bluuebunny/crossref_metadata_2025 huggingface dataset:
Practical aspects of an elastic migration/inversion of crosshole data for reservoir characterization: A Paris basin example
Authors: Beydoun W. B. , Delvaux J. , Mendes M. , Noual G. , Tarantola A.
Published: October 2002

The main obstacle for a detailed study of a reservoir is the lack of geophysical and petrophysical information between producing wells. Crosshole seismic data can aid reservoir geologists and engineers in (1) estimating the volume of oil in place, (2) mapping permeability/porosity barriers, and ultimately (3) monitoring and designing enhanced off‐recovery experiments. Due to their high‐frequency content, crosshole seismic data offer an ideal information link between conventional seismic data (surface reflection, VSPs, etc.) and full‐waveform sonic acoustic logs. Using a weightdrop downhole source located at one depth level, acoustic and multicomponent data at two different wells were collected in the Paris basin with interwell distances of about 100 m. The target zone includes three sand reservoir levels between depths of 575 and 600 m. A 2‐D elastic migration/inversion (M/I) of the scattered S‐S and S‐P crosshole field data produced high‐resolution S‐wave velocity and density depth images of the subsurface, extending information away from wells and identifying reservoirs. The residual energy reduction between synthetic seismograms derived from M/I images and field data is 18 percent, confirming that images contain elastic information. Structural dips obtained are very reasonable, the observed vertical spatial resolution being of the order of 3 m. We believe that this is the first time that such techniques have been applied to crosshole data. Elastic M/I images are generally better than elastic VSP‐CDP and migration images and have the advantage of producing global quality measures of images. Such a technique uses as input a background velocity, e.g., a tomogram obtained by traveltime tomography, and complements the background by recovering subsurface discontinuities and changes in elastic parameters within the signal bandwidth.
Efficient terrain corrections: A geostatistical analysis
Authors: Sprenke Kenneth F.
Published: October 2002

The farther out that terrain corrections for gravity surveys are computed, the more they cost. The conventionally recommended practice is to compute corrections out to a distance of 167 km. Explorationists, however, do not always require this much accuracy. Terrain corrections can be made cost‐effective by optimizing the distance to which they are carried in any given study area. First, the terrain is simulated by a mathematical model with the same statistical properties as the topography in the study area. Geostatistical procedures are then used to estimate the errors in carrying out terrain corrections to distances less than 167 km. As an example of the process, terrain corrections for the central Cascade Mountains of Washington were analyzed. In this type of terrain, terrain corrections within 1 mGal, on the average, of the 167 km standard can be obtained either by carrying corrections out to 21 km and then simply truncating them or by carrying corrections out to 3.5 km and making an analytic adjustment for the more distant terrain. Corresponding results were obtained for other accuracies. The geostatistical method used to obtain these error estimates is universal and can be applied to any region for which the mean elevation and the covariance of elevations can be calculated.
Conjugate complex variables method for the computation of gravity anomalies
Authors: Kwok Yue‐Kuen
Published: October 2002

Using conjugate complex variables, a generalized method is presented to derive formulas to calculate first‐ and higher‐order derivatives of the gravity potential due to selected mass models. Double integrals in the computation of gravity‐gradient anomalies are transformed into complex contour integrals. Analytical expressions for higher‐order derivatives of the gravitational potential in arbitrary directions due to two‐dimensional (2‐D) polygonal mass models are derived. The method is extended to 2‐D polygonal bodies whose density contrasts vary with depth and horizontal distance and can be generalized to deal with 2‐D bodies of any shape. The vertical gravity field and its first derivatives due to a homogeneous radially symmetric body are also computed using conjugate complex variables. Derivation of gravity and gravity gradient formulas generally is greatly simplified by the use of complex variables.
A method for determining shear‐wave anelastic attenuation using the inhole configuration
Authors: Liu Hsi‐Ping
Published: October 2002

Field measurements of seismic shear‐wave attenuation can in principle yield information important to oil and gas exploration such as lithology, porosity, and degree of saturation of rock strata (Toksöz and Johnston, 1981). In practice, measuring seismic‐wave anelastic attenuation is made difficult by the amplitude and shape changes caused by intrabed multiple reflections (O’Doherty and Anstey, 1971; Schoenberger and Levin, 1974, 1978; Spencer et al., 1982).
On: “Seismic contouring: A unique skill” by Paul M. Tucker (GEOPHYSICS, 53, 741–749, June 1988)
Authors: O’Brien Colin F.
Published: October 2002

