bluuebunny/crossref_metadata_2025 huggingface dataset:
Application of seismic refraction methods in groundwater modeling studies in New England
Authors: Haeni F. P.
Published: October 2002

Seismic refraction methods have been extensively used in petroleum, mineral, and engineering investigations, and to some extent for hydrologic applications, during the past 30 years. Recent advances in equipment, sound sources, and computer interpretation techniques make seismic refraction methods highly effective and economical for obtaining data for groundwater modeling studies. Aquifers that can be defined by one or more seismic velocity surfaces, such as alluvial or glacial deposits in consolidated rock valleys, limestone, or sandstone underlain by metamorphic or igneous rock, or saturated unconsolidated deposits overlain by unsaturated unconsolidated deposits, are ideally suited for seismic refraction methods. Seismic refraction allows economical collection of data for one or more model input parameters and provides the basis for efficient collection of subsurface data by test drilling or aquifer tests. Seismic refraction studies were conducted over unconfined glacial aquifers in New England to determine: (1) depth of the underlying bedrock; (2) depth of the water table; (3) saturated thickness of the aquifer in areas not accessible to heavy drilling equipment; (4) areas where thick, unsaturated sediments overlie thickly saturated parts of the aquifer; and (5) locations of test holes and type of drilling equipment needed. These data were used in groundwater models and provided parameter values that required few adjustments during calibration.
On: “The inverse problem of resistivity sounding” by R. L Parker (GEOPHYSICS, 49, 2143–2158, December 1984
Authors: Szaraniec Edward
Published: October 2002

Parker’s paper undoubtedly is an important contribution to the theory and interpretation of resistivity sounding, but I would like to make some criticisms and objections.
A finite‐element solution for the transient electromagnetic response of an arbitrary two‐dimensional resistivity distribution
Authors: Goldman Y. , Hubans C. , Nicoletis S. , Spitz S.
Published: October 2002

We present a numerical method for solving Maxwell’s equations in the case of an arbitrary two‐dimensional resistivity distribution excited by an infinite current line. The electric field is computed directly in the time domain. The computations are carried out in the lower half‐space only because exact boundary conditions are used on the free surface. The algorithm follows the finite‐element approach, which leads (after space discretization) to an equation system with a sparse matrix. Time stepping is done with an implicit time scheme. At each time step, the solution of the equation system is provided by the fast system ICCG(0). The resulting algorithm produces good results even when large resistivity contrasts are involved. We present a test of the algorithm’s performance in the case of a homogeneous earth. With a reasonable grid, the relative error with respect to the analytical solution does not exceed 1 percent, even 2 s after the source is turned off.
Stratigraphic filter theory: Combined effects of parallel bedding and random inhomogeneities
Authors: Lerche I.
Published: October 2002

Mean field renormalization techniques determine the phase distortion (time delay) and effective attenuation as functions of frequency for a plane acoustic wave, normally incident on a layered medium, when the medium also contains a distribution of scattering centers at random sites. The power spectra of impedance fluctuations of both the bedding layers and the random centers contribute to the time delay and mean field effective attenuation. At low frequencies (long wavelengths), the power spectrum of the bedding planes dominates the mean wave response, producing most of the time delay and the effective attenuation. At high frequencies (short wavelengths), the random scattering centers dominate the mean wave behavior. The wave no longer propagates exactly perpendicularly to bedding planes since the random scattering centers cast energy into directions transverse to the normal to the bedding planes. The precise frequency dependence of both the time delay and the effective attenuation of the mean wave are sensitive to the power spectrum of impedance fluctuations of the bedding planes relative to the power spectrum of the random scattering centers.
Analysis of exploitation‐induced gravity changes at Wairakei Geothermal Field
Authors: Allis Richard G. , Hunt Trevor M.
Published: October 2002

Gravity changes (corrected for subsidence) of up to -1 000 (±300) μGal have occurred in the [Formula: see text] area of the production bore field at Wairakei, and smaller decreases extend over a [Formula: see text] surrounding area. The largest part of these decreases occurred during the 1960s; since then the net gravity change for the whole field has been zero, indicating mass flow equilibrium. The principal causes of gravity change have been deep liquid pressure drawdown which resulted in formation of a steam zone, subsequent saturation changes in the steam zone, liquid temperature decline, and groundwater level changes. Gravity models suggest saturation of the steam zone was 0.7 (±0.1) in 1962 and decreased to 0.6 by 1972. Gravity increases in the northern and eastern bore field since the early 1970s are attributed to cool water invading the steam zone.
Nonlinear inversion of gravity data using the Schmidt‐Lichtenstein approach
Authors: Granser Harald
Published: October 2002

