P-wave Reflection Research Articles

P-Wave Reflection Imaging

Abstract The reflection of elastic waves from interfaces is a robust phenomenon extensively used by animals (bats and dolphins), nondestructive techniques, and medical diagnosis. This study addresses the design and implementation of P-wave reflection imaging to evaluate the internal variability in small-scale submerged, soil models. The performance of this technology depends on fundamental aspects of P-wave propagation in soils, the selection of optimal P-wave transducers, and the development of an adequate test methodology. Design issues include transducer directivity, noise, axial resolution, near field effects, and proper thickness of the coupling water layer. The operating frequency is determined by transducer selection and affects the axial and lateral resolution, skin depth, near field, and divergence; high damping transducers permit higher axial resolution. In addition, data gathering must take into consideration temporal and spatial aliasing. Results show that P-wave reflection is a valuable tool to detect subsurface anomalies and layers, to assess phenomena such as slurry sedimentation, and to monitor the evolution of subsurface structures such as soil layers during liquefaction. Gradual changes in impedance, such as in slurry sedimentation, may prevent reflections.

Comparison of ultrasonic wave transmission and reflection measurements with P- and S-waves on early age mortar and concrete

This paper reports on results of round robin tests comparing two nondestructive, ultrasonic techniques: the wave transmission method using P-waves and the wave reflection method using S-waves. The experiments were conducted within the activities of the RILEM Technical Committee TC ATC-185 with the objective to evaluate the ability of these methods to continuously monitor the setting and hardening process of cementitious materials. In total, eight different mortar and concrete mixtures were subjected to the ultrasonic tests. Additionally, experiments were conducted to determine the penetration resistance (ASTM C 403), the in-situ temperature rise, the adiabatic heat release, and the chemical shrinkage (of the cement paste phase) of the investigated materials. The results revealed that, originating from the different wave types, the two ultrasonic methods monitor the setting process of mortar and concrete in significantly different ways. Despite these differences, the comparison of the ultrasonic test results with the development of the adiabatic heat and the chemical shrinkage has proven that P-wave velocity and reflection loss, as the parameters measured by the two methods, have a consistent and direct relationship to the cement hydration process.

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.

Fast P-wave AVO in fluid-saturated porous media with aligned fractures

Based on elastic wave equations of fluid-saturated porous anisotropic media and four continuous conditions for interfaces between dissimilar fluid-saturated porous anisotropic media, a set of methods for calculating elastic wave reflection and transmission coefficients is derived. On the basis of method and models, calculated results show that the fast P-wave reflection coefficient in the recorded line parallel to the fractures is larger than that in the recorded line perpendicular to the fractures, and that the energy of the fast P-waves converts in part to slow P-waves. Due to large attenuation and small reflection energy, slow P-waves are difficult to observe in real data. Fluid type will affect fast P-wave reflection coefficients.

Identifying vertical productive fractures in the Narraway gas field using the envelope of the anisotropic gradient

In this Canadian case study Dragana Todorovic-Marinic, Glenn Larson, David Gray, Scott Cheadle, Greg Soule and Ye Zheng report on their progress with a new seismic attribute in identifying productive vertically aligned fractures, cracks or micro-cracks in gas reservoirs using surface seismic data. Fractures are of great interest for hydrocarbon production. They can either hurt or help production depending on the nature of the reservoir being explored, so knowledge of their distribution and orientation can be critical to exploration success. Vertically aligned fractures, cracks or micro-cracks are known causes of Horizontal Transverse Isotropy (HTI). This type of anisotropy often has a horizontal axis aligned with open vertical fracturing that trends parallel to the maximum horizontal stress and normal to the minimum horizontal stress. It is widely recognised (e.g. Hall et al, 2000; Gray et al, 2002) that HTI anisotropy has a strong effect on the seismic amplitude. This can be measured by fitting the parameters of the Pwave Amplitude Versus Angle and Azimuth (AVAZ) equation of Ruger (1996) to surface seismic data. The outputs are seismic attributes that contain information that may be relevant to the fracturing. The P-wave reflectivity is the response of the rock to compression by the seismic wave and provides information on the rock’s lithology and fluid content. The Swave reflectivity is the response of the rock to shearing by the seismic wave and is comprised primarily of information about the lithology. The anisotropic gradient describes the variations of the AVO gradient with azimuth and is related to the crack density, i.e. to the magnitude of the differential horizontal permeability (Lynn et al, 1996). The azimuth of the anisotropic gradient is the orientation of the symmetry axis of an HTI medium. To the extent that a reservoir with vertical open fractures represents an HTI medium, the azimuth of the anisotropic gradient indicates the orientation of the fractures.

