bluuebunny/crossref_metadata_2025 huggingface dataset:
AVO detectability against tuning and stretching artifacts
Authors: Dong Wenjie
Published: October 2002

The [Formula: see text] of hydrocarbon‐bearing sediments normally deviates from the [Formula: see text] trend of the background rocks. This causes anomalous reflection amplitude variation with offset (AVO) in the seismic data. The estimation of these AVOs is inevitably affected by wave propagation effects and inversion algorithm limitations, such as thin‐bed tuning and migration stretch. A logical point is to determine the minimum [Formula: see text] change required for an anomalous AVO to be detectable beyond the background tuning and stretching effects. Assuming Ricker wavelet for the seismic data, this study addresses this point by quantifying the errors in the intercept/slope estimate. Using these results, two detectability conditions are derived. Denoting the background [Formula: see text] by γ and its variation by δγ, the thin‐bed parameter (thickness/wavelength) by ξ, the maximum background intercept closest to the AVO by |A|max, and the thin‐bed intercept value by |A|thinthe two conditions are [Formula: see text] [Formula: see text] for detectability against stretching and tuning plus stretching, respectively. Tests on synthetic data confirm their validity and accuracy. These conditions provide a quantitative guideline for evaluating AVO applicability and effectiveness in seismic exploration. They can eliminate some of the subjectivity when interpreting AVO results in different attribute spaces. To improve AVO detectability, a procedure is suggested for removing the tuning and stretching effects.
Seismic waveform inversion in the frequency domain, Part 1: Theory and verification in a physical scale model
Authors: Pratt R. Gerhard
Published: October 2002

Seismic waveforms contain much information that is ignored under standard processing schemes; seismic waveform inversion seeks to use the full information content of the recorded wavefield. In this paper I present, apply, and evaluate a frequency‐space domain approach to waveform inversion. The method is a local descent algorithm that proceeds from a starting model to refine the model in order to reduce the waveform misfit between observed and model data. The model data are computed using a full‐wave equation, viscoacoustic, frequency‐domain, finite‐difference method. Ray asymptotics are avoided, and higher‐order effects such as diffractions and multiple scattering are accounted for automatically. The theory of frequency‐domain waveform/wavefield inversion can be expressed compactly using a matrix formalism that uses finite‐difference/finite‐element frequency‐domain modeling equations. Expressions for fast, local descent inversion using back‐propagation techniques then follow naturally. Implementation of these methods depends on efficient frequency‐domain forward‐modeling solutions; these are provided by recent developments in numerical forward modeling. The inversion approach resembles prestack, reverse‐time migration but differs in that the problem is formulated in terms of velocity (not reflectivity), and the method is fully iterative. I illustrate the practical application of the frequency‐domain waveform inversion approach using tomographic seismic data from a physical scale model. This allows a full evaluation and verification of the method; results with field data are presented in an accompanying paper. Several critical processes contribute to the success of the method: the estimation of a source signature, the matching of amplitudes between real and synthetic data, the selection of a time window, and the selection of suitable sequence of frequencies in the inversion. An initial model for the inversion of the scale model data is provided using standard traveltime tomographic methods, which provide a robust but low‐resolution image. Twenty‐five iterations of wavefield inversion are applied, using five discrete frequencies at each iteration, moving from low to high frequencies. The final results exhibit the features of the true model at subwavelength scale and account for many of the details of the observed arrivals in the data.
Fracture detection using azimuthal variation ofP-wave moveout from orthogonal seismic survey lines
Authors: Li Xiang‐Yang
Published: October 2002

An algorithm is proposed for determining the fracture orientation based on the azimuthal variations in the P-wave reflection moveout for a target interval. The differential moveout between orthogonal survey lines from the bottom of a given target shows cos 2ϕ variations with the line azimuth ϕ measured from the fracture strike for a fixed offset. A configuration of four intersecting survey lines may be used to quantify the fracture strike. The four lines form two orthogonal pairs, and the fracture strike can be obtained by analyzing the crossplot of the two corresponding pairs of the differential moveouts. An offset‐depth ratio (x/z) of 1.0 or greater (up to 1.5) is often required to quantify the moveout difference reliably. The sensitivity of the method is further enhanced by low/high impedance contrast at the top target interface but is greatly reduced by high/low impedance contrast. The method may be particularly useful in marine exploration with repeated surveys of various vintages where continuous azimuthal coverage is often not available. A data set from the North Sea is used to illustrate the technique.
Repeatability issues of 3-D VSP data
Authors: Landrø Martin
Published: October 2002