The nonlinear inversion of the gravity from a single density interface can be performed through a power series expansion. The method is based on the Schmidt‐Lichtenstein approach for solving nonlinear integral equations. After expanding the nonlinear integral operator for the gravity effect as an operator power series, the inverse operator series is found by applying a technique formally equivalent to the classical inversion scheme of a scalar power series. Unlike the forward power series expansion, however, the convergence of the inverse series is restricted to a low‐frequency domain, characterized by a cutoff frequency that is dependent upon the amplitude of the gravity anomaly, the magnitude of the density contrast, and the mean depth of the interface. To ensure the stability of the inversion scheme, a suitable low‐pass filtering has to be performed. By taking advantage of the noniterative nature of the inversion scheme and the fast Fourier transform, the method is efficiently applied to invert a profile‐like, simulated model and a 3-D field example (the Malcov gravity anomaly) caused by a small sedimentary basin in the East Slovakian Outer Carpathians.
Physical properties of shale at temperature and pressure
Authors: Johnston David H.
Published: October 2002

Deformational properties, P‐wave and S‐wave velocities, and electrical resistivity were measured for three North Sea Malm shales in the laboratory under pressures to 800 bars and temperatures to 100 °C. These data were used to evaluate how factors such as mineralogy, microstructure, compaction, and pore‐fluid conductivity affect a shale’s seismic and electrical responses. Deformation in the shales is dominated by inelastic processes which cause time‐dependent changes in velocity, resistivity, and pore pressure. Overall, shales are less sensitive to pressure changes as compared to sandstones of similar porosity. However, changes in temperature result in large changes in physical properties as compared to sandstones or shaly sands. P‐wave and S‐wave velocities may decrease by as much as 10 percent over the temperature range studied, and calculated activation energies for surface conduction are nearly twice those observed in shaly sands. These comparisons emphasize fundamental differences in fabric among the clastic rocks and suggest that solid‐liquid physical and electrical interactions may play an important role in controlling a shale’s seismic or electrical response to compaction. The results of this study have impact on the well‐log interpretation of shaly sands and the determination of shale properties from seismic data.
On the theory of sea‐floor conductivity mapping using transient electromagnetic systems
Authors: Cheesman S. J. , Edwards R. N. , Chave A. D.
Published: October 2002

The electrical conductivity of the sea floor is usually much less than that of the seawater above it. A theoretical study of the transient step‐on responses of some common controlled‐source, electromagnetic systems to adjoining conductive half‐spaces shows that two systems, the horizontal, in‐line, electric dipole‐dipole and horizontal, coaxial, magnetic dipole‐dipole, are capable of accurately measuring the relatively low conductivity of the sea floor in the presence of seawater. For these systems, the position in time of the initial transient is indicative of the conductivity of the sea floor, while at distinctly later times, a second characteristic of the transient is a measure of the seawater conductivity. The diagnostic separation in time between the two parts of the transient response does not occur for many other systems, including several systems commonly used for exploration on land. A change in the conductivity of the sea floor produces a minor perturbation in what is essentially a seawater response. Some transient responses which could be observed with a practical, deep‐towed coaxial magnetic dipole‐dipole system located near the sea floor are those for half‐space, the layer over a conductive or resistive basement, and the half‐space with an intermediate resistive zone. The system response to two adjoining half‐spaces, representing seawater and sea floor, respectively, is derived analytically. The solution is valid for all time, provided the conductivity ratio is greater than about ten, or less than about one‐tenth. The analytic theory confirms the validity of numerical evaluations of closed‐form solutions to these layered‐earth models. A lateral conductor such as a vertical, infinite, conductive dike outcropping at the sea floor delays the arrival of the initial crustal transient response. The delay varies linearly with the conductance of the dike. This suggests that time delay could be inverted directly to give a measure of the anomalous integrated conductance of the sea floor both between and in the vicinity of the transmitter and the receiver dipoles.
Periodic motion of a piecewise homogeneous medium with complex damping
Authors: Langhaar H. L. , Boresi A. P.
Published: October 2002

A piecewise homogeneous medium with complex damping (Q damping) is excited by a prescribed periodic motion at the origin. The differential equations for the associated periodic motion of the medium are shown to be of second order in terms of the space‐dependent displacement components. The continuity conditions at an interface between disparate parts require specific conditions on the stress vector and the displacement components. We solve the one‐dimensional case completely, in which the displacement vector depends upon a single space coordinate, for a bilayered medium. An explicit formula for the energy dissipation in a layer, based upon the work input to the medium, shows that the energy dissipated is zero when no damping is present and is positive for positive damping.
On: F. Bryan Davies’ discussion of “Airborne gravity is here,” by S. Hammer, and related discussions with replies by S. Hammer (GEOPHYSICS, 51, 1502–1503, July 1986).
Authors: Steenland Nelson C.
Published: October 2002

Attenuation of potential field anomalies with increasing distance from the source is, of course, independent of sampling, so the Nyquist problem may always be avoided with increased cost, as Davies pointed out. However, attenuation does produce a complete loss of some signal with increasing distance because instruments operate in a discrete or space domain. So all instruments are filters in this sense, and no amount of sampling overcomes this unfortunate characteristic.
Techniques for improving Vibroseis data
Authors: Schrodt Joseph K.
Published: October 2002

