ABSTRACT:

As a portion of a joint University of New Hampshire-Raytheon Company Sea Grant Program, a multi channel towed array was designed, fabricated and tested at sea. The array acquires both normal and oblique incident reflection data which are recorded on magnetic tape for subsequent computer processing. The fixed spatial position of the hydrophones in the array allows highly repeatable determinations of'compressiona1 wave velocity employing a variation of the classical T2X2 approach. Description of the multi channel array and shipboard data acquisition system together with results 9f at-sea system tests and initial analytical results are presented

Introduction:

Since 1970, the University of New Hampshire and the Submarine Signal Division of Raytheon Company have been engaged in a joint Sea Grant Project examining the application of reflected acoustical energy to the remote measurement of physical and engineering properties of continental shelf sediments. Complementing the theoretical investigations being conducted by the University of New Hampshire towards formulating a field theory solution to modeling the reflection process, initial field studies were undertaken to acquire data from a range of "sediment types." Data on the normal and oblique incident response of these sediments were obtained at five discrete frequencies between 3.0 KHz to 16.0. Khz from a number of test sites where extensive physical sampling also was performed.

Experimental results of angular reflectivity and attenuation studies relating these acoustical indices to sediment physical properties have agreed well with several prior studies, both in the functional relationships observed between the variables and in the degree of scatter exhibited. Comparable results were also achieved in a parallel effort employing a 2-element vertical hydrophone array for acquisition of oblique incident data from which sediment velocity was calculated. These velocity computations which were based upon a modified T2XZ approach, however, exhibited large variances indicating the greater dependency of this procedure on precise geometric control and time resolution of the arriving signals. Computer simulation of the data acquisition geometry and analysis of individual source - receiver pair displacements established that a significant portion of the observed scatter could be attributed to small random variations in the position or angular relationships of the acoustical sensors. Subsequent experiments conducted with both source and receivers located on stable, bottom-resting structures confirmed this hypothesis. Extending the above test, a parametric analysis using the above computer simulation, also was performed to assess the magnitude of "allowable" sensor motion for the range of anticipated data acquisition geometries and the reflection process statistics developed during the fixed geometry tests. Allowable motion was defined for the purpose of this investigation as the minimum amount of displacement to cause either a ±2dB variation in reflected signal amplitude or a :3% variation in the T2X2 computed compress ional wave velocity.

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