Stone (1989), in his comments on Tucker’s paper on seismic contouring, concentrates on the inaccuracies which arise from ignoring the migration of data. A good approximation to a correctly migrated data set can be obtained in a fairly straightforward manner (O’Brien, 1988) but that still leaves the problem of how best to contour the data, whether in depth or in time.
Synthetic reflection seismograms in three dimensions by a locked‐mode approximation
Authors: Nolet G. , Sleeman R. , Nijhof V. , Kennett B. L. N.
Published: October 2002

We present a simple algorithm for computing the acoustic response of a layered structure containing three‐dimensional (3-D) irregularities, using a locked‐mode approach and the Born approximation. The effects of anelasticity are incorporated by use of Rayleigh’s principle. The method is particularly attractive at somewhat larger offsets, but computations for near‐source offsets are stable as well, due to the introduction of anelastic damping. Calculations can be done on small minicomputers. The algorithm developed in this paper can be used to calculate the response of complicated models in three dimensions. It is more efficient than any other method whenever many sources are involved. The results are useful for modeling, as well as for generating test signals for data processing with realistic, model‐induced “noise.” Also, this approach provides an alternative to 2-D finite‐difference calculations that is efficient enough for application to large‐scale inverse problems. The method is illustrated by application to a simple 3-D structure in a layered medium.
Exact Green’s functions using leaking modes for axisymmetric boreholes in solid elastic media
Authors: Haddon R. A. W.
Published: October 2002

By choosing appropriate paths of integration in both the complex frequency ω and complex wavenumber k planes, exact Green’s functions for elastic wave propagation in axisymmetric fluid‐filled boreholes in solid elastic media are expressed completely as sums of modes. There are no contributions from branch line integrals. The integrations with respect to k are performed exactly using Cauchy residue theory. The remaining integrations with respect to ω are then carried out partly by using the fast Fourier transform (FFT) and partly by using another numerical method. Provided that the number of points in the FFT can be taken sufficiently large, there are no restrictions on distance. The method is fast, accurate, and easy to apply.
Effects of subsurface characteristics on surface seismic measurements: A simulation study on horizontally layered media
Authors: Byun Bok S. , Young Chi‐Yuh
Published: October 2002

We use seismic modeling of shot records to simulate the effects of acoustic‐wave propagation, P/SV-wave conversion, intrabed multiple reflections, anelastic attenuation, and transverse isotropy in materials representative of a shallow Gulf of Mexico environment. The study focuses on the estimates of three kinematic properties: vertical traveltime, velocity used for depth conversion, and depth to the reflector. The primary tool used for the measurement is velocity spectral analysis along hyperbolic traveltime‐distance curves. Analyses of numerous modeling experiments in the Gulf of Mexico environment lead to the following conclusions: First, the kinematic properties are affected mainly by anelastic attenuation, vertical velocity heterogeneity, and velocity anisotropy. The effects of P/SV-wave mode conversion and intrabed multiples are of secondary significance. Second, anelastic attenuation coupled with heterogeneity contribute up to 3 percent in depth error, mainly in the form of apparent time delays. Transverse isotropy is likely to add up to an additional 2 percent, for a total of 5 percent depth error. Since the depth error magnitude agrees closely with published field observations, we conclude, therefore, that attenuation, heterogeneity, and transverse isotropy account for most of the errors in depth estimation in horizontally layered media.
Interpretation of airborne electromagnetic data using the modified image method
Authors: Bergeron Clyde J. , Ioup Juliette W. , Michel Gus A.
Published: October 2002

The modified image method is used to invert active electromagnetic (AEM) data from a 1984 U. S. Navy survey of Cape Cod Bay. The high‐frequency data (7200 Hz) give a robust value for the altitude of the helicopter‐towed AEM bird and for the first‐layer skin depth and, hence, for the first‐layer conductivity. The inversion of low‐frequency (385 Hz) bottom‐probing signals produces more noise‐sensitive estimates for the water depth and for the conductivity contrast K, the ratio of the bottom to water conductivities. The results show good agreement with “ground‐truth” radar altimeter, sea conductivity, and sonar depth data. To demonstrate the accuracy of the modified image method of inversion, we incorporate ground‐truth measurements along a flight line and the experimental frequencies in a forward Sommerfeld calculation to generate synthetic data which then are inverted using this technique.
Reflection shear‐wave data collected near the principal axes of azimuthal anisotropy
Authors: Lynn H. B. , Thomsen L. A.
Published: October 2002

