Anelastic P-wave, S-wave and Converted-wave AVO Approximations

Converted‐wave amplitude versus offset (AVO) behavior may be fit with a cubic relationship between reflection coefficient and ray parameter. Attributes extracted using this form can be directly related to elastic parameters with low‐contrast or high‐contrast approximations to the Zoeppritz equations. The high‐contrast approximation has the advantage of greater accuracy; the low‐contrast approximation is analytically simpler. The two coefficients of the low‐contrast approximation are a function of the average ratio of compressional‐to‐shear‐wave velocity (α/β) and the fractional changes in S‐wave velocity and density (Δβ/β and Δρ/ρ). Because of its simplicity, the low‐contrast approximation is subject to errors, particularly for large positive contrasts in P‐wave velocity associated with negative contrasts in S‐wave velocity. However, for incidence angles up to 40° and models confined to |Δβ/β| < 0.25, the errors in both coefficients are relatively small. Converted‐wave AVO crossplotting of the coefficients of the low‐contrast approximation is a useful interpretation technique. The background trend in this case has a negative slope and an intercept proportional to the α/β ratio and the fractional change in S‐wave velocity. For constant α/β ratio, an attribute trace formed by the weighted sum of the coefficients of the low‐contrast approximation provides useful estimates of the fractional change in S‐wave velocity and density. Using synthetic examples, we investigate the sensitivity of these parameters to random noise. Integrated P‐wave and converted‐wave analysis may improve estimation of rock properties by combining extracted attributes to yield fractional contrasts in P‐wave and S‐wave velocities and density. Together, these parameters may provide improved direct hydrocarbon indication and can potentially be used to identify anomalies caused by low gas saturations.

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Comparing P-P, P-SV, and SV-P mode waves in the Midland Basin, West Texas

Interpretation ◽

10.1190/int-2015-0170.1 ◽

2016 ◽

Vol 4 (2) ◽

pp. T183-T190 ◽

Cited By ~ 1

Author(s):

Michael V. De Angelo ◽

Bob A. Hardage

Keyword(s):

Vertical Component ◽

P Wave ◽

Seismic Survey ◽

Vertical Force ◽

S Wave ◽

Converted Wave ◽

Midland Basin ◽

West Texas ◽

Exploration Risk ◽

Seismic Acquisition

We acquired 3D multicomponent data in Andrews County, Midland Basin, West Texas with a seismic survey. We extracted direct-SV modes generated by a vertical-force source (an array of three inline vertical vibrators) from the vertical component of multicomponent geophones. This seismic mode, SV-P, was created by reprocessing legacy 2D/3D P-wave seismic data to create converted-wave data and consequently forgoing the need for a multicomponent seismic acquisition program to obtain important S-wave information from the subsurface. We have compared P-P, P-SV, and SV-P traveltime and amplitude characteristics to determine which seismic mode provided better characterization of the targeted reservoirs and reduced exploration risk.

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Converted-wave model building and imaging based on common-focus-point methodology

Geophysics ◽

10.1190/geo2019-0549.1 ◽

2020 ◽

Vol 85 (6) ◽

pp. U139-U149

Author(s):

Hongwei Liu ◽

Mustafa Naser Al-Ali ◽

Yi Luo

Keyword(s):

Elastic Wave ◽

Wave Velocity ◽

Seismic Data ◽

Model Building ◽

P Wave ◽

Rock Properties ◽

S Wave ◽

Converted Wave ◽

Seismic Images ◽

Focus Point

Seismic images can be viewed as photographs for underground rocks. These images can be generated from different reflections of elastic waves with different rock properties. Although the dominant seismic data processing is still based on the acoustic wave assumption, elastic wave processing and imaging have become increasingly popular in recent years. A major challenge in elastic wave processing is shear-wave (S-wave) velocity model building. For this reason, we have developed a sequence of procedures for estimating seismic S-wave velocities and the subsequent generation of seismic images using converted waves. We have two main essential new supporting techniques. The first technique is the decoupling of the S-wave information by generating common-focus-point gathers via application of the compressional-wave (P-wave) velocity on the converted seismic data. The second technique is to assume one common VP/ VS ratio to approximate two types of ratios, namely, the ratio of the average earth layer velocity and the ratio of the stacking velocity. The benefit is that we reduce two unknown ratios into one, so it can be easily scanned and picked in practice. The PS-wave images produced by this technology could be aligned with the PP-wave images such that both can be produced in the same coordinate system. The registration between the PP and PS images provides cross-validation of the migrated structures and a better estimation of underground rock and fluid properties. The S-wave velocity, computed from the picked optimal ratio, can be used not only for generating the PS-wave images, but also to ensure well registration between the converted-wave and P-wave images.

