FROM MINES 2-15 Figure 14. Single-component seismograms ( ,700 kg) from seven, contained, single-fired explosions 360 km from the Pinedale array. All seismograms are scaled to their own peak amplitude. Relative sizes of the peak amplitudes are represented by the height of the rectangles to the far right of each seismogram. The radius of the hexagon to the right of each seismogram is proportional to the amount of explosives, which ranged from 2500 to 22,700 kg. ing regional seismograms. The effect of blasting practice on peak regional amplitudes is summarized in Figure 15. Here the peak P g amplitudes were determined for a number of known explosions from the Black Thunder coal mine. These amplitudes are plotted against the total explosive weight in Figure 15. Three distinct populations are highlighted in Figure 15. [51] The first set is representative of explosives which are emplaced in the coal and detonated for the purpose of fracturing the material to facilitate recovery by mechanical means. These shots are normally 45,000 kg and produce the smallest amplitudes in Figure 15. The cast shot population in Figure 15 uses explosives that are Figure 15. Peak P and L g amplitudes at a singe element of the Pinedale array from a variety of different types of explosions detonated in a single mine in the Powder River Basin. detonated in the material overlying the coal. These explosions, with total explosive weight of 1 3 million kg, are designed to fracture the overburden and cast it into the pit, exposing as much of the coal as possible. Practically, these mining explosions cast only 30% of the overburden to its final resting place. These explosions are all delay-fired with the timing designed to maximize the amount of material cast, while minimizing the local ground motion. There is no increase in peak amplitude with explosive size (Figure 15), as the larger explosions are simply longer in duration. The amplitudes of the regional seismic waves from these large cast blasts are further reduced because the explosives are not well coupled. The final set is the contained, single-fired explosions previously illustrated in Figures 9 and 14. These explosions produce regional amplitudes that increase in amplitude with a power law in yield. Peak amplitudes for this last subset are the only ones that can be used to determine explosive yield. The peak amplitude from the largest contained, single-fired explosion (22,700 kg) is similar in size to those for the largest (2 3 million kg) cast shots Anomalous Mining Explosions [52] Special consideration must be given to chemical explosions that are large and impulsive in nature since it is not possible to distinguish between single-fired chemical and nuclear explosion sources [Denny, 1994]. Typically, mining explosions are delay-fired and thus distribute the seismic energy from their individual explosions over a time window that can range from several hundred