Field Experiments to Study Sea-Floor Seismoacoustic Response

Results of a 5-year program to measure the seismoacoustic properties of unlithified sediments in the seafloor are presented. A field technique utilizing an impulsive source and a geophone array located on the seafloor was used to obtain detailed travel-time curves which permit a number of different kinds of analysis. A new signal source powered by electrically detonated, 8-gauge shotgun shells was used to focus energy into the bottom and eliminate much of the high-frequency water-borne noise that accompanies the usual explosive sources. Both interface waves and wide-angle refractions were analyzed. A variety of data inversion techniques including slant stacking and cross multiplication were used to obtain dispersion curves, and constrained, least-squares inversion using partial derivatives was used to develop an iterative inversion method to obtain velocity and attenuation as a function of depth. Examples of data from a wide range of different kinds of sediment ranging from soft Holocene mud to stiff overconsolidated clay are presented. The resulting geoacoustic models reflect the influence of geologic history, which largely determines the in situ porosity, and the additional effect of the present overburden pressure.