Marine seismics with a pulsed combustion source and Pseudo Noise codes

Abstract

There has been a long-standing debate concerning how dangerous seismic surveys are with respect to marine life. Marine seismic work today is dominated by airgun technology, where high energy is generated by a release of compressed air into the water. The objective of the "Time coded impulse seismic technique" project is to examine whether a new low energy acoustic source can be used for seismic purposes. If the method turns out to be successful, the low output energy and continuous operation will make the source suitable in environmental sensitive areas. The Low level Acoustic Combustion Source (LACS) is a petrol driven pulsed underwater acoustic source. It operates at a few meters depth, and each shot can be digitally controlled from the surface by a computer located in the mother vessel. A presentation of the recorded LACS signal characteristics, the modulation, the Pseudo Noise coding/decoding principles and field test results, is given. The importance of using an optimized code with fine resolution and of using the near field recording as correlator sequence is demonstrated. Clear correlation peaks could then be seen from the bottom and sub bottom reflectors.

A seismic field test with a Low-level Acoustic Combustion Source and Pseudo-Noise codes

“…How successful the correlation is, depends on the timing of a series of pulses which is the code of the sequence. In (Askeland et al, 2006) a proposed coding and decoding principal based on Pseudo Noise (PN) sequences is presented. The LACS is coded with randomly spaced pulses.…”

A Summary of Existing and Future Potential Treatments for Reducing Underwater Sounds from Oil and Gas Industry Activities

“…0-933957-35-1 ©2007 MTS Several alternatives to conventional seismic exploration techniques have been identified. Various acoustic projectors have been prototyped and show large reductions in sound level across the frequency spectrum [2][3][4]. These are compresCsed replacements for conventional airguns or other seismic sources.…”

“…However, in this case the total acoustic energy input into the water is reduced relative to conventional methods, and thus a positive net effect may be realized.) Additional details on these approaches are provided in References [2][3][4]. These sources are still in the development/proof-of-concept stage, but may not be far from being used in commercial applications (one to several year timeframe).…”