bluuebunny/crossref_metadata_2025 huggingface dataset:
3-D seismic modeling for cracked media: Shear‐wave splitting at zero‐offset
Authors: Ramos‐Martínez Jaime , Ortega Andrey A. , McMechan George A.
Published: October 2002

Splitting of zero‐offset reflected shear‐waves is measured directly from three‐component finite‐difference synthetic seismograms for media with intersecting vertical crack systems. Splitting is simulated numerically (by finite differencing) as a function of crack density, aspect ratio, fluid content, bulk density, and the angle between the crack systems. The type of anisotropy symmetry in media containing two intersecting vertical crack systems depends on the angular relation between the cracks and their relative crack densities, and it may be horizontal transverse isotropy (HTI), tetragonal, orthorhombic, or monoclinic. The transition from one symmetry to another is visible in the splitting behavior. The polarities of the reflected quasi‐shear waves polarized perpendicular and parallel to the source particle motion distinguish between HTI and orthorhombic media. The dependence of the measured amount of splitting on crack density for HTI symmetry is consistent with that predicted theoretically by the shear‐wave splitting factor. In orthorhombic media (with two orthogonal crack systems), a linear increase is observed in splitting when the difference between crack densities of the two orthogonal crack systems increases. Splitting decreases nonlinearly with the intersection angle between the two crack systems from 0° to 90°. Surface and VSP seismograms are simulated for a model with several flat homogeneous layers, each containing vertical cracks with the same and with different orientations. When the crack orientation varies with depth, previously split shear waves are split again at each interface, leading to complicated records, even for simple models. Isotropic and anisotropic three‐component S-wave zero‐offset sections are synthesized for a zero‐offset survey line over a 2.5-D model of a carbonate reservoir with a complicated geometry and two intersecting, dipping crack sets. The polarization direction of the fast shear wave, propagating obliquely through the cracked reservoir, is predicted by theoretical approximations for effective properties of anisotropic media with two nonorthogonal intersecting crack sets.
Dip selective 2-D multiple attenuation in the plane‐wave domain
Authors: Liu Faqi , Sen Mrinal K. , Stoffa Paul L.
Published: October 2002

In many geological settings, strong reflections at the air‐water interface contribute to most of the multiple energy in the recorded seismograms. Here, we describe a method for free‐surface multiple attenuation using a reflection operator model of a seismic record, derived using the well‐known invariant embedding technique. We implement this method in the 2-D plane‐wave domain, where lateral variation of the geological structure of the earth is taken into account by the coupling of different ray parameters. In situations where the lateral variations are smooth, the data are well compressed in the 2-D plane‐wave domain and the resultant bandlimited matrices significantly reduce the computation cost. One important feature of the proposed method is its flexibility, which allows for the removal of multiples from selected reflections. To generate multiple free data, wave‐theory‐based multiple attenuation methods attempt to estimate either the source function or the subsurface reflectivity. Our method takes advantage of both approaches, such that we initially predict multiple traveltime using the reflectivity approach and then seek a source function to predict the amplitudes. Synthetic and real data examples show that this method is stable and successful in attenuating the surface multiples.
Guidelines for building a detailed elastic depth model
Authors: Dunne Jarrod C. , Beresford Greg , Kennett Brian L. N
Published: October 2002

We developed guidelines for building a detailed elastic depth model by using an elastic synthetic seismogram that matched both prestack and stacked marine seismic data from the Gippsland Basin (Australia). Recomputing this synthetic for systematic variations upon the depth model provided insight into how each part of the model affected the synthetic. This led to the identification of parameters in the depth model that have only a minor influence upon the synthetic and suggested methods for estimating the parameters that are important. The depth coverage of the logging run is of prime importance because highly reflective layering in the overburden can generate noise events that interfere with deeper events. A depth sampling interval of 1 m for the P-wave velocity model is a useful lower limit for modeling the transmission response and thus maintaining accuracy in the tie over a large time interval. The sea‐floor model has a strong influence on mode conversion and surface multiples and can be built using a checkshot survey or by testing different trend curves. When an S-wave velocity log is unavailable, it can be replaced using the P-wave velocity model and estimates of the Poisson ratio for each significant geological formation. Missing densities can be replaced using Gardner’s equation, although separate substitutions are required for layers known to have exceptionally high or low densities. Linear events in the elastic synthetic are sensitive to the choice of inelastic attenuation values in the water layer and sea‐floor sediments, while a simple inelastic attenuation model for the consolidated sediments is often adequate. The usefulness of a 1-D depth model is limited by misties resulting from complex 3-D structures and the validity of the measurements obtained in the logging run. The importance of such mis‐ties can be judged, and allowed for in an interpretation, by recomputing the elastic synthetic after perturbing the depth model to simulate the key uncertainties. Taking the next step beyond using simplistic modeling techniques requires extra effort to achieve a satisfactory tie to each part of a prestack seismic record. This is rewarded by the greater confidence that can then be held in the stacked synthetic tie and applications such as noise identification, data processing benchmarking, AVO analysis, and inversion.
Gravity inversion by means of growing bodies
Authors: Camacho Antonio G. , Montesinos Fuensanta G. , Vieira Ricardo
Published: October 2002

