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Showing papers in "Geophysics in 2005"


Journal ArticleDOI
TL;DR: The magnetic method is the primary exploration tool in the search for minerals, oil and gas, geothermal resources, and groundwater, and for a variety of other purposes such as natural hazards assessment, mapping impact structures, and engineering and environmental studies as discussed by the authors.
Abstract: The magnetic method, perhaps the oldest of geophysical exploration techniques, blossomed after the advent of airborne surveys in World War II. With improvements in instrumentation, navigation, and platform compensation, it is now possible to map the entire crustal section at a variety of scales, from strongly magnetic basement at regional scale to weakly magnetic sedimentary contacts at local scale. Methods of data filtering, display, and interpretation have also advanced, especially with the availability of low-cost, high-performance personal computers and color raster graphics. The magnetic method is the primary exploration tool in the search for minerals. In other arenas, the magnetic method has evolved from its sole use for mapping basement structure to include a wide range of new applications, such as locating intrasedimentary faults, defining subtle lithologic contacts, mapping salt domes in weakly magnetic sediments, and better defining targets through 3D inversion. These new applications have increased the method’s utility in all realms of exploration — in the search for minerals, oil and gas, geothermal resources, and groundwater, and for a variety of other purposes such as natural hazards assessment, mapping impact structures, and engineering and environmental studies.

467 citations


Journal ArticleDOI
TL;DR: In this paper, the authors presented a new methodology for computing a time-frequency map for nonstationary signals using the continuous-wavelet transform (CWT), which does not require preselecting a window length and does not have a fixed timefrequency resolution over the timefrequency space.
Abstract: This paper presents a new methodology for computing a time-frequency map for nonstationary signals using the continuous-wavelet transform (CWT). The conventional method of producing a time-frequency map using the short time Fourier transform (STFT) limits time-frequency resolution by a predefined window length. In contrast, the CWT method does not require preselecting a window length and does not have a fixed time-frequency resolution over the timefrequency space. CWT uses dilation and translation of a wavelet to produce a time-scale map. A single scale encompasses a frequency band and is inversely proportional to the time support of the dilated wavelet. Previous workers have converted a time-scale map into a time-frequency map by taking the center frequencies of each scale. We transform the time-scale map by taking the Fourier transform of the inverse CWT to produce a time-frequency map. Thus, a time-scale map is converted into a time-frequency map in which the amplitudes of individual frequencies rather than frequency bands are represented. We refer to such a map as the time-frequency CWT (TFCWT). We validate our approach with a nonstationary synthetic example and compare the results with the STFT and a typical CWT spectrum. Two field examples illustrate that the TFCWT potentially can be used to detect frequency shadows caused by hydrocarbons and to identify subtle stratigraphic features for reservoir characterization.

446 citations


Journal ArticleDOI
TL;DR: A seismic attribute is a quantitative measure of a seismic characteristic of interest as mentioned in this paper, which has been integral to reflection seismic interpretation since the 1930s when geophysicists started to pick traveltimes to coherent reflections on seismic field records.
Abstract: A seismic attribute is a quantitative measure of a seismic characteristic of interest. Analysis of attributes has been integral to reflection seismic interpretation since the 1930s when geophysicists started to pick traveltimes to coherent reflections on seismic field records. There are now more than 50 distinct seismic attributes calculated from seismic data and applied to the interpretation of geologic structure, stratigraphy, and rock/pore fluid properties. The evolution of seismic attributes is closely linked to advances in computer technology. As examples, the advent of digital recording in the 1960s produced improved measurements of seismic amplitude and pointed out the correlation between hydrocarbon pore fluids and strong amplitudes (“bright spots”). The introduction of color printers in the early 1970s allowed color displays of reflection strength, frequency, phase, and interval velocity to be overlain routinely on black-and-white seismic records. Interpretation workstations in the 1980s provided...