An ongoing program of field experiments to analyze vibrator‐earth response has verified two major problems in Vibroseis® data acquisition: amplitude and phase control of vibrator output, and suppression of vibrator‐generated harmonic distortion. This study confirms that these problems are more severe on rigid ground (i.e., frozen tundra) than on soil. The test results demonstrate that a large‐amplitude vibrator resonance at 40 Hz and a higher frequency (equal to or greater than 100 Hz) earth‐baseplate resonance dominate the vibrator‐frozen ground response. Adaptive, closed‐loop amplitude control of ground force virtually eliminates baseplate‐ground decoupling at the resonant frequencies while maximizing force output throughout a sweep. Analysis of far‐field downhole signals indicates that phase locking to ground force, along with amplitude control, provides a more stable downgoing wavelet than phase locking to baseplate or reaction mass acceleration. Quantitative spectral analysis of downhole and vibrator accelerometer signals shows severe harmonic distortion and demonstrates the suppression of all even harmonics and certain deleterious odd harmonics after rotating the phase of successive sweeps by a phase increment of less than 180 degrees. ®Trade and service mark of Conoco Inc.
Inversion of reflection traveltimes and amplitudes
Authors: Macdonald C. , Davis P. M. , Jackson D. D.
Published: October 2002

A Bayesian nonlinear inversion scheme is used to invert traveltime and amplitude data from supercritical reflections. Reflection amplitudes are determined using a spherical‐wave model, where the amplitude of a reflected spherical wave is given by a Sommerfeld integral. This integral is evaluated numerically by integrating over a contour in the complex plane. The spherical‐wave model differs from plane‐wave models in the regions around the critical points for P and S refraction. In these regions, the spherical‐wave amplitudes are sensitive to frequency with a dependance on the ratio H/λ, between the reflector depth H and the seismic wave‐length λ. The accuracies of the inversion scheme and the spherical reflection model are tested using control data from the University of Houston physical‐model tank. The solution obtained by inversion agrees with the known solution to within 3 percent for all parameters and the amplitudes predicted by the spherical‐wave model agree remarkably well with the observed data.
Q measurements from compressional seismic waves in unconsolidated sediments
Authors: Badri Mohammed , Mooney Harold M.
Published: October 2002

Several processing methods in both time and frequency domains have been used to compute the in‐situ specific dissipation function 1/Q, or its inverse, the quality factor Q, in water‐saturated unconsolidated sediments. These methods are based on measurements of spectral amplitude ratio, peak‐to‐peak and first‐peak amplitude ratio, rise time, pulse broadening, and the Futterman causal attenuation operator of an attenuating signal. Compressional seismic waves were generated from explosive sources ranging in size from 1 to 64 mg of silver azide and recorded at a depth of 7.62 m below the surface near Wendover, Utah. The medium consisted of silty sandy clays with mean grain size diameter of 7.48 μm and water saturation of 70 percent. The hydrophone receivers were spirally distributed at distances ranging from 25 to 200 m from the source. An average Q value of 26 was obtained using analysis of spectral amplitude ratios over the frequency range 450–725 Hz for the five different sizes of explosive sources. Measurements of peak amplitude ratio produced an average Q value of 123. The computed Q value from the rise‐time method is found to have a wide range of Q values, from 50 to 207. The Q values determined from the pulse‐broadening technique ranged from 25 to 158 for quarter‐cycle measurements and from 26 to 114 for half‐cycle measurements. The Futterman causal attenuation operator yields high Q values, on the order of 200 to 300. The wide variation of the computed Q values indicates that different computational techniques can result in different Q values for the same type of materials. Results show that the rise‐time and pulse‐broadening methods are probably source‐dependent. This makes the spectral amplitude‐ratio method preferable to these two methods since it is independent of the source. A correct geometrical spreading factor appears to be sufficient to account for the observed amplitude decay with distance, which makes the Q value computed from the Futterman operator questionable. This suggests that the Q value computed from the spectral‐ratio method is probably more reliable than values computed by other techniques.
COCORP: Northwest Cordillera and Southern Appalachian regions
Authors: Kaufman S.
Published: October 2002

The Consortium for Continental Reflection Profiling (COCORP) announces the availability of seismic reflection data sheets, map sheets, and digital tapes for two regions: (1) the Northwest Cordillera area covering 532 line‐km consisting of Washington lines 1–5, 7, 8; Idaho lines 1, 2; and Montana lines 1, 2; and (2) the Southern Appalachian area covering 1073 line‐km consisting of Florida lines 1, 2, 4; and Georgia lines 10–21, 24. The COCORP operation is part of the U.S. Geodynamics Program sponsored by the National Academy of Sciences and funded by the National Science Foundation. The executive group of the consortium consists of representatives from Cornell University, Dartmouth College, Princeton University, Rice University, and the University of Wisconsin. Cornell University is the operating institution.
Three‐dimensional model results for an electrical hole‐to‐surface method: Application to the interpretation of a field survey
Authors: Le Masne D. , Poirmeur C.
Published: October 2002