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S-Zero Stack: A converted wave processing to extract subsurface density information

Geophysics ◽

10.1190/geo2013-0029.1 ◽

2014 ◽

Vol 79 (3) ◽

pp. N1-N10

Author(s):

Keshan Zou

Keyword(s):

Incident Angle ◽

Shear Velocity ◽

P Wave ◽

Density Contrast ◽

Richards Equation ◽

Maximum Magnitude ◽

S Wave ◽

Converted Wave ◽

Amplitude Variation With Offset ◽

Conversion Point

Analyzing the Aki-Richards equation for converted waves, I found that it is possible to decouple the effect of density contrast from that of shear velocity contrast. The two terms were mixed when the P-wave incident angle was less than 30°, but they started to separate at a middle angle range (approximately 40°). The term related to S-wave velocity contrast reached zero at an incident angle around 60°. However, the other term, which was related to the density contrast, did not reverse polarity until 90°. Furthermore, this density term reached almost the maximum (magnitude) around 60°. Based on those characteristics, I designed a new method called “S-Zero Stack” to capture the density contrast reliably at the subsurface interface without going to inversion. S-Zero Stack captured subsurface density anomalies using a special stacking method. It is simple but robust, even when there is noise in the common-conversion-point gathers. Combined with the traditional P-wave amplitude-variation-with-offset technique, S-Zero Stack of PS-waves may help discriminate commercial gas from fizz in gas sand and could be a useful tool in shale gas exploration to locate lower-density anomalies (sweet spots).

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Identifying, removing, and imaging P‐S conversions at salt‐sediment interfaces

Geophysics ◽

10.1190/1.1581076 ◽

2003 ◽

Vol 68 (3) ◽

pp. 1052-1059 ◽

Cited By ~ 15

Author(s):

Richard S. Lu ◽

Dennis E. Willen ◽

Ian A. Watson

Keyword(s):

P Wave ◽

Time Interval ◽

S Wave ◽

Converted Wave ◽

Prestack Depth Migration ◽

Depth Migration ◽

Time Migration ◽

Wave Imaging ◽

Wave Image ◽

Converted Waves

The large velocity contrast between salt and the surrounding sediments generates strong conversions between P‐ and S‐wave energy. The resulting converted events can be noise on P‐wave migrated images and should be identified and removed to facilitate interpretation. On the other hand, they can also be used to image a salt body and its adjacent sediments when the P‐wave image is inadequate. The converted waves with smaller reflection and transmission angles and much larger critical angles generate substantially different illumination than does the P‐wave. In areas where time migration is valid, the ratio between salt thickness in time and the time interval between the P‐wave and the converted‐wave salt base on a time‐migrated image is about 2.6 or 1.3, depending upon whether the seismic wave propagates along one or both of the downgoing and upcoming raypaths in salt as the S‐wave, respectively. These ratios can be used together with forward seismic modeling and 2D prestack depth migration to identify the converted‐wave base‐of‐salt (BOS) events in time and depth and to correctly interpret the subsalt sediments. It is possible to mute converted‐wave events from prestack traces according to their computed arrival times. Prestack depth migration of the muted data extends the updip continuation of subsalt sedimentary beds, and improves the salt–sediment terminations in the P‐wave image. Prestack and poststack depth‐migrated examples illustrate that the P‐wave and the three modes of converted waves preferentially image different parts of the base of salt. In some areas, the P‐wave BOS can be very weak, obscured by noise, or completely absent. Converted‐wave imaging complements P‐wave imaging in delineating the BOS for velocity model building.

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Predicting reservoir quality in the Bakken Formation, North Dakota, using petrophysics and 3C seismic data

Interpretation ◽

10.1190/int-2020-0007.1 ◽

2020 ◽

Vol 8 (4) ◽

pp. T851-T868

Author(s):

Andrea G. Paris ◽

Robert R. Stewart

Keyword(s):

North Dakota ◽

Seismic Data ◽

Rock Physics ◽

P Wave ◽

Well Logs ◽

Reservoir Quality ◽

Bakken Formation ◽

S Wave ◽

Converted Wave ◽

Wave Velocities

Combining rock-property analysis with multicomponent seismic imaging can be an effective approach for reservoir quality prediction in the Bakken Formation, North Dakota. The hydrocarbon potential of shale is indicated on well logs by low density, high gamma-ray response, low compressional-wave (P-wave) and shear-wave (S-wave) velocities, and high neutron porosity. We have recognized the shale intervals by cross plotting sonic velocities versus density. Intervals with total organic carbon (TOC) content higher than 10 wt% deviate from lower TOC regions in the density domain and exhibit slightly lower velocities and densities (<2.30 g/cm3). We consider TOC to be the principal factor affecting changes in the density and P- and S-wave velocities in the Bakken shales, where VP/ VS ranges between 1.65 and 1.75. We generate the synthetic seismic data using an anisotropic version of the Zoeppritz equations, including estimated Thomsen’s parameters. For the tops of the Upper and Lower Bakken, the amplitude shows a negative intercept and a positive gradient, which corresponds to an amplitude variation with offset of class IV. The P-impedance error decreases by 14% when incorporating the converted-wave information in the inversion process. A statistical approach using multiattribute analysis and neural networks delimits the zones of interest in terms of P-impedance, density, TOC content, and brittleness. The inverted and predicted results show reasonable correlations with the original well logs. The integration of well log analysis, rock physics, seismic modeling, constrained inversions, and statistical predictions contributes to identifying the areas of highest reservoir quality within the Bakken Formation.