An amplitude-based multiazimuthal approach to mapping fractures using P-wave 3D seismic data

We have tested an amplitude-based multiazimuthal approach for mapping fractures which requires only a simple azimuth-offset sorting process. By displaying the amplitudes of all traces collected within a superbin, the method predicts fractures by mapping P-wave amplitude variations, in which a lineation within the map indicates the presence and the orientation of fractures within the superbin. Test results using physical model and field data sets suggest that the amplitude-based multiazimuthal approach could help to determine the presence of multiple fracture sets in a single layer, which may be expressed through subtle variations in P-wave multiazimuthal seismic reflections. Our experiments with a physical model containing manmade vertical fractures suggest that transmission effects could be one of the dominant factors which control azimuthal amplitude versus offset (AVO) behavior. The technique described in this paper can operate on any 3D P-wave seismic data with wide azimuth and offset distributions.

An introduction—Multicomponent

Multicomponent seismic data provide the potential to better characterize the subsurface than do P-wave data alone. Converted waves (C-waves) and pure shear waves can be used to validate bright spot reflections, provide Poisson's ratio estimates, provide reflection images where the P-wave reflectivity contrast is small, image through gas clouds where the P-wave signal is attenuated/scattered, and detect/characterize fractures through shear-wave splitting.

τ-p analysis in transversely isotropic media

SUMMARY Incorporating the effects of anisotropy in seismic processing is an active area of research. Here we describe a method for the analysis of P-wave seismic reflection data in the plane-wave domain for transversely isotropic media with a vertical axis of symmetry. The generalization of τ–p methods to include transverse isotropy is straightforward in that we simply replace the isotropic vertical slowness term with one appropriate for anisotropy. Exact expressions for the vertical slowness do exist; P and SV require four parameters and SH require two. The P-wave traveltime trajectories are not sensitive to all four parameters and can be expressed as a function of two parameters only. We have developed a series representation for P-wave vertical slowness in terms of increasing powers of horizontal slowness; it is also convenient for interactive moveout analysis. The two-term expression derived from this series is adequate for the analysis of data from weakly anisotropic media and brings out the physics of wave propagation clearly. Thus our development is analogous to the non-hyperbolic normal moveout expression used in the offset-time domain. The τ–p analysis enables us to derive interval anisotropy parameters directly without a two-point ray tracing, which can be used in further processing. We have developed a computer algorithm to iteratively and interactively fit the τ–p trajectories to derive anisotropy parameters. The technique is demonstrated with applications to synthetic and field reflection seismic data from the Gulf of Mexico.

Anisotropic semblance analysis and NMO corrections for long offset data

Sedimentary rocks can be regarded as transversely isotropic (TI) media. One difficulty in seismic processing is how to flatten the events for long offset data. P-wave reflections from horizontal reflectors in transversely isotropic media have non-hyperbolic moveout. For a multi-layered model, the reflection moveout formula is usually expressed as a Taylor series with higher order terms ignored. Alkhalifah and Tsvankin (1995) developed a three term Taylor series formula to calculate reflection travel times from a horizontal reflector in TI media with vertical symmetry axis (VTI). Using this formula, NMO correction works well for short spread lengths, but not so well for long spreads. Zhang and Uren (2001a, b) developed an approximate explicit analytical P-wave ray velocity function for transversely isotropic (TI) media. From this ray velocity function, a reflection travel time formula from a horizontal reflector in TI media was derived. This formula can be used for anisotropic NMO correction. It works well for both large offsets and small offsets. In order to obtain the unknown parameters required for seismic processing, a 3D-semblance analysis technique has been developed. We tested this method with numerical data from TI models with a single horizontal reflector and from multi-horizontal reflector models. The method was also tested with an isotropic model with multi-horizontal reflectors. The results show that the events can be completely flattened even for very large offsets and that both multi-layered TI and isotropic models may appear to be anisotropic.

