A new high‐resolution or deep penetration airgun array

Abstract

A new airgun array has been designed to provide a source for deep penetration or high‐resolution seismic surveys. The airgun array consists of 21 airguns with a total volume of [Formula: see text] operated at a pressure of 2000 psi. The far‐field signatures with the source array at depths of 5 and 10 m were recorded at a depth of 100 m, with a calibrated hydrophone located directly beneath the energy source. The signature was recorded digitally with a 1-msec sample interval, using a presampling filter with a high‐frequency cutoff at 124 Hz and an attenuation rate of 72 dB/octave. The peak‐to‐peak amplitude of the recorded signature is 49.5 bar‐m; the primary‐to‐bubble peak‐to‐peak amplitude ratio is 16.7. The signal frequency band (measured between the 6 dB points) ranges from 9.5 to 90 Hz. For several streamer depths, the effect of the surface reflection at the hydrophone streamer is derived by subtracting the recorded far‐field signature, delayed by the two‐way traveltime from the streamer to the surface, from itself. By studying the derived far‐field signatures and their amplitude spectra, it is seen that for a high‐resolution seismic survey, the airgun array and the hydrophone streamer should both be placed at shallow depths (5–8 m). For a deep penetration seismic survey, where the signal energy in the low‐frequency band is most useful, the airgun array and hydrophone streamer should be placed at medium depths (8–12 m). Far‐field signature tests show that adding a large airgun [Formula: see text] to the airgun array increases the signal energy in the frequency band 0–90 Hz and improves the primary‐to‐bubble peak amplitude ratio from 11.8 to 16.7. Two comparison seismic sections demonstrate that the large airgun improves both resolution and penetration.