D. F. Winterstein

Velocity anisotropy terminology for geophysicsts

Interest in velocity anisotropy increased sharply among exploration seismologists following publication in 1986 of evidence for azimuthal anisotropy in sedimentary rocks. This increased level of interest generated a need for a glossary of anisotropy terminology in the language of geophysicists. This glossary is to: • help geophysicists find proper words • promote correct usage. The introduction preceding the alphabetically organized glossary focuses on concepts that have special importance for anisotropy but may be unfamiliar to most geophysicists. Symmetry concepts receive special emphasis because of their fundamental importance.

Shear-wave polarizations and subsurface stress directions at Lost Hills Field

2 × 2 S-wave data matrix, accomplished by computationally rotating sources and receivers. Although polarization directions obtained by assuming a homogeneous subsurface were moderately consistent with depth, considerable improvement in consistency resulted from analytically stripping off a thin near‐surface layer whose fast S-wave polarization direction was about N 6°E. S-wave birefringence for vertical travel averaged 3 percent in two zones, 200–700 ft and 1200–2100 ft (60–210 m and 370–640 m), which had closely similar S-wave polarizations. Between those zones, the polarization direction changed and the birefringence magnitude was not well defined. S-wave polarizations from two concentric rings of offset VSPs were consistent in azimuth with one another and with polarizations of the near offset VSP. This consistency argues strongly for the robustness of the S-wave polarization technique as applied in this area. The S-wave polarization pattern in offset data fits a model of vertical cracks striking N 55°E...

Changes in shear-wave polarization azimuth with depth in Cymric and Railroad Gap oil fields

Shear‐wave (S-wave) polarization azimuths, although consistent over large depth intervals, changed abruptly and by large amount of various depths in nine-component vertical seismic profiling (VSP) data from the Cymric and Railroad Gap oil fields of the southwest San Joaquin basin. A simple layer‐stripping technique made it possible to follow the polarization changes and determine the S-wave birefringence over successive depth intervals. Because the birefringence and polarization azimuth are related to in‐situ stresses and fracture, information from such analysis could be important for reservoir development. Near offset VSP data from Cymrix indicated that the subsurface could be appproximated roughly as two anisotropic layers. The upper layer, from the surface to 800 ft (240 m), had vertical S-wave birefringence as large was about 6 percent down to 1300 ft (400 m). In the upper layer the polarization azimuth of the fast S-wave was N 60°E, while in the lower layer it was about N 10°E. Refinement of the laye...

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

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 thickne...

Velocity anisotropy in shale determined from crosshole seismic and vertical seismic profile data

Crosshole and vertical seismic profile (VST) data made possible accurate characterization of the elastic properties, including noticeable velocity anisotropy, of a near‐surface late Tertiary shale formation. Shear‐wave splitting was obvious in both crosshole and VSP data. In crosshole data, two orthologonally polarrized shear (S) waves arrived 19 ms in the uppermost 246 ft (75 m). Vertically traveling S waves of the VSP separated about 10 ms in the uppermost 300 ft (90 m) but remained at nearly constant separation below that level. A transversely isotropic model, which incorporates a rapid increase in S-wave velocities with depth but slow increase in P-wave velocities, closely fits the data over most of the measured interval. Elastic constants of the transvesely isotropic model show spherical P- and S2-wave velocity surfaces but an ellipsoidal S1-wave surface with a ratio of major to minor axes of 1.15. The magnitude of this S-wave anisotropy is consistent with and lends credence to S-wave anisotropy magn...

Analysis of conventional and converted mode reflections at Putah sink, California using three‐component data

In 1980 Chevron recorded a three‐component seismic line using vertical (V) and transverse (T) motion vibrators over the Putah sink gas field near Davis, California. The purpose was to record the total vector motion of the various reflection types excited by the two sources, with emphasis on converted P‐S reflections. Analysis of the conventional reflection data agreed with results from the Conoco Shear Wave Group Shoot of 1977–1978. For example, the P‐P wave section had gas‐sand bright spots which were absent in the S‐S wave section. Shot profiles from the V vibrators showed strong P‐S converted wave events on the horizontal radial component (R) as expected. To our surprise, shot records from the T vibrators showed S‐P converted wave events on the V component, with low amplitudes but high signal‐to‐noise (S/N) ratios. These S‐P events were likely products of split S‐waves generated in anisotropic subsurface media. Components of these downgoing waves in the plane of incidence were converted to P‐waves on r...

