There are two types of seismic waves that can be generated to illuminate deep geology – a compressional (P) wave and a shear (S) wave. The majority of seismic programs record only P waves. However, S waves have great value for detecting and estimating rock properties needed for reservoir characterization. In particular, S waves react to azimuthal anisotropy properties of rocks in a much more robust manner than do P waves. When an S wave propagates through an interval that has embedded azimuthal anisotropy, it splits into two daughter waves called the fast-S mode and the slow-S mode. A P-wave can also exhibit fast and slow behavior, but it does so in a much weaker manner than does an S wave. Because of this wave physics, S waves are far more valuable than P waves for detecting and characterizing fractures, subtle faults, and stress fields, all of which induce azimuthal anisotropy in layered rock and are key pieces of geologic information needed to characterize and exploit both conventional and unconventional reservoirs. When both P and S data are available, the velocity ratio VP/VS (VP = P-wave velocity; VS = S-wave velocity) can also be determined across targeted stratigraphic intervals. This velocity ratio is one of the best seismic attributes for distinguishing variations in rock type and is invaluable for mapping lithological variations within reservoir systems. Much more could be said about the value of S waves in geologic interpretations of reservoirs, but these few comments indicate several of the key applications that S waves provide.
Even though S waves contribute valuable information for prospect development, the amount of S-wave seismic data that is recorded worldwide is small compared to the amounts of P-wave data that are acquired across prospective basin areas. There are reasons, both on the data-acquisition side of S-wave technology and on the data-processing side, why there is low usage of S-wave reflection seismology. The purpose of this document is to describe a new approach to S-wave data acquisition that should lead to expanded use of S waves in prospect evaluation and reservoir characterization. This technology not only allows S-wave seismic data to be acquired at low cost, but it also expands the range of earth surface conditions over which S-wave sources can be deployed.
The fundamental principle that will be presented is that downgoing S wavefields are produced directly at a source station by simple vertical-displacement sources (e.g. vertical vibrators, vertical impacts, shot-hole explosives), which are sources that are widely distributed around the globe. These sources are considered by most geophysicists to be only P-wave sources. However, vertical-displacement sources produce robust SV (vertical shear) displacements in addition to P displacements at the point where they apply their force vector to the earth. This direct-S illumination of geology by vertical-displacement sources has simply been ignored for decades. The geometrical shapes and relative amplitudes of direct-P and direct-S modes produced by a vertical-displacement source are illustrated in Figure 1. The SV radiation patterns illustrated in this figure are designated direct-S modes to distinguish them from SV wavefields created by P-to-SV mode conversions at interfaces remote from a source station.
Figure 1. Direct-P and direct-SV body waves produced by a vertical-displacement source in a homogeneous earth. (a) A soft earth. (b) A hard earth. At take-off angle Ф, the amplitude of the direct-P mode is A, and the amplitude of the direct-SV mode is B. For a wide range of surface stiffness, a vertical-displacement source produces more SV-wave energy than P-wave energy.
The Exploration Geophysics Laboratory at The University of Texas at Austin has performed extensive field experiments to determine the P and S radiation patterns and wave-mode strengths of P and S modes produced by all generic types of vertical-displacement sources – vertical vibrators, vertical impacts, and shot-hole explosives. In all tests, the amplitude strength of the direct-S mode produced by a vertical-displacement source was found to be larger than the amplitude strength of the direct-P mode, often by a factor of 5 or greater. In these tests, the direct-S radiation pattern produced by a vertical-displacement source was compared with the direct-S radiation pattern produced by a horizontal vibrator, considered to be the “gold standard” for an S-wave seismic source. We find the direct-S wavefields produced by a vertical vibrator (as well as by all other vertical-displacement sources) are identical to the direct-S wavefields produced by a horizontal vibrator. One such comparison of S wavefields produced by vertical and horizontal vibrators is shown in Figure 2.
Figure 2. (a) Radial-S geophone response to a direct-S wavefield produced by a vertical vibrator (red traces) overlain by the response to the direct-S wavefield produced by a radial horizontal vibrator (blue traces). (b) Transverse-S geophone response to a direct-S wavefield produced by a vertical vibrator (red traces) overlain by the response to the direct-S wavefield produced by a transverse horizontal vibrator (blue traces). Vibrators were positioned at the same surface source station. Data were recorded by the same downhole receiver array without altering receiver orientations or couplings.
Two types of direct-S data are produced by vertical-displacement sources. Type-1 direct-S data are S-S data which involve a downgoing illumination S wave and an upgoing reflected S wave. Type-2 data are SV-P data, which involves a downgoing SV wave and an upgoing P wave. An SV-P mode provides the same S-wave information and S-wave attributes as does the P-SV mode that is used by numerous people to evaluate hydrocarbon reservoirs when data are recorded with 3-component geophones.
