Appendix B
Executive Summary

Reducing the Ambiguity and Visibility of Seismic Signals from Mining Activities: Benefits to the Mining Industries and to the Communities Monitoring the Comprehensive Test Ban Treaty

March 1997 Draft Report of a DOE-Working Group



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--> Appendix B Executive Summary Reducing the Ambiguity and Visibility of Seismic Signals from Mining Activities: Benefits to the Mining Industries and to the Communities Monitoring the Comprehensive Test Ban Treaty March 1997 Draft Report of a DOE-Working Group

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--> EXECUTIVE SUMMARY OVERVIEW Statement of the Problem The Comprehensive Test Ban Treaty (CTBT), which opened for signature September 24, 1996 and has since been signed by many nations, is intended to preclude any future nuclear testing. After the treaty is in force (following ratification by appropriate countries), the main concern of the international community will be to verify that no nuclear test is performed. Because mining activities world-wide can generate seismic events which might appear as treaty-prohibited explosions, the mining community may be asked to explain visible or ambiguous seismic signals. The mining industry generates seismic signals from surface and underground blasting, and from underground mine failures. Seismic magnitudes of the largest mine collapses have exceeded 5.0 (equivalent to the signal from a 10 kiloton contained nuclear explosion), but more commonly, mine-related events range from magnitude 3.0 to 4.0 (the larger equivalent to the signal from a 1 kiloton contained nuclear explosion). Ground motions observed at regional distances are highly correlated with mining practices and ground motions in the mine. Mining practices which result in increased productivity, improved safety and minimized in-mine ground motions also produce smaller and unambiguous regional seismic signals. Minimizing unproductive, blasting induced ground motion also makes industry a better neighbor. Thus, the motivation for industry in following these practices is not in the context of the Treaty but in an attempt to maximize their return on investment in mining resources. Further, mines can minimize false alarms under the CTBT by being less seismically visible, i.e. sending out weaker signals, and being less ambiguous, i.e. making sure that the signals have the characteristics of legitimate mining activities. Verifying international compliance with the Treaty requires the ability to detect and identify small underground nuclear tests. Signals from such an event will have to be discriminated from a background of natural earthquakes, man-made mining and construction activities, and noise from wind and ocean waves. Detection and identification of small seismic events (magnitudes less than 4) remains an area of active research. Mine blasts are explosions of course, but unlike most underground nuclear tests they are normally composed of many small explosions spread out in both time and space which can lead to modulation in the frequency content of the event that can be used to identify some delay-fired mine shots and distinguish them from concentrated blasts. Earthquakes and single-fired explosions do not exhibit this modulation. Mine seismicity from uncontrolled sources (e.g. rockbursts, coal bumps, mine collapses) may have characteristics unlike either earthquakes or explosions. Uncontrolled mine events in the past have been identified by comparing long-period waveforms with computational models. A variation of these waveform matching techniques can be used to identify blasts or mine tremors from a specific mine, to produce a “fingerprint” of the mine. When seismic events are planned to occur at a specific mine, ground truth (specific information obtained at the mine) can be used to determine the event type and characteristics for each of the events.

