Relocation of Wyoming mine production blasts using calibration explosions

An important requirement for a comprehensive seismic monitoring system is t he capability to accurately locate small seismic events worldwide. Accurate event location can improve the probability of determining whether or not a small event, recorded predominantly by local and regional stations, is a n uclear explosion. For those portions of the earth where crustal velocities are not well established, reference event calibration techniques offer a me thod of increased locational accuracy and reduced locational bias. In this study, data from a set of mining events with good ground-truth data in the Powder River Basin region of eastern Wyoming are used to investigat e the potential of event calibration techniques in the area. Results of thi s stud; are compared with locations published in the prototype Internationa l Data Center's Reviewed Event Bulletin (REB). A Joint Hypocenter Determina tion (JHD) method was applied to a set of 23 events. Four of those events w ith superior ground-truth control (mining company report or Global Position ing System data) were used as JHD reference events. Nineteen (83%) of the s olutions converged and the resulting set of station-phase travel-tine corre ctions from the JHD results was then tested. When those travel-time correct ions were applied individually to the four events with good ground-truth co ntrol, the average locational error reduced the original REB location error from 16.1 km to 5.7 km (65% improvement). The JHD locations indicated redu ced locational bias and all of the individual error ellipses enclosed the a ctual known event locations. Given a set of well-recorded calibration events, it appears that the JHD me thodology is a Viable technique for improving locational accuracy of future small events where the location depends on arrival times from predominantl y local and/or regional stations. In this specific case? the international Association of Seismology and the Physics of the Earth's Interior (IASPEI) travel-lime tables, coupled with JHD derived travel-time corrections, may o bviate the need for an accurately known regional velocity structure in the Powder River Basin region.