bluuebunny/crossref_metadata_2025 huggingface dataset:
A seismic analysis of Black Creek and Wabumun salt collapse features, western Canadian sedimentary basin
Authors: Anderson N. L. , Brown R. J.
Published: October 2002

Two Devonian salts of western Canada, those of the Black Creek member (Upper Elk Point subgroup) in northwest Alberta and those of the Wabamun group in southeastern Alberta, were widely distributed and uniformly deposited within their respective basins. Both of these salts are interbedded within predominantly carbonate sequences and both have been extensively leached. They are now preserved as discontinuous remnants of variable thickness and areal extent. These salt remnants and their associated collapse features are often associated with structural or stratigraphic traps. Structural traps typically form where reservoir facies are closed across remnant salts, stratigraphic traps often develop where reservoir facies were either preferentially deposited and/or preserved in salt collapse lows. As a result of these relationships between dissolution and hydrocarbon entrapment, the distribution (areal extent and thickness) of these salt remnants is of significant interest to the explorationist. Both the Black Creek and Wabamun salts have relatively abrupt contacts with the encasing higher velocity, higher density carbonates. Where these salts are sufficiently thick, their top and base typically generate high amplitude reflections, and lateral variations in the salt isopach can be directly determined from the seismic data. Relative salt thicknesses can also be indirectly estimated through analyses of lateral variations in the thicknesses of the encompassing carbonates, time structural drape and velocity pullup. Such seismic information about the thickness and the extent of these salts should be used together with well log control to generate subsurface distribution maps. These maps will facilitate both the delineation of prospective structural and stratigraphic play fairways and the determination of the timing of salt dissolution. In addition, an appreciation of regional salt distribution will decrease the likelihood that remnant salts will be misinterpreted as either reefs and/or faulted structures.
Geometrical ray theory: Line foci and point foci in the anterior surface of seismic systems (second‐order approximation of the traveltimes)
Authors: Bortfeld R. , Kemper M.
Published: October 2002

We discuss focus phenomena that may occur with seismic surveys. There are situations where all rays from a source are concentrated upon a single receiver location; this location is a point focus. There are also situations where all rays from a source are concentrated upon receiver locations distributed along a line; this situation is a line focus. In either case, the signal amplitude may be anamalously high at the focus and anamalously low at other places, making the data difficult to interpret. The point foci are subdivided into three types: the reflection points of the rays from the source to the unique receiver location can fall together; or they can be distributed along a line in the reflecting interface; or they can be distributed over the whole reflecting interface. The line foci are in two types: the reflection points of the rays from the source to any (fixed) receiver location on the line focus can fall together, or along a line in the reflecting interface.
Upwind finite‐difference calculation of traveltimes
Authors: van Trier J. , Symes W. W.
Published: October 2002

Seismic traveltimes can be computed efficiently on a regular grid by an upwind finite‐difference method. The method solves a conservation law that describes changes in the gradient components of the traveltime field. The traveltime field itself is easily obtained from the solution of the conservation law by numerical integration. The conservation law derives from the eikonal equation, and its solution depicts the first‐arrival‐time field. The upwind finite‐difference scheme can be implemented in fully vectorized form, in contrast to a similar scheme proposed recently by Vidale. The resulting traveltime field is useful both in Kirchhoff migration and modeling and in seismic tomography. Many reliable methods exist for the numerical solution of conservation laws, which appear in fluid mechanics as statements of the conservation of mass, momentum, etc. A first‐order upwind finite‐difference scheme proves accurate enough for seismic applications. Upwind schemes are stable because they mimic the behavior of fluid flow by using only information taken from upstream in the fluid. Other common difference schemes are unstable, or overly dissipative, at shocks (discontinuities in flow variables), which are time gradient discontinuities in our approach to solving the eikonal equation.
Rapid map and inversion of P-SV waves
Authors: Stewart Robert R.
Published: October 2002

Multicomponent seismic recordings are currently being analyzed in an attempt to improve conventional P‐wave sections and to find and use rock properties associated with shear waves (e.g. Dohr, 1985; Danbom and Dominico, 1986). Mode‐converted (P-SV) waves hold a special interest for several reasons: They are generated by conventional P‐wave sources and have only a one‐way travel path as a shear wave through the typically low velocity and attenuative near surface. For a given frequency, they will have a shorter wavelength than the original P wave, and thus offer higher spatial resolution; this has been observed in several vertical seismic profiling (VSP) cases (e.g., Geis et al., 1990). However, for surface seismic data, converted waves are often found to be of lower frequency than P-P waves (e.g., Eaton et al., 1991).
A sloping wedge technique for calculating gravity terrain corrections
Authors: Barrows L. J. , Fett J. D.
Published: October 2002