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--> It has long been recognized that detonation of an explosion in an underground cavity significantly reduces (by a factor of 10 to 100) the amplitude of the seismic signal. It is thought that if a nuclear test were decoupled in some circumstances its signal could be small enough to be masked by another seismic signal, such as a normal mine explosion. Calculations have shown that a single concentrated explosion is detectable within a large delay-fired explosion if the single shot energy is more than about 10% of the total energy of the two explosions. With this background, the U.S. government and its National Laboratories have engaged in an extensive partnership and joint research with the U.S. mining industry, to identify mining practices that provide intrinsic benefit to mining companies while naturally minimizing the impact of these operations on the monitoring of the Treaty. CTBT Implementation The Treaty will be enforced by an international organization that includes all signatory countries as members. The CTBT Organization (CTBTO) will consist of the Conference of the States parties, the Executive Council, and the Technical Secretariat. This organization will operate an International Monitoring System (IMS) for treaty compliance using seismological, radionuclide, hydroacoustic, and infrasonic monitoring. Data from the monitoring stations will be transmitted to the International Data Center (IDC) which will process the data and will make results and raw data available to all member countries. Individual member countries may re-analyze the data in any way they wish and may bring questionable events to the attention of the CTBTO. Before requesting an On-Site Inspection (OSI), States Parties are encouraged to “make every effort to clarify and resolve, among themselves or with or through the Organization, any matter which may cause concern about possible non-compliance with the basic obligations of this Treaty.” (United Nations General Assembly, A/50/1027) Two voluntary actions are included in the provisions of the Treaty: (1) Consultation and clarification; and (2) Confidence building measures. Consultation and clarification is the process by which countries can ask another for information that might help resolve a questionable event. This process could include the exchange of data or information about a particular source that generated the signals. Confidence building measures are cooperative actions that can be taken by nations to improve the performance of the monitoring system and eliminate ambiguities that may develop in the interpretation of the resulting data. The voluntary exchange of information related to the national use of all chemical explosives greater than 300 tonnes TNT-equivalent is an example of a suggested confidence building measure. The sole purpose of an OSI is to determine whether or not an ambiguous signal detected on the basis of the IMS data was generated by a nuclear explosion. Initiation of an OSI will require a positive vote from 30 of the Executive Council's 51 members and will therefore be difficult to initiate, minimizing the number of such occurrences. If the OSI is at a mine site, the mining company, cooperating with its government, will be asked to comply with the rules of inspection and facilitate the visit of the inspectors. These inspectors may deploy a variety of instruments and collect different types of samples. They may also want to drill into the subsurface. The actual and political cost of an OSI will be great, thus making them infrequent.

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--> MINING EXPLOSIONS Types of Mine Explosions Blasting operations in surface coal, open-pit operations, and underground operations have the highest potential of being detected at regional seismic stations of an IMS. Seismic visibility would increase proportionally as a function of hole diameter, bench height, explosive quantity per hole, number of holes fired per delay interval, the ground response to the explosion, and the total amount of explosive detonated per unit space and time. Surface coal cast blasting requires heavy explosive loads where individual holes in a shot are fired instantaneously or with very short delays (under 25 ms) between them. Delays between rows of holes can range from 50 to over 250 ms. Powder factors can range from 0.17 to 0.87 kg of explosives per cubic meter (0.5 to 2.5 lb/yd3) of rock blasted, depending on the targeted final muckpile configuration. In open-pit operations holes can be detonated individually, in combination, or simultaneously along a row of holes. Powder factors can range from 0.1 to over 0.67 kg/m3 (0.3 to over 2.0 lbs/yd3) depending on a host of factors ranging from the material hardness to the type of digging equipment used. The harder the material, the higher the powder factor which is required to fragment the material. Underground blasting techniques are dependent on the same parameters as those described for surface operations, in addition to a heavy dependence on the exact nature, thickness and orientation of the ore body being mined. Only sublevel caving, longhole open stopping, and final pillar recovery in the room and pillar method represent a high potential of being detected at far-field seismic stations. Ground Vibration and Airblast Monitoring in the Near-Source Region Current ground vibration monitoring in mines and around blast sites is performed with relatively inexpensive seismographs. Most seismograph stations are located within a kilometer or two of the blasting operation, usually at some point of immediate concern. In any case, the majority of mines monitor seismic/airblast levels with as few instruments as possible while still meeting local ordinances, or state and federal regulations. These local recordings may be useful in answering queries about questionable events through the voluntary Consultation and Clarification process. Anomalous Blasts Anomalous detonations of large amounts of explosives at the same time have been detected at regional seismic stations. They can occur in both surface and underground operations as a result of: direct and indirect lightning strikes; sympathetic detonations whereby a large number of holes or sections of a shot fire prematurely and instantaneously; accidents from improper use of explosives and failure to comply with the safety regulations; or operator errors and/or inexperience in designing and hooking up the shot sequence. These mishaps occur more frequently than one might think (approximately one