Increased repeatability is recognized as one major issue for improving the time‐lapse seismic technology as a reservoir management tool. A 3-D vertical seismic profiling (VSP) data set, acquired over a period of two days, is used to analyze how repeatable a permanent installed geophone array can be and how repeatability changes with inaccuracies in source positioning. It is found that for a frequency range between 3.5 and 50 Hz, the difference root‐mean‐square (rms) level between two recorded traces belonging to two different shots is about 8%. This fact shows that there is a potential for acquiring very accurate time‐lapse seismic data by using a permanently installed downhole geophone array. Repeatability variation with increasing shot separation distances is analyzed, showing a rapid decrease in repeatability as the accuracy of the positioning of the repeat survey decreases. Horizontal geophone components show approximately the same degree of repeatability compared to the vertical component, but horizontal geophone data is slightly more sensitive to positioning errors. The results show that repeated 3-D VSP surveys (preferably using permanently installed geophone arrays) might be an efficient tool for detailed and precise monitoring of fluid and pressure changes within a hydrocarbon reservoir.
Effects of layered sediments on the guided wave in crosswell radar data
Authors: Ellefsen Karl J.
Published: October 2002

To understand how layered sediments affect the guided wave in crosswell radar data, traces are calculated for a model representing a sand layer between two clay layers. A guided wave propagates if the wavelengths in the sand layer are similar to the thickness of the sand layer. The amplitude of the guided wave but not its initial traveltime is affected by the thickness of the sand layer. In contrast, both the amplitude and the initial traveltime are affected by the locations of the transmitting and receiving antennas, the electrical conductivity of the sand layer, and the dielectric permittivity of the sand layer. This permittivity can be estimated from the initial traveltime. The effects of the layering on the waves in these calculated traces also are observed in field traces, which were collected in layered sediments.
Stacked global satellite gravity profiles
Authors: Yale Mara M. , Sandwell D. T.
Published: October 2002

Gravity field recovery from satellite altimetry provides global marine coverage but lacks the accuracy and resolution needed for many exploration geophysics studies. The repeating ground tracks of the ERS-1/2, Geosat, and Topex/Poseidon altimeters offer the possibility of improving the accuracy and resolution of gravity anomalies along widely spaced (∼40-km spacing) tracks. However, complete ocean coverage is usually needed to convert the sea‐surface height (or along‐track slope) measurements into gravity anomalies. Here we develop and test a method for constructing stacked gravity profiles by using a published global gravity grid (Sandwell and Smith, 1997), V7.2, as a reference model for the slope‐to‐gravity anomaly conversion. The method is applied to stacks (averages) of Geosat/ERM (up to 62 cycles), ERS-1/2 (up to 43 cycles), and Topex (up to 142 cycles) satellite altimeter profiles. We assess the accuracies of the ERS-1/2 profiles through a comparison with a gravity model of the northern Gulf of Mexico (profiles provided by EDCON Inc.). The 40 ERS profiles evaluated have a mean rms difference of 3.77 mGal and full wavelength resolution (0.5 coherence) of 24 km. Our processing retains wavelengths as short as 10 km so smaller, large‐amplitude features can be resolved, especially in shallow ocean areas (<1000 m deep). We provide an example of combining these higher resolution profiles with lower resolution gravity data in the Caspian Sea.
Antialiasing methods in Kirchhoff migration
Authors: Abma Ray , Sun James , Bernitsas Nick
Published: October 2002

This paper addresses some practical aspects of antialiasing in Kirchhoff migration. We show that the effective trace spacing related to the aliasing‐frequency limits for directions between the in‐line and cross‐line directions is smaller than values previously used and that the frequency limits for antialiasing may be increased. We present a corrected expression for the migration operator spatial derivative for 3-D time migration that avoids the overfiltering at nonzero offsets produced by a previous approximation. Simple operator truncation is shown to reduce aliasing noise; it is inexpensive but may lead to errors in interpretation. Triangle filtering and Gray’s method both produce similar results, but the relative costs of these two methods will vary with the size of the migration and the computational environment. Gray’s method takes more setup time for each input trace than does triangle filtering, but Gray’s method might be more cost effective when each input trace contributes to a large number of output traces.
Amplitude‐variation‐with‐angle behavior of self‐similar interfaces
Authors: Wapenaar Kees
Published: October 2002

Amplitude‐variation‐with‐angle (AVA) analysis is generally based on the assumption that the medium parameters behave as step functions of the depth coordinate z, at least in a finite region around the interface. However, outliers observed in well logs often behave quite differently from step functions. In this paper, outliers in the acoustic propagation velocity are parameterized by functions of the form [Formula: see text]. The wavelet transform of this function reveals properties similar to those of several outliers in real well logs. Moreover, this function is self‐similar, according to [Formula: see text], for β > 0. Analytical expressions are derived for the acoustic normal incidence reflection and transmission coefficients for this type of velocity function. For oblique incidence, no explicit solutions are available. However, by exploiting the self‐similarity property of the velocity function, it turns out that the acoustic angle‐dependent and frequency‐dependent reflection and transmission coefficients are self‐similar as well. To be more specific, these coefficients appear to be constant along curves described by [Formula: see text], where p is the raypath parameter and ω the angular frequency. The singularity exponent α that is reflected in these curves may prove to be a useful indicator in seismic characterization.
Reflection moveout inversion for horizontal transverse isotropy: Accuracy, limitation, and acquisition
Authors: Al‐Dajani Abdulfattah , Alkhalifah Tariq
Published: October 2002

