Anisotropy effects in P-wave and SH-wave stacking velocities contain information on lithology

Abstract

Depths calculated from S-wave stacking velocities and event times almost always exceed actual depths, sometimes by as much as 25 percent. In contrast, depths from corresponding P-wave information are often within 10 percent of actual depths. Discrepancies in depths calculated from P- and S-wave data are attributed to velocity anisotropy, a property of sedimentary rocks that noticeably affects S-wave moveout curves but leaves the P-wave relatively unaffected. Two careful studies show that discrepancies in depths, and hence in constituent layer thicknesses, correlate with lithology. Discrepancies ranged from an average of 13 percent (Midland basin) to greater than 40 percent (Paloma field) in shales, but were within expected errors in massive sandstones or carbonates. Hence anisotropy effects are indicators of lithology. Analysis of seismic data involved determining interval velocities from stacking velocities, calculating layer thicknesses, and then comparing layer thicknesses from S-wave data with thicknesses from P-wave data. When the S-wave thicknesses were significantly greater than the P-wave (i.e., outside the range of expected errors), I concluded the layer was anisotropic. I illustrate the technique with data from the Paloma field project of the Conoco Shear Wave Group Shoot.

Estimating a shear modulus of a transversely isotropic rock

“…This quantity was calculated using the formula, (Thomsm, 1986), a.nd outliers were omitted. This amount of anisotropy is similar to the amounts found in other sedimentary rocks (Thomsen, 1986;Winterstein, 1986;Winterstein and Paulsson, 1990).…”

“…Some researchers are currently trying to use this anisotropy to improve the exploration for and production of hydrocarbons. For example, Winterstein (1986) found that the effects of transverseisotropy are good indicators of lithology. Geophysicists might need to account for the effects of this anisotropy in some areas of processing and interpretation like reflection a mplitude versus offset (Wright, 1987), migration, and the estimation of the mechanical strength of rocks.…”

Anisotropic velocity analysis1

“…The results presented are not only of interest in velocity model building but may also be useful for lithology determination (Winterstein 1986). A measurement of P-wave velocity in a single direction is insufficient to distinguish silty shales from sandstones.…”

Quasi-shear wave ray tracing by wavefront construction in 3-D, anisotropic media

“…Levin (1978Levin ( , 1979Levin ( , 1980 pointed out that quasi-SV (qSV) waves usually do not have hyperbolic traveltime curves and that moveout velocity varies with distance. Winterstein (1986) concluded that layer thickness estimates obtained from SH-wave data can be significantly thicker than those from P-wave data in the presence of anisotropy, and the percentage of clay in layers dominates the magnitude of anisotropy. Processing to measure shear-wave splitting also provides further information on earth properties that cannot be obtained from quasi-compressional (qP) data.…”

“…For example, Levin (1979), Alkhalifah and Tsvankin (1995), Grechka and Tsvankin (1998), and Grechka et al (2001) described methods for estimating the moveout velocities in transversely isotropic (TI) media, which can be difficult to estimate from surface seismic data. Banik (1984) and Winterstein, (1986) showed that neglecting anisotropy can easily lead to substantial misfits of layer thicknesses when analyzing seismic data. Leslie and Lawton (1999), Grech et al (2002), and Hornby et al (2003) showed that corrections for shale anisotropy are necessary to obtain accurate subsurface seismic images.…”