Converted-wave Data Research Articles

Inversion of shear wave interval velocity on prestack converted wave data

Seismic velocity is important to migration of seismic data, interpretation of lithology and lithofacies as well as prediction of reservoir. The information of shear wave velocity is required to reduce the uncertainty for discriminating lithology, identifying fluid type in porous material and calculating gas saturation in reservoir prediction. Based on Zoeppritz equations, a numeral and scanning method was proposed in this paper. Shear wave velocity can be calculated with prestack converted wave data. The effects were demonstrated by inversion of theoretical and real seismic data.

Data Processing of Marine Multicomponent Seismic-A Case Study of 4-Component 2D OBC data, Offshore Norway-

Multicomponent seismic technology has been improved significantly with higher fidelity acquisition systems and more advanced data analysis methods. Especially in marine seismic, 4-Component Ocean Bottom Cable (OBC) is practically applied for reservoir characterization and exploration at the water depth of more than 1000m. Converted wave processing and interpretation is a fairly new topic and there is a lack of data and expertise around the world. Until now, the most common converted wave data has been essentially 1-D, VSP and OBS (Ocean Bottom Seismometer) records. Recently there is an increasing use of multi-component OBC 2-D and 3-D surveys. They are especially applied to evaluate reservoirs, because it has been revealed that P-S converted waves can give valuable additional lithological information. They also give clear reflections where the P-wave reflections are poor due to a very small acoustic impedance contrast. It is worthwhile that we introduce the actual processing of the 4-C line to discuss on P-S converted wave processing and later to develop new tools and techniques in this rapidly evolving field. Since marine multicomponent seismic survey has not been carried out yet in Japan, we use a 4-Component OBC (4C OBC) 2-D line, acquired by PGS Norway in 1998. The data was purchased by JNOC/TRC in 2002, as a part of the methane hydrate research program, to evaluate the use of P-S converted wave data in determining the properties of the hydrate layer and the BSR. The 4C line was acquired along the track of a previous conventional streamer line which showed a BSR. The location is in the Norwegian Sea, just up-dip of a major sedimentary slide, known as the 'Storegga Slide'. The results of the first investigation of the 4C data were published by Andreassen et al (2001). They presented a very significant difference between the P-wave and the PS converted wave sections in the hydrate layer and around the BSR. In fact, the BSR was not visible on the PS-wave section. The result shows that a lot of additional information is contained in the converted waves. We reprocessed the 4C data and discussed the specific problems.

Rock and reservoir parameters from pre-stack inversion of surface seismic data

Huseyin Ozdemir, Jesper Wissendorf Hansen, and Emma Tyler provide an example of mapping of oil-water contacts where pre-stack AVO inversion to rock physics parameters provided improved delineation whilst post stack inversion procedures including elastic inversion gave inconclusive results. A Paleocene field was discovered in year 2000 in the Danish North Sea on the flank of a salt dome. The field was further appraised by a sidetrack discovery well and two subsequent vertical wells through years 2000 and 2001. Full logging suites were acquired in all wells including shear sonic data. VSP data were acquired in the three vertical wells. The wells were positioned using a 3D seismic data set, which was not specifically processed for Paleocene targets and no AVO processing was performed. Hence it was decided to re-process the data for AVO analysis and as part of this to perform forward modelling and fluid substitution in order to calibrate the seismic anomalies to the results of the forward modelled well data and use the results as an exploration/appraisal tool. The main exploration and development challenge is to find a seismic attribute that can differentiate between hydrocarbon and water zones. Conventional and pre-stack AVA inversion analysis was also made for lithological and pore fluid determinations. The latter analysis resulted in reservoir rock properties such as Poisson’s ratio (m), hl and l. h and are the Lame constants and l is the density. Other combined and weighted attributes have been computed for more accurate pore fill identification. This paper is based on our EAGE presentation (Ozdemir, Hansen, and Robinson/Tyler, 2003), which also includes a brief discussion of pre-stack inversion to acoustic impedance (AI), shear impedance (SI), and density using full Aki-Richards equations (Aki and Richards, 1980; Stewart 1990; Fatti, Smith, Vail, Strauss and Levitt, 1994; Larsen, Margrave, and Lu, 1999). This inversion methodology was first introduced by Ozdemir, Ronen, Olofsson, Goodway, and Young (2001) for the joint inversion of multicomponent seismic p-wave and converted wave data.

Vector imaging of converted wave data in laterally uniform media with VTI anisotropy

A vector imaging method has been developed for PS-converted waves in laterally homogeneous vertically transverse isotropic (VTI) media. It decomposes the converted-wave data into two upgoing quasi-shear waves ([Formula: see text] and [Formula: see text]) within the prestack migration algorithm according to subsurface image and surface receiver locations. Because the decomposition is performed as part of the migration, it is consistent with the dip and polarization of the seismic events, unlike traditional algorithms that use premigration rotations. Two shear-wave images with potentially enhanced resolution are formed simultaneously from the vector migration. The effects of VTI anisotropy on PS-converted wave imaging and the capability of the PS vector imaging algorithm to provide enhanced images are illustrated using a point-scatterer model.

