Effect of source imbalances on the determination of fracture orientations from VSP data

Choi, Sang-Moon; Gangi, Anthony F.

doi:10.1190/1.1889189

Cited by 3 publications

(4 citation statements)

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“…If the cause of the asymmetry can be identified as source coupling only, the errors may be reduced by employing a balancing procedure based upon energies. Choi and Gangi (1991) conclude that consideration of source balancing can reduce the variability of the estimated polarization directions from different source components from degrees to less than degrees. The extent to which the results are affected depends on the magnitude of the polarization direction of the shear-waves and the relative time delay.…”

Section: Source and Geophone Effectsmentioning

confidence: 85%

“…An alternate explanation is furnished by the analysis of Choi and Gangi (1991) on the same data set. They obtain two differently calculated angular values related to in-line and crossline sources, which display similar symmetric undulations.…”

Section: Lost Hills Field Californiamentioning

confidence: 99%

“…An asymmetry in the recorded data matrix may arise because of factors of experimental control such as: geophone coupling to the borehole, coupling of the source pad to the ground, differential gain and phase response between different horizontal geophones, differential amplitudes between different source activations (Lewis, 1989;Choi and Gangi, 1991), directional response of receivers (Ghose and Takahashi, 1991), polarity reversals, and misorientations of the geophone tool (Queen, 1987), misorientations of the source (Queen and Rizer, 1990), borehole deviation, etc. Some of these factors may also be avoided or reduced with careful field practice, although others must be tolerated and corrected in the recorded data by conditioning steps after acquisition, such as amplitude adjustments from source balancing computations (Winterstein and Meadows, 1991a).…”

Section: Introductionmentioning

confidence: 99%

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

et al. 1994

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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 reorientation 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.

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Section: Source and Geophone Effectsmentioning

confidence: 85%

Section: Lost Hills Field Californiamentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

et al. 1994

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“…Typical acquisition factors may include source and geophone misalignment, source imbalance, incorrect polarity and coupling. Several studies have examined the effects of such acquisition errors on the processing of split shear waves (Choi and Gangi 1991;Lewis, Davis and Vuillermoz 1991;Zeng and MacBeth 1993a). Typical processing factors may include inadequate choice of processing flows, false assumptions of the medium properties such as crack strike variation, and false assumptions of the vector wavefields such as orthogonality of the split shear waves.…”

Section: Introductionmentioning

confidence: 99%

Interpreting non‐orthogonal split shear waves for seismic anisotropy in multicomponent VSPS

MacBeth

Crampin

1998

Geophysical Prospecting

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There are two main sources of non‐orthogonality in multicomponent shear‐wave seismics: inherent non‐orthogonal split shear waves arising from substantial ray deviation in off‐symmetry planes due to strong anisotropy or complex overburden, and apparent non‐orthogonal split shear waves in the horizontal plane due to variation of the angle of incidence even if the two shear waves along the raypath are orthogonal. Many techniques for processing shear‐wave splitting in VSP data ignore these kinds of non‐orthogonality of the split shear waves. Assuming inherent non‐orthogonality in zero‐offset VSPs, and apparent non‐orthogonality in offset VSPs, we derive equations for the four‐component data matrix. These can be solved by extending the linear‐transform technique (LTT) to determine the shear‐wave polarizations in zero‐offset and offset VSPs. Both full‐wave synthetic and field data are used to evaluate the technique and to examine the effects of non‐orthogonal polarized split shear waves. If orthogonality is incorrectly assumed, errors in polarization measurements increase with the degree of non‐orthogonality, which introduces a consistent decreasing trend in the polarization measurements. However, the effect of non‐orthogonality on the estimation of geophone orientation and time delays of the two split shear waves is small and negligible in most realistic cases. Furthermore, for most cases of weak anisotropy (less than 5% shear‐wave anisotropy) apparent non‐orthogonality is more significant than inherent non‐orthogonality. Nevertheless, for strong anisotropy (more than 10% shear‐wave anisotropy) with complicated structure (tilted or inclined symmetry axis), inherent non‐orthogonality may no longer be negligible. Applications to both synthetic and real data show that the extended linear‐transform techniques permit accurate recovery of polarization measurements in the presence of both significant inherent and apparent non‐orthogonality where orthogonal techniques often fail.

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Full wavefield inversion to estimate impact-source orientation from multicomponent land seismic data

Ramos-Martinez

McMechan

2002

GEOPHYSICS

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Orientation of an impact source is estimated from multicomponent land data. The seismic wavefield produced by an arbitrarily oriented impact source is represented by the linear combination of the nine independent wavefield components (Green's functions) for a full vector source. Coefficients of this linear combination are expressed in terms of the parameters that define source orientation. The parameters are an azimuthal angle, a vertical angle, and source strength, which are the unknowns of the inverse problem. Once the Green's functions are computed, source parameters are estimated by full wavefield inversion using a grid search over the source parameters. This inverse procedure also provides information about the uniqueness and resolution of the solution. Information on the source orientation is dominated by the relative amplitudes between components, so the inversion can be performed even when the source-time wavelet is not well known. The approach is tested using a synthetic common-source gather. Results for a field data example from northeast Texas show deviations between 11° and 16° from the vertical for an expected effective vertical impact, and between 19° and 23° from the horizontal for an expected horizontal cross-line impact, depending on whether a geophone rotation is performed to reduce the uncertainty produced by geophone misalignment. Results show the potential to quantify and isolate the effects of source misorientation in multicomponent land data.

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Effect of source imbalances on the determination of fracture orientations from VSP data

Cited by 3 publications

References 1 publication

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

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

Interpreting non‐orthogonal split shear waves for seismic anisotropy in multicomponent VSPS

Full wavefield inversion to estimate impact-source orientation from multicomponent land seismic data

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