Acoustic and In-Situ Techniques for Measuring the Spatial Variability of Seabed Geoacoustic Parameters in Littoral Environments

Abstract

Geoacoustic properties of the seabed are required for accurate modeling of acoustic propagation, and hence sonar performance prediction. Characterizing acoustic interaction with the seabed is particularly important in shallower water environments as the propagation typically involves extensive interaction with the sea surface and seabed. DRDC Atlantic is developing acoustic and in-situ techniques for seabed classification and measuring geoacoustic parameters. The acoustic technique uses normal incidence acoustic returns, in the 1 to 10 kHz band, from the seabed and sub-bottom. The transition from interface to volume scattering depends upon frequency and sediment type and can be used to distinguish the composition of near surface marine sediments. An experimental methodology has been developed using a vertical line array of receivers and a downward-looking superdirective projector array. The technique is also being adapted for use with commercial normal incidence sub-bottom profilers. In-situ measurements are being made using the DRDC Atlantic free fall cone penetrometer probe (FFCPT). It has been developed to measure acceleration and dynamic sediment porewater pressure as a function of depth of penetration into the seafloor. It also records hydrostatic pressure and optical backscatter for detection of the mudline. This combination of sensors permits the direct application of geotechnical analysis methods and parametric-based correlations already long established in engineering practice. The FFCPT provides two independent means of calculating the undrained shear strength, as well as other engineering variables that are used to identify the sediment grain size characteristics. The probe has a modular design allowing additional sensor payloads to be integrated, the first of which uses resistivity as a means to determine sediment bulk density. Experimental results, using the acoustic and in-situ techniques, will be presented from two joint US-SACLANTCEN-CAN sea-trials in 2001 at the New Jersey Strataform area and the Scotian Shelf.