The presence of vertically oriented fractures and/or unequal horizontal stresses has created an azimuthally anisotropic earth, in which shear‐wave (SH) data collected along the principal axes of the anisotropy display time and reflection amplitude anomalies. Amoco shot two crossing shear‐wave (SH) lines that were approximately parallel to the orthogonal principal axes of the azimuthal anisotropy. At the tie point, these crossing SH lines display a time‐variant mis‐tie. The tie point also displays reflection‐coefficient anomalies, attributable to azimuthally dependent shear‐wave velocities. Field mapping documented a set of fractures striking N69E which are approximately parallel to the line that exhibited greater traveltimes. Time‐variant mis‐ties and reflection coefficient anomalies are two of the seismic responses theoretically expected of an azimuthally anisotropic earth, i.e., one in which the shear‐wave velocity depends upon the polarization azimuth of the shear wave.
Seismic traveltime inversion for transverse isotropy
Authors: Byun B. S. , Corrigan D.
Published: October 2002

Quantitative measurements of seismic anisotropy can provide a valuable clue to the lithology and degree of stratification in sedimentary rocks with hydrocarbon potential. We present a practical technique for obtaining anisotropy parameters (i.e., five stiffness coefficients A, C, F, L, and M) from seismic traveltime measurements for horizontally layered, transversely isotropic media. The technique is based on the construction of ray‐velocity surfaces in terms of five measurement parameters. An iterative model‐based optimization scheme is then used to invert the traveltime parameters for the five stiffness coefficients in a layer‐stripping mode. Both model and field experiments are performed to demonstrate the feasibility of the method. The model experiment shows that inversion errors (especially in stiffness coefficients A, F, and M) increase with increasing number of layers. Despite these errors, the proposed method does provide a quantitative measure of velocity anisotropy as additional information that cannot be obtained readily from conventional methods. A field VSP data example shows the correlation between the anisotropy parameters and lithology: Chalk and shale exhibited high degrees of anisotropy, and sands showed low degrees of anisotropy.
On thin‐layer telluric modeling of magnetotelluric responses
Authors: Wannamaker Philip E.
Published: October 2002

Modeling of three‐dimensional (3-D) thin‐sheet telluric anomalies has been a popular means of estimating the distortions of magnetotelluric (MT) response functions in the vicinity of an upper crustal resistivity inhomogeneity (Kaikkonen, 1986) because structures with an arbitrary 3-D shape in plan view can be simulated with modest computational effort. In the class of thin‐layer problems of this note, the sheet structure overlies an infinitely resistive basement; and anomalous fields are computed in the long‐period or zero‐frequency (dc) limit (e.g., Hermance, 1982). Under the assumption of no vertical current flow into the basement, this 3-D problem reduces to a two‐dimensional (2-D) dc solution across the sheet. Since only the long‐period limit is considered, however, the conclusions that can be drawn about the effects of general 3-D structures are restricted. The purpose of this note is to clarify restrictions on the use of 2-D MT interpretations in 3-D areas based on just thin‐sheet telluric modeling.
Physical properties of deep crustal reflectors in southern California from multioffset amplitude analysis
Authors: Louie John N.
Published: October 2002

Analysis of reflection waveforms before stack can constrain the physical properties of reflectors in the deep crust. To simplify this analysis, recorded amplitudes are assumed to be reflections from weak elastic heterogeneities. With these assumptions, trends in reflection amplitudes with offset may indicate whether the signs of a reflector’s density and rigidity contrast agree with or oppose the sign of its Lamé’s parameter contrast. The slope of the trend indicates the degree of Poisson’s ratio contrast. No attempt is made to invert for the individual modulus or density contrasts. By examining only gross amplitude‐versus‐offset (AVO) trends, deep reflections constrain some crustal properties. Two seismic reflection surveys in the Mojave Desert recorded deep reflections that show amplitude changes with offset. Both the 1985 Calcrust Ward Valley survey in the eastern Mojave and the 87 km COCORP Mojave line 3 in the western Mojave incorporate long offsets of 10 km or more. Prestack traces are equalized using a quantile technique assuming a constant noise level at large time, then corrected for spherical divergence. Gross AVO trends that are summarized for each survey in amplitude trend stacks suggest that the strongest reflectors in the middle and deep crust represent Poisson’s ratio contrasts of at least 10 percent. In the eastern Mojave, a transition to a basal‐crustal zone, at ∼23 km depth, may include an increase in Poisson’s ratio with depth. Poisson’s ratio may also increase at the Moho.
The inversion of time‐domain airborne electromagnetic data using the plate model
Authors: Keating P. B. , Crossley D. J.
Published: October 2002

