ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

1pAO2. FM sonar characteristics for normal-incidence sediment classification.

Steven G. Schock

Lester R. LeBlanc

Dept. of Ocean Eng., Florida Atlantic Univ., Boca Raton, FL 33431

The prediction of sediment properties from normal-incidence acoustic measurements made with a broadband sub-bottom profiler requires that several acoustic parameters be estimated from the reflection data using independent numerical techniques to reduce the large potential errors of any one parameter. Examples of acoustic parameters that are combined empirically to estimate the vertical profiles of physical sediment properties are acoustic impedance, interlayer volume scattering, and attenuation. From the analysis of normal-incidence data collected in many depositional environments, the characteristics of a quantitative sub-bottom profiler for estimating impedance and volume scattering include: (1) the acoustic bandwidth of the transmitted pulse must be at least 2 oct for the reliable measurement of signal phase from interlayer reflections; (2) the two-way transmission/reception beam should be a cone with a width between 15(degrees) and 20(degrees) at the 3-dB down points to ensure that interlayer reflection amplitude to volume and surface scattering noise ratios are at least 6 dB (narrower beamwidths result in reflection amplitude measurement errors from sensor motion or seafloor slopes less than 5(degrees)); (3) the transmitted pulses contain energy from 500 Hz to 10 kHz to ensure most interlayer impedance gradients can be accurately measured.