3D fracture assessment using AVAz and layer-stripping approach

Subtle variations in P-wave multi-azimuthal seismic reflections can be used to map lineations in a horizon, representing seismic amplitude changes resulting from the presence of vertical fractures. This paper studies the phenomenon of P-wave propagation through vertically fractured media under controlled laboratory conditions, and shows how resulting amplitude maps can be used to predict fractured layers. A scheme of layer-stripping, based on Ruger's reflection coefficient approximations, can reduce the overburden effect on reflection amplitudes of underlying layers. By removal of overburden transmission effects, greater clarity is achieved in mapping any deeper fractured horizons. The effect of fractures is examined using physical model data, and the technique is demonstrated using field data.

Imaging sub-basalt structures using locally converted waves

In many parts of the world large-scale basalt flows associated with continental rifting have blanketed pre-existing sedimentary basins. In recent years considerable attention has turned to exploration beneath these extensive flows. A region of particular interest is the Atlantic margin of northwest Europe which experienced early tertiary magmatism associated with the interaction of the Iceland plume and the opening of the Atlantic Ocean (Barton and White, 1997). Imaging sub-basalt structures is notoriously complicated and there are several reasons why few successful case histories exist. The basalt flows are characterised by rough boundary topography and finely-layered internal structure. Therefore, scattering and attenuation are probably significant. Additionally, basalt can be considerably higher in velocity than the surrounding sediments. Hence, the point of critical P-wave reflection is reached at small incidence angle and only a small amount of P-wave energy penetrates the basalt layer. Finally, short-offset data is contaminated with multiples, again due to the high-impedance contrast with surrounding sediments. The Rockall Trough which lies in deep water west of Northern Ireland is the site of extensive intrusive and extrusive dolerites and basalts. Seismic exploration in this area presents both acquisition and processing challenges. We present a methodology for processing long-offset data recently acquired in this region. Our technique builds on the approach of Emsley et al. (1998) who use locally P-to-S converted waves recorded at far offsets to penetrate the basalt. The use of local mode conversions extends the range of useful offsets for imaging beneath interfaces with large velocity contrasts (Tatham et al., 1983; Tatham and Goolsbee, 1984). We improve upon the approach of Emsley et al. (1998) by also employing the τ-p transform to further isolate converted waves from multiples and head-waves and to increase the signal-to-noise ratio of the extracted waves. In addition, the τ-p transform enables us to isolate arrivals with specific incidence angles, thereby facilitating the distinction between converted-wave and pure-mode energy even further (Tatham et al., 1983; Tatham and Goolsbee, 1984).

