VTI documented

Geophysics ◽  
2001 ◽  
Vol 66 (1) ◽  
pp. 237-245 ◽  
Author(s):  
D. F. Winterstein ◽  
G. S. De
Keyword(s):  
Seismic Data ◽  
Transversely Isotropic ◽  
Seismic Profile ◽  
Oil Field ◽  
Central Basin ◽  
S Wave ◽  
West Texas ◽  
Wave Splitting ◽  
Vertical Propagation ◽  
Central Basin Platform

Shear‐wave (S‐wave) seismic data indicate that rocks of McElroy oil field on the Central Basin Platform of West Texas are transversely isotropic with a vertical symmetry axis (VTI). Although geophysicists from time to time had anticipated or assumed that sedimentary rock was VTI, no one had shown that any sedimentary section actually was VTI. The proof at McElroy comes from a nine‐component, near‐offset vertical seismic profile (VSP) combined with a ring of 15 offset VSPs spaced about 24° apart at offsets of about 460 m. S-wave splitting at VSP frequencies was negligible for vertical propagation to a depth of 885 m but reached about 12 ms for nonvertical propagation from the offset source locations. Crossed‐dipole log data supported the VSP result for vertical propagation but found two layers of vertically birefringent rock at depth whose thicknesses were below VSP resolution.

Geometrical spreading in a layered transversely isotropic medium with vertical symmetry axis

Geophysics ◽  
2003 ◽  
Vol 68 (6) ◽  
pp. 2082-2091 ◽  
Author(s):  
Bjørn Ursin ◽  
Ketil Hokstad
Keyword(s):  
Ray Tracing ◽  
Seismic Data ◽  
Symmetry Axis ◽  
Transversely Isotropic ◽  
Analytical Approximation ◽  
Geometrical Spreading ◽  
S Wave ◽  
Weak Anisotropy ◽  
P Waves ◽  
Amplitude Versus

Compensation for geometrical spreading is important in prestack Kirchhoff migration and in amplitude versus offset/amplitude versus angle (AVO/AVA) analysis of seismic data. We present equations for the relative geometrical spreading of reflected and transmitted P‐ and S‐wave in horizontally layered transversely isotropic media with vertical symmetry axis (VTI). We show that relatively simple expressions are obtained when the geometrical spreading is expressed in terms of group velocities. In weakly anisotropic media, we obtain simple expressions also in terms of phase velocities. Also, we derive analytical equations for geometrical spreading based on the nonhyperbolic traveltime formula of Tsvankin and Thomsen, such that the geometrical spreading can be expressed in terms of the parameters used in time processing of seismic data. Comparison with numerical ray tracing demonstrates that the weak anisotropy approximation to geometrical spreading is accurate for P‐waves. It is less accurate for SV‐waves, but has qualitatively the correct form. For P waves, the nonhyperbolic equation for geometrical spreading compares favorably with ray‐tracing results for offset‐depth ratios less than five. For SV‐waves, the analytical approximation is accurate only at small offsets, and breaks down at offset‐depth ratios less than unity. The numerical results are in agreement with the range of validity for the nonhyperbolic traveltime equations.

The 1998 Valhall microseismic data set: An integrated study of relocated sources, seismic multiplets, and S-wave splitting

Geophysics ◽  
2009 ◽  
Vol 74 (5) ◽  
pp. B183-B195 ◽  
Author(s):  
K. De Meersman ◽  
J.-M. Kendall ◽  
M. van der Baan
Keyword(s):  
Seismic Anisotropy ◽  
Amplitude Ratio ◽  
Temporal Variations ◽  
Oil Field ◽  
P Wave ◽  
Wave Form ◽  
S Wave ◽  
Data Set ◽  
Wave Splitting ◽  
Source Mechanisms

