Executive Summary
The Comprehensive Test Ban Treaty (CTBT) has been signed by 149 nations and bans all nuclear explosions. As of May 1998, the U.S. Senate has yet to ratify the treaty. A global seismic network forms part of the basis of the International Monitoring System (IMS) being established for CTBT compliance. The seismic magnitudes of a small number of mining explosions can be similar in size to those resulting from a small or decoupled nuclear explosion. Mine collapses or rockbursts can also generate similar-sized seismic signals. Not only would some mining-related seismic signals be detectable by (visible to) the IMS, but also some could potentially be misinterpreted as nuclear events and treaty violations.
The Department of Energy (DOE) convened a working group that prepared a draft report, Reducing the Ambiguity and Visibility of Seismic Signals from Mining Activities, in March 1997 (that Executive Summary is in Appendix B). DOE asked the National Research Council (NRC) to conduct a review of the working group's report. That review is the subject of this NRC study. The DOE Working Group approached its topic primarily from a perspective of decreasing the size (i.e., visibility) of seismic signals resulting from mining operations. Within this context, their report suggested several changes in blasting and mining engineering practices. These included decreasing the amount of explosives used in each shot of a multishot blasting sequence and reducing the diameter of the boreholes, which contain the explosives, in the blasting pattern. The changes suggested by the DOE Working Group are technologically feasible and would result in decreased magnitudes of the seismic signals. The DOE Working Group report, however, did not evaluate whether the suggested changes were pragmatic and could be implemented by the mining industry. The NRC's review committee concludes that the cost of implementing the suggested changes could affect the viability of a mine in a highly competitive international environment. Because of this economic concern, the NRC committee recommends that emphasis be given to improving monitoring capabilities to decrease the ambiguity of seismic signals from mining operations, rather than relying on efforts
to alter blasting and mining engineering approaches. The DOE Working Group acknowledged the value of monitoring approaches, but placed greater emphasis on decreasing the visibility of seismic signals.
A majority of mining-induced seismic signals, including those resulting from all blasting at most mines, would be invisible to the CTBT monitoring network because of their small size. Among those signals that are visible to the IMS, most could be clearly associated with legitimate mining activities by a voluntary characterization program, thus substantially reducing any ambiguity. Mining operations that use large explosive yields—that is, greater than the 300 metric tons TNT-equivalent specified in the treaty—could voluntarily submit the time and locations of a few of their larger blasts to the agency (not yet specified) responsible for U.S. CTBT compliance. The resulting data could lead to improved travel-time models and corrections that would enable very accurate locations for signals originating at or near those “calibrated” mines. This ability would greatly reduce the ambiguity of many small seismic signals detected by the CTBT seismic monitoring system and would make it possible to rapidly obtain confidence-building information from the mine to explain any questionable signals. For the very few mines that might emit large signals that are still ambiguous despite the process described above, mine operators may want to take more active voluntary measures and allow installation of an unintrusive, on-site monitoring system such as that discussed in the DOE Working Group report. The benefit to the mining companies would be the avoidance of possible international CTBT on-site inspections of legitimate mining activities.
Seismic signals originating from rock bursts, coal bumps, or mine collapses are more likely to be ambiguous than those originating from blasting operations. For mining operations that had significant geologic events that might generate observable seismic signals, it would be extremely useful for treaty monitoring if these operations would report the location and approximate time of these events. However, this is not included as one of the voluntary measures within the treaty,
The burden of the above measures to decrease ambiguity of seismic signals should not be onerous to the mining community. The NRC review committee believes that U.S. industry cooperation in this important national initiative will be forthcoming.
Such a voluntary approach to decreasing the ambiguity of seismic signals originating from mining operations within the United States could set a positive example for other countries to participate in confidence-building measures under the treaty regime.
