New Tools For Studying Seafloor Geotechnical And Geoacoustic Properties

Three new tools designed to measure certain geotechnical and geoacoustic properties of the sediments just beneath the seafloor are described. One tool is a source-receiver system designed to generate and receive SH waves or Love waves. The source applies a torsional impulse to a small circular area of the seafloor and a linear array of gimballed geophones is used to measure horizontal motion perpendicular to the axis of the array. Energy to generate the torsional pulse is supplied by a rotating flywheel that is suddenly brought to rest. The dispersion of Love waves produced by this method is used as the basis for a least-squares inversion to obtain shear-wave velocity as a function of depth. A second source-receiver system generates vertically polarized shear waves or Scholte waves by applying a vertical impulse to the sediment over a small circular area of the bottom. In this case, the receiving array utilizes gimballed geophones that respond to the vertical motion of the seafloor and the Scholte wave dispersion is the basis for inversion to obtain a sediment model. Energy to generate the vertical pulse is supplied by 22-caliber blank cartridges fired into a chamber that focuses the energy into the seafloor. The third new tool is a cone penetrometer that measures in situ shear strength of the sediment continuously over depths up to 60 cm. Both the cone penetrometer and one or the other of the two seismic sources is mounted on a self-righting sled which is deployed on the seafloor and attached to the linear array of geophones. One of our main objectives is to measure shear-wave velocity and shear strength in situ at the same locations to permit correlation without the usual problems caused by sample disturbance and change of environment which occur in sampling and laboratory testing.