Date of Award

2013

Degree Type

Thesis

Degree Name

Master of Science in Ocean Engineering

Department

Ocean Engineering

First Advisor

Gopu R. Potty

Abstract

Acoustic propagation in littoral waters is greatly affected by seafloor sediment properties. Shear properties of sediments are also directly related to the strength of sediments for geotechnical applications. These factors emphasize the importance of estimating shear wave speeds in semi-consolidated shallow water sediments. One of the most promising approaches to estimate shear speed is to invert the shear speed profile using the dispersion of seismo-acoustic interface (Scholte) waves that travel along the water-sediment boundary. The propagation speed and attenuation of Scholte waves are closely related to the shear wave speed and attenuation over a depth of 1-2 wavelengths into the seabed. Based on this concept, the University of Rhode Island Ocean Engineering Department has developed a geophone-hydrophone system for the measurement of these interface waves, along with an inversion scheme that would invert the dispersion data for sediment shear speed profiles. The objective of this research was to investigate the validity of the system in estimating the shear speed of surface waves at the water-sediment interface. This geophone-hydrophone system was tested at Davisville, RI and the results obtained from this test will be presented. These results will also be compared to correlated values of shear speed from existing boring log data at the site to validate the inversion scheme. The data collected was processed using the Spectral Analysis of Surface Waves (SASW) method and inverted using an inversion scheme based around the Godin-Chapman forward model. The inverted shear speed profile was consistent with the shear speed profiles related to sand and silt, which coincides with the types of sediment encountered in the boring logs. The validity of the inversion scheme will be addressed and future improvements and work will be discussed.

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