ABSTRACT

Untuned large‐volume airgun arrays are now widely used in onshore lakes and rivers to image crustal structure and monitor its temporal variation. However, the property of the airgun source is not well understood. In this article, we investigate the source signature of an untuned airgun array by comparing seismic records from closely located airgun shots, a chemical explosion, and a small earthquake. We find that the airgun signal has a dominant frequency of 2–8 Hz, which is the lowest among the three sources. Because of its relative low frequency, the seismic wave can propagate up to 40  km, which is longer than that for an earthquake with similar magnitude (ML 0.7). After being stacked for 20 times or more, the airgun signal can be detected as far as 150 km. The travel times of different P‐wave phases from the airgun source match that picked from the explosion profile. However, the airgun waveforms are slightly more complicated, likely due to reverberations of the air bubbles. However, deconvolutions of near‐source recording only result in minor improvement in the sharpness of phase arrivals. In addition, we show that the signal‐to‐noise ratios of the stacked trace do not depend strongly on the stacking numbers, making it possible to achieve a high temporal resolution on the order of hours. Although the results in this article are drawn from a case study, they have implications for the use of airgun arrays in onshore exploration in other regions.

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