Observations of compressional, shear and interface waves in the New England Mudpatch

Abstract

In March 2017, instruments including a tetrahedral hydrophone array and vertically gimbaled geophones were deployed during the Seabed Characterization Experiment in the New England Mudpatch south of Martha’s Vineyard. The water depth at the location was about 70 m and a 6-m-thick layer of fine-grained sediments overlays sand layers. The relative positions of the geophones were localized using a time-difference-of-arrival (TDOA) technique applied to signals from Mk-64 SUS charges at various aspects and ranges from 5 to 25 km. In addition, noise from a research vessel provided information on relative depths of the geophones. The geophones were found to have sunk into the soft mud, and this provided a unique opportunity to sense shear waves in the mud and Stoneley waves on the mud-sand interface. In addition, post-Airy Phase arrivals are thought to be shear waves generated at the mud-sand boundary. Interface waves were generated by the interface Wave Sediment Profiler (iWaSP), an instrument with bender beams oriented to shake the seabed. Estimates using iWaSP of shear speed in the mud and sand layers were consistent with inversions using modal dispersion measured on the tetrahedral hydrophone array reported by Potty and Miller (2019). [Work supported by the Office of Naval Research.]