Horizontal transverse isotropy (HTI) is the simplest azimuthally anisotropic model used to describe vertical fracturing in an isotropic matrix. Assuming that the subsurface is laterally homogeneous, and using the elliptical variation of P-wave NMO velocity with azimuth measured in at least three different source‐to‐receiver orientations, we can estimate three key parameters of HTI media: the vertical velocity, anisotropy, and the azimuth of the symmetry axis. Such parameter estimation is sensitive to the angular separation between the survey lines in 2-D acquisition or, equivalently, to source‐to‐receiver azimuths in 3-D acquisition and the set of azimuths used in the inversion procedure. The accuracy in estimating the azimuth, in particular, is also sensitive to the strength of anisotropy, while the accuracy in resolving vertical velocity and anisotropy is about the same for any strength of anisotropy. To maximize the accuracy and stability in parameter estimation, it is best to have the azimuths for the source‐to‐receiver directions 60° apart when only three directions are used. This requirement is feasible in land seismic data acquisition where wide azimuthal coverage can be designed. In marine streamer acquisition, however, the azimuthal data coverage is limited. Multiple survey directions are necessary to achieve such wide azimuthal coverage in streamer surveys. To perform the inversion using three azimuth directions, 60° apart, an HTI layer overlain by an azimuthally isotropic overburden should have a time thickness, relative to the total time, of at least the ratio of the error in the NMO (stacking) velocity to the interval anisotropy strength of the HTI layer. Having more than three source‐to‐receiver azimuths (e.g., full azimuthal coverage), however, provides a useful data redundancy that enhances the quality of the estimates, thus allowing acceptable parameter estimation at smaller relative thicknesses.
GPR modeling study in a contaminated area of Krzywa Air Base (Poland)
Authors: Carcione José M. , Marcak Henryk , Seriani Géza , Padoan Giorgio
Published: October 2002

Krzywa Air Base is a former Soviet military base which is highly polluted because of soil contamination during the dismantling of fuel tanks after the 1990 political changes in the U.S.S.R. The contaminated sites are indicated as magazines 1 and 2 in Figure 1. The area is located within the pre‐Sudetic block, where Tertiary and Quaternary deposits lie on Mesozoic crystalline basement. The deposits consist of sands of different grain sizes, gravels, clays, and interbedded silts, with a total thickness ranging from 80 to 100 m. In the extreme cases, the pollution is so intense that the spilled fuel can be extracted in the system of drainage wells. In general, the contamination can be correlated with the geologic structure. As a prelude to acquisition of field data, we simulate and evaluate the expected ground‐penetrating radar (GPR) responses for uncontaminated and contaminated layers.
A finite‐difference injection approach to modeling seismic fluid flow monitoring
Authors: Robertsson Johan O. A. , Ryan‐Grigor Sarah , Sayers Colin M. , Chapman Chris H.
Published: October 2002

A highly efficient and accurate tool for predicting the seismic response of reservoir fluid flow has been developed which integrates the finite‐difference injection method with a reservoir simulator and a petrophysical model. Finite‐difference methods allow for the full response to be synthesized as the wavefield interacts with a seismic model. This includes wave propagation in arbitrary heterogeneous anisotropic and anelastic media, scattering, and mode conversions. The finite‐difference injection method, in turn, can be used to efficiently synthesize the seismic response from models after local alterations to the model. Thus, it is ideally suited for time‐lapse seismic studies. The modeling methodology is demonstrated on a case study from the Gullfaks field in the North Sea. Six complete marine seismic surveys over the reservoir at different stages during waterflood oil production were synthesized. A total of 180 shot gathers were synthesized with computational savings of a factor of 54 after one single full simulation. The computational savings for the analogous 3-D study are 370 or greater after the initial simulation. The surface seismic response acquired along a towed streamer was processed through to stack and migrated. In a noise‐free environment the replacement of oil by water at a constant pressure caused visible changes in the synthetic seismic response that closely correspond to the impedance changes in the reservoir because of fluid flow. Downhole permanent sensor or vertical seismic profiling configurations were also considered; they provided a particularly suitable acquisition geometry for time‐lapse seismic monitoring. The recorded wavefields during and before production were greatly different (comparable to the magnitude of the wavefield itself). Moreover, multicomponent measurements may allow for elimination of changes attributable to environmental effects in the overburden and source characteristics. The study also indicates that monitoring the phase change of a reflector below a reservoir may provide a fluid flow indicator. The simulation technique thus provides an important tool for designing downhole surveys and deploying permanent sensors.
Principal domains of the trispectrum, signal bandwidth, and implications for deconvolution
Authors: Pflug Lisa A.
Published: October 2002