This paper presents a gravity inversion method for determining the volumes of bodies with pre‐established density contrasts. The method works step‐by‐step on a prismatic partition of the subsurface volume, expanding the anomalous bodies to fit the observed gravity values in a systematic exploration of model possibilities. The process is treated in a 3-D context; at the same time, it can determine a simple regional trend. Moreover, positive and negative density contrasts are simultaneously accepted. The solution is obtained by a double condition: (1) the 𝓁2-fitness to the observed gravity data (model fitness) and (2) the minimization of the total (weighted) anomalous mass (model smoothness). A positive parameter is used to balance the two minimization terms. The method is applied to a simulated example and also to a real example: the volcanic island of Gran Canaria (Canary Islands, Spain). In both cases, the results obtained show the possibilities of the method.
Fault whispers: Transmission distortions on prestack seismic reflection data
Authors: Hatchell Paul J.
Published: October 2002

Transmission distortions are observed on prestack seismic data at two locations in the Gulf of Mexico. These distortions produce anomalous amplitude versus offset (AVO) signatures. The locations of the distortion zones are determined using acquisition geometry and ray tracing. No obvious reflection events, such as shallow gas zones, are observed at the predicted locations of the distortion zones. Instead, the distortion zones correlate with buried faults and unconformities. It is postulated that the distortions are produced by velocity changes across buried faults and unconformities. The distortions result from an interference pattern resulting from seismic waves arriving from different sides of the faults. A simple model is developed to explain many of the characteristics of the distortion pattern.
Phase encoding of shot records in prestack migration
Authors: Romero Louis A. , Ghiglia Dennis C. , Ober Curtis C. , Morton Scott A.
Published: October 2002

Frequency‐domain shot‐record migration can produce higher quality images than Kirchhoff migration but typically at a greater cost. The computing cost of shot‐record migration is the product of the number of shots in the survey and the expense of each individual migration. Many attempts to reduce this cost have focused on the speed of the individual migrations, trying to achieve a better trade‐off between accuracy and speed. Another approach is to reduce the number of migrations. We investigate the simultaneous migration of shot records using frequency‐domain shot‐record migration algorithms. The difficulty with this approach is the production of so‐called crossterms between unrelated shot and receiver wavefields, which generate unwanted artifacts or noise in the final image. To reduce these artifacts and obtain an image comparable in quality to the single‐shot‐per‐migration result, we have introduced a process called phase encoding, which shifts or disperses these crossterms. The process of phase encoding thus allows one to trade S/N ratio for the speed of migrating the entire survey. Several encoding functions and two application strategies have been tested. The first strategy, combining multiple shots per migration and using each shot only once, reduces computation in direct relation to the number of shots combined. The second strategy, performing multiple migrations of all the shots in the survey, provides a means to reduce the crossterm noise by stacking the resulting images. The additional noise in both strategies may be tolerated if it is no stronger than the inherent seismic noise in the migrated image and if the final image is achieved with less cost.
Euler deconvolution of gravity tensor gradient data
Authors: Zhang Changyou , Mushayandebvu Martin F. , Reid Alan B. , Fairhead J. Derek , Odegard Mark E.
Published: October 2002

Tensor Euler deconvolution has been developed to help interpret gravity tensor gradient data in terms of 3-D subsurface geological structure. Two forms of Euler deconvolution have been used in this study: conventional Euler deconvolution using three gradients of the vertical component of the gravity vector and tensor Euler deconvolution using all tensor gradients. These methods have been tested on point, prism, and cylindrical mass models using line and gridded data forms. The methods were then applied to measured gravity tensor gradient data for the Eugene Island area of the Gulf of Mexico using gridded and ungridded data forms. The results from the model and measured data show significantly improved performance of the tensor Euler deconvolution method, which exploits all measured tensor gradients and hence provides additional constraints on the Euler solutions.
Effects of the equatorial electrojet on aeromagnetic data acquisition
Authors: Rigoti Augustinho , Padilha Antonio L. , Chamalaun F. H. , Trivedi Nalin B.
Published: October 2002

In recent years, considerable advances have taken place in aeromagnetic surveying. These improvements involved data acquisition (instruments and survey design), processing, and interpretation. In addition to improved spatial resolution, the high‐resolution aeromagnetics, as applied to oil exploration, attempts to resolve very low amplitude (1 nT or even subnanotesla) magnetic features (Paterson and Reeves, 1985). These features are caused by weak intra‐sedimentary magnetic sources of magnetite and pyrrhotite, which could have been formed as a result of hydrocarbon seepage (e.g., Reynolds et al., 1990, 1991). For such small spatial variations to be meaningful, it is required that similar temporal and spatial variations due to external sources be corrected accordingly.
High‐resolution 3-D seismic survey over a coal mine reserve area in the U.S.—A case study
Authors: Gochioco Lawrence M.
Published: October 2002

