The Low level Acoustic Combustion Source (LACS) is a new type of pulsed underwater acoustic source with an output level of 0.5-1.0 barm peak to peak. The source is based on a combustion engine and intended for shallow seismic surveys. It operates at 5 m depth and is able to fire pulses at a rate of up to 11 shots per second. Because of the source's low output level, it is expected to be more suitable than other sources in environmental sensitive areas. The low pulse level may be compensated for by an integration of energy from a sequence of pulses. This article describes the results of a seismic field test in Byfjorden outside Bergen, Norway. The best seismic result is achieved using deconvolution or an iterative correlation method where the correlator sequence is recorded right below the seismic source. Compared with airgun results along the same survey path, the seismic picture based on the LACS source gives a clearer presentation of the bottom layers.

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 time-coded sequence of source pulses has been designed to allow for increased range to the target and reduced side-lobe energy with correlation. The method was designed for a new type of marine-combustion seismic source that can be fired repeatedly, but the method also can be applied to other acoustic-pulsed sources. The basic concept is that one can reduce autocorrelation side lobes by using semiperiodic sequences to concentrate much of the side-lobe noise into time intervals corresponding to the semiperiod. Signals can then be detected with better sensitivity in the time interval after the first autocorrelation peak and before the first periodic side lobe, because this interval contains only lower-level crosscorrelations between the sequence components. Compared with a source which fires pulses randomly, an 8-dB reduction of autocorrelation side lobes has been simulated for sequences of [Formula: see text] duration.