Type-1 Direct-S Data (S-S Modes)
S-wave data acquisition with vertical-displacement sources provides two advantages. First, the cost of data acquisition is reduced when P and S data are acquired using simple and reliable vertical-displacement sources such as vertical vibrators, vertical impacts, or shot-hole explosives. It is no longer necessary to deploy three sources – a vertical-displacement source to generate direct-P data and two orthogonal horizontal-displacement sources to generate inline and crossline direct-S data. This step alone reduces the cost of S-S data acquisition by a factor of three because the number of deployed sources is reduced from three to only one.
Second, S-wave seismic programs can be implemented across any area where P-wave seismic programs can be done. Specifically, vertical-displacement sources allow S-wave data to be acquired in swamps, marshes, desert dunes, dense timber, and rugged mountain terrains. These surface conditions are areas where the use of horizontal vibrators would not be considered.
Type-2 Direct-S Data (SV-P Modes)
Although the source-side of S-S data acquisition is simplified by utilizing a vertical-displacement source, the receiver-side of S-S data acquisition is unchanged. It will still be necessary to deploy 3-component sensors to acquire S-S data when vertical-displacement sources are used just as it is when horizontal-displacement sources are used. However, even the receiver requirement can be simplified in situations where the only direct-S mode that is needed is SV-P data because SV-P modes are recorded by vertical geophones. When adequate S-wave information can be provided by SV-P data, data acquisition uses exactly the same equipment that is used to acquire P-wave data – a vertical-force source and vertical geophones.
Vertical-displacement sources allow valuable direct-S images to be created from vertical seismic profile (VSP) data. An example of S-S imaging at a Marcellus Shale prospect is shown in Figure 3. A vertical vibrator was positioned at a source station offset 4136 ft (1261 m) from a receiver well and simultaneously generated downgoing direct-P and downgoing direct-S modes. The P-P image made from the downgoing direct-P mode is displayed as Figure 3a. A second vertical vibrator was positioned only 252 ft (79 m) from the receiver well to generate zero-offset VSP data. The P-P image generated from these zero-offset data is shown as Figure 3b.
An S-S image was then constructed from the far-offset data recorded by the transverse geophones in the vertical receiver array. This S-S image is displayed as Figure 3c after the S-S image-time scale is compressed to agree with the P-P image-time scale. The agreement between the P-P and S-S images is good. A particularly impressive aspect of the transverse-S image is the robust nature of the S-S reflections associated with the geology in the time window between 0.9 and 1.1 seconds. The bolder reflections in this data window are from top to bottom, respectively, the tops of the Upper Marcellus, Lower Marcellus, and Onandaga, the major targets of interest at this study site. The downlapping events immediately below 1.2 sec in the S-S image (Fig. 3c) show features of the Salina salt interval that are not revealed by the companion P-P image (Fig. 3a).The images in Figure 3 are the first public examples where VSP images made from direct-P and direct-S modes generated simultaneously by the same vertical vibrator have ever been compared. Previous to Year 2014, direct-S images created by vertical-displacement sources have been shown only to sponsors of the Exploration Geophysics Laboratory at the Bureau of Economic Geology.
Figure 3. Comparison of VSP direct-P and direct-S imaging with a vertical-vibrator source. (a) Far-offset P-P VSP image. (b) Zero-offset P-P image. (c) Far-offset S-S image. Both far-offset images were produced by the same vertical vibrator positioned at the same source station.
Surface-Based Seismic Data
Seismic data acquired across the Wolfberry play in the Midland Basin illustrate how vertical-displacement sources can be utilized to acquire direct-S data with surface-based receivers. These test data were generated by an array of three inline vertical vibrators and were recorded by single 3-C geophones deployed at surface-based receiver stations. Four images constructed from the data are compared in Figure 4. Two images were made from the direct-P mode, these being the P-P and P-SV images in Figures 4a and 4b, respectively. Common practice is to do no more than create only these two images with 3C3D data generated by a vertical-displacement source. However, when attention is focused on the direct-S modes emanating from the vertical-vibrator source stations, two more images can be made, the SV-P and S-S images shown as Figures 4c and 4d. These data are the first public exhibit of the four possible images that can be made with surface-based 3-C data generated by a vertical-displacement source.
Figure 4. Comparison of surface-based direct-P and direct-S images. All images were created from data produced by an array of three inline vertical vibrators. (a) P-P image = direct-P image option 1. (b) P-SV image = direct-P image option 2. (3) SV-P image = direct-S image option 2. (d) S-S image = direct-S image option 1.