Gravity terrain corrections account for the upward pull of topographic features which are higher than a gravity station (hills) and the lack of downward pull from open space which is lower than the station (valleys). In areas of rugged topography or in high precision surveys, the magnitude of the terrain corrections can be comparable to the anomalies being sought and the uncertainties in the terrain corrections can limit the accuracy of the survey. Also, calculating the corrections can require more time and effort than gathering the original field data. Even if terrain corrections are not made, it is necessary to show that their omission does not compromise the integrity of the survey.
High‐resolution velocity and attenuation logs from long‐spaced sonic data
Authors: Sams Mark
Published: October 2002

A long‐spaced sonic survey may be thought of as a special case of ray theoretical tomographic imaging. With such an approach estimates of borehole properties at a resolution of 6 inches (0.15 m) have been obtained by inversion compared with a resolution of 2 ft (0.6 m) from standard borehole‐compensated techniques (BHC). The inversion scheme employs the conjugate gradient technique which is fast and efficient. Unlike BHC, the method compensates for variable refraction angles and provides estimates of errors in the measurements. Results from synthetic data show that these factors greatly improve the imaging of the properties of a finely layered medium, though amplitude decay and coupling are less well defined than velocity and mud traveltime. Results from real data confirm the superior quality of logs from inversion. Furthermore, they indicate that measured amplitudes can be dominated by errors that cause deterioration of BHC estimates of amplitude decay and coupling.
A geophysical tracking and data logging system: Description and case history
Authors: Nyquist Jonathan E. , Blair Michael S.
Published: October 2002

Oak Ridge National Laboratory has developed an Ultrasonic Ranging and Data System (USRADS) and interfaced the system with a Geonics EM31 terrain conductivity meter. The system tracks the EM31 operator’s position by measuring the traveltime of ultrasonic pulses from a transducer carried in a backpack, to microphones mounted on stationary receivers deployed in the survey area. A microprocessor‐controlled radio transmitter, also carried in the backpack, transmits the terrain conductivity data to a mobile base station (van or truck), where the EM31’s X-Y position, and the electromagnetic quadrature and in‐phase readings are automatically recorded each second by a portable computer. USRADS can track the surveyor for distances of up to about 120 m from the base station with an accuracy of about 10 cm. USRADS potentially offers several advantages for surface geophysical surveys: (1) when elevations are not required, USRADS saves time and money because a measurement grid does not have to be surveyed before the geophysical work begins, (2) data are directly recorded by a portable computer and are available for analysis in the field, and (3) refining or expanding the coverage of an anomaly does not require additional surveying to add extra grid points. We discuss a field study where we surveyed roughly [Formula: see text] of Solid Waste Storage Area 4 (SWSA 4) on the Oak Ridge Reservation collecting over 2700 in‐phase and quadrature measurements in 45 minutes. The high density of measurements provided sharp resolution of waste trench boundaries.
Magnetic exploration of ocean crust for craters of impact origin: Model results
Authors: Ochadlick Andrew R.
Published: October 2002

Magnetic data sets over deep ocean areas may contain clues to the existence of craters formed by the impact of an extraterrestrial body with the Earth’s ocean crust. To aid in the magnetic exploration of the ocean crust for oceanic impact craters, basic but effective computations from an impact model are studied from an aeromagnetic point of view. The main assumption of the analysis is that a sufficiently large impact can excavate large volumes of magnetized basalt, vaporize basalt, and raise basalt to temperatures above the Curie temperature (approximately 500°C) to alter the preimpact magnetization of the ocean floor and result in a magnetic anomaly being associated with an oceanic impact crater. In the absence of an existing theory on the influence of impacts on ocean crustal magnetization, the representation of a crater on the ocean floor by a simple potential provides, apparently for the first time, quantitative estimates of the crater’s magnetic anomaly along a horizontal surface. Numerical results from the model suggest that the detection of the anomaly of a Cretaceous‐Tertiary (K-T) type of impact is well within the capabilities of aeromagnetic technology.
Spatial resolution of acoustic imaging with the Born approximation
Authors: von Seggern David
Published: October 2002

The practical limits on resolution of images produced by processing seismic data with a Born approximation depend on the spectrum of the source pulse, the incompleteness of the recording geometry, and the fidelity with which one can model the reference medium. These factors together prevent one from recovering a point image from data produced by a point diffractor in the subsurface. A simple imaging formula quantifies the effect of the source pulse, showing that spatial wavelet extent in an imaged volume is related to the product of the local velocity and the period of the propagating pulse. A general 3-D scheme for estimating the effect on resolution of the recording geometry is applied to various recording geometries in a homogeneous 3-D medium to show the importance of increasing the aperture of recording. A study of layered versus homogeneous media on the image resolution, lateral and horizontal, with 2-D models shows that layering, with a monotonic velocity increase, actually improves the lateral image resolution attainable with surface seismic data. A salt‐dome 2-D model shows how imperfect knowledge of either the velocities or structure (or both) can severely smear the acoustic image. Errors on the order of 5–10 percent in velocity of the layers of the reference model will greatly affect the clarity of depth images. Errors on the order of a wavelength in the position of interfaces will similarly affect the image. The addition of wells to the data acquisition plan for the salt model markedly improves the lateral resolution of the image.
Nonspecular reflections from a curved interface
Authors: Schleicher J. , Hubral P. , Tygel M.
Published: October 2002