Fourth‐order statistics can be useful in many signal processing applications, offering advantages over or supplementing second‐order statistical techniques. One reason is that fourth‐order statistics can discriminate between non‐Gaussian signals and Gaussian noise. Another is that fourth‐order statistics contain phase information, whereas second‐order statistics do not. In the continuing development of the mathematical properties of fourth‐order statistics, several researchers have derived existence conditions and definitions for the unaliased and aliased principal domains of the discrete trispectrum, which is significantly more complex than the power or energy spectrum. The consistencies and inconsistencies of these results are presented and resolved in this paper. The most flexible definitions give four individual principal domains for the discrete trispectrum: two unaliased and two aliased. The most useful combinations are those that combine the two unaliased domains together and the two aliased domains together, which can be done easily from the four individual domains. The relationship between the individual trispectral domains and signal bandwidth is important when using the fourth‐order statistic for applications because they have particular properties that can be detrimental to some deconvolution algorithms. The reasons for this, as well as the validity of proposed solutions to this problem, are explained by the trispectral structure and its origins.
The realizable resistive limit: A new concept for mapping geological features spanning a broad range of conductances
Authors: Smith Richard S.
Published: October 2002

The integral of the step response from zero time to infinite time (the ideal resistive limit) can be used to determine the conductance of the ground, in theory, because the former is directly proportional to the latter. However, in a real time‐domain airborne electromagnetic (AEM) system, it is impossible to measure the step response, or the ideal resistive limit. This is because (1) the off time is finite, being interrupted by the next transmitter pulse; (2) the total effect of all previous transmitter pulses is to reduce the measured response; and (3) the process of removing the primary field during the on time removes a component of the secondary response that has the same shape as the primary response. With a real time‐domain AEM system, it is possible to estimate what is defined as the realizable resistive limit (RRL). The RRL can also be calculated theoretically for a horizontal thin sheet of known conductance. Hence, the measured data can be input into a nonlinear inversion scheme and used to estimate an apparent conductance. RRL is calculated using on‐time data, which is above the noise level between 0.001 S and 100 000 S, so it is possible to map conductances in this eight‐decade range. Traditional methods for deriving conductance use off‐time data only and are restricted to a much smaller range of values (i.e., about two decades). A field example illustrates that, within the resistive areas, the RRL map shows many structural features and lithologies that are not evident on the map of conductance derived using off‐time data. Within the conductive areas, the RRL image shows greater variation; a number of geologically meaningful features are also apparent. Another advantage of RRL images is that artifacts associated with current migration near the edge of conductive features are not as evident as they are in the off‐time‐derived conductance images.
Simulation of anisotropic wave propagation based upon a spectral element method
Authors: Komatitsch Dimitri , Barnes Christophe , Tromp Jeroen
Published: October 2002

We introduce a numerical approach for modeling elastic wave propagation in 2-D and 3-D fully anisotropic media based upon a spectral element method. The technique solves a weak formulation of the wave equation, which is discretized using a high‐order polynomial representation on a finite element mesh. For isotropic media, the spectral element method is known for its high degree of accuracy, its ability to handle complex model geometries, and its low computational cost. We show that the method can be extended to fully anisotropic media. The mass matrix obtained is diagonal by construction, which leads to a very efficient fully explicit solver. We demonstrate the accuracy of the method by comparing it against a known analytical solution for a 2-D transversely isotropic test case, and by comparing its predictions against those based upon a finite difference method for a 2-D heterogeneous, anisotropic medium. We show its generality and its flexibility by modeling wave propagation in a 3-D transversely isotropic medium with a symmetry axis tilted relative to the axes of the grid.
Constrained inversion of seismic refraction data using the controlled random search
Authors: Smith David N. , Ferguson John F.
Published: October 2002

When designing models for 2-D seismic refraction inversion, it is desirable to frame the models as polygonal units with well‐defined geophysical characteristics. Such inversion is strongly nonlinear. Random search methods for nonlinear inversion, such as the genetic algorithm and simulated annealing, have received much attention in recent years. Such methods are well suited when the model space is large and contains multiple global minima to the misfit function. Constraints are necessary in the automatic construction of polygonal models. Controlled random search (CRS) is a member of this class of methods that has not been widely exploited. CRS is an iterative process based on the polytope algorithm, which offers a means by which linear constraints may be used. The CRS samples large model spaces, resists local minima, and can find multiple global minima. A new program has been developed for inversion of 2-D seismic refraction data which incorporates geologic compatibility constraints for polygon‐based models. A theoretical example and practical applications of this technique to environmental investigations illustrate its characteristics.
Rapid VSP-CDP mapping of 3-D VSP data
Authors: Chen Genmeng , Peron Janusz , Canales Luis
Published: October 2002