An important objective of borehole geophysics is to maximize information about inhomogeneities in the vicinity of existing boreholes. Three‐dimensional models (parallelepipedic inhomogeneities in a homogeneous half‐space) are used here to characterize the responses of an electrical hole‐to‐surface method ELECENT (ELECtrode ENTerrée) where conductive or resistive inhomogeneities (orebodies) occur in the vicinity of a borehole. Two types of surface measurements are carried out simultaneously at each station: electrical potential V (as in mise à la masse), and associated electrical field E. These two measured parameters give rise to three other parameters, two apparent resistivities (one for each V and E case) and another parameter quantifying the orientation of the E field. The influence of various geometrical and electrical parameters (such as the horizontal and vertical distances between the current source, the stations, and the inhomogeneity, and the size and resistivity of the inhomogeneity) on the five parameters above appears to be important. Values and positions of the extrema of these five parameters are characteristic of the geometrical and electrical parameters of the models. The orientation of the E field, for instance, is found to be critical to determining the shape of the inhomogeneity and its depth relative to the current source. The theoretical results obtained are used to interpret a field survey carried out around a borehole at the Beauvain prospect in France. Field maps of the five parameters show characteristics similar to the models described in the theoretical part of the study.
Effect of source spectrum on seismic attenuation measurements using the pulse‐broadening method
Authors: Liu Hsi‐Ping
Published: October 2002

Numerical examples of one‐dimensional (1-D) wave propagation, using realistic source time functions in an anelastic material characterized by frequency‐independent internal friction, demonstrate that (a) the source time function strongly influences the dependencies of initial rise time τ and pulse width w on internal friction [Formula: see text] and distance x; (b) in general, τ and w have different functional dependencies on [Formula: see text] and x; and (c) the slope ∂τ/∂x for particle displacement computed for a band‐limited source time function can be either greater than or less than the corresponding value computed for a delta‐function displacement source time function. Result (a) corroborates the result for τ given by Blair and Spathis, which implies that the commonly used linear rise time and distance relation, [Formula: see text], where T is the traveltime and C a source‐independent constant, is an oversimplification of 1-D anelastic wave propagation; the source spectrum must be considered when inferring seismic attenuation from changes in rise time. Result (c) contradicts the assertion by Blair and Spathis that ∂τ/∂x of wave trains generated by a band‐limited source time function is always less than the corresponding value generated by a delta‐function source time function. By matching first arrival times and rise times obtained by modeling 1-D wave propagation in an anelastic medium to those obtained in the field, seismic attenuation can be determined.
Estimation of formation temperature from bottom‐hole temperature measurements: COST ♯1 well, Norton Sound, Alaska
Authors: Cao S. , Hermanrud C. , Lerche I.
Published: October 2002

We recently developed a numerical method, the Formation Temperature Estimation (FTE) model, to determine formation temperatures by inversion of borehole temperature (BHT) measurements (Cao et al., 1988a). For more than two BHT measurements, the FTE model can estimate (1) true formation temperature [Formula: see text], (2) mud temperature [Formula: see text] at the time the mud circulation stops, (3) thermal invasion distance R into the formation before the formation is at the true formation temperature, (4) formation thermal conductivity K perpendicular to the borehole, and (5) efficiency factor F for mud heating in the borehole after mud circulation has stopped. The method optimizes three free parameters: τ (diffusion time‐scale), ε (scaling parameter related to the thermal invasion distance R), and [Formula: see text] (normalized efficiency factor for mud heating.
Aeromagnetic regional survey of onshore Australia
Authors: Tucker D. H. , Hone I. G. , Downie D. , Luyendyk A. , Horsfall K. , Anfiloff V.
Published: October 2002

The Australian Bureau of Mineral Resources (BMR) is responsible for the National Airborne Magnetic Database. This data base consists of results from approximately 3 500 000 line‐km of regional survey flying carried out over 35 years, recording total magnetic intensity. The magnetic data base is one of the most important geophysical data bases for Australia and is used extensively by the minerals and petroleum exploration industries. First‐pass coverage of onshore Australia is aimed for completion in 1992. This coverage contains data from surveys with a wide range of specifications, resulting in a wide range of data quality; some of the areas covered by poorer quality data may be reflown later. For the most part, the intention has been to acquire data at a continuous ground clearance of 150 m and with a line spacing of 1500 m. However, over some sedimentary basins, the line spacing is in excess of 3200 m. New color and grey‐scale (image processed type) digital magnetic maps (pixel maps) are in preparation; these will supersede the 1976 digital magnetic map of Australia, which was gridded on a 1.2 minute mesh (2000 m) mostly by digitizing contours on maps. The new map, produced from flight‐line data, will have a grid size of 0.25 minutes. Initially, a series of maps will be produced with each one covering a block of 4 degrees latitude by 6 degrees longitude, coinciding with standard 1 : 1 000 000 map sheets. An example included for the Adelaide 1 : 1 000 000 map sheet in Southern Australia shows a dramatic increase in the number of anomalies over those that were evident in earlier contour presentations.
Low‐frequency tube waves in permeable rocks
Authors: Chang S. K. , Liu H. L. , Johnson D. L.
Published: October 2002