Quantifying 3D acquisition adequacy for azimuthal AVO analysis

Azimuthal AVO (AZAVO) refers to the directional variation of P-wave reflection amplitude at a fixed offset. There are several possible causes for this azimuthal amplitude variation: reservoir property, structural change of the reflector, near-surface effects, overburden property variation, acquisition footprint, processing artifacts, source/receiver radiation/reception patterns, etc. While paying close attention to all these causes is necessary for meaningful azimuthal AVO analysis, geophysicists are most interested in the azimuthal amplitude variation caused by a particular reservoir property: fracturing of the reservoir. A decade of theoretical study and some well-documented case histories have brought widespread acceptance that aligned open fractures will introduce azimuthal anisotropy to the host rock and can therefore be detected from analyzing P-wave reflection amplitude variation as a function of azimuth. Analysis of fracture-induced anisotropy in reservoirs requires 3D seismic data with suitable offset and azimuth distribution. Wide-azimuth data and longer offset will in general increase the chance of success for azimuthal AVO analysis. However, because of the associated cost in collecting/processing/analyzing these “wide” azimuth data, a quantitative assessment of acquisition adequacy becomes necessary to minimize the acquisition effort while achieving the goals of azimuthal AVO analysis for fractures. In this article, a method of quantifying the adequacy of a planned or completed 3D survey for azimuthal AVO analysis for fracture orientation and intensity is described. This new method is actively used by ExxonMobil operating companies for business and technical evaluation of azimuthal AVO potential of their planned 3D surveys and existing 3D data. The P-wave reflection coefficient at the interface between an isotropic medium and an azimuthally anisotropic medium depends on both the offset and azimuth formed by the source-receiver pairs. The simplest azimuthal anisotropy medium is an isotropic host rock with a single set of open and aligned vertical fractures. This type of medium has been referred …

Anisotropic semblance analysis and NMO corrections for long-offset data

Sedimentary rocks can be regarded as transversely isotropic (TI) media. P-wave reflections from horizontal reflectors in transversely isotropic media have non-hyperbolic moveout. One difficulty in seismic processing is how to flatten such reflection events for long-offset data. For a multi-layered model, the reflection moveout formula is usually expressed as a Taylor series with higher order terms ignored. Alkhalifah and Tsvankin (1995) developed a three-term Taylor-series formula to calculate reflection travel times from a horizontal reflector in TI media with a vertical symmetry axis (VTI). Using this formula, NMO correction works well for short spread lengths, but not so well for long spreads.Zhang and Uren (2001a, b) developed an approximate explicit analytical P-wave ray velocity function for transversely isotropic (TI) media. From this ray velocity function, a reflection travel-time formula for a horizontal reflector in TI media was derived. This formula can be used for anisotropic NMO correction. It works well for both large offsets and small offsets. In order to obtain the unknown parameters required for seismic processing, a 3D-semblance analysis technique has been developed. We tested this method on numerical data from TI models with a single horizontal reflector and from multiple horizontal reflector models. The method was also tested on an isotropic model with multiple horizontal reflectors. The results show that the events can be completely flattened even for very large offsets, and that both multi-layered TI and isotropic models may appear to be anisotropic.

3D fracture assessment using AVAz and a layer-stripping approach

Subtle variations in P-wave multi-azimuthal seismic reflections can be used to map lineations in a horizon, representing seismic amplitude changes resulting from the presence of vertical fractures. This paper studies the phenomenon of P-wave propagation through vertically fractured media under controlled laboratory conditions, and shows how resulting amplitude maps can be used to predict fractured layers. A scheme of layer-stripping, based on Ruger’s reflection coefficient approximations, can reduce the overburden effect on reflection amplitudes of underlying layers. By removal of overburden transmission effects, greater clarity is achieved in mapping any deeper fractured horizons. The effect of fractures is examined using physical model data, and the technique is demonstrated using field data.

Forward Modeling and Inversion Methods Based on Generalized Elastic Impedance in Seismic Exploration

AbstractConventional seismic trace inversion method is based on the assumption of Pwave striking interface normally. However, conventional multi‐channel seismic data records are oblique reflections, reflection amplitude is the average value of CDP gathers. Using conventional seismic trace inversion method, we can not obtain reliable acoustic impedance (AI) or reservoir properties. Following the idea of Patrick Connolly's (1999) Elastic Impedance (EI), this paper further approximated the Zoeppritz equation. derived the P‐wave reflectivity recursive formula suitable the post‐stack and seismic data of oblique reflection, and proposed Generalized Elastic Impedance (GEI). The key problem of forward modeling and inversion of seismic trace is solved better than ever. GEI not only includes AI, but also includes VP, VS and other lithological parameters. It will have high value when applied in practice. By means of GEI inversion, we can obtain more and more reliable information of fluids, porosity, sand to mud ratio and other lithological. It is helpful to interpret the pitfall in conventional seismic trace inversion and trace integration. It is also helpful to decrease non‐uniqueness in conventional seismic trace inversion and improve the accuracy of reservoir prediction.