Airborne electromagnetic (EM) methods were developed in the early 1950s, mostly by Canadian mining exploration companies as a means of economically carrying out prospecting for sulfide deposits associated with volcanics in resistive shield areas. Present interpretation techniques are based on the use of nomograms but the approach is easily amenable to digital processing. For highly accurate interpretation, however, it is necessary to develop quantitative interpretation techniques that can make full use of all the data available. Inverse theory has been used for interpretation with great success in most geophysical disciplines; however, in airborne EM surveying, inversion has only been used for the interpretation of airborne EM data using half‐space and one‐layer models. By introducing some approximations to the rectangular thin‐plate model, it is now possible to apply inverse theory to the interpretation of time‐domain EM data. This approach provides estimates of the parameter errors, the correlation matrix, and a means of assessing the validity of the model. Synthetic profile data are used to demonstrate the validity of the method. The results of the inversion of real anomalies are compared with ground geophysical survey interpretation and drillhole data. The inversion results agree with the known geology of the area and the ground geophysical survey interpretation.
Noniterative three‐dimensional inversion of magnetic data
Authors: Pustisek A. M.
Published: October 2002

The interpretation of magnetic or aeromagnetic data often requires the inverse problem’s solution of the structure of the magnetization interface. This nonlinear inverse problem of mapping the basement topography from potential field data was first discussed by Peters (1949).
Enhancements to prestack frequency‐wavenumber (f-k) migration
Authors: Li Z. , Lynn W. , Chambers R. , Larner Ken , Abma Ray
Published: October 2002

Prestack frequency‐wavenumber (f-k) migration is a particularly efficient method of doing both full prestack time migration and migration velocity analysis. Conventional implementations of the method, however, can encounter several drawbacks: (1) poor resolution and spatial aliasing noise caused by insufficient sampling in the offset dimension, (2) poor definition of steep events caused by insufficient sampling in the velocity dimension, and (3) inadequate handling of ray bending for steep events. All three of these problems can be mitigated with modifications to the prestack f-k algorithm. The application of linear moveout (LMO) in the offset dimension prior to migration reduces event moveout and hence increases the bandwidth of non‐spatially aliased signals. To reduce problems of interpolation for steep events, the number of constant‐velocity migrations can be economically increased by performing residual poststack migrations. Finally, migration with a dip‐dependent imaging velocity addresses the issue of ray bending and thereby improves the positioning of steep events. None of these enhancements substantially increases the computational effort of f-k migration. Prestack f-k migration possesses a limitation for which no solution is readily available. Where lateral velocity variation is modest, steep events (such as fault‐plane reflections in sediments) may not be imaged as well as by other migration approaches. This shortcoming results from the restriction that, in the prestack f-k approach, a single velocity field must serve to perform two different functions: imaging and stacking. Nevertheless, in areas of strong velocity variation and gentle to moderate dip, the detailed velocity control afforded by the prestack f-k method is an excellent source of geologic information.
Azimuthal variations in scattering amplitudes induced by transverse isotropy
Authors: Pelissier M. A. , Thomas‐Betts Anna , Vestergaard Peter D.
Published: October 2002

The study of amplitude variations of reflected and transmitted seismic waves due to anistropy has received considerable attention in recent years, but most investigations have concentrated on the effect of transverse isotropy with the symmetry axis either vertical or horizontal. The published results on the whole tend to exclude mode conversions. Amplitudes of all reflected and transmitted wave modes are addressed for [Formula: see text]-waves incident on boundaries between isotropic and transversely isotropic media, the symmetry axis of which is oriented at 45 degrees to the interface. The results cover the full range of incidence angles and all “acquisition azimuth” in the plane of the interface. When the anistropy axis is not normal to the interface, the scattering coefficients are shown to be highly dependent on the azimuth. The pattern of azimuthal variation is especially complicated in the case of mode conversion, and scattering coefficient profiles that are 180 degrees apart are not the same. This has the implication that source‐receiver interchangeability does not hold and could have serious consequences to amplitude studies in split‐spread surveys. Both the [Formula: see text] and [Formula: see text] reflections show strong azimuthal variations, dependent on both the dip and the strike of the anisotropy axis. It may be possible to recover shown that the scattered amplitude patterns are dominantly controlled by the value of the elastic modulus [Formula: see text].
On: “Spectral analysis of airborne electromagnetics” by D. C. Bartel and A. Becker (GEOPHYSICS, 55, 1338–1346, October 1990).
Authors: O’Connell Michael , Carson Michael
Published: October 2002