Low‐frequency tube waves in permeable boreholes are of interest because their propagation depends upon in‐situ rock permeability. In this paper, we use low‐ frequency asymptotics to unify two approaches that both describe tube waves. The first approach is due to White, in which the low‐frequency tube‐wave velocity and attenuation are expressed explicitly as functions of frequency. The expressions include contributions from three factors: the borehole fluid compressibility, the wall rigidity, and the wall impedance due to the permeability of the solid. The second approach is the full solution of tube‐wave properties based on Biot theory and the appropriate boundary conditions between the fluid and the porous solid. By taking the low‐frequency approximations to the full solution, we derive an explicit expression for the tube‐wave properties, similar to White’s expression. White’s formulation is hereby corrected to include the elasticity of the solid frame around the borehole. For a 20 cm diameter borehole, the corrected low‐frequency expression is quite accurate below 300 Hz for the four cases shown, when compared with the full solution. Above that frequency, one should use the full solution to obtain an accurate expression of tube‐wave properties in permeable rocks.
Two‐dimensional modeling of a towed in‐line electric dipole‐dipole sea‐floor electromagnetic system: The optimum time delay or frequency for target resolution
Authors: Edwards R. N.
Published: October 2002

Towed in‐line transient electric dipole‐dipole systems are being used to map the electrical conductivity of the sea floor. The characteristic response of a double half‐space model representing conductive seawater and less conductive crustal material to a dipole‐dipole system located at the interface consists of two distinct parts. As time in the transient measurements progresses, two changes in field strength are observed. The first change is caused by the diffusion of the electromagnetic field through the resistive sea floor; the second is caused by diffusion through the seawater. The characteristic times at which the two events occur are measures of sea‐floor and seawater conductivity, respectively. Entirely equivalent responses are observed in a frequency‐domain measurement as frequency is swept from high to low. The simple double half‐space response is modified when the towed array crosses over a conductivity anomaly. I evaluate the magnitude of the anomalous response as a function of delay time and frequency using a two‐dimensional theory and a vertical, plate‐like target. If the ratio of the conductivity of the seawater to that of the sea floor is greater than unity, then an optimum time delay or frequency can be found which maximizes the response. For large conductivity contrasts, the optimum response is greater than the response at late time or zero frequency by a factor of the order of the conductivity ratio.
A method of ground‐roll elimination
Authors: Saatçilar Ruhi , Canitez Nezihi
Published: October 2002

Amplitude‐ and frequency‐modulated wave motion constitute the ground‐roll noise in seismic reflection prospecting. Hence, it is possible to eliminate ground roll by applying one‐dimensional, linear frequency‐modulated matched filters. These filters effectively attenuate the ground‐roll energy without damaging the signal wavelet inside or outside the ground roll’s frequency interval. When the frequency bands of seismic reflections and ground roll overlap, the new filters eliminate the ground roll more effectively than conventional frequency and multichannel filters without affecting the vertical resolution of the seismic data.
Threshold algorithms for the prediction of reflection coefficients in a layered medium
Authors: Koltracht Israel , Lancaster Peter
Published: October 2002

An algorithm is presented for the solution of the inverse problem of reflection seismology in the presence of noise. The algorithm is based on a new representation of reflection coefficients in terms of the recorded seismogram. This representation allows use of matrix perturbation methods for the analysis of error magnification in the recursive reconstruction of a stratified acoustic medium. Our analysis indicates that one of the main reasons for uncontrollable noise magnification is the assignment of significant values to very small reflection coefficients, most of which reflect only noise in the data rather than reflection information. Our analysis also allows one to decide when a small computed reflection coefficient should be set to zero. The strategy of setting small reflection coefficients to zero, which is called thresholding, has a stabilizing effect on inverse scattering algorithms. The threshold algorithm also permits adaptive change of noise barriers, which can be used for more detailed exposure of certain parts of a seismic section at the expense of its less important parts. These properties of the threshold algorithm are demonstrated on both synthetic examples and sets of seismic survey data. The general version of the threshold algorithms allows efficient implementation on modern computer architectures (such as parallel or pipelined computers). In particular, the algorithm can be implemented with linear complexity on parallel processors. Simplified versions of the general algorithm for special surface conditions are also presented.
Approximating subsurface structure in areas of severe subsea erosion—A nonprocessing technique
Authors: Lishman Fred W. , Christos Michael N.
Published: October 2002