Vertical seismic profiling‐common depth point (VSPCDP) mapping with rapid ray tracing in a horizontally layered velocity model is used to create 3-D image volumes using Blackfoot and Oseberg 3-D vertical seismic profiling (VSP) data. The ray‐tracing algorithm uses Fermat’s principle and is specially programmed for the layered model. The algorithm is about ten times faster than either a 3-D VSP-CDP mapping program with an eikonal traveltime computation method or a 3-D VSP Kirchhoff migration program. The mapping method automatically separates the image zone from the nonimage zone within the 3-D image volume. The Oseberg data example shows that the lateral extent of the image zone created by the 3D VSP-CDP mapping is larger than that created by 3-D VSP Kirchhoff migration. The same sample result also provides high‐frequency events at target zones. We include an analysis of the imaging error induced from using a horizontally layered model for the Oseberg data, indicating that the method is reliable in the presence of gently dipping structure.
Special Section on mining geophysics—Introduction
Authors: Asten Michael
Published: October 2002

The Exploration 97 Conference held in Toronto in 1997 was a decennial event which provided a forum for review of advances in geophysics applied to mineral exploration and resource evaluation. This Special Section of Geophysics publishes a set of 16 papers, originally presented at that conference, which demonstrate recent advances in mineral exploration and mine geophysics.
Seismic reflection imaging of mineral systems: Three case histories
Authors: Drummond Barry J. , Goleby Bruce R. , Owen A. J. , Yeates A. N. , Swager C. , Zhang Y. , Jackson J. K.
Published: October 2002

Mineral deposits can be described in terms of their mineral systems, i.e., fluid source, migration pathway, and trap. Source regions are difficult to recognize in seismic images. Many orebodies lie on or adjacent to major fault systems, suggesting that the faults acted as fluid migration pathways through the crust. Large faults often have broad internal zones of deformation fabric, which is anisotropic. This, coupled with the metasomatic effects of fluids moving along faults while they are active, can make the faults seismically reflective. For example, major gold deposits in the Archaean Eastern Goldfields province of Western Australia lie in the hanging‐wall block of regional‐scale faults that differ from other nearby faults by being highly reflective and penetrating to greater depths in the lower crust. Coupled thermal, mechanical, and fluid‐flow modeling supports the theory that these faults were fluid migration pathways from the lower to the upper crust. Strong reflections are also recorded from two deeply penetrating faults in the Proterozoic Mt. Isa province in northeastern Australia. Both are closely related spatially to copper and copper‐gold deposits. One, the Adelheid fault, is also adjacent to the large Mt. Isa silver‐lead‐zinc deposit. In contrast, other deeply penetrating faults that are not intrinsically reflective but are mapped in the seismic section on the basis of truncating reflections have no known mineralization. Regional seismic profiles can therefore be applied in the precompetitive area selection stage of exploration. Applying seismic techniques at the orebody scale can be difficult. Orebodies often have complex shapes and reflecting surfaces that are small compared to the diameter of the Fresnel zone for practical seismic frequencies. However, if the structures and alteration haloes around the orebodies themselves, seismic techniques may be more successful. Strong bedding‐parallel reflections were observed from the region of alteration around the Mt. Isa silver‐lead‐zinc orebodies using high‐resolution profiling. In addition, a profile in Tasmania imaged an internally nonreflective bulge within the Que Hellyer volcanics, suggesting a good location to explore for a volcanic hosted massive sulfide deposit. These case studies provide a pointer to how seismic techniques could be applied during mineral exploration, especially at depths greater than those being explored with other techniques.
3-D inversion of induced polarization data
Authors: Li Yaoguo , Oldenburg Douglas W.
Published: October 2002

We present an algorithm for inverting induced polarization (IP) data acquired in a 3-D environment. The algorithm is based upon the linearized equation for the IP response, and the inverse problem is solved by minimizing an objective function of the chargeability model subject to data and bound constraints. The minimization is carried out using an interior‐point method in which the bounds are incorporated by using a logarithmic barrier and the solution of the linear equations is accelerated using wavelet transforms. Inversion of IP data requires knowledge of the background conductivity. We study the effect of different approximations to the background conductivity by comparing IP inversions performed using different conductivity models, including a uniform half‐space and conductivities recovered from one‐pass 3-D inversions, composite 2-D inversions, limited AIM updates, and full 3-D nonlinear inversions of the dc resistivity data. We demonstrate that, when the background conductivity is simple, reasonable IP results are obtainable without using the best conductivity estimate derived from full 3-D inversion of the dc resistivity data. As a final area of investigation, we study the joint use of surface and borehole data to improve the resolution of the recovered chargeability models. We demonstrate that the joint inversion of surface and crosshole data produces chargeability models superior to those obtained from inversions of individual data sets.
Poststack migration deconvolution
Authors: Hu Jianxing , Schuster Gerard T. , Valasek Paul A.
Published: October 2002

Reflectivity images of the earth are calculated by migrating discrete grids of seismic traces. Typically, such traces are spatially undersampled on a recording grid with limited aperture width and so give rise to migration noise sometimes referred to as the acquisition footprint. For poststack migration images, we show how to partly deconvolve the acquisition footprint by applying a deblurring filter to the migration section, where the filter is the approximate inverse to the migration Green’s function. Results with synthetic and field data show that post‐stack migration deconvolution can noticeably improve the spatial resolution of migration images, decrease the strength of migration artifacts, and improve the quality of the migration image. We conclude that migration deconvolution can be a viable alternative to some of the other postmigration processing procedures based on statistics and ad hoc parameter choices.
A MECHANICAL DEVICE FOR COMPUTING SEISMIC PATHS
Authors: Wolf Alexander
Published: October 2002