437 citations


Journal ArticleDOI
TL;DR: The 3D analytic signal amplitude of a total magnetic intensity (TMI) map, denoted by | A |, has the added advantage of being independent of the orientation of magnetization of the source bodies as discussed by the authors.
Abstract: The 3D analytic signal amplitude of a total magnetic intensity (TMI) map, introduced by Roest et al. (1992), is widely used in magnetic interpretation as a means of positioning anomalies directly over their sources. This technique is most important at low magnetic latitudes, where reduction to the pole distorts anomalies to the point where they often become uninterpretable: the reduction operator does not converge if the magnetization and regional field are truly horizontal (Baranov, 1957). Methods have been devised to suppress the artifacts appearing in low-latitude reduction to the pole, but no method can reduce such data without distortion (e.g., Silva, 1986; Hansen and Pawlowski, 1989), which becomes severe for inclinations less than 20°. The amplitude of the analytic signal, denoted by | A |, has the added advantage of being independent of the orientation of magnetization of the source bodies. It reaches a maximum over magnetic contacts, and thus, in theory, can be used to trace the outline of magnetic bodies. In practice, especially in the case of aeromagnetic data at high instrument-source separation, | A | is high over magnetic bodies, but is not sufficient to resolve body edges. This appears to be true even with higher-order analytic signal derivatives (Debeglia and Corpel, 1997, their Figure 11).

340 citations


Journal ArticleDOI
TL;DR: This work investigates the nonorthogonality of the Fourier basis on an irregularly sampled grid and proposes a technique called “antileakage Fourier transform” to overcome the spectral leakage and demonstrates the robustness and effectiveness of this technique.
Abstract: Seismic data regularization, which spatially transforms irregularly sampled acquired data to regularly sampled data, is a long-standing problem in seismic data processing. Data regularization can be implemented using Fourier theory by using a method that estimates the spatial frequency content on an irregularly sampled grid. The data can then be reconstructed on any desired grid. Difficulties arise from the nonorthogonality of the global Fourier basis functions on an irregular grid, which results in the problem of “spectral leakage”: energy from one Fourier coefficient leaks onto others. We investigate the nonorthogonality of the Fourier basis on an irregularly sampled grid and propose a technique called “antileakage Fourier transform” to overcome the spectral leakage. In the antileakage Fourier transform, we first solve for the most energetic Fourier coefficient, assuming that it causes the most severe leakage. To attenuate all aliases and the leakage of this component onto other Fourier coefficients, the data component corresponding to this most energetic Fourier coefficient is subtracted from the original input on the irregular grid. We then use this new input to solve for the next Fourier coefficient, repeating the procedure until all Fourier coefficients are estimated. This procedure is equivalent to “reorthogonalizing” the global Fourier basis on an irregularly sampled grid. We demonstrate the robustness and effectiveness of this technique with successful applications to both synthetic and real data examples.

326 citations


Journal ArticleDOI
TL;DR: The gravity method was the first geophysical technique to be used in oil and gas exploration and has continued to be an important and sometimes crucial constraint in a number of exploration areas as discussed by the authors.
Abstract: The gravity method was the first geophysical technique to be used in oil and gas exploration. Despite being eclipsed by seismology, it has continued to be an important and sometimes crucial constraint in a number of exploration areas. In oil exploration the gravity method is particularly applicable in salt provinces, overthrust and foothills belts, underexplored basins, and targets of interest that underlie high-velocity zones. The gravity method is used frequently in mining applications to map subsurface geology and to directly calculate ore reserves for some massive sulfide orebodies. There is also a modest increase in the use of gravity techniques in specialized investigations for shallow targets. Gravimeters have undergone continuous improvement during the past 25 years, particularly in their ability to function in a dynamic environment. This and the advent of

274 citations


Journal ArticleDOI
TL;DR: In this paper, an intensely fractured limestone quarry is used as a test site for a 100-MHz 3D ground-penetrating radar (GPR) survey with 0.1 m × 0.2 m trace spacing.
Abstract: Noninvasive 3D ground-penetrating radar (GPR) imaging with submeter resolution in all directions delineates the internal architecture and processes of the shallow subsurface. Full-resolution imaging requires unaliased recording of reflections and diffractions coupled with 3D migration processing. The GPR practitioner can easily determine necessary acquisition trace spacing on a frequency-wavenumber (f-k) plot of a representative 2D GPR test profile. Quarter-wavelength spatial sampling is a minimum requirement for full-resolution GPR recording. An intensely fractured limestone quarry serves as a test site for a 100-MHz 3D GPR survey with 0.1 m × 0.2 m trace spacing. This example clearly defines the geometry of fractures in four different orientations, including vertical dips to a depth of 20 m. Decimation to commonly used half-wavelength spatial sampling or only 2D migration processing makes most fractures invisible. The extra data-acquisition effort results in image volumes with submeter resolution, both ...

