Remote Determination of in Situ Sediment Properties Using Love Waves

In-situ properties of near-bottom, seafloor sediments are determined using an experimental technique based on the dispersive properties of horizontally polarized surface waves, better known as Love waves. This technique basically consists of generating Love waves, analyzing the recorded signals and finally, mathematically inverting the dispersion curves to obtain a geoacoustic model of the sediment. In the field experiments, Love waves are generated using a torsional source and the generated signals are recorded using a linear array of gimballed geophones. Then experimental dispersion curves (wave velocity vs frequency) are obtained from the recorded seismograms. These seismograms are usually composed of overlapping wavetrains representing multipath propagation. It is demonstrated that this multipath energy causes a high frequency modulation in the experimental phase velocity dispersion curve. In order to reduce this effect, the use of a phase-matched filter or the use of a low-pass filter is proposed. Finally, the filtered dispersion curve is mathematically inverted, using a nonlinear inverse based on a constrained least-squares method. In the inversion, the sediment is idealized as being composed of a stack of homogeneous layers overlaying a half space. It is shown that the maximum parameter resolution depth can be estimated from the generalized inverse. Also, an estimate of the model uncertainty is obtained. The results of an experiment carried-out in New York Harbor are presented.