Fine-scale measurements of sediment roughness and subbottom variability

As part of the Asian Seas International Acoustics Experiment (ASIAEX), this paper describes the results of fine-scale characterization of surficial sediments at a site in the East China Sea using a conductivity probe. This probe is a modified version of a system originally developed for high-frequency acoustic backscatter applications [see Tang et al., "Fine-scale volume heterogeneity measurements in sand" IEEE J. Oceanic Eng., vol. 27, pp. 546-560, 2002, and Briggs et al., "Characterization of interface roughness of rippled sand off Fort Walton Beach, Florida," IEEE J. Oceanic Eng., vol. 27, pp. 505-514, 2002]. The new system has a single conductivity sensor that can move in both vertical and horizontal directions with controlled steps while taking data, thus resulting in a two-dimensional (2-D) grid on which medium conductivity is measured. Because sediment conductivity is much smaller than that of the overlaying seawater, bottom roughness is determined by selecting the grid points at which conductivity has a sudden drop along each vertical profile. Subbottom density variations are obtained by converting conductivity data to density using Archie\´s law and core density data. The spatial resolution of the measurements is designed to obtain environmental data suitable for modeling acoustic backscatter in the midfrequency (3-4 kHz) range. The results of this paper include the spectrum of sediment interface roughness and subbottom density gradient profiles. Error analysis is conducted to provide bounds on the results. It is found that bottom roughness is dominated by small features caused by bottom-dwelling organisms. A model is developed to simulate the random distributions of these bottom features. The model employs a superposition of discrete features, which results in a power spectrum that is consistent with the measured power spectrum. Potentially, this kind of model can provide a remote sensing means to estimate bottom biological populations through measuring sound backscatter from the bottom. Although the subbottom heterogeneity spectrum is not obtained at the present time because of measurement errors, it is found that sediment density variance is less than 0.5%, implying that bottom backscatter may be caused primarily by bottom roughness.