3D description and inversion of reflection moveout of PS-waves in anisotropic media

Common-midpoint moveout of converted waves is generally asymmetric with respect to zero offset and cannot be described by the traveltime series t2(x2) conventionally used for pure modes. Here, we present concise parametric expressions for both common-midpoint (CMP) and common-conversion-point (CCP) gathers of PS-waves for arbitrary anisotropic, horizontally layered media above a plane dipping reflector. This analytic representation can be used to model 3D (multi-azimuth) CMP gathers without time-consuming two-point ray tracing and to compute attributes of PS moveout such as the slope of the traveltime surface at zero offset and the coordinates of the moveout minimum. In addition to providing an efficient tool for forward modelling, our formalism helps to carry out joint inversion of P and PS data for transverse isotropy with a vertical symmetry axis (VTI media). If the medium above the reflector is laterally homogeneous, P-wave reflection moveout cannot constrain the depth scale of the model needed for depth migration. Extending our previous results for a single VTI layer, we show that the interval vertical velocities of the P- and S-waves (VP0 and VS0) and the Thomsen parameters e and δ can be found from surface data alone by combining P-wave moveout with the traveltimes of the converted PS(PSV)-wave. If the data are acquired only on the dip line (i.e. in 2D), stable parameter estimation requires including the moveout of P- and PS-waves from both a horizontal and a dipping interface. At the first stage of the velocity-analysis procedure, we build an initial anisotropic model by applying a layer-stripping algorithm to CMP moveout of P- and PS-waves. To overcome the distorting influence of conversion-point dispersal on CMP gathers, the interval VTI parameters are refined by collecting the PS data into CCP gathers and repeating the inversion. For 3D surveys with a sufficiently wide range of source–receiver azimuths, it is possible to estimate all four relevant parameters (VP0, VS0, e and δ) using reflections from a single mildly dipping interface. In this case, the P-wave NMO ellipse determined by 3D (azimuthal) velocity analysis is combined with azimuthally dependent traveltimes of the PS-wave. On the whole, the joint inversion of P and PS data yields a VTI model suitable for depth migration of P-waves, as well as processing (e.g. transformation to zero offset) of converted waves.

P-wave stacking-velocity tomography for VTI media

A major complication caused by anisotropy in velocity analysis and imaging is the uncertainty in estimating the vertical velocity and depth scale of the model from surface data. For laterally homogeneous VTI (transversely isotropic with a vertical symmetry axis) media above the target reflector, P-wave moveout has to be combined with other information (e.g. borehole data or converted waves) to build velocity models for depth imaging. The presence of lateral heterogeneity in the overburden creates the dependence of P-wave reflection data on all three relevant parameters (the vertical velocity VP0 and the Thomsen coefficients e and δ) and, therefore, may help to determine the depth scale of the velocity field. Here, we propose a tomographic algorithm designed to invert NMO ellipses (obtained from azimuthally varying stacking velocities) and zero-offset traveltimes of P-waves for the parameters of homogeneous VTI layers separated by either plane dipping or curved interfaces. For plane non-intersecting layer boundaries, the interval parameters cannot be recovered from P-wave moveout in a unique way. Nonetheless, if the reflectors have sufficiently different azimuths, a priori knowledge of any single interval parameter makes it possible to reconstruct the whole model in depth. For example, the parameter estimation becomes unique if the subsurface layer is known to be isotropic. In the case of 2D inversion on the dip line of co-orientated reflectors, it is necessary to specify one parameter (e.g. the vertical velocity) per layer. Despite the higher complexity of models with curved interfaces, the increased angle coverage of reflected rays helps to resolve the trade-offs between the medium parameters. Singular value decomposition (SVD) shows that in the presence of sufficient interface curvature all parameters needed for anisotropic depth processing can be obtained solely from conventional-spread P-wave moveout. By performing tests on noise-contaminated data we demonstrate that the tomographic inversion procedure reconstructs both the interfaces and the VTI parameters with high accuracy. Both SVD analysis and moveout inversion are implemented using an efficient modelling technique based on the theory of NMO-velocity surfaces generalized for wave propagation through curved interfaces.

