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

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.

Download Full-text

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

The APPEA Journal ◽

10.1071/aj13109 ◽

2014 ◽

Vol 54 (2) ◽

pp. 536

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

Download Full-text

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.

Download Full-text

Three-component amplitude versus offset analysis

Exploration Geophysics ◽

10.1071/eg989257 ◽

1989 ◽

Vol 20 (2) ◽

pp. 257

Author(s):

D.R. Miles ◽

G. Gassaway ◽

L. Bennett ◽

R. Brown

Keyword(s):

Seismic Data ◽

P Wave ◽

S Wave ◽

Converted Wave ◽

P Waves ◽

S Waves ◽

Avo Analysis ◽

Avo Inversion ◽

Amplitude Versus Offset ◽

Converted Waves

Three-component (3-C) amplitude versus offset (AVO) inversion is the AVO analysis of the three major energies in the seismic data, P-waves, S-waves and converted waves. For each type of energy the reflection coefficients at the boundary are a function of the contrast across the boundary in velocity, density and Poisson's ratio, and of the angle of incidence of the incoming wave. 3-C AVO analysis exploits these relationships to analyse the AVO changes in the P, S, and converted waves. 3-C AVO analysis is generally done on P, S, and converted wave data collected from a single source on 3-C geophones. Since most seismic sources generate both P and S-waves, it follows that most 3-C seismic data may be used in 3-C AVO inversion. Processing of the P-wave, S-wave and converted wave gathers is nearly the same as for single-component P-wave gathers. In split-spread shooting, the P-wave and S-wave energy on the radial component is one polarity on the forward shot and the opposite polarity on the back shot. Therefore to use both sides of the shot, the back shot must be rotated 180 degrees before it can be stacked with the forward shot. The amplitude of the returning energy is a function of all three components, not just the vertical or radial, so all three components must be stacked for P-waves, then for S-waves, and finally for converted waves. After the gathers are processed, reflectors are picked and the amplitudes are corrected for free-surface effects, spherical divergence and the shot and geophone array geometries. Next the P and S-wave interval velocities are calculated from the P and S-wave moveouts. Then the amplitude response of the P and S-wave reflections are analysed to give Poisson's ratio. The two solutions are then compared and adjusted until they match each other and the data. Three-component AVO inversion not only yields information about the lithologies and pore-fluids at a specific location; it also provides the interpreter with good correlations between the P-waves and the S-waves, and between the P and converted waves, thus greatly expanding the value of 3-C seismic data.

Download Full-text

Scalar reverse‐time depth migration of prestack elastic seismic data

Geophysics ◽

10.1190/1.1487098 ◽

2001 ◽

Vol 66 (5) ◽

pp. 1519-1527 ◽

Cited By ~ 106

Author(s):

Robert Sun ◽

George A. McMechan

Keyword(s):

Seismic Waves ◽

Velocity Model ◽

P Wave ◽

Common Source ◽

Reverse Time ◽

S Wave ◽

Wave Source ◽

Reflected Waves ◽

S Waves ◽

Elastic Data

Reflected P‐to‐P and P‐to‐S converted seismic waves in a two‐component elastic common‐source gather generated with a P‐wave source in a two‐dimensional model can be imaged by two independent scalar reverse‐time depth migrations. The inputs to migration are pure P‐ and S‐waves that are extracted by divergence and curl calculations during (shallow) extrapolation of the elastic data recorded at the earth’s surface. For both P‐to‐P and P‐to‐S converted reflected waves, the imaging time at each point is the P‐wave traveltime from the source to that point. The extracted P‐wave is reverse‐time extrapolated and imaged with a P‐velocity model, using a finite difference solution of the scalar wave equation. The extracted S‐wave is reverse‐time extrapolated and imaged similarly, but with an S‐velocity model. Converted S‐wave data requires a polarity correction prior to migration to ensure constructive interference between data from adjacent sources. Synthetic examples show that the algorithm gives satisfactory results for laterally inhomogeneous models.

Download Full-text

Elastic-wavefield interferometry using P-wave source VSPs at the Wamsutter field, Wyoming

Geophysics ◽

10.1190/geo2011-0162.1 ◽

2012 ◽

Vol 77 (2) ◽

pp. Q27-Q36 ◽

Cited By ~ 1

Author(s):

James Gaiser ◽

Ivan Vasconcelos ◽

Rosemarie Geetan ◽

John Faragher

Keyword(s):

