Is it optimal to tow air guns shallow to enhance low frequencies?

Abstract

In marine seismic acquisition, it used to be commonly accepted that it is optimal to tow the source deep to enhance the low-frequency content in the seismic data. However, Mayne and Quay found in 1971 that the low-frequency response of air guns actually improves as the source depth decreases. We evaluated a simple ad hoc theory that demonstrates that two effects are counteracting each other: The free-surface effect favors deep-towed sources, whereas the bubble time period (increasing with decreasing source depth) favors shallower tow depths. From a fjord test, we found that combining several source depths in an air gun array might flatten and improve the low-frequency part of the source spectrum. The experiment confirms that various source depths result in local, characteristic maxima in the low-frequency spectrum, corresponding to the bubble time period of the air gun.