Fine-scale sedimentary structure: implications for acoustic remote sensing

Detailed measurements of sediment properties and acoustic scattering were made at a carbonate sand–silt–clay site off Dry Tortugas in the Florida Keys. The sediment is characterized by varying scales of biologically controlled random roughness and heterogeneity as well as surficial stratification on centimeter scales. The interface roughness was determined from stereo photogrammetric digitization and parameterized by a power spectrum, whereas sediment volume heterogeneity was determined from core measurements and parameterized by first-order autoregressive models for sound speed and density fluctuations. In contrast to previous investigations, fine-scale sediment bulk density fluctuations were examined in sediment cores with computerized tomography in addition to the standard gravimetric technique. Furthermore, the strongly delineated sediment density and sound velocity transition layer was parameterized by piecewise linear fits. These characterizations of the random and deterministic properties were used in acoustic scattering models in an effort to determine the feasibility of remotely measuring fine-scale features of the seafloor. The result was negative: older, low-resolution models gave moderately good fits to the acoustic data, but the fit did not improve for newer, higher-resolution models. It is suggested that scattering due to shell fragments must be included to account for all features observed in the scattering data at this site.