Seismic Signals from Mining Operations and the Comprehensive Test Ban Treaty: Comments on a Draft Report by a Department of Energy Working Group (1998)

Additional research could further contribute to reducing the visibility and ambiguity of seismic signals resulting from mining activities. Specific research needs include issues of sympathetic detonations during mine blast initiation, the development and application of precise electronic detonators and instrumentation, and continued work in the areas of ground failures and seismic discrimination. Research is also needed in discrimination of seismic signals from mine blasts and ground failures from other seismic events.

Sympathetic detonation of explosives is the unintended detonation of an explosive charge by the detonation of another explosive charge in the proximity. Sympathetic detonations can produce undesirable fragmentation, undesirable vibration, backbreak, and flyrock resulting in adverse cost and safety impacts. Many of the causes for sympathetic detonations that occur during blasting are not well understood. Under ideal conditions, with unconfined explosive charges, standard tests such as the flash-over and gap tests have been developed to document the stand-off distances between two explosive charges of specific diameters and sensitivities. Under these conditions, it is generally the shock pressure from the donor charge that initiates the acceptor charge. However, the occurrence of sympathetic detonations between loaded blastholes has been documented in the field. In such instances, the sudden burst of shock pressure and heat can emanate from a detonating hole through openings along joints and bedding planes. These pressures can be of the level required to initiate a detonator and may, in some cases, initiate the priming high explosive.

Sympathetic detonations can also occur in coal mine blasting when heat and open flames of initiating explosives react with coal dust or methane gas pockets.

In addition, there are documented occurrences in which the heat from the oxidation of pyrite in the vicinity of the main explosive charge of ANFO (ammonium nitrate-fuel oil mixtures) within the borehole has prematurely detonated holes within a shot.

The case presented in the DOE Working Group report, in which 40,000 lb. of explosives simultaneously detonated at a large western surface coal mine, has illustrated this issue. Sympathetic detonations of such large quantities of explosives are rare; those related directly to the quality and reliability of explosive products or devices are even rarer. However, external geological and environmental conditions that may contribute to sympathetic detonations are of interest for future study.

Research in the area of programmable detonator use as a means of improving the precision of explosive initiation times may be of interest. In reality, however, the improvement of timing accuracies they promise may only marginally improve errors inherent to existing electric and nonelectric detonators. These errors in timing of explosive initiation are currently only a few percent of the design delay times. Resulting marginal improvements may not be detectable at far-field distances.

Nonetheless, the degree of improvement does need to be quantified. Several U.S. companies are developing programmable detonators. They are a few years away from marketing these devices. More field-scale and production tests are necessary to verify the accuracy of the devices. Research in this area may be limited to providing access to mine sites for detonator manufacturers in order to conduct production testing. This is an area of research in which the mining industry should take the lead and coordinate with the explosive manufacturers.

Further research is needed to understand and control the mechanisms associated with planned and unplanned failures. The following two approaches should be considered: (1) controlled tests conducted in the field and analysis of these results using geomechanical principles and models, and (2) an expansion of efforts to record, predict, and analyze unplanned ground failures. These efforts would be beneficial for developing seismic fingerprints of these events, particularly unplanned failures such as bumps and rock bursts. In addition, such research could provide important information for improving mine design and for developing warning systems prior to the occurrence of these failures.

One area of research that could greatly benefit the mining industry is the development of affordable monitoring equipment with a higher resolution level

than is currently available. Another is the provision of a more sophisticated means (i.e., through software development) of analyzing the spectral content of ground motion wave forms consistent with the methods used in seismology. Through such effort, a common analytical approach to wave form analysis among seismologists and mining engineers should be sought.

The availability of source information from mining blasts in the United States would allow for the improvement of travel-time models and earthquake location algorithms. One goal of further work in this area would be the development of regional velocity models that could be used to greatly improve seismic event locations. Improved earthquake locations could benefit earthquake characterizations, and in some cases, damage assessments and emergency response.