A Tale of Two Countries, Three Discriminants, and Many Mining Explosions
Abstract
With the advent of not only the IMS, but the contribution of high-quality stations and networks throughout the globe, nuclear explosion monitoring strategies have shifted from teleseismic to regional scales. At regional distances, the event detection threshold is greatly decreased, meaning mining explosions must be included in the event identification process. The addition of this event class leads to several questions which much be addressed in the discrimination process: (1) can delay-fired mining explosions be discriminated from earthquakes; (2) can the understanding of mining discriminants illuminate the explosion discrimination process in regions where there are no known nuclear explosions, and (3) are mining discriminants regionally dependent, and are there corrections to account for this dependence? To address these questions, we have assembled a database of ~2500 mining explosions and ~1300 earthquakes recorded at a number of regional stations, comprising a total of ~150,000 waveforms. The database is focused on two regions, the Western United States (WUS) and the Altai-Sayan (AS) region of Russia, which are both areas of prolific mining activity. Due to extensive collaboration with the largest coal mine in Wyoming and the nation, we have detailed shot information for ~1000 mining events, classified into six distinct blast types. We have limited information for events in the AS based upon contacts with the AS Seismological Expedition. We have applied 3 discriminants to data from 11 stations and 1 array in the WUS. The first discriminant, time- of-day (TOD), assesses the event origin time. Mining events occur between 9 am and 6 pm, while earthquakes are randomly distributed in time. The second discriminant, spectral ratios (SR), exploits differences between regional phases due to source type. Results are highly station centric, although the largest mining events separate from earthquakes that are < 250 km from the mine; as the earthquake dataset expands spatially, discrimination performance degrades. 1D path corrections provide improvement, but additional calibrations are necessary to optimize this discriminant. The third discriminant, time-frequency (TF), capitalizes on the unique spectral signature of delay-fired mining events as a function of time. This discriminant separates the larger types of blasts with the longest source duration at all stations. Smaller blasts do not discriminate because of the shorter shot durations. We use classification trees to combine these discriminants, which yield an intuitive decision grid for classifying events based on the discriminant value for each of the 3 discriminants. In the AS region, we calculated these same discriminants at for ~260 earthquakes and ~850 mining events. TOD results are similar to the WUS in that presumed mining events fall within working hours. The SR discriminant shows significant overlap of the earthquake and mining populations. Certain events do separate, but we have no information on what makes these events unique. Similar results are seen for the TF discriminant. We do not know if the discriminant itself fails, or if the majority of our data points are from smaller shots that have shorter time durations. These unanswered questions illustrate the need for detailed ground-truth information. Future studies of mining discrimination, particularly where large datasets are to be acquired, should involve cooperation with mine operators in order to address ambiguities such as those identified in the AS study.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.S11B1746A
- Keywords:
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- 7219 Seismic monitoring and test-ban treaty verification