MEMS sensors: Some issues for consideration

The development of purpose-built multicomponent data acquisition systems based on MEMS (Micro Electro Mechanical System) digital accelerometers has resulted in improved data quality, more efficient field operations, and diminished multicomponent acquisition costs. This, coupled with advances in processing and interpretation of converted-wave (PS) multicomponent seismic data, has renewed enthusiasm for using multicomponent data to address issues poorly resolved by P-wave information alone.

Processing of anisotropic data in the τ-pdomain: II — Common-conversion-point sorting

Common-conversion-point (CCP) sorting of P-SV converted-wave data is conventionally done by first sorting data into common asymptotic-conversion-point (CACP) gathers and then computing the involved CCP shifts from analytic approximations. I explore an alternative method where the latter step is replaced by an entirely data-driven approach. Moveout curves of correlated P-P and P-SV reflections in collocated CMP and CACP gathers are first scanned for points of equal slowness. A common-source slowness indicates that the downgoing branches of the P-P and P-SV waves overlap if the conversion occurs at the reflecting interface. The P-SV conversion point is then assumed to be situated underneath the associated P-P wave midpoint. A migration of amplitudes from CACP to CCP gathers is straightforward once the exact CCP position is known. This data-driven approach requires kinematic information only and is exact for laterally homogeneous media with arbitrary strength of anisotropy if horizontal symmetry planes are present at all depths. Both time-offset and τ-p domain implementations are possible, although the latter are preferred.

Some comments on common-asymptotic-conversion-point (CACP) sorting of converted-wave data in isotropic, laterally inhomogeneous media

Common-midpoint (CMP) sorting of pure-mode data in arbitrarily complex isotropic or anisotropic media leads to moveout curves that are symmetric around zero offset. This greatly simplifies velocity determination of pure-mode data. Common-asymptotic-conversion-point (CACP) sorting of converted-wave data, on the other hand, only centers the apexes of all traveltimes around zero offset in arbitrarily complex but isotropic media with a constant P-wave/S-wave velocity ratio everywhere. A depth-varying CACP sorting may therefore be required to position all traveltimes properly around zero offset in structurally complex areas. Moreover, converted-wave moveout is nearly always asymmetric and nonhyperbolic. Thus, positive and negative offsets need to be processed independently in a 2D line, and 3D data volumes are to be divided in common azimuth gathers. All of these factors tend to complicate converted-wave velocity analysis significantly.

Anadarko Basin survey shows value of multicomponent acquisition

In this case study of a survey in the Anadarko Basin, Oklahoma, Steve Roche, Mark Wagaman, and Howard Watt of Veritas DGC in Houston show that using both P-wave and converted shear wave data provides better gas production estimates. Interpretation of multicomponent data holds great promise for the exploration and development of oil and gas. Shear-wave propagation is sensitive only to rigidity and density, while compressional-wave propagation is sensitive to rigidity, density and compressibility. Interpreting both P- and S-wave reflectivity offers the ability to discriminate lithology, porosity, fractures and possibly fluid content. Recent advances in seismic recording systems, sensor technology and data processing methods are such that the use of multicomponent data for exploration and development is now an economic reality. Specific case studies such as the Anadarko Basin data show the viability of recording converted-wave data. As the industry moves towards 3C recording there also needs to be assurance that P-wave data quality will not be compromised. These data demonstrate that 3C single-sensor P-wave data quality will meet, and possibly exceed, data recorded using conventional P-wave only surveys, if proper attention is given to the signal-to-noise characteristics in the survey area. This may require higher station density and/or a finer group interval in many cases. This paper has three parts, a description of the North Emerald 3C3D test and resulting P-wave (PP) and converted-wave (PS) data quality, relating the P-wave and converted-wave reflectivity to natural gas production from the Springer Formation, and comparing single sensor MEMS recording to six element geophone array data.

Chałupki Dębniańskie Field: Improving drilling success in shallow gas reservoirs with VectorSeis

Edward Gruszczyk, Zygmunt Trzesnioswki and Piotr Misiaczek of Geofizyka Krakow, Poland with Paul Brettwood (UK) and Jon Tessman (USA) of Input/Output provide a case study comparing the performance in the field of I/O VectorSeis sensor technology with conventional analogue geophone systems. In the spring of 2002, a field experiment was conducted over a gas field in southeast Poland. The primary objective was to characterize the response of the I/O VectorSeis sensor in a direct comparison with conventional analogue geophone systems. Both dynamite and Vibroseis sources were tested during the course of the experiment. Analysis of the acquired datasets demonstrated clearly that, in this relatively shallow (160 m - 800 m depth range) clastic depositional environment, VectorSeis sensors not only matched the results of the analogue geophones but also produced datasets with superior vertical resolution. In addition to the conventional compressional datasets, converted wave data were also acquired. These demonstrated that high quality converted wave data could be acquired in a cost-effective manner and that they could solve a number of imaging problems directly related to the presence of multiple stacked gar reservoirs. Mapping of the PS data into PP time also demonstrated that the PS data had similar apparent frequency response when compared to analogue geophone compressional data.