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--> of every 20 heavily instrumented shots are estimated to show such effects, F. Chiappetta, personal communication). Regional Signals from Mining Explosions Seismic data at near-source distances are consistent with regional data, suggesting that contained nuclear explosions cannot be distinguished from contained chemical explosions that are simultaneously detonated (Denny, 1994). Mining and construction explosions that have characteristics similar to large, simultaneously detonated chemical explosions could be problematic in the context of monitoring a CTBT. The degree to which mining explosions will be of interest to the CTBT verification process will depend on the visibility or size of the signals and the degree to which the character of the signal reduces ambiguity with a nuclear explosion. In what follows, we first discuss the coupling of mining explosion to regional seismic waves affecting their visibility. This is followed by a discussion of techniques to uniquely identify the signals as being a legitimate mining explosion. Mining explosions, as illustrated, are rarely detonated simultaneously, often emplaced in relatively incompetent near-surface layers and designed to fracture and/or cast the materials in which they are detonated. All these characteristics result in a reduction in amplitude relative to a contained, single detonation. Comparison of contained single-fired explosions to large delay-fired cast blasts in the same geology suggests coupling differences between the two source types that are frequency dependent. Coupling differences as small as a factor of 10 (contained shot more effective coupling) at long periods and as great as a factor of 100 at high frequencies have been observed. Some of the largest operations in the blasting community involve the emplacement and detonation of explosives to cast the overburden rock into the pit and expose relatively shallow coal seams. This type of blast does not produce the distinctive impulsive character in the radiated compressive energy noted for fragmentation shots. Further, the long duration of cast blasts produce distinctive regional signals compared to a single-fired explosion. Open pit fragmentation explosions designed to fragment hard rock for mineral excavation and recovery form a second subset of the possible types of explosions that will have to be identified. The blasting patterns tend to be simpler than those in cast blasting; such as by using simultaneous detonation of rows. The resulting banded nature of the regional seismic data provides a good discriminant from a singly detonated explosion. The fact that the amplitudes for a particular type of blast are similar to one another despite spanning large ranges in yield, illustrates that normal blasting operations designed to minimize in-mine motions will likewise minimize the amplitude of signals recorded at regional seismic stations. By implication, problematic blasting practices from the perspective of the mine operator will also be a problem in regional CTBT monitoring as the events can be large and with anomalous signal character, possibly similar to a small nuclear explosion. GROUND FAILURES IN UNDERGROUND MINES

