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

Geophysics ◽  
1991 ◽  
Vol 56 (9) ◽  
pp. 1349-1364 ◽  
Author(s):  
D. F. Winterstein ◽  
M. A. Meadows
Keyword(s):  
Shear Wave ◽  
Lower Layer ◽  
Oil Fields ◽  
Data Consistency ◽  
Data Matrix ◽  
Wave Polarization ◽  
S Wave ◽  
Polarization Azimuth ◽  
Uppermost Layer ◽  
Vsp Data

Shear‐wave [Formula: see text]-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 [Formula: see text]-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 [Formula: see text]-wave birefringence as large was about 6 percent down to 1300 ft (400 m). In the upper layer the polarization azimuth of the fast [Formula: see text]-wave was N 60°E, while in the lower layer it was about N 10°E. Refinement of the layer stripping showed that neither layer was anisotropically homogenous, and both could be subdivided into thinner layers. Near offset [Formula: see text]-wave VSP data from the Railroad Gap well also show high birefringence near the surface and less birefringence deeper. In the uppermost layer, which extends down to 1300 ft (400 m), the [Formula: see text]-wave birefringence was 9 percent, and the lag between the fast and slow [Formula: see text]-waves exceeded 60 ms at the bottom of the layer. Seven layers in all were needed to accommodate [Formula: see text]-wave polarization changes. The most reliable azimuth angle determination as judged from the data consistency were those of the uppermost layer, at N 46°E, and those from depths 2900–3700 ft (880–1130 m) and 3900–5300 ft (1190–1610 m), at N 16°E and N 15°W, respectively. Over those intervals the scatter of calculated azimuths about the mean was typically less than 4 degrees. The largest birefringence at both locations occurred in the same formation, the Pliocene Tulare sands and Pebble Conglomerate. In those formations the azimuth of the fast [Formula: see text]-wave polarization was roughly orthogonal to the southwest. In the deeper Antelope shale, [Formula: see text]-wave polarization directions in both areas were close to 45 degrees from the fault. Confidence in the layer stripping procedure was bolstered by major improvement in data quality that resulted from stripping. Before stripping, wavelets of the two [Formula: see text]-waves sometimes had very different waveforms, and it was often impossible to come close to diagonalizing the 2 × 2 S‐wave data matrix by rotating sources and receivers by the same angle. After stripping, wavelets were more similar in shape, and the S‐wave matrix was more nearly diagonalizable by rotating with a single angle.

Interpreting data matrix asymmetry in near‐offset, shear‐wave VSP data

Geophysics ◽  
1994 ◽  
Vol 59 (2) ◽  
pp. 176-191 ◽  
Author(s):  
Colin MacBeth ◽  
Xinwu Zeng ◽  
Gareth S. Yardley ◽  
Stuart Crampin
Keyword(s):  
Shear Wave ◽  
Shear Wave Splitting ◽  
Synthetic Seismograms ◽  
Data Matrix ◽  
Depth Range ◽  
Wave Polarization ◽  
Polarization Azimuth ◽  
Polarization Change ◽  
Wave Splitting ◽  
Vsp Data