Severe subsea erosion distorts seismic reflection times and velocity analyses and makes determining subsurface structure difficult. Although data reprocessing is the logical solution for removing these distortions, reprocessing can be expensive. We present a case history describing a nonprocessing depth‐conversion technique using a geologic erosional model. A grid of common‐midpoint seismic data located in and around several submarine canyons was used for this study. Establishing a geologic erosional model requires an accurate representation of the sea floor, which we obtain by map migration of the sea‐floor reflection. A velocity model was developed using only those analyses not adversely affected by sea‐floor erosion. To remove the effects of erosion from the arrival times of a mapped horizon, static corrections (velocity replacement and compaction) were developed. We replaced the water velocity in the eroded section with depth‐equivalent rock velocities from the velocity model. The compaction correction, which was derived empirically, is based on the assumption that porosity restoration occurred in the sediments beneath the canyons when erosion reduced the overlying pressure. Compaction correction in conjunction with velocity replacement produced structure maps (time and depth) that exhibit only minor effects of erosion. These results were further improved by applying dynamic corrections obtained by ray tracing a subsurface model to determine the traveltime through the water for the reflection from the mapped horizon. Our final structure maps demonstrate that a geologically reasonable structural interpretation in depth can be made in areas of severe subsea erosion without reprocessing the data.
Transient response of some borehole mandrel tools
Authors: Anderson Barbara , Chew Weng Cho
Published: October 2002

The response of transient mandrel tools in realistic well‐logging environments is simulated by first solving the problem analytically in the frequency domain using an efficient high‐speed computer model. Gaussian pulses of reasonable bandwidths are assumed. The geometry modeled includes a borehole, invasion layers, and bed boundaries. Both TE and TM type sources are considered. The resulting receiver voltages, displayed in variable‐density log (VDL) format, yield visually interpretable information regarding signal amplitude and attenuation. The speed and generality of the analytical model makes this approach practical for analyzing tool response in a wide variety of logging geometries. Preliminary results generated by this model demonstrate that transient electromagnetic logging tools have the potential to yield useful information for characterizing the conductivity and dielectric constant of beds penetrated by a wellbore. In the future, the model could be helpful in validating signal processing and data inversion algorithms.
On: “Accurate depth migration by a generalized phase‐shift method” by Dan Kosloff and David Kessler (GEOPHYSICS 52, 1074–1084, August 1987).
Authors: Pai D. M.
Published: October 2002

I would like to point out that the “generalized phase‐shift method” (Kosloff and Kessler, 1987) introduced for solution continuation — and referred to as a new method — is essentially identical to the “generalized Haskell matrix/layer eigenstate propagator (GHM/LEP)” method I presented earlier (Pai, 1985). Unfortunately, Kosloff and Kessler did not reference that paper.
Field array performance: Theoretical study of spatially correlated variations in amplitude coupling and static shift and case study in the Paris Basin
Authors: Berni A. J. , Roever W. L.
Published: October 2002

Near‐surface propagation anomalies degrade the performance of field arrays. We studied this problem by modeling the signal detected by a field array. In our model, the signal arrival time and amplitude were each varied with distance along the array according to some arbitrary spatial trend. Given the intensity and the correlation distance of the signal variations, both wavenumber selectivity for noise rejection and frequency response for desired signal can be calculated. We begin by describing diagnostic graphs that show an array’s attainable signal bandwidth and noise rejection capability. Next, we discuss the mathematical relationships between the graphs and observable quantities such as correlations, array lengths, geophone spacing, etc. Exponential correlation functions are used in the modeling study for illustrative purposes. The same diagnostics are then generated from measured correlations derived from experimental data acquired in the Paris Basin with a densely sampled geophone spread. We found that the bandwidth diagnostic was useful and easy to calculate for this data set. Data sets with stronger noise waves should allow an accurate calculation of noise rejection capability. The diagnostic graphs can help in choosing the number of channels, array length, and weighting in a particular exploration area.
VpVs from mode‐converted P-SV reflections
Authors: Iverson William P. , Fahmy Bill A. , Smithson Scott B.
Published: October 2002

P-SV reflections are generated by a compressional‐wave source and result from P waves that are converted to shear (SV) waves upon reflection. Recording both the P and SV components yields compressional and shear data simultaneously. Verifying that the easily detected events really are P-SV reflections is accomplished by noting the good correlation of surface CDP data with vertical seismic profile (VSP) reflections. Stacking velocities from P-SV CDP gathers determine the [Formula: see text] product when source‐to‐receiver offset is less than the depth of the reflector but data from synthetic models show that P-SV reflections are nonhyperbolic for shallow reflections or when source‐to‐receiver offset is too large. Shear velocity [Formula: see text] can be calculated from P-SV reflections by one of two techniques: comparison of stacked section P-P and P-SV reflection times or by using the P-P and P-SV stacking velocities. Unfortunately, most P-SV reflections on a P-SV seismic section do not necessarily originate from exactly the same depth as P-P reflections. When this depth discrepancy occurs, the reflection‐time comparison technique fails. In addition, [Formula: see text] cannot be calculated from P-SV reflections, and we must settle for the [Formula: see text] product from P-SV reflection stacking velocities. When P-SV stacking velocities are input to the familiar Dix equation, the resulting interval velocities yield the [Formula: see text] product.
On: “A rapid graphical method for the interpretation of the self‐potential anomaly over a two‐dimensional inclined sheet of finite depth extent” by H. V. Ram Babu and D. Atchuta Rao (GEOPHYSICS, 53, 1126–1128, August 1988).
Authors: Eskola L. , Hongisto H.
Published: October 2002