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--> The Pervasiveness of Ground Failures Underground mine failures that may produce regional seismic signals can be categorized as follows: planned failures which are controlled (blasting of existing pillars to create a mass of rock rubble); planned failures which are not controlled (initial and subsequent caving behind a new longwall in coal); and unplanned failures (pillar failures, coal bumps, and rockbursts). Seismic Signals from Engineered Collapses Many mining techniques (e.g. longwall mining, block caving, pillar robbing or blasting) leave the roof unsupported and expect collapse as a result. These types of ground failure events typically produce seismic magnitudes less than 3.5. Details of three engineered collapses provide a quantification of this source type: the White Pine copper mine, in Michigan (room-and-pillar); the Twentymile coal mine, in Colorado (longwall): and the Henderson molybdenum mine, in Colorado (block caving). At the White Pine Mine, 72 pillars, each with an average cross-section of 74 m2 were fragmented using a delay-fired sequence of 325 ms and using 59 tons of explosives. Near-source monitoring of the induced mine collapse showed that the individual explosive charges emplaced in the pillars did not produce strong seismic signals; however, the failure of the pillars and of the material above the working level did produce regionally detected signals. A regional magnitude upper bound estimate of 3.1 was determined for this event. At Twentymile Mine, the seismicity associated with the mining of a 25,000 m2 new panel, in a 3 m thick seam, beginning with the “first cave” of the panel, and continuing with the monitoring of aftershocks and subsequent collapses was recorded. There were five events between magnitude 2 and 3. The larger seismic events emanating from this longwall mine have a point-source seismic mechanisms similar to the larger unplanned collapses (see below). The Henderson Mine experiments provided an opportunity to observe seismic signals generated by a block caving collapse. Both the explosion and caving events were quite small (M<2) and likely would not be detectable by the CTBT monitoring system. The seismic events associated with this process are deficient in high frequencies compared with similar sized earthquakes Seismic Signals from Unplanned Collapses Some of the largest seismic signals associated with mining activities (M>3.5) are due to accidental failures in mines. Several have been documented in the U.S., and they have reached magnitudes as large as ML=5.2 i.e. the 1995 Solvay Mine collapse in Wyoming. Specific discussion of two case histories are included: a coal mine at Gentry Mountain, Utah, and the trona mine of Solvay Minerals, near Green River, Wyoming. Careful analysis of the regional seismic signals from this class of events suggests a pattern of behavior similar to earthquakes for some seismic measures and similar to explosions for others. The current number of known collapses that have been studied is limited and more work needs to be done in this area to define the seismic criteria for identifying collapses separately. Studies of the larger collapses may indicate a reasonable agreement between the area of collapse as measured by seismic moment and that observed geodetically.

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--> Seismic Signals from Coal Bumps and Rockbursts Coal bumps occur frequently in the U.S. and in other countries such as India, Poland and South Africa. The most seismically active coal districts in the U.S. have been in Kentucky and Utah. Seismic evidence from large coal bumps in Kentucky indicates that they have similar source mechanisms to the large unplanned collapse events discussed earlier. Further research is needed to understand the precise mechanism of the failure generating the coal bump signals. While little work has been performed to identify and discriminate these events, on the basis of their mechanism and shallow depth we suggest they will have a behavior similar to that of the large collapse events. The largest rockbursts (possible magnitude in excess of 5) have historically been related to shear motion on faults occasioned by mining. Most rockbursts are thus earthquakes and would not be expected to cause false alarms. However, some rockbursts are similar to implosional or explosional events and thus could be of concern to CTBT verification. RECOMMENDED MEASURES TO REDUCE VISIBILITY AND AMBIGUITY Measures Concerning Explosive Usage Visibility and Transparency Comparison of near-source and regional waveforms from individual mining explosions has shown that practices designed to control ground motions in the strong ground motion region around the mine also control motions at regional distances. Poorly controlled shots are empirically found to be the ones that produce anomalously large regional amplitudes. Further, predictability of regional amplitudes from mining operations is apparently dependent upon the type and style of blasting. There are three major parameters which can significantly affect ground vibration amplitudes: the amount of explosives per delay, the delay period, and the detonation firing-time accuracy. Reducing the amount of explosive detonated per delay is perhaps the single most important factor controlling ground motion. A 20% to 50% reduction of the explosive would be required before measurable reductions in vibration levels can be realized. Sizable reductions in the explosive weight per delay can be achieved by smaller borehole diameters, smaller bench heights and hole depths, multiple explosive decks within a single borehole, decoupling the explosive charge from the borehole wall, more stemming, and less subgrade drilling. The delay periods used in blast designs can significantly influence the vibration outputs in terms of the amplitude and predominant frequencies. One of the ways for mine operators to determine the best possible delays in a shot is by performing a single hole signature analysis. By minimizing undesirable vibration in the near-field, these techniques have been shown to reduce vibration at the far-field regional stations. The accuracy of pyrotechnic detonators has in general been steadily improving, but there are definite limits as to how far these improvements can advance. Precise, programmable electronic detonators hold considerable promise in the control of blast induced ground vibrations by eliminating the inherent scatter in pyrotechnic detonator firing times and providing unlimited choice of delay intervals.