We relocate 303 microseismic events recorded in 1998 by sensors in a single borehole in the North Sea Valhall oil field. A semiautomated array analysis method repicks the P- and S-wave arrival times and P-wave polarizations, which are needed to locate these events. The relocated sources are confined predominantly to a [Formula: see text]-thick zone just above the reservoir, and location uncertainties are half those of previous efforts. Multiplet analysis identifies 40 multiplet groups, which include 208 of the 303 events. The largest group contains 24 events, and five groups contain 10 or more events. Within each multiplet group, we further improve arrival-time picking through crosscorrelation, which enhances the relative accuracy of the relocated events and reveals that more than 99% of the seismic activity lies spatially in three distinct clusters. The spatial distribution of events and wave-form similarities reveal two faultlike structures that match well with north-northwest–south-southeast-trending fault planes interpreted from 3D surface seismic data. Most waveform differences between multiplet groups located on these faults can be attributed to S-wave phase content and polarity or P-to-S amplitude ratio. The range in P-to-S amplitude ratios observed on the faults is explained best in terms of varying source mechanisms. We also find a correlation between multiplet groups and temporal variations in seismic anisotropy, as revealed by S-wave splitting analysis. We explain these findings in the context of a cyclic recharge and dissipation of cap-rock stresses in response to production-driven compaction of the underlying oil reservoir. The cyclic nature of this mechanism drives the short-term variations in seismic anisotropy and the reactivation of microseismic source mechanisms over time.

Full‐waveform processing and interpretation of kilohertz cross‐well seismic data

Geophysics ◽  
1993 ◽  
Vol 58 (9) ◽  
pp. 1248-1256 ◽  
Author(s):  
Ashraf A. Khalil ◽  
Robert R. Stewart ◽  
David C. Henley
Keyword(s):  
Oil Recovery ◽  
Seismic Data ◽  
Shear Waves ◽  
Synthetic Seismograms ◽  
Oil Field ◽  
S Wave ◽  
Sonic Log ◽  
The Cross ◽  
Interval Velocities ◽  
The Individual

High‐frequency, cross‐well seismic data, from the Midale oil field of southeastern Saskatchewan, are analyzed for direct and reflected energy. The goal of the analysis is to produce interpretable sections to assist in enhanced oil recovery activities ([Formula: see text] injection) in this field. Direct arrivals are used for velocity information while reflected arrivals are processed into a reflection image. Raw field data show a complex assortment of wave types that includes direct compressional and shear waves and reflected shear waves. A traveltime inversion technique (layer stripping via ray tracing) is used to obtain P‐ and S‐wave interval velocities from the respective direct arrivals. The velocities from the cross‐well inversion and the sonic log are in reasonable agreement. The subsurface coverage of the cross‐well geometry is investigated; it covers zones extending from the source well to the receiver well and includes regions above and below the source/receiver depths. Upgoing and downgoing primary reflections are processed, in a manner similar to the vertical seismic profiling/common‐depth‐point (VSP/CDP) map, to construct the cross‐well images. A final section is produced by summing the individual reflection images from each receiver‐gather map. This section provides an image with evidence of strata thicknesses down to about 1 m. Synthetic seismograms are used to interpret the final sections. Correlations can be drawn between some of the events on the synthetic seismograms and the cross‐well image.

The 1949 algorithm for the rapid interpretation of aeromagnetic data

Geophysics ◽  
1996 ◽  
Vol 61 (2) ◽  
pp. 608-611
Author(s):  
Nelson C. Steenland
Keyword(s):  
Oil And Gas ◽  
Magnetic Anomalies ◽  
Central Basin ◽  
General Area ◽  
Aeromagnetic Data ◽  
West Texas ◽  
Central Basin Platform ◽  
Topographic Relief

In the early 1920s, magnetic anomalies were thought to be caused by topographic relief at the base of the sedimentary section. The drilling of the Hobbs anomaly on the Central Basin Platform in West Texas was successful, but the anomaly, primarily intrabasement in origin, could not quantitatively be related to the structure that was found. Moreover the drilling of similar anomalies in the general area proved unsuccessful as far as oil and gas were concerned, so the method acquired a bad reputation rather quickly (Heiland, 1940).

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