If an incident wavefield hits a curved interface that possesses certain inflection points, there may exist “nonspecular” events in the reflected field that cannot be explained by real ray theory. The magnitude of such events can reach the order of the specular ones and can be expressed in terms of specular reflections at certain points on the analytic continuation of the interface. In fact, specular reflected “complex rays,” connecting complex reflection points with the observation point, are used to explain such events. Previous results obtained for acoustic calculations, involving an incident plane wave and a perfectly soft reflector, are extended to arbitrary velocity and density contrasts, as well as to an incident far‐field cylindrical wavefield. Moreover, the agreement between analytic results and independent computations using a finite‐differences scheme is shown. It confirms the existence of nonspecular reflections. The interpreter of a seismic section should, therefore, be aware of not attributing a subsurface interface to a nonspecular reflection, e.g., at a flank of a saltdome.
Effects of fluid viscosity on shear‐wave attenuation in partially saturated sandstone
Authors: Vo‐Thanh Dung
Published: October 2002

Shear‐wave attenuation and velocity have been measured in the kiloHertz frequency range at temperatures varying from −80°C to 80°C in a sample of Berea sandstone partially saturated with glycerol. I investigated 7 saturation states ranging from 0 to 62 percent of the pore space. Plots of attenuation versus temperature show squirt and viscous shear peaks, even at low saturation. Their amplitudes and half‐widths increase with increasing saturation. The maxima of the peaks progressively move to higher temperatures (about 4°C for viscous shear peak and 30°C for squirt peak) with increasing saturation from 7 to 62 percent. The velocity dispersion between −80°C and 80°C progressively increases from 700 to 1200 m/s with increasing saturation from 7 to 62 percent. By introducing the crack saturation parameter, a simple viscoelastic model based on O’Connell and Budiansky and using a Cole‐Cole distribution of cracks, is proposed for calculating the shear modulus in partially saturated rocks. This model partially interprets the experimental data.
A simple method for decomposing fracture‐induced anisotropy
Authors: Hood J. A.
Published: October 2002

A heterogeneous mixture of isotropic elements may appear homogeneous and anisotropic when the scale of its fabric is smaller than the seismic wavelengths that measure it (Backus, 1962). These fabrics can result from thin layering or from oriented microcracks or fractures. In many situations the anisotropy is quite complex, resulting, for example, from fractures in previously anisotropic media. Since these anisotropies combine, the effect of each must be separated to permit the study of either the fractures or the preexisting anisotropy.
Changes in shear‐wave polarization azimuth with depth in Cymric and Railroad Gap oil fields
Authors: Winterstein D. F. , Meadows M. A.
Published: October 2002

Shear‐wave [Formula: see text]-wave) polarization azimuths, although consistent over large depth intervals, changed abruptly and by large amount of various depths in nine-component vertical seismic profiling (VSP) data from the Cymric and Railroad Gap oil fields of the southwest San Joaquin basin. A simple layer‐stripping technique made it possible to follow the polarization changes and determine the [Formula: see text]-wave birefringence over successive depth intervals. Because the birefringence and polarization azimuth are related to in‐situ stresses and fracture, information from such analysis could be important for reservoir development. Near offset VSP data from Cymrix indicated that the subsurface could be appproximated roughly as two anisotropic layers. The upper layer, from the surface to 800 ft (240 m), had vertical [Formula: see text]-wave birefringence as large was about 6 percent down to 1300 ft (400 m). In the upper layer the polarization azimuth of the fast [Formula: see text]-wave was N 60°E, while in the lower layer it was about N 10°E. Refinement of the layer stripping showed that neither layer was anisotropically homogenous, and both could be subdivided into thinner layers. Near offset [Formula: see text]-wave VSP data from the Railroad Gap well also show high birefringence near the surface and less birefringence deeper. In the uppermost layer, which extends down to 1300 ft (400 m), the [Formula: see text]-wave birefringence was 9 percent, and the lag between the fast and slow [Formula: see text]-waves exceeded 60 ms at the bottom of the layer. Seven layers in all were needed to accommodate [Formula: see text]-wave polarization changes. The most reliable azimuth angle determination as judged from the data consistency were those of the uppermost layer, at N 46°E, and those from depths 2900–3700 ft (880–1130 m) and 3900–5300 ft (1190–1610 m), at N 16°E and N 15°W, respectively. Over those intervals the scatter of calculated azimuths about the mean was typically less than 4 degrees. The largest birefringence at both locations occurred in the same formation, the Pliocene Tulare sands and Pebble Conglomerate. In those formations the azimuth of the fast [Formula: see text]-wave polarization was roughly orthogonal to the southwest. In the deeper Antelope shale, [Formula: see text]-wave polarization directions in both areas were close to 45 degrees from the fault. Confidence in the layer stripping procedure was bolstered by major improvement in data quality that resulted from stripping. Before stripping, wavelets of the two [Formula: see text]-waves sometimes had very different waveforms, and it was often impossible to come close to diagonalizing the 2 × 2 S‐wave data matrix by rotating sources and receivers by the same angle. After stripping, wavelets were more similar in shape, and the S‐wave matrix was more nearly diagonalizable by rotating with a single angle.
Interpretation of velocity estimates from coherency inversion
Authors: Landa E. , Thore P. , Sorin V. , Koren Z.
Published: October 2002

