High-resolution measurements of volume heterogeneity in sandy sediments

Abstract

As part of SAX 99, a multi-investigator study focused on the penetration and scattering of high-frequency (10–300 kHz) sound in marine sediments, two independent measurements of sediment volume heterogeneity (SVH) were made. The first tool used to measure SVH was an in situ resistivity profiler (IRP) that logs sediment resistivity at 0.25-mm vertical intervals to depths of 15 cm. Robust empirical calibrations are used to calculate sediment porosity. The second method used to measure SVH was a digital x-ray system that comprises a constant-potential x-ray source and an amorphous silicon x-ray detector. The IRP data indicate little between-site variability and bottom-type (i.e., ripple crest, ripple trough, mound) specificity. Profiles are characterized by a zone from 0- to 3-mm subbottom of exponentially decreasing porosity, underlain by a zone of roughly constant porosity characterized by 5%–10% fluctuations. The digital x radiographs indicate three sources of SVH: (1) biogenic structures, (2) mud inclusions, and (3) shell fragments. Of these, the shell fragments are likely to be the most important sound scatterers. Estimates of shell fragment concentrations, distributions, and orientations will be discussed. [Work supported by ONR.]