Measuring dispersion and attenuation in sediment at low frequency

Abstract

Much work has been done at frequencies above 1 kHz to measure sediment dispersion and attenuation and these results have been analyzed within the context of various models such as Biot-Stoll, Buckingham, and BICSQS. Some effort has been made to establish the frequency dependence of sediment attenuation below 1 kHz, but few results are available concerning dispersion in that frequency band. The present work examines a technique for measuring low frequency sediment dispersion and attenuation with a real dataset. Recently, data were collected off the coast of Florida using implosive broadband sources to perform geo-acoustic inversions. Previous inversion studies with this data ignored any dispersion effects. [Stotts et al, J. Acoust. Soc. Am. 115, 1078–1102 (2004).] The environment was shown to consist of thin, hard, porous material less than 1 m thick overlying sand sediment. A ray trace plus plane-wave reflection coefficient model was used for the analysis. Here, dispersion is represented phenomenologically over a finite frequency band in the model by allowing both the sound speed and attenuation to be frequency dependent in both layers of a two-layer sediment plus substrate model.