Determination of the seabed porosity and shear modulus profiles using a gravity wave inversion

Abstract

SUMMARY A previously developed Bottom Shear Modulus Profiler (BSMP) theory (Yamamoto & Torii 1986) allows the seabed shear modulus versus depth profile to be extracted by inverting measurements of seabed motion and water wave-induced pressures at one point on the seafloor. Preliminary BSMP experiments (Trevorrow et af. 1988b; Badiey et af. 1988), compared to existing geologic borehold data at two inner continental shelf sites, showed that this method can accurately predict the magnitude and depth structure of the sediment shear modulus. An improved version of BSMP instrumentation system has been developed and deployed in deeper water at outer continental shelf sites. Gravity water wave-induced bottom pressures are measured in a period band from 7 to 200s, whereas measurements of gravity water wave-induced ground motion are limited to a period band between 7 and 30 s, due to the limited sensitivity of the seismometers used. It is also found that burial of a BSMP in the seabed improved the seabed seismometer coupling significantly. The BSMP inversions of these data extract the shear modulus versus depth profile of the seabed with a depth resolution of a few meters and penetrating as much as 200m into the seabed. The shear modulus of a sediment at a given depth of burial is a unique function of the porosity of the sediment. Using this relation, the porosity versus depth profile of the seabed is calculated from the shear modulus profile obtained by the BSMP inversion. Excellent agreements are shown between the porosity profiles obtained from the BSMP inversion and the borehole porosity logs. The pressure data indicate that the potential penetration depth of the BSMP inversion is about 2 km into the continental shelf and 20 km into the deep seafloor.