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--> A number of characteristics observed in the regional seismic data are proving to be useful for the purposes of identifying (or characterizing) a signal source as a standard mining explosion. When an explosion performs as designed, the resulting seismograms contain the effects in the frequency domain which are indicative of delay-firing. Time varying spectra often exhibit well defined constructive and destructive interference effects (modulation), called spectral scalloping. One unique physical characteristic of cast blasts is their scaling of shot duration with total explosive weight. Long period energy around 10 s has been observed propagating from cast blasts. These seismic waveforms could prove to be characteristic of large, long time duration cast blasts. Recent studies have shown evidence that mining explosions (and theoretically, mine collapses) register very large and characteristic signals at infrasonic stations. One of the biggest differences between a possible nuclear test and a mining explosion is that, in all likelihood, a nuclear test will be conducted underground to eliminate or minimize the release of radioactive materials or gas into the atmosphere. Much work remains to be done in assessing the utility of combined acoustic and seismic data sets for identifying mining explosions, but preliminary work suggests a possible synergy of the two technologies for source identification. Measures Concerning Ground Failures Three different approaches to mine design are described to reduce the risk of violent pillar collapses (Cascading Pillar Failure-CPF), namely 1) containment of failure, 2) prevention of failure, and 3) full extraction mining. The containment approach limits the spread of a potential CPF with barrier pillars. This approach is a noncaving room-and-pillar method that uses low width to height (W/H) ratio panel pillars surrounded by high W/H ratio barrier pillars. The prevention approach “prevents” CPF from ever occurring by using panel pillars with proper mechanical characteristics. The prevention approach is another noncaving room-and-pillar system; however, it uses high W/H ratio panel pillars and optional barrier pillars. The full extraction approach avoids the possibility of CPF altogether by ensuring complete opening closure (and surface subsidence) upon completion of retreat mining. By-and-large, the reduction of coal bump and rockburst occurrences is an on-going endeavor in many countries including the U.S.; it is as much an art as a science. There is no clear-cut solution to the problem. Clearly, mines that reduce their unplanned collapses cannot but enhance the safety of their operations. Cooperative Measures for Calibration Keys to the success of any monitoring system are the location and identification of the source of the seismic waves. Empirical procedures are suggested for calibration or fingerprinting of signals from active mines in order to minimize the impact of the verification system on mine operations by minimizing false alarms. The approach to obtaining ground truth information for calibration/validation of a regional monitoring system is to use a set of simple portable instruments which could be deployed and operated by one or two people with a minimum of effort in the near-source region. This approach would provide a cost effective methodology for calibration, using sources of opportunity such as those available in an active mining region. Near-source data gathered by this

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--> calibration system can also be used to assess important source processes that lead to regional signals that will be detected by the International Monitoring System. POTENTIAL BENEFITS Benefits to Industry Utilization of data collected and techniques developed to assess and understand mining explosions for CTBT monitoring purposes provide the additional opportunity to address problems of direct interest to the mining industry, particularly in the areas of assessment of blast performance. Determination of differences between designed and actual detonation times is critical for control of in-mine ground motion as well as proper fragmentation and cast of material. A number of tools have been developed for the assessment of these effects including high speed film, video with de-interlacing and interpolation, ground motion measurements, velocity of detonation measurements in the explosive boreholes and connection of shock tube from the downhole delay to the surface. Integrated data sets from explosive sources can also be used to assess the coupling of seismic energy by explosive sources. Visualizations, animated in time, can be used to identify how the explosive sequence progressed and the effect on the amplitude of the near-source wavefield. Development of physical models of the blasting process can be used to help assess new blasting techniques. A variety of physical models of blasting are being developed that provide an improved and more comprehensive understanding of the physics, mechanisms and factors influencing mine-generated seismic disturbances. Benefits to CTBT Monitoring For the CTBT verification community, reducing ambiguity of detected signals through information exchange and research has many benefits including: (1) Unambiguously identified mine blasts can provide calibration for a number of important seismological measures such as phase identification, travel time, and amplitude decay rate; and (2) A more complete listing of the number and sizes of seismic events generated by mine blasting and ground failures will allow the CTBT monitoring community to better assess the number of false alarms caused by mines and will help prevent OSI's from potentially being called on legitimate mine operations. The verification system will be more effective if the number of false alarms is kept to a minimum. A call for an On-Site Inspection will come at great economic and political cost, particularly if the event of concern proves not to be a nuclear explosion. For this reason, the detonation of single-fired, contained explosions in the same geology as that where standard delay-fired mining explosions are conducted can provide relevant empirical data for identifying differences between standard mining explosions and those that are simultaneously detonated. Thanks to the cooperation of several U.S. mining companies, cost-effective techniques for conducting these calibration events are being developed. These techniques can be executed in areas that prove to be problematic from a false alarm perspective, so as to improve event identification.