253 citations


Journal ArticleDOI
TL;DR: The cumulative change in two-way traveltimes with depth is referred to as time-lapse time shift as mentioned in this paper, which is a common term for changes in stress and strain fields of the rock material both inside and outside the reservoir.
Abstract: Pressure depletion as a result of oil and gas production creates changes in the stress and strain fields of the rock material both inside and outside the reservoir. Seismic waves propagating through these rocks have different traveltimes before and after depletion because the layer thicknesses change and the seismic velocity is altered by the changes in the stress and strain fields. The cumulative change in two-way traveltimes with depth is referred to as time-lapse time shift.

206 citations


Journal ArticleDOI
TL;DR: The North American gravity database as well as data bases from Canada, Mexico, and the United States are being revised to improve their coverage, versatility, and accuracy as discussed by the authors, and the most striking revision is the use of the internationally accepted terrestrial ellipsoid for the height dataum of gravity stations rather than the conven- tionally used geoid or sea level.
Abstract: The North American gravity database as well as data- bases from Canada, Mexico, and the United States are being revised to improve their coverage, versatility, and accuracy. An important part of this effort is revising pro- cedures for calculating gravity anomalies, taking into ac- count our enhanced computational power, improved ter- rain databases and datums, and increased interest in more accurately defining long-wavelength anomaly components. Users of the databases may note minor differences be- tween previous and revised database values as a result of these procedures. Generally, the differences do not impact the interpretation of local anomalies but do improve re- gional anomaly studies. The most striking revision is the use of the internationally accepted terrestrial ellipsoid for the height datum of gravity stations rather than the conven- tionally used geoid or sea level. Principal facts of gravity observations and anomalies based on both revised and pre- vious procedures together with germane metadata will be available on an interactive Web-based data system as well as from national agencies and data centers. The use of the revised procedures is encouraged for gravity data reduc- tion because of the widespread use of the global position- ing system in gravity fieldwork and the need for increased accuracy and precision of anomalies and consistency with North American and national databases. Anomalies based on the revised standards should be preceded by the adjec- tive "ellipsoidal" to differentiate anomalies calculated us- ing heights with respect to the ellipsoid from those based on conventional elevations referenced to the geoid.

195 citations


Journal ArticleDOI
TL;DR: In this paper, a common-shot Gaussian beam migration (CGBM) algorithm was proposed to handle multipathing in a natural way, allowing the migration of data sets that can include a variety of azimuths, and allowing a simplified treatment of near-surface issues.
Abstract: Gaussian beam migration is a depth migration method whose accuracy rivals that of migration by wavefield extrapolation — so-called “wave-equation migration” — and whose efficiency rivals that of Kirchhoff migration. This migration method can image complicated geologic structures, including very steep dips, in areas where the seismic velocity varies rapidly. However, applications of prestack Gaussian beam migration either have been limited to common-offset common-azimuth data volumes, and thus are inflexible, or suffer from multiarrival inaccuracies in a common-shot implementation. In order to optimize both the flexibility and accuracy of Gaussian beam migration, I present a common-shot implementation that handles multipathing in a natural way. This allows the migration of data sets that can include a variety of azimuths, and it allows a simplified treatment of near-surface issues. Application of this method to model data typical of Canadian Foothills structures and to model data that includes a complicate...

188 citations


Journal ArticleDOI
TL;DR: In this paper, the authors describe a recently developed true-amplitude implementation of modified one-way operators and present some numerical examples, which are used in the standard wave-equation migration imaging condition.
Abstract: One-way wave operators are powerful tools for forward modeling and migration. Here, we describe a recently developed true-amplitude implementation of modified one-way operators and present some numerical examples. By “true-amplitude” one-way forward modeling we mean that the solutions are dynamically correct as well as kinematically correct. That is, ray theory applied to these equations yields the upward- and downward-traveling eikonal equations of the full wave equation, and the amplitude satisfies the transport equation of the full wave equation. The solutions of these equations are used in the standard wave-equation migration imaging condition. The boundary data for the downgoing wave is also modified from the one used in the classic theory because the latter data is not consistent with a point source for the full wave equation. When the full wave-form solutions are replaced by their ray-theoretic approximations, the imaging formula reduces to the common-shot Kirchhoff inversion formula. In this sense...

