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 200m 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 (<400m s-') and low Q, (400) 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.

Reflection/transmission coefficients and azimuthal anisotropy in marine seismic studies

“…An alternative is to use shear-wave sources on the bottom (e.g. Berge et al 1990) recorded by a multicomponent ocean-bottom seismometer (OBS). Although OBS technology is well advanced, ocean-bottom sources, especially shear-wave sources, are still in an early stage of development.…”

Constraints on elastic parameters and implications for lithology in VTI media

“…Velocity surfaces showing wavefront shapes in the sagittal plane are plotted in Figure 1 Eor these VTI media The sedimentary rocks in Table 1 were described by Thomsen (19%). The marine silty clay properties were determined in an ocean-bottom refaction experiment in unconsolidated sediments, as described by Berge et al (1991).…”

Effects of transverse isotropy on modes and mode coupling in shallow water

“…At the highest frequencies it was necessary to employ quadruple precision for the first few eigenvalues and eigenfunctions. Our shallow water model was taken from Berge et al 33 who analyzed multicomponent seismic data from an experiment conducted in 21 m of water about 10 km east of New Jersey.…”

“…The data analyzed by Berge et al 33 were collected in 1986 by Roundout Associates Inc. and Woods Hole Oceanographic Institution. Initial attempts at modeling the data assuming isotropy of the bottom material were not considered adequate, 33 and led to further modeling efforts employing an anisotropic reflectivity program.…”