The paper describes a mechanical device for the computation of the circular seismic paths occurring in a medium in which the velocity increases uniformly in one direction.
THE PERIODIC VARIATIONS OF THE GRAVITATIONAL FORCE. I.
Authors: Lockenvitz Arthur E.
Published: October 2002

The expression for the variation of the gravitational field, at a particular place on the earth’s surface, due to a tide producing body is put into a form which does not neglect terms or factors which will produce changes in the gravitational field of as much as 0.001 milligal. A method of carrying out routine calculations by means of a series of graphs is described.
Extended elastic impedance for fluid and lithology prediction
Authors: Whitcombe David N. , Connolly Patrick A. , Reagan Roger L. , Redshaw Terry C.
Published: October 2002

Constant angle projections of seismic sections can be designed to provide maximum discrimination between fluids or lithologies. The optimum projection for a noise‐free, isotropic environment can be obtained using an extension to the elastic impedance concept, which itself is an extension of acoustic impedance (AI) to nonzero angles of incidence. To achieve this, we modify the definition of elastic impedance (EI) beyond the range of physically meaningful angles by substituting tanχ for sin2 θ in the two‐term reflectivity equation. The primary variable now becomes χ rather than θ. We allow it to vary between −90° and +90°, which gives an extension of EI for any combination of intercept and gradient. We refer to this form of elastic impedance as extended elastic impedance (EEI).

In this paper we demonstrate that EEI can be tuned using different χ values to be approximately proportional to a number of elastic parameters, and we give EEI expressions for shear impedance (SI), bulk modulus, shear modulus, Lamé's parameter, and Vp/Vs. This leads to the identification of different areas of EEI space that tend to be optimum for fluid and lithology imaging. Having identified an appropriate χ value, the equivalent seismic section can be obtained from combinations of intercept and gradient stacks from routine AVO processing.
Kirchhoff image propagation
Authors: Adler Frank
Published: October 2002

Seismic imaging processes are, in general, formulated under the assumption of a correct macrovelocity model. However, seismic subsurface images are very sensitive to the accuracy of the macrovelocity model. This paper investigates how the output of Kirchhoff inversion/migration changes for perturbations of a given 3‐D laterally inhomogeneous macrovelocity model. The displacement of a reflector image point from a perturbation of the given velocity model is determined in a first‐order approximation by the corresponding traveltime and slowness perturbations as well as the matrix. of the Beylkin determinant. The required traveltime derivatives can be calculated with ray perturbation theory.

Using this result, a new, computationally efficient Kirchhoff inversion/migration technique is developed to predict in parallel a series of subsurface images for perturbations of a given macrovelocity model during a single inversion/migration process applied to the unmigrated seismic data. These images are constructed by superposition of the seismic data at predicted image point locations which lie on surfaces that expand from the initial image point as a function of the velocity perturbation. Because of the analogy to Huygens wavefronts in wave propagation, the technique is called Kirchhoff image propagation.

A 2‐D implementation of Kirchhoff image propagation requires about 1.2 times the computation time of a single migration to calculate a set of propagated images. The propagated images provide good approximations to remigrated images and are applied to migration velocity analysis.
Successful production application of 3‐D amplitude variation with offset: The lessons learned
Authors: Soroka William L. , Fitch Thomas J. , Van Sickle Kirk H. , North Philip D.
Published: October 2002

Amplitude variation with offset (AVO) analysis was successfully performed on a 3‐D prestack seismic volume. Important conclusions were that AVO results could improve field development and production, that 3‐D AVO results were more useful than 2‐D AVO results, and that reliable AVO results could be generated on land.

The AVO results were used to help develop an infill drilling program to increase production. AVO information lowered the risk of finding hydrocarbons by helping to identify seismic events that had a higher probability of being gas‐saturated sands. The 3‐D seismic survey covered known gas zones and potential new reserves. The AVO calibration work showed that positive AVO gas responses (classes 2 and 3) were observed for 90% of the zones associated with known production. One 15‐ft‐thick gas reservoir below seismic resolution did not give a positive AVO anomaly. A well drilled to an untested zone displaying a positive AVO anomaly encountered commercial quantities of gas. Production from this new zone at the initial flow rate increased the total production rate in this 25‐year‐old field by >50%.

