Shear Modulus Measurement of Bottom Sediments Using Boundary Waves

Knowing the shear modulus is an essential element in the estimation of the strength of sediments and rocks. Theory and experiments show that the propagation of boundary waves (Scholte-, Rayleigh-, Love- or Stoneley-type) along a liquid/solid or solid/solid interface is highly dependent on the shear moduli. Measurement of the velocity of boundary waves at the interface between the water and the sea floor can thus be used for the determination of the shear modulus of the bottom material. An experimental equipment for the excitation and detection of boundary waves in the sea floor has been developed. The equipment consists mainly of a piston vibrator as a source and a number of geophones as receivers a few meters from the source. Several experiments on land and on the sea floor have been conducted, and the equipment has proved to be able to produce signals of sufficient strength and reliability. The measurements on land have been compared with conventional cross-hole measurements with good results. The experience gained so far indicates that the technique can easily be made into an operational technique for the determination of the shear modulus of the upper layer of the sea floor.