Synthetic Experiments Dealing with Anisotropic Model Building

It has been shown in the past that the interval-NMO velocity and the non-ellipticity parameter ν largely control the P-wave reflection time moveout of VTI media. To invert for these two parameters, one needs either reasonably large offsets, or some structure in the subsurface in combination with relatively mild lateral velocity variation. This paper deals with a simulation of an inversion approach, building on the assumption that accurately measured V NMO, as defined by small offset asymptotics for a particular reflector, were available. Instead of such measurements we take synthetically computed data. First, an isotropic model is constructed which explains these V NMO. Subsequently, residual moveout in common image gathers is modelled by ray tracing (replacing real data), along with its sensitivity for changes in the interval-NMO velocity and the non-ellipticity parameter under the constraint that V NMO is preserved. This enables iterative updating of the non-ellipticity parameter and the interval-NMO velocity in a layer that can be laterally inhomogeneous. This approach is successfully applied for a mildly dipping reflector at the bottom of a layer with laterally varying medium parameters. With the exact V NMO assumed to be given, lateral inhomogeneity and anisotropy can be distinguished for such a situation. However, for another example with a homogeneous VTI layer overlying a curved reflector with dip up to 30°, there appears to be an ambiguity which can be understood by theoretical analysis. Consistently with existing theory using the NMO-ellipse, the presented approach is successfully applied to the latter example if V NMO in the strike direction is combined with residual moveout in dip direction.

A case study on using AVO walkaway VSP data - Barrow Sub-basin, Western Australia

In December 1999 the TL/1 JV partners acquired a 2D walkaway VSP survey in the Monty-2 well in the Barrow Sub-basin, Western Australia. The survey was designed to analyse AVO effects and possible later calibration of surface seismic data. The survey was recorded using CGG?s 12 level array tool, the SST-500, and comprised a total of 4 walkaway lines, with 2 azimuths at each of two array positions. Each of the 4 walkaway lines had a length of 6000 meters. Walkaway VSP data was acquired by positioning the array tool just above the two target reflectors, the North Rankin and the Mungaroo ``B' sandstone formations. The VSP data was processed using an `isotropic' sequence, which allowed for a perfect preservation of the amplitude and phase relationships between the three receiver components. AVO estimates were computed from the reflected P-wavefield after shaping deconvolution, derived from the down going P-wavefield. P-wave polarisation information was used to compute reflection angles at the target horizon and the local reflector dip. Finally, synthetic P-wave reflectivity curves were computed for the North Rankin and the Mungaroo ``B' sandstone formation and compared with the borehole seismic measurements. The AVO/AVA response at the top North Rankin reflection was found to be strongly affected by a thin overlaying carbonate layer. The best match between measured AVO curves and isotropic modeling therefore was found for a shale/carbonate model rather than a carbonate/sandstone model. In contrast, the AVO response of the Mungaroo ``B' reflection was found to be in very good agreement with isotropic modeling results.

Numerical and physical modelling of P-wave AVO response for fractured media

In this study we investigate variations of P-wave reflection coefficients with offset (AVO) and with azimuth (AVA) for a transversely isotropic medium with a horizontal axis of symmetry (HTI) in contact with water, and also with another HTI medium. Aligned, equidistant, vertical fractures, inbedded in an isotropic background, were used to represent a HTI medium. To grasp the effect of fractures their contribution was computed separately and then added to the isotropic background. The total response of a medium is then obtained by adding the two.An approximate equation is used to compute partitioning of seismic energy at layer interfaces. The solution compares well to the results obtained in physical modelling experiments. To analyse AVO and AVA effects we used 3-D multi-azimuth transmission and reflection surveys. The results show that AVA effects are significant even at moderate angles whilst they are dominant for large incident angles. Routine 2-D AVO analysis, as commonly performed, is clearly unsuitable for HTI media.