The AVO method was shown to be applicable onshore and to provide useful results in more consolidated geologic environments with classes 2 and 3 AVO responses. For the successful use of AVO, greater effort and extra care in acquisition and processing were needed than in a normal seismic program.
A generic 1-D imaging method for transient electromagnetic data
Authors: Christensen Niels Bøie
Published: October 2002

This paper presents a fast approximate 1-D inversion algorithm for transient electromagnetic (EM) data that can be applied for all measuring configurationsand transmitter waveforms and for all field components. The inversion is based on an approximate forward mapping in the adaptive Born approximation. The generality is obtained through a separation of the forward problem into a configuration-independent part, mapping layer conductivities into apparent conductivity, and a configuration-dependent part, the half-space step response. The EM response from any waveform can then be found by a convolution with the time derivative of the waveform. The approach does not involve inherently unstable deconvolution computations or nonunique transformations, and it is about 100 times faster than ordinary nonlinear inversion. Nonlinear model responses of the models obtained through the approximate inversion fit the data typically within 5%.
Accurate interpolation with high‐resolution time‐variant Radon transforms
Authors: Trad Daniel O. , Ulrych Tadeusz J. , Sacchi Mauricio D.
Published: October 2002

It is well known that a sparse hyperbolic Radon transform (RT) can be used to extend the aperture of aperture limited data, filter noise, and fill gaps. In the same manner, an elliptical RT can achieve similar results when applied to slant stack sections. A problem with these transformations is that they have a time‐variant kernel that results in slow implementation. By defining a model space in terms of an irregularly sampled velocity space to minimize the number of unknowns during the inversion and using sparse matrices, however, the computation time can be kept low enough for practical application. We implement hyperbolic and elliptical time domain RTs by using inversion via weighted conjugate gradient methods with a sparseness constraint. The hyperbolic RT performs accurate interpolation in common‐midpoint (CMP) gathers, while the elliptical RT attenuates sampling artifacts in slant stack sections obtained from CMP gathers with poor sampling and gaps.
Discussions of “An Evaluation of Basement Depth Determination from Airborne Magnetometer Data,” by Peter Jacobsen, Jr.
Authors: Bean R. J. , Fillippone Walter R. , Paterson Norman R. , Zietz Isidore
Published: October 2002

In his discussion of the magnetic interpretations, Mr. Jacobsen rightly distinguishes between the determination of basement depth and configuration by analysis of anomalies originating from magnetization contrast within the basement and the delineation of local relief or faulting at the basement surface by analysis of smaller anomaly trends. It cannot be emphasized too strongly that the principal purpose of conducting aeromagnetic surveys is to outline the extent and depth of sedimentary basins, and the calculation of depth to basement by quantitative analysis of anomalies has progressed to the point where reliable results can be obtained by skilled interpreters.
Reply by Alan T. Herring to discussion by Lewis R. Tucker
Authors: Herring Alan T.
Published: October 2002

The focal point of Mr. Tucker’s criticism is a feeling that we have not properly identified our assumptions or given the probable uncertainties of our interpretation proper attention. I disagree; over 75 percent of the text of our note is devoted to the discussion of probable sources of error. Mr. Tucker’s discussion brings to light no source of error which has not been mentioned already in our note. Mr. Tucker is quite right to point out, as we have, the assumptions and uncertainties inherent in any geophysical interpretation. The determination of a regional gravity field is indeed a critical step, but as we have pointed out, a step which can be constrained by geologic knowledge and reasonable geologic assumptions.
Reply by authors to discussion by Lloyd S. Grearson
Authors: Lafehr T. R. , Nettleton L. L.
Published: October 2002

Grearson is apparently confusing relative and absolute accuracy. Our purpose, as stated in the third paragraph on p. 116, was to determine the ability of the platform meter to measure relative gravity effects, which are the important product of gravity exploration work. Absolute gravity accuracy is a function of the reference values and their accuracies as well as instrument performance.
Reply by the authors to the Discussion by B. Biswas
Authors: Davis T. L. , Jackson G. M.
Published: October 2002

28 Hz geophones without a low‐cut filter provided a very similar amplitude (and phase) response to the 10 Hz geophones combined with a 25 Hz low‐cut filter. Combining 28 Hz geophones with a 15 or 20 Hz low‐cut filter would produce a record intermediate between Figure 4b and c. There is, however, a tradeoff between low‐frequency noise attenuation and the bandwidth of the seismic wavelet. Before stacking and deconvolution, the more severe low‐cut filtering produces dramatic noise reduction (Figure 4). After deconvolution and stacking, this improvement is much less dramatic. It was decided not to attenuate frequencies in the 10 to 25 Hz range too severely as this could decrease the signal bandwidth and provide a more “ringy,” if marginally cleaner, section.
3‐D electromagnetic anisotropy modeling using finite differences
Authors: Wang Tsili , Fang Sheng
Published: October 2002

Electric anisotropy is considered an important property of hydrocarbon reservoirs. Its occurrence has great influence on estimation of formation water saturation and other properties derived from electromagnetic (EM) measurements. Conventional tools using coaxial coils often underestimate formation resistivity and thus overestimate water saturation. Multicomponent EM sensors provide the additional information needed for better resistivity‐based formation evaluation. We have developed a finite‐difference method to simulate multicomponent EM tools in a 3‐D anisotropic formation. The new method can model inhomogeneous media with arbitrary anisotropy. By using the coupled Maxwell’s equations, our method consumes about the same computational time to model an anisotropic formation as it would take to model an otherwise isotropic formation. We have verified the finite‐difference method using layered‐earth models that are typically encountered in hydrocarbon exploration and development. Our results show that the newly developed simulation algorithm produces accurate results for different borehole and formation conditions.
Reply to A. R. Brown by the author
Authors: Maxant J.
Published: October 2002

