IONOSPHERIC SIGNATURE OF SURFACE MINE BLASTS FROM GLOBAL POSITIONING SYSTEM MEASUREMENTS

Sources such as atmospheric or buried explosions and shallow earthquak es are known to produce infrasonic pressure waves in the atmosphere. B ecause of the coupling between neutral particles and electrons at iono spheric altitudes, these acoustic and gravity waves induce variations of the ionospheric electron density. The Global Positioning System (GP S) provides a way of directly measuring the total electron content in the ionosphere and, therefore, of detecting such perturbations in the upper atmosphere. In July and August 1996, three large surface mine bl asts (1.5 Kt each) were detonated at the Black Thunder coal mine in ea stern Wyoming. As part of a seismic and acoustic monitoring experiment , we deployed five dual-frequency GPS receivers at distances ranging f rom 50 to 200 km from the mine and were able to detect the ionospheric perturbation caused by the blasts. The perturbation starts 10 to 15 m in after the blast, lasts for about 30 min, and propagates with an app arent horizontal velocity of 1200 m s(-1). Its amplitude reaches 3 x 1 0(14) el m(-2) in the 7-3 min period band, a value close to the ionosp heric perturbation caused by the M = 6.7 Northridge earthquake (Calais & Minster 1995). The small signal-to-noise ratio of the perturbation can be improved by slant-stacking the electron content time-series rec orded by the different GPS receivers taking into account the horizonta l propagation of the perturbation. The energy of the perturbation is c oncentrated in the 200 to 300 s period band, a result consistent with previous observations and numerical model predictions. The 300 s band probably corresponds to gravity modes and shorter periods to acoustic modes, respectively. Using a I-D stratified velocity model of the atmo sphere we show that linear acoustic ray tracing fits arrival times al all GPS receivers, We interpret the perturbation as a direct acoustic wave caused by the explosion itself. This study shows that even relati vely small subsurface events can produce ionospheric perturbations tha t are above the detection threshold of the GPS technique. By sensing d erivative signals, which can be detected over a relatively broad regio n, it appears that GPS might be particularly useful for source charact erization within the first acoustic quiet zone where infrasound would probably be ineffective. This suggests that dual-frequency GPS monitor ing could contribute to Comprehensive Nuclear Test Ban Treaty verifica tion.