Journal ArticleDOI
TL;DR: In this paper, high-resolution electrical resistivity imaging is used to delineate the geometry of complex landslides in the Lucanian Apennine chain of southern Italy, to identify the discontinuity between the landslide material and bedrock, and to locate possible surfaces of reactivation.
Abstract: We use high-resolution electrical resistivity imaging to delineate the geometry of complex landslides in the Lucanian Apennine chain of southern Italy, to identify the discontinuity between the landslide material and bedrock, and to locate possible surfaces of reactivation. The Lucanian Apennine chain is characterized by high hydrogeological hazard and shows a complete panorama of mass movements. In this area, all typologies of landslides markedly predisposed and tightly controlled by the geostructural characteristics, are found: rotational and translational slides, rototranslational slides, earth and mudflows, as well as deep-seated gravitational slope phenomena with a predominance of rototranslational slides evolving as earthflow slides. Three test sites, characterized by complex geology and a high hydrogeologic hazard, are studied. The Giarrossa and Varco Izzo earthflow slides are located to the west and east of the town of Potenza, whereas the Latronico slide is located close to the town of Latronico....

Journal ArticleDOI
TL;DR: In this article, the determinant of the impedance tensor is used for inverse modeling of radiomagnetotelluric (RMT) impedance tensors in a shallow conductors in crystalline basement overlain by a thin sedimentary cover.
Abstract: Recent developments in the speed and quality of data acquisition using the radiomagnetotelluric (RMT) method, whereby large amounts of broadband RMT data can be collected along profiles, have prompted us to develop a strategy for routine inverse modeling using 2D models. We build a rather complicated numerical model containing both 2D and 3D elements believed to be representative for shallow conductors in crystalline basement overlain by a thin sedimentary cover. We then invert the corresponding synthetic data on selected profiles, using both traditional MT approaches, as well as the proposed approach, which is based on the determinant of the MT impedance tensor. We compare the estimated resistivity models with the true models along the selected profiles and find that the traditional approaches often lead to strongly biased models and bad data fit, in contrast to those using the determinant. In this case, much of the bias is removed and the data fit is improved. The determinant of the impedance tensor is independent of the chosen strike direction, and once the a priori model is set, the best fitting model is found to be practically independent of the starting model used. We conclude that the determinant of the impedance tensor is a useful tool for routine inverse modeling.

Journal ArticleDOI
TL;DR: In this article, a wave-equation record, the response to a linear or planar source, can be synthesized from all the common-shot records using slant-stack processing, or applying delays to the various shots.
Abstract: For 3D seismic imaging in structurally complex areas, the use of migration by wavefield extrapolation has become widespread. By its very nature, this family of migration methods operates on data sets that satisfy a wave equation in the context of a single, physically realizable field experiment, such as a common-shot record. However, common-shot migration of data recorded over dipping structures requires a migration aperture much larger than the recording aperture, resulting in extra computations. A different type of wave-equation record, the response to a linear or planar source, can be synthesized from all the common-shot records. Synthesizing these records from common-shot records involves slant-stack processing, or applying delays to the various shots; we call these records delayed-shot records. Delayed-shot records don't suffer from the aperture problems of common-shot records since their recording aperture is the length of the seismic survey. Consequently, delayed-shot records hold potential for eff...

Journal ArticleDOI
TL;DR: The idea that the propagation of elastic waves can be anisotropic, i.e., that the velocity may depend on the direction, is about 175 years old as mentioned in this paper, and the first steps are connected with the top scientists of that time, people such as Cauchy, Fresnel, Green and Kelvin.
Abstract: The idea that the propagation of elastic waves can be anisotropic, i.e., that the velocity may depend on the direction, is about 175 years old. The first steps are connected with the top scientists of that time, people such as Cauchy, Fresnel, Green, and Kelvin. For most of the 19th century, anisotropic wave propagation was studied mainly by mathematical physicists, and the only applications were in crystal optics and crystal elasticity. During these years, important steps in the formal description of the subject were made. At the turn of the 20th century, Rudzki stressed the significance of seismic anisotropy. He studied many of its aspects, but his ideas were not applied. Research in seismic anisotropy became stagnant after his death in 1916. Beginning about 1950, the significance of seismic anisotropy for exploration seismics was studied, mainly in connection with thinly layered media and the resulting transverse isotropy. Very soon it became clear that the effect of layer-induced anisotropy on data ac...