A high‐resolution three‐dimensional (3-D) seismic survey was conducted in advance of coal mine development in the Illinois basin in May 1989 to better define a geologic structure with the potential to adversely affect longwall mining conditions. The 3-D seismic data indicate that an abrupt change in seam elevation, or roll, encountered near the northern property line trends south into the reserve area and then turns southeast. A personal computer‐based workstation was used to integrate borehole and seismic data for modeling in which 3-D block diagrams of the calculated seam elevations were generated. The block diagrams show a steep slope on the west flank of the roll that gradually decreases as the roll turns to the southeast. The survey also reveals a geologic structure beneath the roll at an estimated depth of 46–62 m. Horizontal time‐slice sections of this feature suggest the presence of a paleochannel that meanders on a similar course as the roll, which apparently was connected to a larger paleochannel system. A Conoco high‐frequency vibroseis unit was successfully used as the seismic source to generate the high frequencies necessary to detect and resolve the thin coal beds.
An efficient method for calculating finite‐difference seismograms after model alterations
Authors: Robertsson Johan O. A. , Chapman Chris H.
Published: October 2002

Seismic modeling, processing, and inversion often require the calculation of the seismic response resulting from a suite of closely related seismic models. Even though changes to the model may be restricted to a small subvolume, we need to perform simulations for the full model. We present a new finite‐difference method that circumvents the need to resimulate the complete model for local changes. By requiring only calculations in the subvolume and its neighborhood, our method makes possible significant reductions in computational cost and memory requirements. In general, each source/receiver location requires one full simulation on the complete model. Following these pre‐computations, recalculation of the altered wavefield can be limited to the region around the subvolume and its neighborhood. We apply our method to a 2-D time‐lapse seismic problem, thereby achieving a factor of 15 reduction in computational cost. Potential savings for 3-D are far greater.
Electrical properties of sedimentary rocks having interconnected water‐saturated pore spaces
Authors: Chinh Pham Duc
Published: October 2002

Permeable sedimentary rocks can often be modeled as an impermeable rock matrix cut by a system of an irregular system of interconnected, irregularly shaped, water‐saturated pore spaces. I represent this system by a multiphase effective medium that is compatible with Archie’s Law for electrical conductivity. My effective medium is an extention of the self‐similar Sen, Scalar, and Cohen model which characterizes sedimentary rocks as a water suspension of spherical solid grains. My generalized multiphase model includes two important components: open water spherelike pockets, which significantly increase the porosity but add little to the electrical conductivity, and thin films surrounding the grains and water‐filled cracks, which contribute little to the porosity but significantly to the electrical conductivity. By perturbing the relative balance between these two model components, I am able to represent a range of aggregates for which I can construct effective media that are consistent with the electrical conductivity predicted by Archie’s Law.
Understanding LOTEM data from mountainous terrain
Authors: Hördt Andreas , Müller Martin
Published: October 2002

Long‐offset transient electromagnetic (LOTEM) data from the Vesuvius volcano, in Italy, show that the EM response of the topography is a potential cause of data distortions. A modeling study was carried out to simulate the effect of mountainous terrain on vertical magnetic‐field time derivatives using a 3-D finite‐difference code. The objectives were to assess the importance of topographic effects and to help identify them in existing field data. The total effect of topography on the LOTEM response can be considered as a combination of four distortions of the corresponding responses for a flat terrain. First, the receiver is at some height above the flat surface. Second, the mountain acts as a conductive body displacing air. Third, large loop receivers are nonhorizontal and sense a combination of horizontal and vertical magnetic fields. Finally, the electromagnetic coupling between the mountain and deeper‐lying structure modifies the structure response. Each of the effects can be identified in field data recorded at Mount Vesuvius. The topographic induced distortions for the model used in this study are moderate in the sense that 1-D inversions of the theoretical data still recover the gross conductivity structure, albeit with small deviations from the true parameters. Although this result might imply that topography might be ignored during the first stage of an interpretation, no simple correction method is evident, so topography will have to be included in any 2-D or 3-D inversion attempt.
Prestack phase‐shift migration of separate offsets
Authors: Alkhalifah Tariq
Published: October 2002

