Geophys. J. Int. zyxwvutsrqpon (1989) zyxwvutsrq 98, 173-182 zyxwvutsrq Determination of the seabed porosity and shear modulus profiles using a gravity wave inversion Tokuo Yamamoto, Mark V. Trevorrow, Mohsen Badiey and Altan Turgut Geo-Acoustics Laboratory, Division of Applied Marine Physics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, zy 4600 Rickenbacker Causeway, Miami, FL 33149, USA Accepted 1989 February 2. Received 1989 February 1; in original form 1988 September 8 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 zyxw 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. zy Key words: seabed, porosity, shear modulus, inversion 1 INTRODUCTION The determination of the physical properties of the seabed is an important and challenging problem of geophysics. Such information is badly needed for modelling the wave propagation in the ocean. Such information is also important for the design of foundations for offshore structures. Our understanding of geologic processes in various continental shelf provinces also requires such information. Among all of the elastic moduli, the shear modulus is a particularly sensitive indicator of the skeletal structure of sediments. Sedimentological parameters like grain size, porosity, compaction, cementation, etc., are therefore closely related to shear modulus. Recently, the petroleum industry has actively pursued shear-wave seismology in land exploration because of this reason (Danbom & Domenico 1987). On the other hand, active shear-wave seismology has been limited in the marine environment partly because shear waves are difficult to excite acoustically in the marine sediments (Rauch & Schmalfeldt 1983). Recently, an entirely new passive geophysical inverse method called the Bottom Shear Modulus Profiler (BSMP) has been developed as an alternative to active seismic methods. The BSMP theory (Yamamoto & Torii 1986) enables seabed shear modulus profiles to be extracted by inverting measurements of seabed motion and water wave-induced pressure. Experimental verification of the BSMP theory has recently been made through comparisons between the BSMP inversion of wave-induced seabed motion measured by an OBS and existing borehole data at the inner continental shelf off the New Jersey coast (Trevorrow et af. 1988a,b) and at the Great Bahama Bank (Badiey zyx et af. 1988). Due to the limited sensitivity of the seismometers used in these preliminary experiments, the penetration depth of BSMP inversion was limited to 50m into the seabed at water depths up to 12m. An improved version of BSMP instrumentation system was built and tested on the outer continental shelf off the coast of New Jersey in the summer of 1987 (Turgut et af. 173 Downloaded from https://academic.oup.com/gji/article-abstract/98/1/173/621562 by guest on 26 May 2020