Velocity estimation by coherency inversion provides a velocity‐depth macro‐model of the subsurface. The accuracy of this model is critical for imaging techniques such as depth migration and inversion. Therefore, the reliability of seismic interpretation is strongly dependent on the accuracy of the derived model. Uncertainties in the velocity and depth estimates for the model can be calculated using a combination of the response surface technique and the Dix equation. In the layer‐stripping strategy, the method takes into account the local errors connected mainly to the current layer and the global errors connected to the derived overburden. Examples using synthetic and real data show that the coherency inversion produces accurate macro‐models.
An evaluation of short‐wave‐infrared (SWIR) data from the AVIRIS and GEOSCAN instruments for mineralogical mapping at Cuprite, Nevada
Authors: Hook Simon J. , Elvidge Christopher D. , Rast Michael , Watanabe Hiroshi
Published: October 2002

An evaluation was performed on SWIR (2000–2400 nm) data from two airborne remote sensing systems for discriminating and identifying alteration minerals at Cuprite, Nevada. The data were acquired by the NASA Airborne Visible/InfraRed Imaging Spectrometer (AVIRIS) and the GEOSCAN Mk II multispectral scanner. The evaluation involved comparison of processed imagery and image‐derived spectra with existing alteration maps and laboratory spectra of rock samples from Cuprite. Results indicate that both the AVIRIS and GEOSCAN data permit the discrimination of areas of alunite, buddingtonite, kaolinite, and silicification using color composite images formed from three SWIR bands processed with either the decorrelation stretch or a log residual algorithm. The laboratory spectral features of alunite, kaolinite and buddingtonite could be seen clearly only in the log residual processed AVIRIS data. However, this does not preclude their identification with the GEOSCAN data.
The equivalent force system of a monopole source in a fluid‐filled open borehole
Authors: Ben‐Menahem Ari , Kostek Sergio
Published: October 2002

One of the major contributors to the complexity of boundary‐value problems pertaining to theoretical modeling of exploration elastodynamics is the presence of both vertical and horizontal discontinuities. It has been known for a long time that it is sometimes possible to replace certain boundaries by a system of images, provided the extra stresses and displacements induced by these images could indeed mimic the discontinuities caused by the said boundaries (e.g., Ben‐Menahem and Singh, 1981). In this vein, we show that part of the field created by a monopole source acting on the axis of a fluid‐filled open borehole surrounded by a homogeneous and isotropic formation, can be reconstructed with the aid of an equivalent force system (EFS) that mimics the geometrodynamic effects of the borehole. The advantages of the EFS are twofold. In the first place, it simplifies the physical setup and brings many seemingly different problems into a common denominator in a sense that they are reduced to fields of known basic force systems. Second, and this is not less important, much computer time is saved and numerical complexities are avoided.
Approximate calculation of low‐frequency three‐dimensional magnetotelluric responses using a multiple plate model
Authors: Flores Carlos , Edwards R. Nigel
Published: October 2002

The numerical methods now used to calculate the magnetotelluric (MT) response of three‐dimensional (3-D) conductive inhomogeneities are still expensive in terms of computing cost. This is a more severe problem in many actual field conditions where multiple conductors exist. There is then a clear need for the development of faster algorithms for data interpretation. We can rapidly compute the 3-D MT response of a multiple‐conductor structure embedded in a layered earth at low frequencies using an approximate algorithm where each conductor is simulated by a set of plates. There are two limitations to the technique. The first low‐frequency limitation is a requirement that the induction number of any plate in the model be small compared with unity. The second condition specifies that the range from an observation point on the surface of the earth to a typical plate must be small compared with the skin depth in the layered background medium. These conditions allow us to ignore local self‐induction within the plates and mutual induction between the plates and the host medium. The secondary anomalous fields generated by any plate are assumed to be due to a distribution of current dipoles in the plane of the plate. The strengths of the distributed dipoles are found by solving a Fredholm integral equation of the second kind with the layered earth MT electric field at depth as the driving function. MT responses obtained with our approximate algorithm for three prismatic models compare favorably with previously published data obtained with complete algorithms for the same models. The use of the technique to model multiple conductors is illustrated with controlled source MT data collected at the Cavendish Test Site.
Marine PSSP reflections with a bottom velocity transition zone
Authors: Kim N. W. , Seriff A. J.
Published: October 2002