Prestack phase‐shift migration is implemented by evaluating the offset‐wavenumber ([Formula: see text]) integral using the stationary‐phase method. Thus, the stationary point along [Formula: see text] must be calculated prior to applying the phase shift. This type of implementation allows for migration of separate offsets, as opposed to migration of the whole prestack data when using the original formulas. For zero‐offset data, the stationary point ([Formula: see text]) is known in advance, and, therefore, the phase‐shift migration can be implemented directly. For nonzero‐offset data, we first evaluate the [Formula: see text] that corresponds to the stationary point solution either numerically or through analytical approximations. The insensitivity of the phase to [Formula: see text] around the stationary point solution (its maximum) implies that even an imperfect [Formula: see text] obtained analytically can go a long way to getting an accurate image. In transversely isotropic media, the analytical solutions of the stationary point ([Formula: see text]) include more approximations than those corresponding to isotropic media (i.e., approximations corresponding to weaker anisotropy). Nevertheless, the resultant equations, obtained using Shanks transform and perturbation theory, produce accurate migration signatures for strong anisotropy (η ≈ 0.3) and even large offset‐to‐depth ratios (>2). The analytical solutions are particularly accurate in predicting the nonhyperbolic moveout behavior associated with anisotropic media, a key ingredient to performing an accurate nonhyperbolic moveout inversion for strongly anisotropic media. Although the prestack correction achieved using the phase‐shift method can also be obtained using a cascade of NMO correction, dip‐moveout (DMO) correction, and zero‐offset time migration, the prestack approach can handle sharper velocity models more efficiently. In addition, the resulting operator is sharper than that obtained from the DMO method. Synthetic, including the Marmousi, data applications of the proposed prestack migration demonstrate its usefulness.
An overlooked aspect of the parabolic Radon transform
Authors: Maeland Einar
Published: October 2002

Linear integral transforms play an important role in many fields in physics. A transform of this kind is given by the general formula.
Modulus—porosity relations, Gassmann’s equations, and the low‐frequency elastic‐wave response to fluids
Authors: Nolen‐Hoeksema Richard C.
Published: October 2002

Gassmann’s equations relate the low‐frequency drained and undrained elastic‐wave response to fluids. This tutorial explores how different modulus—porosity relationships affect predictions of the low‐frequency elastic‐wave response to fluids based on Gassmann’s equations. I take different modulus—porosity relations and substitute them into Gassmann’s equations through the framework moduli. The results illustrate the range of responses to fluids and can be summarized in a nomograph of the effective fluid coefficient, which quantifies the change in the pore‐space modulus ([Formula: see text]) in response to a change in fluid modulus ([Formula: see text]). Two ratios control the effective fluid coefficient: the ratio of the fluid modulus to the solid‐grain modulus ( [Formula: see text]) and the ratio of the Biot coefficient to porosity ([Formula: see text]). The effective fluid coefficient nomograph is a convenient tool for estimating how low‐frequency elastic‐wave properties will respond to changes in reservoir fluids.
Seismic and potential‐field imaging of the Guichon Creek batholith, British Columbia, Canada, to delineate structures hosting porphyry copper deposits
Authors: Roy Baishali , Clowes Ron M.
Published: October 2002

The Guichon Creek batholith (GCB), located in south‐central British Columbia, contains several large, low‐grade copper deposits of considerable economic importance. The surface geology of the Guichon batholith and its surrounding region have been well mapped; however, little information about subsurface features is available. The batholith consists of four major phases, emplaced radially outward, which can be separated on the basis of their texture and composition. Previous interpretation of gravity data suggests a mushroom‐shaped structure for the batholith. Data from Lithoprobe seismic reflection line 88-11, acquired across the batholith in 1988, reveal weakly coherent east‐dipping reflections on the west side and west‐dipping reflections on the east in the upper 10 km. To determine if these are related to structures associated with the batholith, we reprocessed the upper 6 s with particular emphasis on applications of signal enhancement techniques (e.g., pattern recognition methods, refraction statics, dip moveout corrections) and correlation of the improved subsurface images with the geological environment associated with porphyry copper deposits. Low near‐surface velocities correlate well with the phases of the batholith hosting the major copper deposits, which structurally lie in faulted and brecciated regions. Although the top 1.5 km cannot be imaged by the regional‐scale seismic reflection data, the reprocessed seismic section helps define the edges of the batholith, its various concentric phases, and the stem in the depth range of 1.5 to 10 km. The seismic results are complemented by 2.5-D (profile sense) modeling and 3-D inversion of regional‐scale gravity and high‐resolution aeromagnetic data. These show a low‐density and low‐magnetic‐susceptibility region associated with the batholith that extends to more than 10 km depth. The region of active mining interest lies above a circular low‐susceptibility area at 2 km depth and a low‐velocity region. Integrated interpretation of geophysical results and geological observations indicates the GCB is a funnel‐shaped feature in which mineralization is located above the stem of the batholith.
Interpretation of reflection attributes in a 3-D GPR survey at Vallée d’Ossau, western Pyrenees, France
Authors: Sénéchal Pascale , Perroud Hervé , Sénéchal Guy
Published: October 2002