Stationary Phase ◽

Wave Velocity ◽

P Wave ◽

Phase Point ◽

S Wave ◽

Wave Source ◽

S Waves ◽

Wave Splitting ◽

Zero Offset ◽

Tube Wave

In this study, elastic-wavefield interferometry was used to recover P- and S-waves from the 3D P-wave vibrator VSP data at Wamsutter field in Wyoming. S-wave velocity and birefringence is of particular interest for the geophysical objectives of lithology discrimination and fracture characterization in naturally fractured tight gas sand reservoirs. Because we rely on deconvolution interferometry for retrieving interreceiver P- and S-waves in the subsurface, the output fields are suitable for high-resolution, local reservoir characterization. In 1D media where the borehole is nearly vertical, data at the stationary-phase point is not conducive to conventional interferometry. Strong tube-wave noise generated by physical sources near the borehole interfere with S-wave splitting analyses. Also, converted P- to S-wave (PS-wave) polarity reversals occur at zero offset and cancel their recovery. We developed methods to eliminate tube-wave noise by removing physical sources at the stationary-phase point and perturbing the integration path in the integrand based on P-wave NMO velocity of the direct-arrival. This results in using nonphysical energy outside a Fresnel radius that could not have propagated between receivers. To limit the response near the stationary-phase point, we also applied a weighting condition to suppress energy from large offsets. For PS-waves, a derivative-like operator was applied to the physical sources at zero offset in the form of a polarity reversal. These methods resulted in effectively recovering P-wave dipole and PS-wave quadrupole pseudosource VSPs. The retrieved wavefields kinematically correspond to a vertical incidence representation of reflectivity/transmissivity and can be used for conventional P- and S-wave velocity analyses. Four-component PS-wave VSPs retrieve S-wave splitting in transmitted converted waves that provide calibration for PS-wave and P-wave azimuthal anisotropy measurements from surface-seismic data.

Download Full-text

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

Download Full-text

Automatic P and S velocity determination from full waveform digital acoustic logs

Geophysics ◽

10.1190/1.1441444 ◽

1983 ◽

Vol 48 (12) ◽

pp. 1631-1644 ◽

Cited By ~ 17

Author(s):

Mark E. Willis ◽

M. Nafi Toksöz

Keyword(s):

P Wave ◽

Common Source ◽

S Wave ◽

S Waves ◽

Threshold Detection ◽

Full Waveform ◽

Automatic Methods ◽

Correlation Process ◽

Sonic Logs ◽

Fine Adjustment

Automatic methods of determining P and S velocities from full waveform acoustic logs are studied and compared. The suggested P‐wave method is an event detector which is based on threshold detection in a window near the previous picks and fine adjustment by a semblance correlation. The moveouts found by the correlation process are used to find common source P velocities as well as effective borehole compensated (BHC) P velocities. Compensated velocities are derived from waveforms from complementing tool positions which compare favorably to velocities from standard BHC sonic logs. We call the most reliable method for S waves the P‐correlated S‐method. It consists of correlating the P waveform with the rest of the record to find the S arrival. The S waveform is then correlated with the next record to determine the S velocity. This method is compared with published techniques and proves more stable and reliable. The method does not necessitate a distinct S‐wave arrival and can utilize the existence of the reflected conical wave whose phase velocity is controlled by the S velocity of the formation.

Download Full-text

A simple method for resolving large converted‐wave (P-SV) statics

Geophysics ◽

10.1190/1.1443426 ◽

1993 ◽

Vol 58 (3) ◽

pp. 429-433 ◽

Cited By ~ 27

Author(s):

Peter W. Cary ◽

David W. S. Eaton

Keyword(s):

P Wave ◽

S Wave ◽

Converted Wave ◽

P Waves ◽

Simple Method ◽

Near Surface ◽

S Waves ◽

Static Solutions ◽

Surface Fluctuations

The processing of converted‐wave (P-SV) seismic data requires certain special considerations, such as commonconversion‐point (CCP) binning techniques (Tessmer and Behle, 1988) and a modified normal moveout formula (Slotboom, 1990), that makes it different for processing conventional P-P data. However, from the processor’s perspective, the most problematic step is often the determination of residual S‐wave statics, which are commonly two to ten times greater than the P‐wave statics for the same location (Tatham and McCormack, 1991). Conventional residualstatics algorithms often produce numerous cycle skips when attempting to resolve very large statics. Unlike P‐waves, the velocity of S‐waves is virtually unaffected by near‐surface fluctuations in the water table (Figure 1). Hence, the P‐wave and S‐wave static solutions are largely unrelated to each other, so it is generally not feasible to approximate the S‐wave statics by simply scaling the known P‐wave static values (Anno, 1986).

Download Full-text

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

Geophysics ◽

10.1190/1.1442877 ◽

1990 ◽

Vol 55 (6) ◽

pp. 646-659 ◽

Cited By ~ 23

Author(s):

C. Frasier ◽

D. Winterstein

Keyword(s):

Time Window ◽

Structural Features ◽

Low Noise ◽

P Wave ◽

Bright Spot ◽

High Signal ◽

Converted Wave ◽

Gas Field ◽

Seismic Line ◽

S Waves

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 reflection and arrived at receivers in a low‐noise time window ahead of the S‐S waves. The two types of converted waves (P‐S and S‐P) were first stacked by common midpoint (CMP). The unexpected S‐P section was lower in true amplitude but much higher in S/N ratio than the P‐S section. The Winters gas‐sand bright spot was missing on the converted wave sections, mimicking the S‐S reflectivity as expected. CRP gathers were formed by rebinning data by a simple ray‐tracing formula based on the asymmetry of raypaths. CRP stacking improved P‐S and S‐P event resolution relative to CMP stacking and laterally aligned structural features with their counterparts on P and S sections. Thus, the unexpected S‐P data provided us with an extra check for our converted wave data processing.

Download Full-text