In Situmeasurement of transverse isotropy in shallow-water marine sediments

Abstract

The interleaving of parallel isotropic lamellae of contrasting mineralogical composition makes almost all marine sediments anisotropic, the form of anisotropy being transverse isotropy with a vertical axis of symmetry. Conventional marine seismic experiments, however, cannot quantify the anistropy because they do not record unconverted shear waves. In 1986, Rondout Associates, Inc. (RAI) and Woods Hole Oceanographic Institution (WHOI) recorded direct shear waves in shallow marine sediments by using a newly developed ocean-bottom shear source and a multicomponent on-bottom receiver. No single isotropic model could be adequately fit to the data, implying anisotropy. the seismic experiment was conducted in 21 m deep water about 10 km east of the New Jersey coast. In this paper, we describe the anisotropy in the top 50m of marine sediments beneath two of the RAI/WHOI refraction profiles. We use an anisotropic reflectivity program to produce synthetic seismograms to estimate the five independent elastic stiffnesses necessary for describing the transverse isotropy. Our synthetics fit the vertical and two horizontal components of the data for both profiles. the two intersecting refraction profiles are 150 and 200 m long. These profiles are not long enough to constrain compressional wave velocities and anisotropy, but are quite adequate to find the shear wave anisotropy. A nearby drill hole showed that the sediments are interbedded silty clays, clays, and sands. the data require low shear velocities (<400 m s-1) and low Qs (<100) in about the top 30 m of the sediments. In the top 10 m of the sediments, silty clay exhibits ∼12–15 per cent anisotropy for shear waves.