To study the attributes of reflected radar waves, a 3-D ground‐penetrating radar (GPR) data set was recorded, processed with ProMAX 3-D seismic processing software, and analyzed with Stratimagic 3-D seismic interpretation software. The zone studied was an old glacial valley (Vallée d’Ossau, western Pyrenees) where fluvio‐glacial deposits (8–10 m thick) overlay a karstic limestone bedrock. Reflections were picked to 250 ns (9 m depth with velocity υ = 0.07 m/ns). Analysis of the isochron map of the bedrock surface reflector showed north‐south undulations, corresponding to the displacement of the old glacier, together with sharp discontinuities, interpreted as local faults or karstic zones. Amplitude variations along the reflector were interpreted as being from water saturation and the nearby north‐south alignment of springs at the surface. In the fluvio‐glacial deposits, paths of old channels were also imaged using amplitude analysis. The radar‐wave amplitude attenuation was also characterized within the deposits by analyzing the average absolute amplitude for different time windows. The strong amplitude attenuations were interpreted as the result of water‐saturated zones. As a conclusion, we correlated the surface karstic springs with bedrock structures and followed the water path across the glacial sedimentary filling. The recognized features were very consistent with a 2-D dc resistivity profile made through the GPR data block.
Application of gravity and magnetic methods to assess geological hazards and natural resource potential in the Mosida Hills, Utah County, Utah
Authors: Benson Alvin K. , Floyd Andrew R.
Published: October 2002

Gravity and magnetic data were collected in the Mosida Hills, Utah County, Utah, at over 1100 stations covering an area of approximately 58 km2(150 mi2) in order to help define the subsurface geology and assess potential geological hazards for urban planning in an area where the population is rapidly increasing. In addition, potential hydrocarbon traps and mineral ore bodies may be associated with some of the interpreted subsurface structures. Standard processing techniques were applied to the data to remove known variations unrelated to the geology of the area. The residual data were used to generate gravity and magnetic contour maps, isometric projections, profiles, and subsurface models. Ambiguities in the geological models were reduced by (1) incorporating data from previous geophysical surveys, surface mapping, and aeromagnetic data, (2) integrating the gravity and magnetic data from our survey, and (3) correlating the modeled cross sections. Gravity highs and coincident magnetic highs delineate mafic lava flows, gravity lows and magnetic highs reflect tuffs, and gravity highs and magnetic lows spatially correlate with carbonates. These correlations help identify the subsurface geology and lead to new insights about the formation of the associated valleys. At least eight new faults (or fault segments) were identified from the gravity data, whereas the magnetic data indicate the existence of at least three concealed and/or poorly exposed igneous bodies, as well as a large ash‐flow tuff. The presence of low‐angle faults suggests that folding or downwarping, in addition to faulting, played a role in the formation of the valleys in the Mosida Hills area. The interpreted location and nature of concealed faults and volcanic flows in the Mosida Hills area are being used by policy makers to help develop mitigation procedures to protect life and property.
An approach to upscaling for seismic waves in statistically isotropic heterogeneous elastic media
Authors: Gold N. , Shapiro S. A. , Bojinski S. , Müller T. M.
Published: October 2002

For complex heterogeneous models, smoothing is often required for imaging or forward modeling, e.g., ray tracing. Common smoothing methods average the slowness or squared slowness. However, these methods are unable to account for the difference between scattering caused by fluctuations of the different elastic moduli and density. Here, we derive a new smoothing method that properly accounts for all of the parameters of an isotropic elastic medium. We treat the geophysical problem of optimum smoothing of heterogeneous elastic media as a problem of moving‐window upscaling of elastic media. In seismology, upscaling a volume of a heterogeneous medium means replacing it with a volume of a homogeneous medium. In the low‐frequency limit, this replacement should leave the propagating seismic wavefield approximately unchanged. A rigorous approach to upscaling is given by homogenization theory. For randomly heterogeneous models, it is possible to reduce the problem of homogenization to calculating the coherent wavefield (mean field) in the low‐frequency limit. After deriving analytical expressions for the coherent wavefield in weakly heterogeneous and statistically isotropic random media, we obtain a smoothing algorithm. We apply this algorithm to random media and to deterministic models. The smoothing algorithm is frequency dependent, i.e., for different dominant frequencies, different smooth versions of the same medium should be considered. Several numerical examples using finite differences demonstrate the advantages of our approach over common smoothing schemes. In addition, using a numerical eikonal equation solver we show that, in the case of complex heterogeneous media, appropriate initial smoothing is important for high‐frequency modeling.
Development of 3-D seismic exploration technology for deep nickel‐copper deposits—A case history from the Sudbury basin, Canada
Authors: Milkereit Bernd , Berrer E. K. , King Alan R. , Watts Anthony H. , Roberts B. , Adam Erick , Eaton David W. , Wu Jianjun , Salisbury Matthew H.
Published: October 2002

Following extensive petrophysical studies and presite surveys, the Trill area of the Sudbury basin was selected for conducting the first 3-D seismic survey for mineral exploration in North America. The 3-D seismic experiment confirms that in a geological setting such as the Sudbury Igneous Complex, massive sulfide bodies cause a characteristic seismic scattering response. This provides an excellent basis for the direct detection of massive sulfides by seismic methods. The feasibility study suggests that high‐resolution seismic methods offer a large detection radius in the order of hundreds to thousands of meters, together with accurate depth estimates.
Reservoir geophysics
Authors: Pennington Wayne D.
Published: October 2002

