NEW HIGH-RESOLUTION OR DEEP PENETRATION AIRGUN ARRAY

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 4165 inch**3 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).