Bartel and Becker present original and academically challenging ideas; however, to state that INPUT® is basically the same as GEOTEM® is only correct with respect to the transmitter‐to‐receiver geometry and the transmitter waveform (McMullan et al., 1989).
Advances in three‐dimensional magnetotelluric modeling using integral equations
Authors: Wannamaker Philip E.
Published: October 2002

Recent progress in integral equation modeling of three‐dimensional magnetotelluric responses includes the ability to simulate 3-D structures which outcrop (excluding topography), which transect layer interfaces, and which extend indefinitely in one or more directions. The most important factor in achieving this capability is an accurate treatment of the electric surface charge. In particular, a previous integro‐difference formulation for evaluating charges has been abandoned in favor of true surface integrations over the source cells with potential differencing across the field cells in the 3-D body. The new procedure constitutes a good approximation to Galerkin’s method while preserving internal consistency in terms of pulse basis functions. To verify outcropping structures, juxtaposed conductive and resistive prisms at the surface are simulated and compared to 2-D results. An elongate version of the 3-D model shows good agreement with both transverse electric and transverse magnetic modes of the 2-D response at both high and low frequencies. Apparent finite strike length effects for a short version of the 3-D body are physically reasonable. To verify structures cut by layer interfaces, an elongate tabular conductor at the base of a resistive overburden layer, which is in immediate contact with a tabular resistor at the top of a conductive basal half‐space is considered. All 3-D field components are in good agreement with 2-D transverse electric and transverse magnetic calculations from the surface down through or beside the body into the conductive basement below. The proper discontinuity of the normal component of the electric field across inhomogeneity boundaries is simulated well by the algorithm, but not exactly because of the pulse nature of the basis functions. Finally, outcropping and infinitely extended structures are checked by comparison of an offset basin model with very low frequency, thin‐layer calculation. In general, best results are obtained at cell body centers or face centers, but are not reliable over cell corners. From the offset basin study, we show also that 2-D transverse magnetic mode modeling of corresponding 3-D responses cannot be carried out indiscriminately over arbitrary structures without propagating errors to substantial depths in the 2-D model cross‐section.
Asymptotically linear velocity analysis with high resolution in time domain
Authors: Katz Simon
Published: October 2002

A new method of velocity analysis based on the joint use of a linear velocity analysis technique and a nonlinear penalty function is described. The penalty function is defined as a monotonic continuous function of the signal‐to‐noise ratio estimate, tending to a constant or to zero with this estimate converging respectively to infinity or to zero. The signal‐to‐noise ratio estimate is obtained as the ratio of the estimates of the energy of the signal with given velocity and the energy of the residual field. The resulting velocity analysis technique is characterized by higher noise suppression and higher resolution in the velocity and the time domain compared to the semblance based velocity analysis or the velocity analysis based on the slant stacking of input data.
Ship navigation for geophysical applications using Kalman‐filtered GPS fixes
Authors: Genrich J. F. , Minster J.‐B.
Published: October 2002

Based on a state vector representation of ship position and velocity, we have developed a Kalman filter that provides accurate navigation from position fixes supplied by a Global Positioning System (GPS) receiver. Quasi‐constant position offsets occurring for fixes associated with switches in observed satellite constellation are modeled by including constellation biases as state vector components. A proper choice of statistics for state propagation and measurement noise leads to improved positioning. However, it may also increase the statistical dependence of Kalman estimates over characteristic time periods. Since a quantitative measure of this dependence is generally important for further processing, we have derived general expressions for the autocovariance of state vector Kalman estimates. These expressions contain products of propagation matrices and Kalman gains and, hence, relate directly to stability properties of the filter. For many applications, only certain components of the state vector are of interest. We show that autocovariances for state vector projections can be computed from corresponding projections of propagation matrices and Kalman gains, provided the state transition and measurement matrices satisfy certain reducibility conditions. Application of the filter and of the autocovariance expressions is illustrated using GPS and gravity measurements collected on board the Scripps Institution of Oceanography’s R/V Thomas Washington during Leg 1 of the Roundabout expedition.
Removal of static shift in two dimensions by regularized inversion
Authors: deGroot‐Hedlin Catherine
Published: October 2002