The concept of petroleum reservoir geophysics is relatively new. In the past, the role of geophysics was largely confined to exploration and, to a lesser degree, the development of discoveries. As cost‐efficiency has taken over as a driving force in the economics of the oil and gas industry and as major assets near abandonment, geophysics has increasingly been recognized as a tool for improving the bottom line closer to the wellhead. The reliability of geophysical surveys, particularly seismic, has greatly reduced the risk associated with drilling wells in existing fields, and the ability to add geophysical constraints to statistical models has provided a mechanism for directly delivering geophysical results to the reservoir engineer.
Finite‐difference modeling of scalar‐wave propagation in cracked media
Authors: van Baren Gerben B. , Mulder Wim A. , Herman Gérard C.
Published: October 2002

We discuss a finite‐difference modeling technique for the simplified case of scalar, two‐dimensional wave propagation in a medium containing a large number of small‐scale cracks. The cracks are characterized by an explicit (Neumann) boundary condition whereas the embedding medium can be heterogeneous. The boundaries of the cracks are not represented in the finite‐difference mesh, but the cracks are incorporated as distributed point sources. This enables the use of grid cells that are considerably larger than the crack sizes. We compare our method to an accurate integral‐equation solution for the case of a homogeneous embedding and conclude that the finite‐difference technique is accurate and computationally fast.
Quasi‐shear waves in inhomogeneous weakly anisotropic media by the quasi‐isotropic approach: A model study
Authors: Pšenčík Ivan , Dellinger Joe A.
Published: October 2002

In inhomogeneous isotropic regions, S-waves can be modeled using the ray method for isotropic media. In inhomogeneous strongly anisotropic regions, the independently propagating qS1- and qS2-waves can similarly be modeled using the ray method for anisotropic media. The latter method does not work properly in inhomogenous weakly anisotropic regions, however, where the split qS-waves couple. The zeroth‐order approximation of the quasi‐isotropic (QI) approach was designed for just such inhomogeneous weakly anisotropic media, for which neither the ray method for isotropic nor anisotropic media applies. We test the ranges of validity of these three methods using two simple synthetic models. Our results show that the QI approach more than spans the gap between the ray methods: it can be used in isotropic regions (where it reduces to the ray method for isotropic media), in regions of weak anisotropy (where the ray method for anisotropic media does not work properly), and even in regions of moderately strong anisotropy (in which the qS-waves decouple and thus could be modeled using the ray method for anisotropic media). A modeling program that switches between these three methods as necessary should be valid for arbitrary‐strength anisotropy.
On: “Multicomponent Kirchhoff migration” (K. Hokstad, GEOPHYSICS, 65, 861–873).
Authors: Tygel Martin
Published: October 2002

Dr. Hokstad introduces a multicomponent true‐amplitude Kirchhoff migration formula, equation (29). This expression has a factor [Formula: see text], calculated at the image depth point. This factor is determined by means of a limiting procedure described in Appendix B. I show that this limiting process produces zero, identically. As a consequence, the imaging expression that uses [Formula: see text] as a factor should also identically vanish.
Seismic data acquisition—The new millennium
Authors: Roche Steve
Published: October 2002

As we enter the new millennium, seismic data acquisition is in an interesting position. Because of overcapacity of seismic acquisition crews related to the downturn in the oil and gas industry, acquisition technology is essentially “frozen” in place. Companies previously active in seismic data acquisition research have limited these activities, or eliminated them. Some advances related to improving the resolution of seismic data through improved acquisition methods are being made, but much more effort is being directed towards improving the efficiency of acquisition.
Ground‐penetrating radar: Postmigration stacking ofn-fold common midpoint profile data
Authors: Hermance John F.
Published: October 2002

Diffraction and nonvertical side‐looking reflection patterns are typical features of most commercial ground‐penetrating radar (GPR) surveys. While a number of techniques are used by GPR workers to migrate such signals back to the proper position of the subsurface reflector, these have not been generally applied to multifold data nor to conditions of extreme noise. A procedure known among seismologists as wavefront migration is adapted here for processing multifold common midpoint (CMP) GPR data; it appears to be promising for both noise‐free and noisy data. The algorithm is largely geometrical and nonmathematical, and it is readily implemented on a personal computer. An example of synthetic data with extreme levels of noise illustrates that migrating prestack multifold CMP data, followed by postmigration, n-fold stacking, leads to a substantial improvement in image quality over unmigrated or single‐fold migrated data.
Constrained inversion of gravity fields for complex 3-D structures
Authors: Moraes Roberto A. V. , Hansen R. O.
Published: October 2002

As part of a research program to develop gravity interpretation tools that can be merged with seismic techniques, a full 3-D complex structural inversion scheme for (possibly multibody) polyhedral models has been developed. The forward modeling algorithm was adopted from previous work. Because the inverse problem is generally very ill posed, several methods of regularizing the inversion were investigated and a combination of the most useful was adopted. The combination includes (i) a structured matrix formulation for the system equations, (ii) an analytical expression for the Jacobian calculation, (iii) first‐derivative damping, (iv) a choice of damping parameter based on a variation of the trust region method, (v) a weighted scheme for parameter correction, and (vi) complete freezing of degrees of freedom found not to influence the gravity field significantly. This combination yields a robust inversion which was successfully demonstrated on data over the Galveston Island salt dome, offshore Texas. Variations of the technique should be applicable to magnetic data, which would make the method useful for mining problems and petroleum exploration settings involving volcanic structures.
Automatic interpretation of magnetic dike parameters using the analytical signal technique
Authors: Bastani Mehrdad , Pedersen Laust B.
Published: October 2002

