Ionospheric signature of surface mine blasts from Global Positioning System measurements

Abstract

Summary Sources such as atmospheric or buried explosions and shallow earthquakes are known to produce infrasonic pressure waves in the atmosphere Because of the coupling between neutral particles and electrons at ionospheric altitudes, these acoustic and gravity waves induce variations of the ionospheric electron density. The Global Positioning System (GPS) 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 blasts (1.5 Kt each) were detonated at the Black Thunder coal mine in eastern Wyoming. As part of a seismic and acoustic monitoring experiment, we deployed five dual‐frequency GPS receivers at distances ranging from 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 min after the blast, lasts for about 30 min, and propagates with an apparent horizontal velocity of 1200 m s− 1. Its amplitude reaches 3 × 1014 el m− 2 in the 7–3 min period band, a value close to the ionospheric 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 recorded by the different GPS receivers taking into account the horizontal propagation of the perturbation. The energy of the perturbation is concentrated 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 1‐D stratified velocity model of the atmosphere we show that linear acoustic ray tracing fits arrival times at all GPS receivers. We interpret the perturbation as a direct acoustic wave caused by the explosion itself. This study shows that even relatively small subsurface events can produce ionospheric perturbations that are above the detection threshold of the GPS technique. By sensing derivative signals, which can be detected over a relatively broad region, it appears that GPS might be particularly useful for source characterization within the first acoustic quiet zone where infrasound would probably be ineffective. This suggests that dual‐frequency GPS monitoring could contribute to Comprehensive Nuclear Test Ban Treaty verification.

The shock-acoustic waves generated by earthquakes

“…This procedure is better suited to the detection of a single pulse signal (N -wave) than the frequently used band-pass filter (Li et al, 1994;Minster, 1995, 1996;Fitzgerald, 1997;Calais et al, 1998). A limitation of the band-pass filter is the oscillatory character of the response which prevents it from reconstructing the form of the N -wave.…”

“…In particular, the substitution of the earthquake zone for a point source turns out to be fruitful when describing long-period internal gravity waves at a very long (thousands of kilometers) distance from the epicenter (Row, 1967). The visual resemblance of ionospheric disturbances at short (hundreds of kilometers) distances from the earthquake epicenter to disturbances from surface explosions is discussed in Calais et al (1998).…”

“…It should be remembered, however, that taking into account this factor can be very important in the analysis of the complete picture of TEC disturbances above the earthquake or explosion source (see, for example, Calais et al, 1998).…”

“…The oscillation period of the ionospheric response of SAWs varied from 30 to 300 s, and the propagation velocity fluctuated from 700 to 1200 m/s (Nagorsky, 1998;Fitzgerald, 1997;Calais et al, 1998;Afraimovich et al, 1984;Blanc and Jacobson, 1989).…”

“…Currently, some authors have embarked on an intense development of methods for detecting the ionospheric response of strong earthquakes (Calais and Minster, 1995), rocket launchings (Calais and Minster, 1996), and industrial surface explosions (Fitzgerald, 1997;Calais et al, 1998). In the cited references, the SAW phase velocity was determined by the 'crossing' method, by estimating the time delay of SAW arrival at subionospheric points corresponding to different GPS satellites observed at a given time.…”