Marine shear‐wave reflection methods using the conventional data acquisition system (i.e., source and receiver in water) rely on two mode conversions at the water bottom to produce shear reflections such as PSSP. Some theoretical considerations and the results of a marine check shot survey conducted in the Gulf of Mexico demonstrate that the difficulty in observing PSSP events is attributable to weak P-S and S-P conversion at the bottom in regions with very low shear velocity (a few hundred ft/s or less) sediments at the bottom. For a simple water bottom with a low shear‐wave velocity, water over a uniform half space, the PS conversion factor is proportional to [Formula: see text] and the SP conversion factor is proportional to [Formula: see text], where [Formula: see text] is the bottom shear velocity. For [Formula: see text] their product gives PSSP reflections that can be comparable in amplitude to typical PPPP events. For [Formula: see text], the PSSP events should be about 30 dB weaker and probably not visible. For typical Gulf of Mexico sediments with a shear velocity transition zone several tens of feet thick at the bottom, the situation is even worse, since the velocities start near zero and may not reach 500 ft/s. This condition is common in many areas of recent sedimentations.
The application of high‐resolution seismics in Polish coal mining
Authors: Pietsch Kaja , Ślusarczyk Ryszard
Published: October 2002

Use of high‐resolution seismics were applied to the exploration and development of coal deposits in the Upper Silesian and the Lublin Coal Basins in Poland. High‐resolution seismic surveys were carried out in various geological and mining conditions. The objectives were to map commercial coal seams, locate coal‐less zones and faults, and solve some problems associated with mining conditions, e.g., determining the height of caving roof zone. Theoretical modeling studies of seismic wave response for several geological models representing the coal‐basin structures were used to address these problems. Results from these successful surveys helped improved designs of new mines and/or expand existing ones to provide more productive and safer coal mines.
Generalization of traveltime inversion
Authors: Herman Gérard C.
Published: October 2002

A nonlinear inversion method is presented, especially suited for the determination of global velocity models. In a certain sense, it can be considered as a generalization of methods based on traveltimes of reflections, with the requirement of accurately having to determine traveltimes replaced by the (less stringent and less subjective) requirement of having to define time windows around main reflections (or composite reflections) of interest. It is based on an error norm, related to the phase of the wavefield, which is directly computed from wavefield measurements. Therefore, the cumbersome step of interpreting arrivals and measuring arrival times is avoided. The method is applied to the reconstruction of a depth‐dependent global velocity model from a set of plane‐wave responses and is compared to other methods. Despite the fact that the new error norm only makes use of data having a temporal bandwidth of a few Hz, its behavior is very similar to the behavior of the error norm used in traveltime inversion.
Topographic responses in magnetometric resistivity modeling
Authors: Yang Chieh‐Hou , Tseng Hung‐Wen
Published: October 2002

The magnetometric resistivity (MMR) topographic responses due to earth topography were simulated using a finite‐element method. An algorithm was developed and the computer program was verified by comparison with analytic responses for half‐space and contact models. The topographic responses for different rugged surfaces were computed, and the model results indicate topographic effects can affect MMR sounding interpretation. In general, MMR topographic responses do depend on surface form; the more rugged the ground surface is, the larger the MMR topographic anomaly will be. These topographic effects will decrease as the distance between the source (and/or receiver) position and the high relief area is increased. We only address the problem of determining MMR anomalies over a two‐dimensional (2-D) topography. A numerical example illustrates an effective means of reducing the terrain effects for a 45‐degree dipping fault model incorporating a 45‐degree ramp surface, suggesting that the finite‐element modeling technique does provide a means of determining topographic correction for MMR sounding data.
Canceling stationary sinusoidal noise
Authors: Linville A. Frank , Meek Robert A.
Published: October 2002

Sinusoidal noise often contaminates seismic data. When this noise is large compared to seismic signals, it adversely affects prestack seismic processing and subsequent interpretation. We develop a digital least‐squares filtering algorithm for canceling stationary sinusoidal noise in seismic data. The method effectively cancels sinusoidal noise when the noise is stationary, which is typical for recordings of a few seconds in length. This procedure differs from the usual notch‐filtering techniques because the sinusoidal noise is canceled without notching the signal spectrum. Since the method requires that the line frequency be accurately known, the algorithm can automatically search the trace spectrum to find the exact sinusoidal frequency value needed for filter design. The algorithm is highly automated and requires no input parameters when the interference comes from power lines or generators. We use model and field data to quantify the algorithm’s performance.
Spectral IP studies of Finnish ore prospects
Authors: Vanhala Heikki , Peltoniemi Markku
Published: October 2002