The analytical signal of the magnetic field is used to automatically determine the source parameters of dikelike structures. The method is particularly useful for interpreting large amounts of data collected during airborne surveys because it makes full use of the high density of data along the flight lines while simultaneously checking for two‐dimensionality and strike directions by searching for coherent signals in neighboring profiles. The maximum horizontal curvature of the amplitude of the analytical signal is used to locate the dikes along a given profile. Tests with synthetic data show that the dike’s horizontal position is resolved accurately. Magnetic data from the Siljan impact structure in Sweden show that the estimated strikes are reliable and that dip, depth, and width estimates are coherent, especially for well‐isolated dikelike structures.
AVA analysis and interpretation of a carbonate reservoir: northwest Java basin, Indonesia
Authors: Adriansyah  , McMechan George A.
Published: October 2002

A detailed analysis is performed of amplitude variation with angle (AVA) observations in six common‐midpoint gathers with reflection points that are over and beside carbonate reefs in the Parigi Formation in the northwest Java basin. Both empirical analysis and full‐wavefield modeling of the AVA data suggest that the presence of gas affects AVA by reducing the bulk density of the reservoir, decreasing of the overburden [Formula: see text] ratio and by local attenuation caused by gas sieving through the overlying sediments. The slopes of AVA curves for reflections from the top of the Parigi are negative for brine saturation and strongly positive for gas saturation.
SVD for multioffset linearized inversion: Resolution analysis in multicomponent acquisition
Authors: Lebrun D. , Richard V. , Mace D. , Cuer M.
Published: October 2002

Acquisition of the full elastic response (compressional and shear) of the subsurface is an important technology in the seismic industry because of its potential to improve the quality of seismic data and to infer accurate information about rock properties (fluid type and rock lithology). In the framework of 3-D propagation in 1-D media, we propose a computational tool to analyze the information about elastic parameters contained in the amplitudes of reflected waves with offset. The approach is based on singular value decomposition (SVD) analysis of the linearized elastic inversion problem and can be applied to any particular seismic data. We applied this tool to examine the type of information in the model space that can be retrieved from sea‐bottom multicomponent measurements. The results are compared with those obtained from conventional streamer acquisition techniques. We also present multiparameter linearized inversion results obtained from synthetic data that illustrate the resolution of elastic parameters. This approach allows us to investigate the reliability of the elastic parameters estimated for different offset ranges, wave modes, data types, and noise levels involved in data space.
Comprehensive assessment of seismic acquisition geometries by focal beams—Part II: Practical aspects and examples
Authors: Volker A. W. F. , Blacquière G. , Berkhout A. J. , Ongkiehong L.
Published: October 2002

The acquisition geometry of a 3-D seismic survey should be designed in such a way that it allows high‐quality images and fulfills economical constraints. Focal source beams and focal detector beams are used to analyze and design field geometries. An attractive property is that the focal source and detector beams can be computed and evaluated separately. Seismic quality attributes such as resolution, noise suppression rate, angle‐averaged amplitude, and angle‐dependent amplitude information can be obtained efficiently from those beams. Examples confirm the theory that source and detector distributions may complement each other to achieve a high resolution. This important property can be used to design cost‐effective geometries. The acquisition design criteria for reliable amplitude versus ray parameter information turn out to be more stringent and may require a compromise in resolution.
A NOTE ON A RATIONALIZED VELOCITY DEPTH EQUATION
Authors: Sparks Neil R.
Published: October 2002

An average velocity depth equation of the form [Formula: see text] is suggested and its properties relative to the generalized velocity variation within the sedimentary column are discussed.
AN ANALYSIS OF ABNORMAL REFLEXIONS
Authors: Deacon L. E.
Published: October 2002

During the course of a reflexion survey of an area in Southwest Texas, a group of seismograms was obtained on which were recorded a number of events having abnormal apparent velocities. The dips computed from these events on the assumption that they were conventional reflexions in the plane of the profile were much larger than the anticipated stratigraphic dip. This paper presents a discussion of the possible origins of these events and an analysis which affords considerable evidence that they originated on fault planes.
NOTES ON EARTH TIDES
Authors: Lambert Walter D.
Published: October 2002

Attention is called to the problem of the zero point for earth tides; this involves the so‐called permanent tide or increase in the flattening of the earth. A suggestion is made for the use of existing tabulations for computing theoretical earth tides. Attention is also called to the effect of the varying distance of the tide‐producing body and to the secondary effects of the oceanic tides. Finally the author requests to be kept informed of current developments.
MAINTAINING AN ADEQUATE LEVEL OF GEOPHYSICAL EXPLORATION
Authors: Goldstone F.
Published: October 2002