Journal ArticleDOI
TL;DR: In this paper, the authors measured the saturation effects on the acoustic properties in carbonates and the results question some theoretical assumptions, such as Gassmann's assumption of a constant shear modulus.
Abstract: In laboratory experiments we measured the saturation effects on the acoustic properties in carbonates and the results question some theoretical assumptions. In particular, these laboratory experiments under dry and wet conditions show that shear moduli do not remain constant during saturation. This change in shear modulus puts Gassmann's assumption of a constant shear modulus into question and also explains why velocities predicted with the Gassmann equation can be lower or higher than measured velocities.

Journal ArticleDOI
TL;DR: In this paper, a method for estimating interval velocity using the kinematic information in defocused diffractions and reflections is proposed, where velocity information from defocused migrated events is extracted by analyzing their residual focusing in physical space (depth and midpoint) using prestack residual migration.
Abstract: We propose a method for estimating interval velocity using the kinematic information in defocused diffractions and reflections. We extract velocity information from defocused migrated events by analyzing their residual focusing in physical space (depth and midpoint) using prestack residual migration. The results of this residual-focusing analysis are fed to a linearized inversion procedure that produces interval velocity updates. Our inversion procedure uses a wavefield-continuation operator linking perturbations of interval velocities to perturbations of migrated images, based on the principles of wave-equation migration velocity analysis introduced in recent years. We measure the accuracy of the migration velocity using a diffraction-focusing criterion instead of the criterion of flatness of migrated common-image gathers that is commonly used in migration velocity analysis. This new criterion enables us to extract velocity information from events that would be challenging to use with conventional veloci...

Journal ArticleDOI
TL;DR: In this paper, a formalism was developed to describe elastic moduli and anisotropy of rocks as nonlinear functions of confining stress and pore pressure. But the authors only considered compliant porosity.
Abstract: Elastic properties of rocks depend on tectonic stress. Using the theory of poroelasticity as a constraint, we analyze features of these dependencies related to changes in rock pore-space geometry. We develop a formalism describing elastic moduli and anisotropy of rocks as nonlinear functions of confining stress and pore pressure. This formalism appears to agree with laboratory observations. To a first approximation, elastic moduli and seismic velocities as well as porosity depend only on the difference between the confining tectonic stress and pore pressure. However, in general, both the confining stress tensor and the pore pressure must be taken into account as independent variables. The stress-dependent geometry of the pore space fully controls the stress-induced changes in elastic moduli and seismic velocities. Specifically, the compliant porosity plays the most important role, despite the fact that in many rocks the compliant porosity is a very small part of total porosity. Changes in compliant porosi...

Journal ArticleDOI
TL;DR: In this article, the benefits of using 90°-phase wavelets in stratigraphic and lithologic interpretation of seismically thin beds are discussed, in a two-part article, in which seismic models of Ricker wavelets with selected phases are constructed to assess interpretability of composite waveforms in increasingly complex geologic settings.
Abstract: We discuss, in a two-part article, the benefits of 90°-phase wavelets in stratigraphic and lithologic interpretation of seismically thin beds. In Part 1, seismic models of Ricker wavelets with selected phases are constructed to assess interpretability of composite waveforms in increasingly complex geologic settings. Although superior for single surface and thick-layer interpretation, zero-phase seismic data are not optimal for interpreting beds thinner than a wavelength because their antisymmetric thin-bed responses tie to the reflectivity series rather than to impedance logs. Nonsymmetrical wavelets (e.g., minimum-phase wavelets) are generally not recommended for interpretation because their asymmetric composite waveforms have large side lobes. Integrated zero-phase traces are also less desirable because they lose high-frequency components in the integration process. However, the application of 90°-phase data consistently improves seismic interpretability. The unique symmetry of 90°-phase thin-bed response eliminates the dual polarity of thin-bed responses, resulting in better imagery of thin-bed geometry, impedance profiles, lithology, and stratigraphy. Less amplitude distortion and less stratigraphy-independent, thin-bed interference lead to more accurate acoustic impedance estimation from amplitude data and a better tie of seismic traces to lithology-indicative wireline logs. Field data applications are presented in part 2 of this article.

