Abstract:

Acoustic sounding of sediment properties is usually carried out with waves at more or less normal incidence. This method, however, fails to detect thin layers (d<<(lambda)) of undissolved fluid covering the sea-bottom due to the small difference in impedance compared to water. But the spherical wave reflection coefficient is strongly affected at grazing incidence and allows for the detection of these layers. Laboratory measurements in model scale as well as model calculations are carried out in order to determine the capabilities of the new measuring method. Experiments were done in a water tank with a hydrophone as a spherical broadband sound source. Various liquid chemicals of density greater than water are poured on the sand in small amounts, forming layers of less than 3-mm thickness. Measured excess attenuation of the pure sediment is in very good agreement with simulations carried out with a fast field program (FFLAGS) that incorporates Biot theory [S. Tooms et al., J. Acoust. Soc. Am. 93, 173--181 (1993)]. The excess attenuation of the sediment coated by a thin layer differs significantly. In particular, the interference pattern is broadened and shifted toward lower frequencies. This behavior is explainable by FFLAGS as well. [Work supported by BMBF.]