Ram Babu and Atchuta Rao (1988a, b) presented a graphical algorithm for the interpretation of a self‐potential anomaly over a sheet. Ram Babu and Atchuta Rao (1988b) also presented an inversion method based on iterative optimization for the self‐potential anomalies caused by spherical, cylindrical, and sheetlike bodies. The theoretical models on which the algorithms are based are very simple: for the sphere, an electrostatic dipole; for the cylinder, a line dipole; and for the sheet two line poles, the negative one along the upper edge of the sheet and the positive one along its lower edge.
Thermal infrared exploration in the Carlin trend, northern Nevada
Authors: Watson Kenneth , Kruse Fred A. , Hummer‐Miller Susanne
Published: October 2002

Experimental Thermal Infrared Multispectral Scanner (TIMS) aircraft data have been acquired for the Rodeo Creek NE 7 1/2 minute quadrangle, Eureka County, northern Nevada, covering the Carlin gold mine. A simple model has been developed to extract spectral emissivities for mapping surface lithology and alteration based on the physical properties of geologic materials. Emissivity‐ratio images were prepared that allow generalized lithologic discrimination, identification of areas with high silica content, and the first reported detection of the carbonate secondary reststrahlen feature. Together with thermal‐inertia images, they permit identification of areas of subtle lithologic variation not shown on conventional geologic maps. Emissivity‐ratio spectra extracted from the data compare favorably with modeled laboratory spectra and establish for the first time the link among theory, measured physical properties, and color‐composited TIMS images. Identification of previously unknown silicification in the Tertiary volcanic rocks and mapping of a silicified structure and possible extensions in alluvium indicate that TIMS data can be a valuable tool for detecting subtle alteration associated with mineralization, particularly in less accessible or less well‐known areas.
Subsurface imaging using reversed vertical seismic profiling and crosshole tomographic methods
Authors: Chen S. T. , Zimmerman L. J. , Tugnait J. K.
Published: October 2002

A combined reversed vertical seismic profile (RVSP), crosshole, and surface seismic experiment was performed to image the subsurface between two 1000 ft wells separated by 600 ft. Explosives were used as downhole sources and a 24-channel hydrophone streamer provided the downhole receivers. Surface shots were 1/3 lb dynamite charges and the surface receivers were vertical geophones. RVSP data, consisting of 48 offsets ranging from 25 ft to 1200 ft, were processed with a ray‐tracing method to produce an equivalent stacked seismic section. The crosshole data were processed with an SIRT traveltime transmission tomography algorithm to produce a velocity cross‐section. The stacked RVSP subsurface image and the crosshole tomography velocity cross‐section both have higher resolution than does the surface seismic image. For example, the velocity map from tomography exhibits a rapid lateral velocity change that is not resolved by surface seismic. However, all three methods agree within the limits of their common resolution. The results indicate that RVSP and crosshole tomography can potentially be used to image steeply dipping beds or complex structures, such as small faults, pinchouts, steam‐flooded zones, reef boundaries, or the flanks of salt domes.
Reduction of potential field data to a horizontal plane
Authors: Pilkington Mark , Urquhart W. E. S.
Published: October 2002

Most existing techniques for potential field data enhancement and interpretation require data on a horizontal plane. Hence, when observations are made on an irregular surface, reduction to a horizontal plane is necessary. To effect this reduction, an equivalent source distribution that models the observed field is computed on a mirror image of the observation surface. This irregular mirror image surface is then replaced by a horizontal plane and the effect of the equivalent sources is computed on the required horizontal level. This calculated field approximates the field reduced to a horizontal plane. The good quality of this approximation is demonstrated by two‐dimensional synthetic data examples in which the maximum errors occur in areas of steep topographic gradients and increased magnetic field intensity. The approach is also applied to a portion of a helicopter‐borne aeromagnetic survey from the Gaspé region in Quebec, Canada, where the results are a horizontal shifting of anomaly maxima of up to 150 m and changes in anomaly amplitudes of up to 100 nT.
Schlumberger soundings near Medicine Lake, California
Authors: Zohdy Adel A. R. , Bisdorf Robert J.
Published: October 2002

The use of direct current resistivity soundings to explore the geothermal potential of the Medicine Lake area in northern California proved to be challenging because of high contact resistances and winding roads. Deep Schlumberger soundings were made by expanding current electrode spacings along the winding roads. Measured apparent resistivities were corrected using the geometric factor for the exact array geometry instead of a linear array geometry. For horizontally stratified, laterally homogeneous media, the apparent resistivities measured with a nonlinear Schlumberger array are equal to those measured with a linear Schlumberger array provided that (a) distances from the current electrodes to the center of the array are equal, and (b) the proper geometric factor is used to calculate the apparent resistivity. Corrected sounding data were interpreted using an automatic interpretation method. Forty‐two maps of interpreted resistivity were calculated for depths extending from 20 to 1000 m. Computer animation of these 42 maps revealed that (a) certain subtle anomalies migrate laterally with depth and can be traced to their origin, (b) an extensive volume of low‐resistivity material underlies the survey area, and (c) the three areas (east of Bullseye Lake, southwest of Glass Mountain, and northwest of Medicine Lake) may be favorable geothermal targets. Six interpreted resistivity maps and three cross‐sections illustrate the above findings.
Density‐based reflectivity in seismic exploration for coal in Alberta, Canada
Authors: Lawton D. C. , Lyatsky H. V.
Published: October 2002