Poor experimental control in shear‐wave VSPs may contribute to unreliable estimates of shear‐wave splitting and possible misinterpretation of the medium anisotropy. To avoid this, the acquisition and processing of multicomponent shear‐wave data needs special care and attention. Measurement of asymmetry in the recorded data matrix using singular‐value decomposition (SVD) provides a useful way of examining possible acquisition inaccuracies and may help guide data conditioning and interpretation to ensure more reliable estimates of shear‐wave polarization azimuth. Three examples demonstrate how variations in shear‐wave polarization and acquisition inaccuracies affect the SVD results in different ways. In the first example, analysis of synthetic seismograms with known depth changes in the polarization azimuth show how these may be detected. In the second example, a known source re‐orientation and polarity reversal is detected by applying SVD to near‐offset, shear‐wave VSP data, recorded in the Romashkino field, Tatar Republic. Additional information on a polarization change in the overburden is also obtained by comparing the SVD results with those for full‐wave synthetic seismograms. The polarization azimuth changes from N160°E in the overburden to N117°E within the VSP depth range. Most of the shear‐wave splitting is built up over the VSP depth range. The final example is a near‐offset, shear‐wave VSP data set from Lost Hills, California. Here, most of the shear‐wave splitting is in the shallow layers before the VSP depth range. SVD revealed a known correction for horizontal reorientation of the sources, but also exhibited results with a distinct oscillatory behavior. Stripping the overburden effects reduces but does not eliminate these oscillations. There appears to be a polarization change from N45°E in the overburden to N125°E in the VSP section. The details in these examples would be difficult to detect by visual inspection of the seismograms or polarization diagrams. Results from these preliminary analyses are encouraging and suggest that it may be possible to routinely use this, or a similar technique, to resolve changes in the subsurface anisotropy from multicomponent experiments where acquisition has not been carefully controlled.

Virtual shear source makes shear waves with air guns

Geophysics ◽  
2007 ◽  
Vol 72 (2) ◽  
pp. A7-A11 ◽  
Author(s):  
Andrey Bakulin ◽  
Albena Mateeva ◽  
Rodney Calvert ◽  
Patsy Jorgensen ◽  
Jorge Lopez
Keyword(s):  
Shear Wave ◽  
Shear Waves ◽  
Shear Velocity ◽  
Velocity Profiles ◽  
Virtual Source ◽  
S Wave ◽  
Converted Wave ◽  
Source Method ◽  
Vsp Data ◽  
Walkaway Vsp

We demonstrate a novel application of the virtual source method to create shear-wave sources at the location of buried geophones. These virtual downhole sources excite shear waves with a different radiation pattern than known sources. They can be useful in various shear-wave applications. Here we focus on the virtual shear check shot to generate accurate shear-velocity profiles in offshore environments using typical acquisition for marine walkaway vertical seismic profiling (VSP). The virtual source method is applied to walkaway VSP data to obtain new traces resembling seismograms acquired with downhole seismic sources at geophone locations, thus bypassing any overburden complexity. The virtual sources can be synthesized to radiate predominantly shear waves by collecting converted-wave energy scattered throughout the overburden. We illustrate the concept in a synthetic layered model and demonstrate the method by estimating accurate P- and S-wave velocity profiles below salt using a walkaway VSP from the deepwater Gulf of Mexico.

Shear-wave anisotropy of active tectonic regions via automated S-wave polarization analysis

1989 ◽  
Vol 165 (1-4) ◽  
pp. 279-292 ◽  
Author(s):  
M.K. Savage ◽  
X.R. Shih ◽  
R.P. Meyer ◽  
R.C. Aster
Keyword(s):  
Shear Wave ◽  
Wave Polarization ◽  
Polarization Analysis ◽  
S Wave ◽  
Active Tectonic

Shear‐wave polarizations and subsurface stress directions at Lost Hills field

Geophysics ◽  
1991 ◽  
Vol 56 (9) ◽  
pp. 1331-1348 ◽  
Author(s):  
D. F. Winterstein ◽  
M. A. Meadows
Keyword(s):  
Symmetry Axis ◽  
Transversely Isotropic ◽  
Data Matrix ◽  
Wave Polarization ◽  
Polarization Direction ◽  
S Wave ◽  
Near Surface ◽  
Polarization Technique ◽  
The Matrix ◽  
Vertical Travel

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 in a weakly transversely isotropic matrix, where the infinite‐fold symmetry axis of the matrix is tilted 10 degrees from the vertical towards N 70°E. Such a model is of monoclinic symmetry.

