A Sensitivity Study for Full-field Inversion of Geo-acoustic Data with a Towed Array in Shallow Water

Inversion of acoustic data for estimating bottom acoustic parameters has been the subject of a considerable number of studies. Usually, signals are received on vertical arrays of sensors and transmitted from sound sources being towed away from the array location in order to form a syntethic aperture array. That configuration is greatly dependent on the knowledge of the sourcereceiver distance which is, in practice, relatively difficult to measure with the required precision. Also, since the vertical array is generally moored, or slowly drifting, the area that can be surveyed with such a method is limited to a tenth of a mile in shallow water. Changing area requires the recover and redeployement of the whole system. This paper explores the possibility of using an horizontal array and a sound source simultaneously towed by a single ship where the source-receiver distance is constant. It has been shown that sensitivity to sound speed variations is higher on the first bottom layers and it increases with array length. Density and attenuation (both compressional and shear) have in general small influence on the acoustic field structure and are therefore difficult to estimate. Increasing the signal frequency bandwidth by incoherent module averaging has no significant influence on sensitivity. Mismatch cases, mainly those related to array/source relative position, showed that deviations of more than λ/2 in range and λ/5 in depth may give erroneous extremum location and therefore biased final estimates.