CTBT Monitoring Methods

An International Monitoring System (IMS) will be established for monitoring compliance of the CTBT. A major component of the IMS is a worldwide network of seismic stations. Seismic signals that will be detectable by (visible to) the IMS will originate from both natural and man-made sources; an IMS goal is to be able to detect events with a magnitude of about 3.0 (Mb). Naturally occurring events include earthquakes and volcanic eruptions, most of which are deeper than man-made seismic sources. Accurate locations of such events to hypocentral depths greater than 10 kilometers makes further consideration of these events unnecessary in the context of monitoring compliance. More enigmatic from the monitoring perspective are man-made or induced seismic signals resulting from (1) applications using conventional explosives for mining and excavation, and (2) rock bursts and collapses associated with surface or underground mining operations. 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 rock bursts can also generate similar-sized seismic signals. Uncertainties in the seismically determined location3 of such an event may not allow assignment of an event to a specific mine unless local data are available. By itself, determination of location may not be sufficient to identify the source type, and event identification will need to rely on distinguishing features of the seismic wave forms, possibly in conjunction with other monitoring technologies. Of the other monitoring technologies to be used by the IMS, infrasonics or detection of atmospheric signals are the most germane to monitoring explosions from mining operations.

Recognizing that mining blasts represent a significant source of small-magnitude seismic signals, the CTBT calls for voluntary exchanges of information on large mining explosions as part of the treaty's confidence-building measures. These include annual surveys to determine which mines in each country detonate explosives over 300 metric tons (TNT-equivalent), notifications of blasts in which 300 metric tons or more of explosives are detonated in a single explosion4, and dedicated calibration explosions. These measures, although voluntary and not mandated by the treaty, could significantly improve the performance of the monitoring system and reduce the ambiguity of some mine-related seismic signals by allowing for the identification and calibration of mine event locations and signal types.

3  

A discussion of locational errors associated with seismic sources is given in Appendix C of the 1997 NRC report, Research Required to Support Comprehensive Nuclear Test Ban Treaty Monitoring.

4  

The section of the CTBT dealing with confidence-building measures does not explicitly differentiate between one single explosion and a delay-fired blast where hundreds of boreholes filled with explosives may be detonated in a sequence within a few seconds. The largest delayed-fired blast (Powder River Basin, Wyoming) detonated about 3,600 metric tons of explosives over a 4- or 5-second period.



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