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Estimating shear‐wave velocities from P-wave and converted‐wave data

Geophysics ◽

10.1190/1.1444556 ◽

1999 ◽

Vol 64 (2) ◽

pp. 504-507 ◽

Cited By ~ 1

Author(s):

Franklyn K. Levin

Keyword(s):

Wave Velocity ◽

Seismic Data ◽

P Wave ◽

S Wave ◽

Converted Wave ◽

Wave Data ◽

Wave Velocities ◽

Shear Wave Velocities ◽

P Wave Velocity ◽

Growing Body

Tessmer and Behle (1988) show that S-wave velocity can be estimated from surface seismic data if both normal P-wave data and converted‐wave data (P-SV) are available. The relation of Tessmer and Behle is [Formula: see text] (1) where [Formula: see text] is the S-wave velocity, [Formula: see text] is the P-wave velocity, and [Formula: see text] is the converted‐wave velocity. The growing body of converted‐wave data suggest a brief examination of the validity of equation (1) for velocities that vary with depth.

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Examining the processing differences between P and P-S waves in a Rocky Mountain Foothills model

The APPEA Journal ◽

10.1071/aj13077 ◽

2014 ◽

Vol 54 (2) ◽

pp. 504

Author(s):

Sanjeev Rajput ◽

Michael Ring

Keyword(s):

Rocky Mountain ◽

P Wave ◽

S Wave ◽

Converted Wave ◽

Wave Source ◽

Depth Migration ◽

S Waves ◽

Full Waveform ◽

Fluid Saturation ◽

Converted Waves

For the past two decades, most of the shear-wave (S-wave) or converted wave (P-S) acquisitions were performed with P-wave source by making the use of downgoing P-waves converting to upgoing S-waves at the mode conversion boundaries. The processing of converted waves requires studying asymmetric reflection at the conversion point, difference in geometries and conditions of source and receiver, and the partitioning of energy into orthogonally polarised components. Interpretation of P-S sections incorporates the identification of P-S waves, full waveform modeling, correlation with P-wave sections and depth migration. The main applications of P-S wave imaging are to obtain a measure of subsurface S-wave properties relating to rock type and fluid saturation (in addition to the P-wave values), imaging through gas clouds and shale diapers, and imaging interfaces with low P-wave contrast but significant S-wave changes. This study examines the major differences in processing of P and P-S wave surveys and the feasibility of identifying converted mode reflections by P-wave sources in anisotropic media. Two-dimensional synthetic seismograms for a realistic rocky mountain foothills model were studied. A Kirchhoff-based technique that includes anisotropic velocities is used for depth migration of converted waves. The results from depth imaging show that P-S section help in distinguishing amplitude associated with hydrocarbons from those caused by localised stratigraphic changes. In addition, the full waveform elastic modeling is useful in finding an appropriate balance between capturing high-quality P-wave data and P-S data challenges in a survey.

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Advantages of Shear Wave Seismic in Morrow Sandstone Detection

International Journal of Geophysics ◽

10.1155/2011/958483 ◽

2011 ◽

Vol 2011 ◽

pp. 1-16 ◽

Cited By ~ 3

Author(s):

Paritosh Singh ◽

Thomas Davis

Keyword(s):

Shear Wave ◽

Pure Shear ◽

High Amplitude ◽

Compressional Wave ◽

Side Lobe ◽

P Wave ◽

S Wave ◽

Converted Wave ◽

P Waves ◽

Wave Data

The Upper Morrow sandstones in the western Anadarko Basin have been prolific oil producers for more than five decades. Detection of Morrow sandstones is a major problem in the exploration of new fields and the characterization of existing fields because they are often very thin and laterally discontinuous. Until recently compressional wave data have been the primary resource for mapping the lateral extent of Morrow sandstones. The success with compressional wave datasets is limited because the acoustic impedance contrast between the reservoir sandstones and the encasing shales is small. Here, we have performed full waveform modeling study to understand the Morrow sandstone signatures on compressional wave (P-wave), converted-wave (PS-wave) and pure shear wave (S-wave) gathers. The contrast in rigidity between the Morrow sandstone and surrounding shale causes a strong seismic expression on the S-wave data. Morrow sandstone shows a distinct high amplitude event in pure S-wave modeled gathers as compared to the weaker P- and PS-wave events. Modeling also helps in understanding the adverse effect of interbed multiples (due to shallow high velocity anhydrite layers) and side lobe interference effects at the Morrow level. Modeling tied with the field data demonstrates that S-waves are more robust than P-waves in detecting the Morrow sandstone reservoirs.

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Acquiring S‐wave velocity using VSP converted wave of P‐wave source

10.1190/1.3255740 ◽

2009 ◽

Author(s):

Weifeng Geng ◽

Aiyuan Hou ◽

Wenbo Zhang ◽

Na Lei

Keyword(s):

Wave Velocity ◽

P Wave ◽

S Wave ◽

Converted Wave ◽

Wave Source

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