X. Detection and Estimation Theory Academic Research Staff

Multiple reflections in the water column often make the detection of deep seismic reflectors in oceanographic exploration very difficult. In this report we describe a novel method of modeling the unwanted feedback generating these multiples. This model formulates the removal of the multiples as a system-identification problem, and uses the maximum-likelihood method of statistics to derive an algorithm for estimating the model parameters. This algorithm can be implemented on a small onboard computer and yields an enhanced performance over comparable methods previously used. Experimental results of applying the algorithm using data taken aboard the R/V Atlantis II during Woods Hole Oceanographic Institute's International Decade of Oceanographic Exploration cruise to the coast of Southwest Africa are presented. In seismic profiling, a short pulse of acoustic energy is generated by the sudden release of compressed air (2000 psi) or by the discharge of a high-voltage capacitor (15-20 kV) through a submerged electrode. This energy is reflected at the sea floor and beneath it at interfaces of seismic layers (reflectors), where there is a finite change in the acoustic impedance of the medium. Over a sequence of returns, the geologic structure is indicated by the horizons generated by these reflectors. In many situations, particularly in shallow water, this reflected energy reverberates in the water column via multiple reflections off the sea floor and sea surface. This energy often appears with the same travel time as reflections from deeper layers in the primary return so as to create a very low signal-to-reverberation level. The net result is that detection of the deeper interfaces is made very difficult by these "multiples." Numerous studies have been directed toward eliminating this reverberation. Generally, the methods can be classed as those which exploit the temporal structure of the signals and those which use the spatial structure by combining outputs of elements in a trailing streamer array. 1, 2 With the temporal methods it is recognized that the multiple