The data windows used in these image comparisons span deep geology that starts near the base of the Wolfberry interval and extends into non-productive basement beneath the Ellenburger (depth of deepest marked reflection is more than 15,000 ft [4570 m]). The identification of key formations in the P-P image (Fig. 4a) was done using a local synthetic seismogram. The transfer of these P-P reflection events to their proper positions in the three companion image spaces (P-SV, SV-P, S-S) was done using the equations written on the data panels. These equations specify how depth intervals in P-P image space are related to equivalent depth intervals in P-SV, SV-P, and S-S image spaces. The VP/VS velocity ratio involved in these data-space transitions is an average of VP/VS velocity ratios observed in three local wells. The identification of formation tops in each image space is not precise but is sufficiently accurate to justify image comparisons of depth-equivalent geology.
Several points can be emphasized about the data examples in Figure 4. First, the quality of direct-S images (Figs. 4c, 4d) is equivalent to the quality of direct-P images (Figs. 4a, 4b). Second, direct-S modes image geology at deep depths equivalent to the depths of direct-P imaging. For example, basement event E labeled in Figure 4 is at a depth of more than 15,000 ft (4570 m). Third, the resolution of direct-S images is equivalent to the resolution of direct-P images. Fourth, direct-S modes produced by a vertical-displacement source are just as valuable for imaging geology as are direct-P modes produced by the same source. Fifth, the SV-P mode (Fig. 4c) provides the option of extracting valuable S-wave information and images from legacy P-wave data generated by a vertical-displacement source and recorded with only single-component vertical geophones.
VSP data have been acquired with 3-component geophones for decades. Thus the option for creating S-S and SV-P images from VSP data acquired with vertical-force sources is now available to users of VSP data. The possibility of reprocessing legacy VSP data to create S-S and SV-P images should be particularly attractive. Such legacy VSP data can provide valuable S-wave information without any investment in new VSP data acquisition.
Up to this point, when explorationists have worked with 3-component seismic data acquired with vertical-displacement sources, imaging options have been confined to just P-P and P-SV data (Figs. 4a and 4b). However, direct-S modes produced by vertical-displacement sources allow imaging options for 3-component data to be expanded to include S-S and SV-P images (Figs. 4c and 4d). Geologic interpretations will be more robust if these two direct-S images are included in the list of data that interpreters examine to analyze rock and fluid properties across prospects. In particular, the availability of S-S data can be quite important. Because S-S data are processed using common-midpoint (CMP) concepts, there will be some data-acquisition geometries where S-S images may be more reliable than P-SV data processed using common-conversion-point (CCP) concepts. Even in cases where there is no justification for questioning P-SV data processing, it is wise strategy to have a second S-wave image to integrate into an interpretation effort. The S-S images provided by vertical-force sources allow such a backup strategy.
The direct-S technology described here was developed at the Exploration Geophysics Laboratory at the Bureau of Economic Geology in Austin, Texas. Developers of this technology have concluded that SV-P data are an excellent choice for providing lower-cost S-wave information to the global seismic community. Their attraction to the SV-P mode is based on the following facts:
- Vertical-displacement land sources (vertical vibrators, shot-hole explosives, and vertical impacts) produce direct-SV modes as well as direct-P modes. Thus the seismic sources needed to generate SV-P data are common P-wave sources that are widely spread around the globe.
- Because the upgoing raypath of an SV-P mode is a P wave, SV-P data are recorded by vertical geophones. In contrast, P-SV data must be recorded by horizontal geophones, which requires that 3C geophones be deployed. There can be significant cost savings in the data-acquisition phase of a seismic program when data are acquired with vertical geophones rather than with 3C geophones. Also, many field-oriented geophysicists observe that vertical-geophone data are superior to horizontal-geophone data because vertical geophones usually couple to the earth better than do horizontal geophones.
- Because SV-P modes reside in data generated by common P-wave sources and recorded by single-component vertical geophones, there is a huge amount of untapped SV-P data in legacy P-wave seismic data (both surface-based data and VSP data) preserved in seismic-data libraries in many countries. Thus for many prospect areas, interpreters may be able to produce valuable S-wave images from legacy P-wave data and will not even have to acquire new seismic data.
The combination of these facts leads to the conclusion that a focus on SV-P data will be the lowest cost and most widely available way to provide S-wave information to the global seismic community.
The author and The University of Texas at Austin are part owners of VertiShear, a company that licenses technology based on the vertical-displacement -source concepts discussed here. The terms of this VertiShear ownership have been reviewed and approved by The University of Texas at Austin in accordance with the University’s policy on objective research. Any seismic data-processing company interested in providing this new S-wave imaging technology to clients can contact VertiShear for licensing information [firstname.lastname@example.org]. Any group interested in applying this new S-wave imaging concept can also contact VertiShear for advice, expanded information, or recommendations about contacting licensed data-processing shops [email@example.com].
Chesapeake Energy provided access to valuable VSP data, and Halliburton provided VSP data-processing support. Fasken Oil and Ranch Ltd provided the data shown in Figure 4, and FairfieldNodal processed the data.
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Bob A. Hardage
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