A comparison between airguns and explosives as wide-angle seismic sources

The relative merits of a 48-gun, 9324 cu. in. (153 litre) airgun array and a 200 kg explosive source are considered for the purposes of long-range (0–400 km) refraction seismic work, with particular reference to traveltime modelling. Theoretical source calculations indicate that in the frequency range 2.5–12.0 Hz, the airgun source will produce an RMS pressure ～ 8% of that produced by the explosive source and an initial burst pressure ～17% of that produced by the explosive source. Observed data support these calculations at short ranges and illustrate the greater attenuation of the airgun signal with range due to its lack of very low frequency (< 5 Hz) content. At short offsets, the airgun array provides a preferable seismic source to the explosives, due to densely spaced shots and a consistent waveform resulting in excellent trace-to-trace coherence. With increasing offsets, it may be necessary to stack the airgun data to enhance its signal-to-noise ratio: here we use a 4-fold stack. Large explosive shots, although more powerful, produce a less consistent waveform and are more widely spaced due to operational constraints. The offset at which airguns provide a preferable source is dependent on the ambient noise. This practical comparison of real sources demonstrates that, even without advanced processing, a well-tuned airgun array may provide a preferable source to explosives at offsets up to 160 km, under favourable experimental conditions.