NEW TOOLS FOR STUDYING SEA-FLOOR GEOTECHNICAL AND GEOACOUSTIC PROPERTIES

Three new tools designed to measure certain geotechnical and geoacoust ic properties of the sediments just beneath the seafloor are described . One tool is a source-receiver system designed to generate and receiv e 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 ge ophones 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-s quares 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 motio n of the seafloor and the Scholte wave dispersion is the basis for inv ersion to obtain a sediment model. Energy to generate the vertical pul se is supplied by 22-caliber blank cartridges fired into;a chamber tha t focuses the energy into the seafloor. The third new tool is a cone p enetrometer that measures in situ shear strength of the sediment conti nuously 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 Sl ed which is deployed on the seafloor and attached to; the linear array of geophones. One of our main objectives is to measure shear-wave vel ocity and shear strength in situ at the same locations to permit corre lation without the usual problems caused by sample disturbance and cha nge of environment which occur in sampling and laboratory testing.