Shear‐wave studies in glacial till

Geophysics ◽  
1998 ◽  
Vol 63 (4) ◽  
pp. 1273-1284 ◽  
Author(s):  
Bradley J. Carr ◽  
Zoltan Hajnal ◽  
Arnfinn Prugger
Keyword(s):  
Shear Wave ◽  
P Wave ◽  
Vertical Resolution ◽  
S Wave ◽  
Glacial Deposits ◽  
Seismic Line ◽  
S Waves ◽  
Additional Information ◽  
Vsp Data ◽  
Clay Percentage

Within a high‐resolution shallow reflection survey program in Saskatchewan, Canada, S-waves were produced using a single seismo‐electric blasting cap and were found to be distinguishable from surface wave phases. The local glacial deposits have average velocities of 450 m/s. [Formula: see text] ratios average 3.6 in these sequences, but they vary laterally, according to the velocity analyses done in two boreholes drilled along the seismic line. Vertical resolution for S-wave reflections are 0.75 m [in the vertical seismic profiling (VSP) data] and 1.5 m (in the CDP data). Yet, the S-wave CDP results are still better than corresponding P-wave data, which had a vertical resolution of 2.6 m. S-wave anisotropy is inferred in the glacial deposits on the basis of particle motion analysis and interpretations of S-wave splitting. However, the amount of observed splitting is small (∼2–6 ms over 5–10 m) and could go undetected for seismic surveys with larger sampling intervals. VSPs indicate that S-wave reflectivity is caused by both distinct and subtle lithologic changes (e.g., clay/sand contacts or changes in clay percentage within a particular till unit) and changes in bulk porosity. Migrated S-wave sections from line 1 and line 2 image reflections from sand layers within the tills as well as the first “bedrock” sequence (known as the Judith River Formation). Shear wave images are not only feasible in unconsolidated materials, but provide additional information about structural relationships within these till units.

scholarly journals Estimation of azimuthal anisotropy from VSP data using multicomponent S-wave velocity analysis

Geophysics ◽  
2011 ◽  
Vol 76 (5) ◽  
pp. D1-D9 ◽  
Author(s):  
Roman Pevzner ◽  
Boris Gurevich ◽  
Milovan Urosevic
Keyword(s):  
Shear Wave ◽  
Transverse Isotropy ◽  
Symmetry Plane ◽  
Azimuthal Anisotropy ◽  
Velocity Analysis ◽  
Apparent Velocity ◽  
S Wave ◽  
North West ◽  
Vsp Data ◽  
Zero Offset

Observation of azimuthal shear wave anisotropy can be useful for characterization of fractures or stress fields. Shear wave anisotropy is often estimated by measuring splitting of individual shear wave events in vertical seismic profile (VSP) data. However, this method may become unreliable for zero-offset (marine) VSP where the seismogram often contains no strong individual shear events, such as direct downgoing shear wave, but often contains many low-amplitude PS mode converted waves. We have developed a new approach for estimation of the fast and slow shear wave velocities and orientation of polarization planes based on the multicomponent linear traveltime moveout velocity analysis. This technique is applicable to zero-offset VSP data, and should take advantage of the presence of a large number of shear wave events with the same apparent velocity (which, for a horizontally layered medium, should be close to the interval velocity). The approach assumes that the VSP data are acquired in a vertical well drilled in an orthorhombic medium with a horizontal symmetry plane (including horizontal transverse isotropy). The main idea is to estimate the dominant apparent velocity for a given polarization direction by measuring the coherency of the seismic signal of a large number of events as a function of the apparent velocity. The algorithm was tested on marine three-component (3C) VSP acquired in the North West Shelf of Australia, and on land 3C VSP acquired with different sources in the same borehole located in Otway Basin, Victoria. These tests show good agreement between anisotropy parameters (magnitude and orientation) derived from the VSP and cross-dipole sonic log data.

Analysis of Shear-Wave Polarization in VSP Data: A Tool for Reservoir Development

1995 ◽  
Vol 10 (04) ◽  
pp. 223-232 ◽  
Author(s):  
D.F. Winterstein ◽  
M.A. Meadows
Keyword(s):  
Shear Wave ◽  
Wave Polarization ◽  
Reservoir Development ◽  
Vsp Data
Sign in / Sign up

Export Citation Format

Share Document