Head wave data inversion for geoacoustic parameters of the ocean bottom off Vancouver Island

Abstract

Head wave signals from small explosive charges have been analyzed to determine a geoacoustic profile for a shallow water site off the West coast of Vancouver Island. The experimental design contained a vertical aperture, provided by a vertical line array, and a synthetic horizontal aperture from shots at different ranges from the vertical line array. The simultaneous use of travel time and amplitude data for both apertures provided improved accuracy and vertical resolution of the estimated geoacoustic profile. Travel time vs range data from head waves from three distinct sub-bottom interfaces were used to measure compressional speeds in the layers below the interfaces and estimate the layers’ thicknesses. Spatial filtering of the vertical line array data was necessary to resolve the head wave arrival from the shallowest layers. Analysis of the interference structure of the head wave power spectra at the vertical line array provided the means to measure the travel time of the head wave from the deepest layer. In addition, the head wave power spectra were used to estimate the compressional wave attenuation, using a method based on the variation with range of the logarithmic derivative with respect to frequency of the head wave spectrum.