Spectral‐induced polarization (IP) field measurements, both in the frequency domain and in the time domain, were made at six early Proterozoic sulfide, oxide, and graphitic‐gneiss deposits in Finland. Core samples were also measured. The textures of the mineralizations were studied from thin and polished sections. Deposits with large differences in texture, such as graphitic gneiss and coarse‐grained disseminated sulfide, can be separated on the basis of their phasespectra time constants. A good correlation was found between the observed grain size in thin sections and the grain size calculated from the apparent, field‐survey phase spectra in the case of homogeneous, disseminated textures. The measured frequency‐domain phase spectra, and phase spectra calculated from the time‐domain Cole‐Cole parameters are very similar in a comparable frequency band. For both techniques, the phase maximum lies beyond the lowest measured frequency. The possibilities to expand the frequency band to levels low enough to reach the phase maximum seem to be restricted. However, the results indicate that contrasts between spectra from different ore‐texture types also appear at higher frequencies.
The influence of fluids on grain contact stiffness and frame moduli in sedimentary rocks
Authors: Tutuncu Azra N. , Sharma Mukul M.
Published: October 2002

The frame moduli of sedimentary rocks are strongly influenced by the properties of the grain contacts. A modified Hertz contact theory is presented for the self consistent calculation of deformation, equilibrium separation distance (film thickness), and contact area of deformation for two spherical asperities in contact and subjected to an external load. We show that surface forces, i.e., electrostatic repulsion, Born, structural, and Van der Waals forces can be incorporated into the contact deformation problem. From the results presented, it is evident that surface forces play an important role in determining seismic wave velocities and attenuations at low confining stresses. The velocities and attenuations computed from the model are compared with measured values for glass beads, Navajo, Berea, Obernkirchner, and Fort Union sandstones. The velocities and attenuations calculated as functions of stress, frequency, fluid type, and saturation are in good agreement with reported experimental data.
VSP measurement of shear‐wave velocities in consolidated and poorly consolidated formations
Authors: O’Brien John
Published: October 2002

Mode conversion in the subsurface can generate shear waves with sufficient amplitude so that they can be used to measure shear‐wave propagation effects. Significant mode conversion can occur even at near vertical incidence if there is sufficient contrast in Poisson’s ratio across the interface. This can be exploited to measure shear‐wave velocities in the underlying section in the course of vertical seismic profile (VSP) acquisition. The technique is effective even in poorly consolidated formations with low shear‐wave velocities where sonic waveform logging fails. Where shear‐wave velocity data are available from sonic waveform logs, the VSP data can be used to verify the wireline data and to calibrate these data to seismic frequencies. The technique is illustrated with a case study from the North Slope, Alaska, in which several shear‐wave events are observed propagating downward through the subsurface. The seismic source is a vertical‐motion vibrator; shear waves are generated via mode conversion in the subsurface and also radiated from the source at the surface, and they are observed with both far‐ and near‐source offsets. The shear‐wave events are strong even on the near‐offset data, which is attributed to the contrast in Poisson’s ratio at the interfaces where mode conversion occurs. The technique is not limited to the hard surfaces of the Arctic and should work in any well, either land or marine, that penetrates shallow interfaces where mode conversion can occur.
Sideswipe removal via null steering
Authors: Duren Richard E. , Morris Stan V.
Published: October 2002

Null steering refers to the removal (or zeroing) of interferences at specified dips by creating receiving patterns with nulls that are aligned on the interferences. This type of beamforming is more effective than forming a simple crossline array and can be applied to both multistreamer and swath data for reducing out‐of‐plane interferences (sideswipe, boat interference, etc.) that corrupt two‐dimensional (2-D) data (the desired signal). Many beamforming techniques lead to signal cancellation when the interferences are correlated with the desired signal. However, a beamforming technique that has been developed is effective in the presence of signal correlated interferences. The technique can be effectively extended to prestack and poststack seismic data. The number of interferences and their dips are identified by a visual examination of the plotted data. This information can be used to design filters that are applied to the total data set. The resulting 2-D data set is free from the crossline interferences with the inline 2-D data remaining unaltered. Model and real data comparisons between null steering and simple crossline array summation show that: (1) null steering significantly attenuates crossline interference, and (2) 2-D inline data, masked by sideswipe, can be revealed once sideswipe is attenuated by null steering. The real data examples show the identification and effective attenuation of interferences that could easily be interpreted as inline 2-D data: (1) an apparent steeply dipping event, and (2) an apparent flat “bright spot.”
Case history: Seismic exploration in Egypt’s Eastern Desert
Authors: Mark Norman
Published: October 2002