Journal ArticleDOI
TL;DR: In this paper, the authors consider induced microseismicity in geothermal and tight-gas sandstone reservoirs and find that the probability of induced earthquakes occurring is very well described by the relaxation law of pressure perturbation in fluids filling the pore space in rocks.
Abstract: The use of borehole fluid injections is typical for exploration and development of hydrocarbon or geothermal reservoirs. Such injections often induce small-magnitude earthquakes. The nature of processes leading to triggering of such microseismicity is still not completely understood. Here, we consider induced microseismicity, using as examples two case studies of geothermal reservoirs in crystalline rocks and one case study of a tight-gas sandstone reservoir. In all three cases, we found that the probability of induced earthquakes occurring is very well described by the relaxation law of pressure perturbation in fluids filling the pore space in rocks. This strongly supports the hypothesis of seismicity triggered by pore pressure. Moreover, this opens additional possibilities of using passive seismic monitoring to characterize hydraulic properties of rocks on the reservoir scale with high precision.

Journal ArticleDOI
TL;DR: In this paper, a unified description is proposed that relates both types of algorithms based on wave theory, which can be applied in a model-driven or a data-driven way.
Abstract: Removal of surface and internal multiples can be formulated by removing the influence of downward-scattering boundaries and downward-scattering layers. The involved algorithms can be applied in a model-driven or a data-driven way. A unified description is proposed that relates both types of algorithms based on wave theory. The algorithm for the removal of surface multiples shows that muted shot records play the role of multichannel prediction filters. The algorithm for the removal of internal multiples shows that muted CFP gathers play the role of multichannel prediction filters. The internal multiple removal algorithm is illustrated with numerical examples. The conclusion is that the layer-related version of the algorithm has significant practical advantages.

Journal ArticleDOI
TL;DR: In this article, the authors derived a simple equation to estimate the depth of investigation for handheld EM sensors based on layered half-space models and showed that the depth is approximately proportional to the square root of the skin depth in the host for a given detection threshold and conductivity contrast between the target and host.
Abstract: The depth of investigation in electromagnetic (EM) soundings is a maximum depth at which a given target in a given host can be detected by a given sensor. It is of primary interest in EM exploration, particularly for small EM sensors having negligible separation between the transmitter and receiver coils. The depth of investigation is related to many factors, such as sensor sensitivity, precision, operating frequencies, ambient noise level, target and host properties, and the techniques used in data processing and interpretation. Quantitative understanding of the relationships between the depth of investigation and these factors will help users meet their geologic objectives, avoid unnecessary survey expenses, and display meaningful geologic features. Simple equations to estimate the depth of investigation for handheld EM sensors have been derived from analyzing the EM response based on layered half-space models. The results show that the depth of investigation is approximately proportional to the square root of the skin depth in the host for a given detection threshold and conductivity contrast between the target and host. For a given skin depth, the depth of investigation increases with the target conductivity and conductivity contrast and decreases with the detection threshold. Choosing a threshold mainly depends on the S/N ratio of the EM data if the sensor setup, data acquisition methods, and processing techniques are well established. A high threshold such as 20% or 30% is recommended for resistive targets or in areas where environmental noise is high or where terrain conductivity is low (<50 mS/m). In contrast, a threshold as low as 5% or 10% can be used for conductive targets in quiet areas. Field examples are presented to illustrate how to use the depth of investigation in data interpretation and presentation.

Journal ArticleDOI
TL;DR: In this article, the authors examined field seismic data to test the benefits of 90°-phase wavelets in thin-bed interpretation that are predicted by seismic modeling in part 1 of this paper.
Abstract: We examine field seismic data to test the benefits of 90°-phase wavelets in thin-bed interpretation that are predicted by seismic modeling in part 1 of this paper. In an interbedded sandstone-shale Miocene succession in the Gulf of Mexico basin, a 90°-phase shift of nearly zero-phase seismic data significantly improves lithologic and stratigraphic interpretation. A match between seismic and acoustic impedance (AI) profiles results in a better tie between seismic amplitude traces and lithology-indicative logs. Better geometric imaging of AI units that does not use dual-polarity seismic events results in easier and more accurate reservoir delineation. Less amplitude distortion and the stratigraphy-independent nature of thin-bed interference significantly improves stratigraphic resolution and seismic stratigraphic profiling. For a Ricker-like wavelet having small side lobes, stratigraphic resolution of 90°-phase data is considerably higher than that of zero-phase data. In this specific case, stratigraphic re...