At a coal field in central Alberta, Canada, the acoustic reflectivity of shallow coal seams was found to be dominated by the density contrast between coal and host bentonitic sediments. Sonic logs and a check‐shot survey showed that the compressional‐wave velocity is almost constant through the coal zone and the overlying sediments, and ranges in value between 2000 m/s and 2350 m/s over different parts of the coal field. The average coal density is [Formula: see text], whereas the density of the sediments is about [Formula: see text]. Results are illustrated using logs from a typical drillhole in the coal field. At this location, the time reflectivity sequence based on both the density and sonic logs is very similar to that obtained when the density log only is used, with a constant velocity assumed through the coal zone. At another drillhole location in the coal field, where reflection seismic data had been acquired, a synthetic seismogram generated from the density log closely matches the stacked seismic section.
Case study: The importance of gas leakage in interpreting amplitude‐versus‐offset (AVO) analysis
Authors: Sengupta M. K. , Rendleman C. A.
Published: October 2002

The amplitude‐versus‐offset (AVO) method has been shown to indicate the presence of gas sands if the reflection amplitude from the seal/reservoir‐sand interface, measured in a common midpoint (CMP) gather, increases rapidly with increasing shot‐to‐geophone distance (or offset). However, in a few instances, it has been observed that the seismic reflection amplitude does not increase with offset and may even decrease if there is widespread gas leakage above the hydrocarbon reservoir causing partial gas saturation in the overburden sediments. Gas‐charged sediments are known to attenuate seismic energy. Depending on the size and shape of this gas leakage zone, there may be higher attenuation of seismic amplitudes with increasing offset. We present one such case that involves a prominent “bright‐spot” amplitude anomaly corresponding to a 56‐ft‐thick (17 m‐thick) gas sand in the Gulf of Mexico slope. The reflection amplitude for the sand top was found to decrease with increasing offset. There is also evidence of gas leakage into the sediments above the reservoir. Color amplitude displays of the seismic section show a low‐amplitude diffused zone above the bright‐spot amplitude anomaly, which suggests gas leakage. Further evidence of gas leakage can be inferred from the significant gas content (including heavier hydrocarbons) observed in the mud log. Gas leakage is also confirmed by gather modeling in which the effects of leakage‐caused attenuation are accounted for in matching the variation of seismic amplitude with offset.
Seismic traveltime equations for azimuthally anisotropic and isotropic media: Estimation of interval elastic properties
Authors: Sena Arcangelo G.
Published: October 2002

I derive analytical expressions giving traveltime‐offset curves for multilayered weakly azimuthally isotropic and anisotropic media on terms of the elastic properties of each layer. This method is based upon an approximate skewed hyperbolic moveout formula involving three measured bulk velocities for each reflector. The primary benefits of this technique are: (1) it allows for fast traveltime computation; (2) it makes possible an extremely rapid estimation of the interval elastic parameters; and (3) it provides physical insight into the wave propagation in anisotropic media. I establish a traveltime inversion algorithm which allows the estimation of the five elastic constants together with the orientation of the axis of symmetry for each layer. In the isotropic limit, this algorithm reduces to the conventional one used for determining the interval velocities from stacking velocity measurements in isotropic media. The inversion technique is applied to surface seismic measurements and VSP field surveys. In both cases, the method provides very good estimates of these six parameters, even in highly anisotropic materials. The results also show that the orientation of the horizontal axis of symmetry, for a given azimuthally anisotropic layer at depth, can be obtained using only quasi‐P‐wave information. This technique can be easily incorporated into conventional velocity analysis algorithms. Interval anisotropic properties can then be estimated with little additional effort.
Borehole effect on gravimetry
Authors: Sezginer A.
Published: October 2002

Borehole gravimetry measurements are affected by the presence of the borehole at the top and the bottom of the borehole. Less frequently recognized is a borehole effect in the vicinity of the formation bed boundaries across which density varies. The borehole effect is usually insignificant. In typical oil well conditions with 1-m station spacing, the borehole effect is on the order of 1 μgal, just below the sensitivity of the traditional gravimeters. However, the borehole effect can be significant when the station spacing is not much larger than the borehole radius and also in applications with small tolerance to error. For example, determination of oil saturation accurate to 10 percent requires on the order of 0.3 μgal/meter sensitivity. New gravimeter technologies (Prothero and Goodkind, 1968) and developments in gradiometers (Chan, Moody, and Paik, 1987) promise higher sensitivity and resolution, which call for more detailed environmental corrections. The purpose of this note is to examine the size of the borehole effect in this context.