I thank Alan R. Brown for his comments, although I was surprised by the nature of his note. Under the section titled “Variation of density with formation,” I compared density maps to corresponding lithofacies maps compiled by McCrossan and Glaister (1964). Since some of the latter (reproduced in Figures 7–12 of my paper) showed the presence of large oil and gas fields covering tens or hundreds of square miles, it would have been “illogical” to ignore any possible relationship between these fields and density.
Reply by the authors to the discussion by M. Poddar
Authors: Pellerin L. , Johnston J. M.
Published: October 2002

We have reviewed the comments and model results from M. Poddar and find both interesting. The differences in model geometry between the models used in his and our studies are significant. The lateral extent of both components of the anomalous structure, the reservoir and the cap, are much greater in our model in both absolute terms and the relative size of the cap to the reservoir; the anomalous bodies in our model are half again as deep as those in Poddar’s model while the ratio in lateral extent between the caps of the two models is close to ten. The anomalous bodies of Poddar’s model reside in a layered host where ours are hosted by a uniform half space. In addition to these model differences we have used different algorithms to simulate the earth’s response. It is not surprising that there are differences in the responses of the models. Because of the differences in model structure we do not believe that electromagnetic scaling is a basis for discounting differences between the models.
Reply by the authors to the discussion by A. J. Calvert
Authors: Calvert A. J.
Published: October 2002

The authors of “Suppression of sea‐floor‐scattered energy using a dip‐moveout approach—Application to the mid‐ocean ridge environment” chose not to reply to the discussion by.
Reservoir characterization using seismic waveform and feedforword neural networks
Authors: An P. , Moon W. M. , Kalantzis F.
Published: October 2002

Feedforward neural networks are used to estimate reservoir properties. The neural networks are trained with known reservoir properties from well log data and seismic waveforms at well locations. The trained neural networks are then applied to the whole seismic survey to generate a map of the predicted reservoir property. Both theoretical analysis and testing with synthetic models show that the neural networks are adaptive to coherent noise and that random noise in the training samples may increase the robustness of the trained neural networks. This approach was applied to a mature oil field to explore for Devonian reef‐edge oil by estimating the product of porosity and net pay thickness in northern Alberta, Canada. The resulting prediction map was used to select new well locations and design horizontal well trajectories. Four wells were drilled based on the prediction; all were successful. This increased production of the oil field by about 20%.
Empirical estimation of viscoelastic seismic parameters from petrophysical properties of sandstone
Authors: Koesoemadinata Adam P. , McMechan George A.
Published: October 2002

Viscoelastic seismic parameters are expressions of underlying petrophysical properties. Theoretical and empirically derived petrophysical/seismic relations exist, but each is limited in the number and the range of values of the variables used. To provide a more comprehensive empirical model, we combined lab measurements from 18 published data sets and well log data for sandstone samples, and determined least‐squares coefficients across them all. The dependent variables are the seismic parameters of bulk density (ρ), compressional and shear wave velocities ([Formula: see text] and [Formula: see text]), and compressional and shear wave quality factors ([Formula: see text] and [Formula: see text]). The independent variables are effective pressure, porosity, clay content, water saturation, permeability, and frequency. As the derived expressions are empirical correlations, no causal relations should be inferred. Prediction of ρ is based on volumetric mixing of the constituents. For [Formula: see text] and [Formula: see text] predictions, separate sets of coefficients are fitted for three water saturation conditions: dry, partially saturated, and fully saturated. Predictions of [Formula: see text] and [Formula: see text] are fitted as functions of porosity, clay content, effective pressure, saturation, and frequency. Predictions of [Formula: see text] are fitted as a function of porosity, clay content, permeability, saturation, frequency, and pressure. Interactions between effective pressure, saturation, and frequency are included. Predictions of [Formula: see text] are obtained from [Formula: see text] and [Formula: see text]. The result is a composite model that is more comprehensive than previous models and that predicts seismic properties from the petrophysical properties. Empirically estimated values of ρ, [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] for the composite data over all saturations predict the measurements with correlation coefficients [Formula: see text] that range from a low of 0.65 (for [Formula: see text],) to a high of 0.90 (for [Formula: see text]). As the fitted relations have been derived from data with limited parameter ranges, extrapolation is not advised, and they are not intended to substitute for locally derived relations based on site‐specific data. Nevertheless, the derived expressions produce representative values that will be useful when approximate, internally consistent predictions are sufficient. Potential future applications include building of seismic reservoir models from petrophysical data and analysis of the sensitivity of seismic data to changes in reservoir properties.