Real time processing and display of near-bottom acoustic reflectivity measurements

Abstract
Development of an automated system for quantitative processing of ocean-bottom reflected echo sounder signals is described. This system incorporates a mini computer for digitization, processing and display in real time of the reflection data. The system was developed in conjuction with the Deep Tow project of the Marine Physical Laboratory of Scripps Institution of Oceanography and has been utilized to process data from a 4-kHz bottom profiler system on the Deep Tow Instrument. This Instrument is normally towed close to the bottom (100 meters altitude) by means of an armored coaxial cable, thus greatly improving the lateral resolution of the profiler system. The processing system digitizes the raw 4-kHz signal, stores the digitized data for future reference, and displays a normalized and smoothed version of the data on a facsimile-type graphic recorder. Typically, a display of instantaneous energy and displays of cumulative energy for selected vertical sections are presented simultaneously. In addition, the system has been utilized to calculate reflected energy returned from buried layers as a function of depth of burial in an effort to estimate acoustic attenuation in the overlying sediments. Processed data from several areas of the Pacific has revealed abrupt changes in sedimentary reflectivity of over 7dB, small-scale spatial variations in reflectivity of several dB over horizontal distances less than 100 meters, and several dB variations in reflected energy due to small-scale topographic features of buried reflectors. In addition, attenuation calculations have determined values for acoustic attenuation in calcareous equatorial sediments which are almost 1/3 of those determined for North Pacific sediments of comparable porosity.

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