Experimental design for sediment characterization in the southern New England Continental Shelf

The properties of the sea bottom sediments impact the acoustic propagation in shallow water to a great extent. Estimation of the geoacoustic properties of shallow water sediments has been a major area of research in underwater acoustics during the past decade. Several inversion approaches have been developed to estimate the range and depth dependent compressional and shear wave speeds and attenuation of sediments. This study focuses on the design of an experiment to estimate the shear wave properties (shear wave speed and attenuation) at a location in the southern New England Continental Shelf called the “New England Mud Patch”. The mud patch is a 13,000 square kilometer area covered by fine-grained sediment. Previous surveys in this area have estimated the thickness of this fine grained sediment deposit to be as much as 13 m. This layer of sediment rests on a reflector that is geomorphically similar to and continuous with the Holocene transgressive sand sheet, which is exposed on the shelf to the west of this area. This fine-grained sediment layer, which is oriented in an east-west direction seaward of the 55-65 m isobaths, is the only area on the entire eastern United States outer shelf where surficial sediments contain more than 30 percent silt plus clay. Hence this area offers a unique location to apply the inversion techniques to investigate the geoacoustic properties of fine-grained sediments. The inversion technique to estimate the shear properties is based on collecting interface wave (Scholte wave) data using geophones on the sea bottom. The properties of the interface wave are closely dependent on the shear wave profile in the sediment. Hence we can use the Scholte wave data to infer shear properties of the sediment down to 1-2 wavelengths into the bottom. The data for the geoacoustic inversion consists of Scholte wave phase velocity dispersion calculated from the geophone array data. The inversion technique iteratively finds the best shear speed profile that provides the best match to this dispersion data. The phase velocity dispersion of a given shear speed profile in the sediment is calculated using a forward model. Two different forward models have been successfully used to predict the phase velocity dispersion: Chapman-Godin model and Dynamic Stiffness Matrix approach. The model developed by Chapman and Godin uses a power law depth variation of shear speed whereas the model based on dynamic stiffness method models the sediment as layers. In addition to the shear speed, attenuation of shear waves can also be calculated using spectral ratios. The present study aims at applying this Scholte wave based shear wave inversion technique to the mud patch area. Appropriate source to receiver ranges will be explored based on simulations. Most of the deployment of the shear measurement system has been in hard sediments until now and the presence of the soft layer poses some challenges in deployment, data collection and measurement geometry. The engineering challenges associated with deploying the system in a soft seabed will be investigated and design modifications, if required, will be proposed. The simulations will be based on historic sediment data.