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--> FUTURE TRENDS AND OUTSTANDING ISSUES Evidence exists supporting the contention that typical mining explosions couple energy into the seismic wavefield far less efficiently than a contained explosion, such as a nuclear test. These coupling differences are probably a function of the frequency of the observed seismic waves as well as of the particular blasting practices employed. These require further documentation and quantification. Source models exist for earthquakes and explosions that allow one to assess the area or volume of the source region as well as the amount of displacement in the source region from a seismic signal. Development of similar models for ground failure in underground mines is needed. Models that include the important source phenomenology of mine explosions such as shock coupling, spall, and material cast need to be developed and refined. These models should utilize both the regional seismic signals that these events generate as well as close-in observations of the phenomena in the mines. Analysis of infrasonic signals with an emphasis on differences from nuclear tests may provide additional tools for reducing the ambiguity of mine-related signals. The synergy of infrasonic data in combination with seismic data may be very important. Consideration of similarities and differences of U.S. mining practices with those used internationally is needed. New technologies must be assessed as they are introduced. One technology that is in the process of being introduced is electronic detonators with precise initiation time control. The Comprehensive Test Ban Treaty has been successfully negotiated and awaits ratification and entry into force. Implementation of the treaty will require the resolution of many issues such as, the format of an on-site inspection; how questionable or ambiguous events will be handled; and what will constitute a questionable or ambiguous event. The results of research conducted in cooperation with the U.S. mining industry have identified a number of practices and tools which, if widely adopted and practiced, would likely reduce the false alarms that may be registered by an International Monitoring System. REPORT ORGANIZATION This report focuses on two general classes of mining events that will generate regional seismic signals. In Chapter 2 (Mining Explosions) explosion sources are discussed from both the perspective of the mine operator in the near-source (within the mine) and the regional observations. The link is made between blasting practices in the mine and the resulting regional seismic signatures. Chapter 3 (Ground Failures in Underground Mines) focuses on ground failures in underground mines and their resulting seismic signatures. Characteristics and sizes of the regional seismic observations from these different source types are documented. Recommended measures for reducing the size of the regional signals from the different source types or assuring that the resulting signals are easily recognizable as mining related are discussed in Chapter 4 (Recommended Measures to

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--> Reduce the Seismic Visibility and Ambiguity). Possible active procedures for assuring that no mining operation produces a signal that is questionable are addressed. The techniques and analysis tools that have been assembled to characterize the seismic signals from mining operations have intrinsic value for the mining industry, in addition to being essential for Treaty verification. These dual use items are discussed in Chapter 5 (Potential Benefits). Cooperation with the U.S. mining industry has been the central reason for success in the quantification of seismic issues associated with the monitoring of a CTBT. Chapter 5 also documents the benefits that have been gained by this interaction. Finally in Chapter 6 (Future Trends in Relevant Mining Activities, and Outstanding Issues) a list of outstanding issues is drawn and possible approaches to their quantification are documented. These issues should be addressed in the future, as CTBT verification is implemented.

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