Although oil exploration has been performed in the Eastern Desert of Egypt for over a century, seismic reflection techniques have only been in use for less than a fourth of that time. In an effort to improve seismic imaging of geologic targets, many styles of acquisition and processing have been tested, accepted, or discarded. Over the last twenty‐four years, seismic data acquisition has evolved from low‐channel analog to high‐channel digital recordings. The most difficult exploration problems encountered in these efforts have been the low‐frequency and high‐energy ground roll and depth of penetration when imaging the oil producing Pre‐Miocene sandy reservoirs below the highly reflective salt and evaporites. Efforts have been focused on developing seismic processing procedures to enhance the seismic data quality of recently acquired seismic data and developing new acquisition methods to improve seismic data through acquisition and processing. In older acquisition, the new processing has improved the seismic quality (vertical and lateral resolution), but it still retains a low‐frequency character. In the newly acquired seismic data, however, there is improved reflection continuity, depth of penetration, and resolution. We attribute this result to the change from low‐fold (6–24 fold), long receiver and source patterns (50 to 222 m) to high fold (96 fold) short receiver and source group (25 m), and spectral balancing in the processing. The most recent acquisition and processing have greatly improved the quality of the shallow seismic reflections and the deeper reflections that have helped unravel the structural and stratigraphic style of the deeper portions of the basin.
An analytical study of a two‐layer transient thermal conduction problem as applied to soil temperature surveys
Authors: Larson T. H. , Hsui A. T.
Published: October 2002

The soil temperature survey is an inexpensive exploration method in groundwater and geothermal resource investigations. In its simplest form, temperatures measured in shallow holes are analyzed to deduce variations in material properties. Typical interpretation schemes are based on simple, one‐layer solutions to the Fourier conduction equation using the annual solar cycle as a surface heat source. We present a solution to the more complicated two‐layer problem that can be computed using inexpensive personal computers and spreadsheet software. The most demanding mathematical requirement is the ability to manipulate a [Formula: see text] matrix. Testing the solution over a range of thermal diffusivity values expected in common soils and rocks reveals that the solution is very sensitive to variations in the thermal diffusivity of the surface layer and to the depth of the interface with the lower layer. When the boundary to the lower layer is less than about 10 m deep, a soil temperature survey is expected to be sensitive to the diffusivity variations in the lower layer. Because variations in shallow thermal properties often can be significant, this two‐layer method should be useful in areas with distinct shallow layering, (e.g., where there is a shallow water table or a thin soil layer).
Using constraints to address the instabilities of automated prestack velocity analysis
Authors: Stork Christof , Clayton Robert W.
Published: October 2002

Generalized prestack velocity analysis methods that use an automated approach to resolve laterally variable interval velocity fields are beset by a series of problems. The problem of resolving lateral velocity variations has inherent complications that prevent automated methods from being robust enough to be applied routinely to data from a variety of geologic provinces. The use of automated prestack velocity analysis methods will not eliminate the step of carefully producing an initial velocity model derived from regional geologic information and an interpretation of a conventionally processed section. For the methods to regularly produce useful additional information, the unique characteristics of each application must be input into the prestack velocity analysis with the use of inversion constraints. These constraints serve either to adapt the generalized prestack velocity analysis to a focused objective in a particular area or to provide iterative, interpretational tools that help the user produce a velocity model.
Adaptive noise cancellation for time‐domain EM data
Authors: Olsen Kim Bak , Hohmann Gerald W.
Published: October 2002

A typical time‐domain electromagnetic (TDEM) survey is carried out by recording the transient response of the target area due to currents induced by the abrupt turnoff of the current in a large ungrounded loop at the surface. The depth of exploration is controlled by the strength of the transient signal at late times, when the natural fields become comparable in amplitude to the controlled‐source response (Spies, 1989). Nichols et al. (1988) claim that it should be possible to reduce the natural magnetic fields in controlled source electromagnetic data by 40–60 dB because the fields are coherent over large distances, thus reducing the needed transmitter power and averaging time dramatically. This study tests a method using adaptive filtering of remotely measured magnetic fields to improve the signal‐to‐noise ratio for the late‐time response.
Gravitational attraction of a rectangular prism with depth‐dependent density
Authors: García‐Abdeslem J.
Published: October 2002

The gravity effect produced by two and three‐dimensional bodies with nonuniform density contrast has been treated by several authors. One of the first attempts in this direction made by Cordell (1973), who developed a method to compute the gravity effect due to a two‐dimensional prism whose density decreases exponentially with depth. A different approach was proposed by Murthy and Rao (1979). They extended the line‐integral method to obtain the gravity effect for bodies of arbitrary cross‐sections, with density contrast varying linearly with depth. Chai and Hinze (1988) have derived a wavenumber‐domain approach to compute the gravity effect due to a vertical prism whose density contrast varies exponentially with depth. Recently, Rao (1990) has developed a closed expression of the gravity field produced by an asymmetrical trapezoidal body whose density varies with depth following a quadratic polynomial.
Principles of spatial surface electric field filtering in magnetotellurics: Electromagnetic array profiling (EMAP)
Authors: Torres‐Verdín Carlos , Bostick Francis X.
Published: October 2002