Practical approach to joint imaging of multicomponent data

Recent advances in the ocean-bottom cable (OBC) acquisition technology allow recording of high quality multi-component data in the marine environment. These data have been used to image reservoirs obscured by gas clouds (e.g., Valhall) and reservoirs with low P -wave impedance contrast that are hard to see in conventional streamer data (e.g., Alba). Further benefits of using multicomponent data in exploration may come from analyzing P -wave ( PP ) and converted-wave ( PS ) data jointly to obtain more information about a reservoir than is available from PP data alone. Joint interpretation of multicomponent data is complicated by the fact that PP data are traditionally imaged in PP time and PS data are imaged in PS time. Thus, the PP and PS images have different vertical scales. To reconcile these scales, an interpreter has to identify events in both images that correspond to the same reflector and then stretch one of the images to match the other. The event identification is not always straightforward because some interfaces generate PP and PS reflections of the same polarity and other interfaces of the opposite polarity. Thus, there may not be a natural choice of troughs or peaks to correlate. To eliminate the need to stretch PP and PS images and to facilitate joint interpretation, we developed a methodology for joint imaging of multicomponent data in depth. We image PP and PS data in depth by anisotropic pre-stack depth migration. For PS data, our migration algorithm combines P -wave propagation from a source and S -wave propagation from a receiver to a reflection point. The algorithm also handles the OBC acquisition geometry. Prestack depth migration of multicomponent data requires a complete transversely isotropic (TI) velocity model for the subsurface. This model consists of four parameters: P -wave velocity, S -wave velocity, and the Thomsen anisotropy parameters, …

Shear-wave elastic impedance

It is well known that if we are able to estimate acoustic impedance (AI) and a parameter related to shear-wave velocity from seismic data, our ability to discriminate between different lithologies and fluid phases will increase. Prestack inversion on individual CDP gathers and inverting directly for VP , VS , and density have been tested in several ways, but the estimated parameters are often poorly determined. A more robust approach is to apply poststack inversion on partial stacks. For inversion of the near-offset stack, AI can be calculated directly from well logs. However, for the far-offset stack, we need to derive an equivalent of the acoustic impedance that can be used to calibrate the non-zero-offset seismic reflectivity. Connolly (1998, 1999) derived such an equivalent—elastic impedance (EI)—and demonstrated how, by using elastic impedance logs (which requires shear-wave logs), he was able to perform well calibration and inversion of far-offset data. This article describes a new function—shear-wave elastic impedance (SEI)—for linking converted-wave stacks to wells using a linearization of the Zoeppritz equations. SEI is similar to EI but adapted to P-S converted seismic data. It can be computed from acoustic log data ( P - and S -wave velocities and density) and used for well calibration, wavelet estimation, and inversion of P-S reflectivity data leading to improved interpretability of converted wave data. (Equation 4 in Appendix 1 is the key mathematical formula in this approach to SEI). To test the SEI technique, we used a 3-D four-component (4-C) ocean-bottom cable (OBC) survey acquired in 1997 that covers approximately 10 km2 of the Statfjord Field (Rogno et al., 1999). Both P -wave and converted-wave data were 3-D prestack time-migrated. The hydrophone ( P ) and the vertical geophone ( Z ) component were summed to attenuate receiver side water-layer reverberations and both data sets ( PZ …

Decomposition and Inversion of Elastic Reflection Data; First-Order Angular Dependence and Applications

Abstract The elastic wave displacement equation is transformed into pressure-stress coordinates, where the Born approximation of the Lippman-Schwinger equation in the Fourier-transform domain is employed to decompose the observed fields into their scattered components: P-P, P-S, S-P, and S-S. Triple Fourier transforms of the scattered elastic wave data are linear combinations of the double Fourier transforms of the relative changes in the medium properties. Angular-dependent reflection coefficients for each of the scattering modes are constructed, and an inversion algorithm is outlined. Inversion of the observed elastic wave fields is accomplished in a manner similar to the acoustic problem. Density, bulk modulus, and shear modulus variations in an elastic earth can be recovered by utilizing the angular-dependent information present in the observed wave fields. Examples illustrate these points. Transforming the elastic wave data back to displacement coordinates and assuming a compressional source, an analysis of recorded amplitudes yields some practical answers about converted-wave data. Significant amounts of P → S data should typically be generated by compressional sources, with significant contributions at smaller angles, However, signal-to-noise calculations suggest that more sweeps and more geophone channels at longer offsets will typically be necessary to get P-S sections of comparable quality to P-P sections.

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

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.

Three-component amplitude versus offset analysis

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.