On firing an air gun very shallow

Abstract

Recently, two of the authors asked the question “Is it optimal to tow air guns shallow to enhance low frequencies?” By combining recordings of signatures from sources deployed at depths from 3 to 20 m, they concluded that for an air gun array with sources at several depths, more low frequencies would be introduced into the source signature. We have found the significant and notable additional experimental result that firing a [Formula: see text] air gun at approximately 1.3 m in depth gives a broadband signature with low and high frequencies and with notable downgoing amplitude strength. The experiment was not dedicated to the 1.3 m gun depth test, and only one shot was fired at this depth. Nevertheless, we found that the air gun at approximately 1 m depth produces significantly low frequencies less than 2 Hz compared with the guns fired deeper. We suggest that very shallow firing of large volume guns is of interest for VSPs as well as for site surveys due to the high frequencies that are generated and the almost bubble free signature.