Electromagnetic Array Profiling (EMAP) is an adaptation of magnetotellurics to overcome spatial aliasing effects associated with the sampling of the surface electric field. Undersampling lateral electric field variations can result in misleading geoelectric interpretations of the subsurface, particularly under the common presence of static distortion. In the EMAP field procedure, electric dipoles are positioned end‐to‐end along a continuous survey path; this configuration, in addition to reducing aliasing effects, lends itself to low‐pass filtering of the lateral electric field variations. We show that lengthening an electric dipole can reduce the static effect due to confined resistivity anomalies smaller than a dipole length. This modification of the sensor characteristics involves a spatial filtering process in which the cutoff wavenumber is inversely proportional to the length of the dipole. However, excessively long dipoles may not prove appropriate at high frequencies where the objective is to sense geoelectric features smaller than a dipole length. Thus, an array of adjacent dipoles along the survey path allows us to control the characteristics of the low‐pass filter employed to reduce static effects. A standard dipole length should be comparable with the shallowest depth of penetration at the highest frequency. The survey path, on the other hand, should be long enough to allow the suppression of static effects due to geoelectric structure of size comparable to the depth of penetration at the lowest frequency. Previously, we showed that a profile of the subsurface resistivity distribution can be estimated by inverting the inductive part of the surface electric field response derived from spatial filtering. For this purpose, the suggested wavenumber filter is one for which the cutoff wavenumber decreases with decreasing values of frequency, just in the same way the subsurface resistivity distribution is naturally averaged at depth to produce its inductive surface electric field response. Hence, the EMAP field procedure for the measurement of lateral electric field variations is suitable for an inversion of this type. We introduce a data‐adaptive spatial filtering technique that responds to changes in local average resistivity at a given frequency to estimate both effective depth of response and resistivity along the survey path. Application of this filtering procedure shows encouraging results in the interpretation of data simulated numerically from two‐dimensional models possessing different degrees of structural complexity. We also present results from a field study carried out in the northern basin and range geological province of Nevada.
Residual shot profile migration
Authors: Lee Wook B. , Zhang Lin
Published: October 2002

We have developed a residual shot profile migration technique, which consists of dip‐corrected residual normal moveout (NMO) and depth restretching. The dip‐corrected residual NMO equation and the depth restretching equation were derived by generalizing Al‐Yahya’s residual NMO equation. Using the dip‐correction residual NMO equation, velocity errors can be estimated more accurately than without the dip correction term in the residual NMO equation. Residual shot profile migration was applied to migrated prestack data in a manner similar to conventional processing by cascading residual velocity analysis, residual NMO, and stack and depth stretching. With residual migration, we can either avoid remigration of the original prestack data or reduce the number of iterations required to produce a satisfactory image. Residual migration is efficient enough to be implemented on a workstation. Significant improvement in imaging to cases (a pinchout and salt top and bottom) is demonstrated using synthetic and field data examples.
Using downward continuation to reduce memory requirements in reverse‐time migration
Authors: Harris Christopher E. , McMechan George A.
Published: October 2002

In conventional reverse‐time depth migration, successive time slices from a stacked seismic section are inserted as boundary values at the top of the image space, and the wavefield is extrapolated backward in time until time = 0. The resulting fixed‐time snapshot of the propagating wavefield, in space, is the migrate d image.
Singular value decomposition of the velocity‐reflector depth tradeoff, Part 2: High‐resolution analysis of a generic model
Authors: Stork Christof
Published: October 2002

The symmetries of a block circulant matrix significantly reduce the computational expense of the singular value decomposition (SVD) of the variable velocity inverse problem for a generic reflection seismology model. As a result, the decomposition does not suffer from edge effects or parameterization artifacts that are associated with small model spaces. Using this approach, we study the eigenvector and eigenvalue characteristics for a generic model of a size as large as is used with a variety of iterative inversion techniques (>100 000 parameters). Singular value decomposition of the raypath inverse problem of a discretized generic seismic model having one reflector indicates that the eigenvalue distribution for the inverse problem is nonuniform, with a large concentration near 0 and a gap near 0.4. All but the long horizontal wavelength reflector‐depth variations cannot be uniquely resolved from velocity variations. Lateral velocity variations serve to significantly reduce the ability of seismic data to resolve reflector depth for most of the horizontal wavelength components shorter than twice the cable length. As a result, automatic velocity analysis methods may not be able to resolve reflector variations when the velocity field is allowed to take on an arbitrary structure.
A linear filter approach to designing off‐diagonal damping matrices for least‐squares inverse problems
Authors: Squires Livia J. , Cambois Guillaume
Published: October 2002

Many geophysical data processing applications are formulated as either linear or linearized‐nonlinear inverse problems. Usually, the linear systems that describe these problems are ill‐posed. For example, seismic deconvolution is an ill‐posed inverse problem because the seismic wavelet is band limited. Borehole transmission tomography is ill‐posed because of insufficient ray coverage around the tomographic model (Worthington, 1984). Surface‐consistent statics’ estimation is intrinsically ill‐posed because of the geometrical relationships between the parameters defined by the surface-consistent model (Taner et al., 1974).