Elastic anisotropy in marine sedimentary rocks

Abstract

Measurements of four of the five elastic stiffnesses of marine calcareous rocks and estimates of the fifth, c•3, allow more detailed discussion of elastic wave propagation in these rocks than previously possible. The constant c• 3, which is seldom measured and was not measured in the rocks of this study, was derived by equating the Gassmann and Hashin-Shtrikman estimates of the bulk moduli of chalk and limestone and then solving for the single unknown c•3. For chalk, the measured constants, in N/m 2 x 10 •ø, are c• = 1.01, %3 = 0.94, c,•,• = 0.18, and c66 = 0.21. For limestone, the measured constants, also in N/m 2 x 10 •ø, are c• = 2.41, %3 = 2.09, c,•,• = 0.47, and c66 = 0.60. Three physically possible values orca3 were computed for chalk ( 0.48, 0.52, and 0.63 N/m 2 x 10 •ø) and for limestone (0.73, 1.05, and 1.24 N/m 2 x 10•ø). Heretofore, the only statements which could be made about c•3 in these rocks arelcfii < 0.87 x 10 •ø N/m 2 (chalk), and Ic•3 [ < 1.94 x 10 •ø N/m 2 (limestone). The calculations require the assumption that anisotropy in these rocks is caused by mineral alignment and the simplification of monomineralogy (calcite). On the basis of these measurements and estimates, the effect of elastic anisotropy on seismic reflection determinations of vertical compressional wave speed in calcareous rocks below the earth's surface is small ( + 6%), whereas similar determinations of vertically polarized shear wave speeds may be unreliable. Relationships are provided to convert seismic refraction measurements of horizontal compressional wave velocity to vertical velocity. These relationships are independent of the estimated values of c• 3-

Estimating shallow shear velocities with marine multicomponent seismic data

“…In land surveys with vibroseis or buried explosive sources, however, they are apparent and may provide useful information complementary to the Rayleigh waves. For example, with observations of both Rayleigh and Love waves one can infer information about transverse isotropy e.g., Bachman, 1983;Odom et al, 1996. If information from Love w aves is desired, sources on towed sleds have b e e n d e v eloped by marine geophysicists to generate them Stoll et al, 1994a and a few studies have beencompleted on Love waves in marine sediments e.g., Bautista and Stoll, 1995. It should bementioned, however, that Love waves complicate the seismograms such that unambiguous simultaneous determination of Rayleigh and Love w ave dispersion may bechallenging.…”

Analysis of Seismic Anisotropy of the Typical Slate from the Gaoligong Mountains, Yunnan Province, China

“…There is no difficulty to see that the values in the X and Y directions are quite close and apparently higher than that in the Y direction, no matter how we calculate mechanic parameters using either velocity measurement of slate or S-wave splitting. According to related studies (Li et al, 2006;Song, et al, 2004;Lekhnitskii, 1963;Bachman, 1983;Lo et al, 1986;Hao et al, 2006;Daley et al, 1977), we can conclude that slate can be regarded as one typical transverse isotropic (TI) medium and can be used to calculate seismic velocity based on relevant models or formula. Routinely, we employed the strain ellipsoid to fit 3D limited strain within deformed rocks.…”

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

“…1,2 The vertical gradients of the shear speed can be quite large, and velocity anisotropy is an almost universal feature of marine sediments, with transverse isotropy ͑TI͒ being the most common type of anisotropy. [3][4][5][6] Strong range dependence causes energy in an initially unidirectional propagating signal to be redistributed among forward and backward discrete and continuum modes. Inclusion of finite shear speed in the sediment is necessary to model the Stoneley wave propagating at the water sediment interface and to properly account for the component of transmission loss due to conversion to shear waves.…”