Comparison of P- and S-wave velocities and Q's from VSP and sonic log data

We compared P‐ and S‐wave velocities and quality factors (Q’S) from vertical seismic profiling (VSP) and sonic log measurements in five wells, three from the southwest San Joaquin Basin of California, one from near Laredo, Texas, and one from northern Alberta. Our purpose was to investigate the bias between sonic log and VSP velocities and to examine to what degree this bias might be a consequence of dispersion. VSPs and sonic logs were recorded in the same well in every case. Subsurface formations were predominantly clastic. The bias found was that VSP transit times were greater than sonic log times, consistent with normal dispersion. For the San Joaquin wells, differences in S‐wave transit times averaged 1–2 percent, while differences in P‐wave transit times averaged 6–7 percent. For the Alberta well, the situation was reversed, with differences in S‐wave transit times being about 6 percent, while those for P‐waves were 2.5 percent. For the Texas well, the differences averaged about 4 percent for both P...

Supercritical reflections observed in P- and S- wave data

We have observed a conspicuous example of supercritical reflection in both P- and SH- wave seismic data. Data were recorded in the Midland Basin (Texas) Project of the Conoco Shear Wave Group Shoot in 1977–1978. P- and S- wave critical angle phenomena, as observed in the data, are remarkably similar. Event amplitudes are small or undetectable at offsets out to about 2 000 ft, but at offsets from 2 500 to 3 600 ft amplitudes are higher than those of any other event. Head waves originating at the critical distance are weak but detectable. Long path multiplies of the supercritical parts of P and SH events appear at expected times and offsets. Constant velocity moveout corrections helped identify them. Sonic logs in combination with a knowledge of the lithology made it possible to model P- wave critical angle phenomena. Agreement of model results with the data was good when we assumed cylindrical wavefronts. As expected, modeling based on plane waves was unable to match observed phase and amplitude behavior. ...

Twelve years of vertical birefringence in nine‐component VSP data

Since 1986, when industry scientists first publicly showed data supporting the presence of azimuthal anisotropy in sedimentary rock, we have studied vertical shear‐wave (S-wave) birefringence in 23 different wells in western North America. The data were from nine‐component vertical seismic profiles (VSPs) supplemented in recent years with data from wireline crossed‐dipole logs. This paper summarizes our results, including birefringence results in tabular form for 54 depth intervals in 19 of those 23 wells. In the Appendix we present our conclusions about how to record VSP data optimally for study of vertical birefringence. We arrived at four principal conclusions about vertical S-wave birefringence. First, birefringence was common but not universal. Second, birefringence ranged from 0–21%, but values larger than 4% occurred only in shallow formations (<1200 m) within 40 km of California’s San Andreas fault. Third, at large scales birefringence tended to be blocky. That is, both the birefringence magnitude...

Quantitative Comparison Between a Dipole Log And VSP In Anisotropic Rocks From Cymric Oil Field, California

Multicomponent VSP and seismic measurements are traditionally used to measure shear-wave birefringence in anisotropic rocks. Crossed-dipole shear logs measure flexural mode birefringence which, at low frequencies, should be identical to shear-wave birefringence in the absence of borehole related phenomena such as alteration, stress-relief, ellipticity, etc. A quantitative comparison between a multicomponent VSP and crossed-dipole logs in rocks previously reported to exhibit large shear-wave birefringence and depth-variable fast shear-wave polarization azimuths is made. It is found that the crossed-dipole measurements agree quantitatively with the VSP measurements with respect to the fast and slow shear-wave transit times and the fast shear-wave polarization direction. The crossed-dipole measurements reveal significant vertical variability of the anisotropy parameters.