Journal ArticleDOI
TL;DR: Banerjee and Gupta as discussed by the authors presented a full derivation for computing the gravitational attraction caused by a rectangular prism of constant density, which is identical to the one later derived by Nagy and Gupta.
Abstract: Over the last two centuries, the right rectangular prism has played an important role in both 3D forward and inverse gravity problems. This occurs because complex geometries are easy to build using a finite collection of prisms, and analytic solutions are available to compute the gravitational attraction of the prisms. In an article by Nagy (1966), a full derivation is presented for computing the gravitational attraction caused by a rectangular prism of constant density. The article by Nagy (1966) drew attention to earlier solutions of the same problem. Corbato (1966) pointed out that Everest (1830, 94-97) calculated closed-form expressions equivalent to those of Nagy (1966) for computing the horizontal and vertical gravitational effects of a prism, and he used them to estimate the deflection of the plumb bob caused by the Satpura Range in India. De Bremaecker (1966) called attention to the textbook by MacMillan (1930) where a formula for computing the gravitational potential caused by a prism is derived, along with a simple method to compute its derivatives along a coordinate axis. In addition, Nagy (1966) cited work by Sorokin (1951) and Haaz (1953) indicating that these authors had developed an equivalent solution, which turns out to be identical to the one later derived by Banerjee and Gupta (1977).

Journal ArticleDOI
TL;DR: In this paper, an enhanced local-wavenumber (ELW) method is proposed for the interpretation of profile magnetic data, which uses the traditional and phase-rotated local wavenumbers to produce a linear equation as a function of the model parameters.
Abstract: This paper presents an enhancement of the local-wavenumber method (named ELW for “enhanced local wavenumber”) for interpretation of profile magnetic data. This method uses the traditional and phase-rotated local wavenumbers to produce a linear equation as a function of the model parameters. The equation can be solved to determine the horizontal location and depth of a 2D magnetic body without specifying a priori information about the nature of the sources. Using the obtained source-location parameters, the nature of the source can then be inferred. The method was tested using theoretical simulations with random noise over a dike body. It was able to provide both the location and an index characterizing the nature of the source body. The practical utility of the method is demonstrated using field examples over dikelike bodies from Canada and Egypt.

Journal ArticleDOI
TL;DR: In this paper, the authors examined workflows to separate diffraction from reflections that allow enhancement of diffraction-like signals and suppression of reflections, and observed that the most effective techniques are the decomposition of seismic gathers into eigensections and flows based on Radon transformations.
Abstract: Seismic diffractions are often considered noise and are intentionally or implicitly suppressed during processing. Diffraction-like events include true diffractions, wave conversions, or fracture waves which may contain valuable information about the subsurface and could be used for interpretation or imaging. Using synthetic and field data, we examine workflows to separate diffractions from reflections that allow enhancement of diffraction-like signals and suppression of reflections. The workflows consist of combinations of standard processing modules. Most workflows apply normal moveout corrections to flatten reflection hyperbolas, which eases their removal. We observe that the most effective techniques are the decomposition of seismic gathers into eigensections and flows based on Radon transformations.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the electrical response of reactive iron barriers by making measurements on zero valent iron (Fe0)-sand columns under the following conditions: (1) variable Fe0 surface area (0.1-100% by volume Fe0) under constant electrolyte chemistry; (2) variable electrolyte activity ( 0.01-1 mol/liter), valence (mono trivalent), and pH under constant Fe0-sand composition; and (3) forced precipitation of iron hydroxides and iron carbonates on the Fe0surface
Abstract: We investigate the electrical response (0.1–1000 Hz) of reactive iron barriers by making measurements on zero valent iron (Fe0)-sand columns under the following conditions: (1) variable Fe0 surface area (0.1–100% by volume Fe0) under constant electrolyte chemistry; (2) variable electrolyte activity (0.01–1 mol/liter), valence (mono trivalent), and pH under constant Fe0-sand composition; and (3) forced precipitation of iron hydroxides and iron carbonates on the Fe0 surface. We model the measurements in terms of conduction magnitude, polarization magnitude, and polarization relaxation time. Our key findings are: (a) Polarization magnitude exhibits a linear relation to the surface area of Fe0, whereas conduction magnitude is only weakly dependent on the Fe0 concentration below 30% by volume Fe0. (b) Polarization magnitude shows a power law relation to electrolyte activity, with exponents decreasing from 0.9 for monovalent solutions to 0.7 for trivalent solutions. (c) The relaxation time parameter depends on ...

