Velocity variabilities and other physical properties of marine sediments measured by crosswell acoustic tomography

Abstract

High-resolution images of the compressional wave velocity structure within various bottom sediments have been determined from high-frequency crosswell acoustic tomography. The three-dimensional wave-number spectra of the velocity variabilities were determined for each measured velocity image. The sediment velocity variability spectra are anisotropic in general. The variability frequency in the vertical direction is much higher than in the horizontal direction. The aspect ratio of the two is found to be approximately 5.0. In addition, the major and minor axes of the anisotropy are often tilted from the vertical and the horizontal direction, respectively. The angle of tilt called dip is found to be as much as 33 deg. The measured velocity power spectra approximate a power law. The spectral strength depends on the sediment type and the location. Based on the rock mechanics transformation procedure by Yamamoto et al. [Geophysics 59, 1530-1541 (1994)], the sediment porosity, density, permeability, and shear strength images have been transformed from the velocity images. These sediment properties, determined from the acoustic crosswell tomography, have been supported by direct measurements on cores and boreholes. The density variabilities of the sediment are found to be similar to the velocity variabilities. Unlike in ocean water, the relative density variabilities in the sediments are much larger than the relative velocity variabilities. ¸ 1995 Acoustical Society of America. PACS numbers' 43.30.Ft, 43.30.Gv, 43.30.Ky, 43.30.Ma els are available in the open literature, 9-•5 application of these models requires more comprehensive data of velocity and density structure within the media such as 3-D spectra of the velocity and density variabilities. A sediment volume scattering model by the author •6 has explained these grazing and azimuthal angle dependences of acoustic scattering as the effects of anisotropy and dip of the velocity and density variabilities which are characteristic of shallow-water sediments as will be shown in this paper. The geology of shallow-water seabeds is complicated because of the physical, chemical, and biological activities which took place though the long geological time spans. •7 Measurements of the velocity and density variabilities within the ocean are relatively easy because one can place instruments in the ocean. On the other hand, measurements of the variabilities within the solid earth are difficult because it is difficult to place instruments within the solid earth. The two most common ways of measuring the seabed structure are coring and seismic reflection. Cores only give the vertical variabilities of the seabed properties. Undisturbed cores are difficult to obtain from sedi-ments, especially sandy sediments. Seismic reflection provides the interfaces of strong seismic reflectors within the seabed which are heavily dis-2235

Modeling of monostatic bottom backscattering from three-dimensional volume inhomogeneities and comparisons with experimental data

“…Recently, a power-law type of correlation function has attracted more interest [53,63]. Meanwhile, Yamamoto, in his measurement of velocity variability using crosswell acoustic tomography, found the measured power spectra approximate a power law [64]. Still it is hardly conclusive that the power-law type power spectral density is the best description of velocity variations in the sediment.…”

“…However, it is just t he starting point of our parameter search. In the meantime, the standard deviation a of the sound speed fluctuation can range from 1.5% to 8% and (3, the ratio between density and sound speed fluctuations, between 1 to 10 according to Yamamoto [64]. Also in Yamamoto's estimation, both sound speed and density fluctuations were characterized by a power law distribution with v degree grazing angle for all the selected frequencies except 500Hz, there is a small bump.…”

Acoustic scattering in the ocean from velocity and density fluctuations in the sediments

“…1 In order to resolve this problem, we have developed a high-resolution crosswell tomography technique to determine the three-dimensional ͑3-D͒ power spectra of velocity and density fluctuations within the sediment volume ͑Yamamoto et al͒. 20,21 In addition, Yamamoto 22 presented a method to determine 3-D velocity and density spectra from sediment cores. The 3-D velocity and density spectral data for five shallow water sites and five deep water sites given in the companion paper 22 closely approximate a power law and show that the inhomogeneities of ocean sediments are anisotropic and the principal axis of the 3-D spectrum is tilted from the vertical due to dip.…”

“…20,21 In addition, Yamamoto 22 presented a method to determine 3-D velocity and density spectra from sediment cores. The 3-D velocity and density spectral data for five shallow water sites and five deep water sites given in the companion paper 22 closely approximate a power law and show that the inhomogeneities of ocean sediments are anisotropic and the principal axis of the 3-D spectrum is tilted from the vertical due to dip. The effects of spectral exponent, anisotropy and dip on acoustic scattering will be investigated in this paper.…”

An integrated modal approach to surface and volume scattering in ocean acoustic waveguides

“…This spectrum has been used in modeling rough surface scattering [47], and is chosen to reflect the idea that we expect variations in nature at many length scales. Also, it describes a power-law roll off at high wavenumbers like that measured by Yamamoto [85]. spectrum, and wavenumbers outside the I k I= kw circle correspond to waves which are evanescent in both the water and sediment.…”