At the time of the Denver meeting last year, many geologists and geophysicists were frankly in doubt as to where their duty lay in the war effort. This year there should be much less uncertainty as we have been given a task on the home front which will tax our resources to the utmost. The task, to locate 4500 wildcat wells so judiciously that neither steel nor manpower will be wasted in unjustified drilling. The facts which seem to make necessary this enormous wildcat drilling program in the face of a national shortage of critical materials and manpower have been widely publicized in the trade journals, and I will only briefly recapitulate them.
REPORT OF THE WAR EFFORTS ADVISORY COMMITTEE
Authors: Weatherby B. B.
Published: October 2002

As Chairman of the War Efforts Advisory Committee of the Society of Exploration Geophysicists, and as a member of the National Service Committee of the American Association of Petroleum Geologists, I have been listening for some time to reports on the seriousness of the personnel situation in the exploration field. Recent comments have shown that the problem has become very acute and some action must be taken if a sharp decline in activity is to be averted. In order to arrive at the facts of the case, questionnaires were sent to 28 geophysical companies or departments of companies representing 90% of all geophysical activity. Twenty‐one replies were received covering 77% of the total volume of work being done. Consequently the combined experience of this group may be fairly taken as indicative of the situation in the whole industry.
PRIORITIES AND SUPPLIES
Authors: Alcock E. D.
Published: October 2002

In any discussion of priorities it should be kept in mind that the situation is continually changing and that the statements made at this time may no longer hold true by the time this article appears in print.
THE CONTRIBUTION OF GEOPHYSICAL LABORATORIES
Authors: Born W. T.
Published: October 2002

To obtain the information upon which this brief report is based, the following questions were submitted to the executives responsible for the laboratory activities of twenty‐four commercial geophysical prospecting organization.
SAND AND GRAVEL PROSPECTING BY THE EARTH RESISTIVITY METHOD
Authors: Wilcox Stanley W.
Published: October 2002

The four electrode earth‐resistivity method is applied to the search for sand and gravel deposits in the glaciated areas of the United States. This technique differentiates between clays and granular materials enabling the prospector to outline and roughly estimate the quantities of each without test pits or bore holes. Several examples of field surveys are given.
THE EFFECT OF CHARGE SIZE ON REFLECTION RECORDS
Authors: Sharpe Joseph A.
Published: October 2002

Examples are presented showing the effect of charge size on: the ratio of the amplitude of reflected motion to that of surface waves; the ratio of the amplitude of reflected motion to that of random interference; the frequency spectrum of reflected motion. The similarity between these observations and the effects of changing depth of shooting are discussed. It is concluded that the use of a larger or a smaller charge in a particular circumstance may be the means of effecting a useful improvement in a record.
THE GEOPHYSICAL HISTORY OF THE CAYUGA FIELD, ANDERSON COUNTY, TEXAS
Authors: Peacock H. B.
Published: October 2002

Original seismic maps based on the correlation method of reflection shooting are presented, together with a geological structure map. Cross sections show a comparison of the seismic and the well data. Results of a gravity‐meter survey are also shown.
INDEX OF WELLS SHOT FOR VELOCITY
Authors: Swan B. G.
Published: October 2002

This index lists 811 wells in which seismic velocity surveys have been made. In so far as possible to obtain, information has been listed concerning the location, survey depth, by whom and when made, and the company or co‐operative association responsible for the survey. It is intended that the index be brought up to date at approximately six‐month intervals in future issues of Geophysics.
Depth characterization of shallow aquifers with seismic reflection, Part I—The failure of NMO velocity analysis and quantitative error prediction
Authors: Bradford John H.
Published: October 2002

As seismic reflection data become more prevalent as input for quantitative environmental and engineering studies, there is a growing need to assess and improve the accuracy of reflection processing methodologies. It is common for compressional‐wave velocities to increase by a factor of four or more where shallow, unconsolidated sediments change from a dry or partially water‐saturated regime to full saturation. While this degree of velocity contrast is rare in conventional seismology, it is a common scenario in shallow environments and leads to significant problems when trying to record and interpret reflections within about the first 30 m below the water table. The problem is compounded in shallow reflection studies where problems primarily associated with surface‐related noise limit the range of offsets we can use to record reflected energy. For offset‐to‐depth ratios typically required to record reflections originating in this zone, the assumptions of NMO velocity analysis are violated, leading to very large errors in depth and layer thickness estimates if the Dix equation is assumed valid. For a broad range of velocity profiles, saturated layer thickness will be overestimated by a minimum of 10% if the boundary of interest is <30 m below the water table. The error increases rapidly as the boundary shallows and can be very large (>100%) if the saturated layer is <10 m thick. This degree of error has a significant and negative impact if quantitative interpretations of aquifer geometry are used in aquifer evaluation such as predictive groundwater flow modeling or total resource estimates.