Journal ArticleDOI
TL;DR: In this article, traveltime inversion and forward modeling of multicomponent wide-angle seismic data result in detailed P- and S-wave velocities of hydrate and gas-bearing sediment layers.
Abstract: Geophysical evidence for gas hydrates is widespread along the northern flank of the Storegga Slide on the mid-Norwegian margin. Bottom-simulating reflectors (BSR) at the base of the gas hydrate stability zone cover an area of approximately 4000 km 2 , outside but also inside the Storegga Slide scar area. Traveltime inversion and forward modeling of multicomponent wide-angle seismic data result in detailed P- and S-wave velocities of hydrate- and gas-bearing sediment layers. The relationship between the velocities constrains the background velocity model for a hydrate-free, gas-free case. The seismic velocities indicate that hydrate concentrations in the pore space of sediments range between 3% and 6% in a zone that is as much as 50 m thick overlying the BSR. Hydrates are most likely disseminated, neither cementing the sediment matrix nor affecting the stiffness of the matrix noticeably. Average free-gas concentrations beneath the hydrate stability zone are approximately 0.4% to 0.8% of the pore volume, assuming a homogeneous gas distribution. The free-gas zone underneath the BSR is about 80 m thick. Amplitude and reflectivity analyses suggest a rather complex distribution of gas along specific sedimentary strata rather than along the base of the gas hydrate stability zone (BGHS). This gives rise to enhanced reflections that terminate at the BGHS. The stratigraphic control on gas distribution forces the gas concentration to increase slightly with depth at certain locations. Gas-bearing layers can be as thin as 2 m.

Journal ArticleDOI
TL;DR: In this article, an extension to 3C Kirchhoff prestack depth migration is presented, where the migration operator is restricted to the Fresnel volume of the specular reflected raypath.
Abstract: If the aperture of a seismic reflection experiment is strongly limited, Kirchhoff migration suffers from strong artifacts attributable to incomplete summation. This can be overcome by restricting the migration operator to the region that physically contributes to a reflection event. Examples of such limited-aperture experiments include data acquisition in boreholes, tunnels, and mines. We present an extension to three-component (3C) Kirchhoff prestack depth migration, where the migration operator is restricted to the Fresnel volume of the specular reflected raypath. We use the measured polarization direction at a 3C receiver to determine points of specular reflection. In homogeneous media, the polarization angle of 3C data can be used directly to decide whether a certain image point belongs to the Fresnel volume of a specular reflection. In heterogeneous media, the Fresnel volume around an image point is approximated by means of paraxial ray tracing. The method is tested on a synthetic vertical seismic profiling experiment with strongly limited aperture. Migration artifacts and crosstalk effects from converted waves are strongly reduced compared with standard migration schemes. The method is successfully applied to seismic data acquired in a tunnel.

Journal ArticleDOI
TL;DR: The World Stress Map (WSM) project as mentioned in this paper is a collaborative project between academia, industry, and government that is building a comprehensive global database of present-day stress information to better understand the state and sources of contemporary tectonic stress in the lithosphere.
Abstract: Knowledge of the present-day tectonic stress is essential for numerous applications in petroleum exploration and production and in civil and mining engineering, such as improving the stability of boreholes and tunnels and enhancing petroleum production through natural or induced fractures. The World Stress Map (WSM) Project is a collaborative project between academia, industry, and government that is building a comprehensive global database of present-day stress information to better understand the state and sources of contemporary tectonic stress in the lithosphere (Figure 1).