A common problem in magnetotelluric (MT) sounding is the presence of static shifts in the data, i.e., a vertical shifting of the log‐apparent‐resistivity versus period curves relative to regional values (Jones, 1988; Jiracek, 1990; Berdichevsky et al., 1989). These static shifts are due to the presence of small‐scale, shallow conductivity anomalies near the measurement site. Electric charge builds up on near‐surface anomalies that are small in comparison to the skin depth of the electromagnetic (EM) fields. The charge buildup produces a perturbation of the measured electric fields from their regional values that persists to arbitrarily low frequencies. Incorrect removal of these local distortions leads to incorrect interpretation of the deeper targets of investigation.
Resolution comparison of conventional and recursively inverted signals
Authors: Szulyovszky I.
Published: October 2002

The recursive inversion for acoustic impedance results in pseudoacoustic impedance sections that approximate integration of the seismogram. Consequently, besides the well‐known benefits, the resolution decreases after recursive inversion. My model investigation shows that the resolution, concerning absolute amplitude maximum, changes from 0.25 to 0.29 of an incident wavelength. The thickness of the thin layer below the tuning thickness can be estimated from the pseudoacoustic impedance response, but the error of the estimation becomes greater than that for a conventional signal. For comparison, I used the well‐known resolution and calibration graph of Kallweit and Wood (1982).
Crossvalidation method for crosswell seismic tomography
Authors: Peterson J. E. , Davey A.
Published: October 2002

Crosswell seismic tomography is used to determine the variation of elastic wave velocity or attenuation between two boreholes and, if possible, boreholes and the surface from which they are drilled. In a transmission tomographic survey, traveltimes or amplitudes are measured for many raypaths between the boreholes and the surface. The data are inverted for velocity and attenuation, respectively. In this paper we only discuss traveltimes, but the methods are equally applicable to amplitude inversions.
Waveform inversion and digital filter theory
Authors: Cardimona S.
Published: October 2002

For many years, we have used seismic data processing techniques such as digital filter theory to enhance the signal‐to‐noise (S/N) ratio of the observed data to obtain the best possible image of the subsurface. With our increased knowledge and the increasingly sophisticated computers at our disposal, modern methods of data analysis are tending to be inversions through model fitting, i.e., actually finding the best model of the subsurface which would produce the observed data.
The use of drill‐bit energy as a downhole seismic source
Authors: Rector J. W. , Marion B. P.
Published: October 2002

A new wellbore seismic technique uses the vibrations produced by a drill bit while drilling as a downhole seismic energy source. The technique is described as “inverse” VSP because the source and receiver positions of conventional VSP are reversed. No downhole instrumentation is required to obtain the data and the data recording does not interfere with the drilling process. These characteristics offer a method by which borehole seismic data can be acquired, processed, and interpreted while drilling. Interchanging the conventional VSP source and receiver positions improves the efficiency of recording multioffset surveys for imaging a 3-D data volume in the borehole vicinity. The continuous signals generated by the drill bit are recorded by a pilot sensor attached to the top of the drillstring and by receivers located at selected positions around the borehole. The pilot signal is crosscorrelated with the receiver signals to compute traveltimes of the arrivals and to attenuate incoherent noise. Deconvolution and time shifts of the pilot signal compensate for the effects of propagation from the drill bit to the top of the drillstring. By repeating this process for an interval of the well, a VSP‐equivalent data set is generated. Results from a test well demonstrate that the processed drill‐bit data are comparable to conventional VSP data.
Borehole‐to‐surface electrical resistivity monitoring of a salt water injection experiment
Authors: Bevc D. , Morrison H. F.
Published: October 2002

A field experiment was conducted at the University of California Richmond Field Station to demonstrate the sensitivity of borehole‐to‐surface resistivity measurements in groundwater investigations. A quantity of saline water was injected into a fresh water aquifer while the resistivity was monitored using a multichannel borehole‐to‐surface system. Two experiments were conducted using pole‐pole and pole‐dipole receiver electrode arrays. The data from the pole‐pole experiment were superimposed to simulate a dipole‐pole array and the data from the pole‐dipole array were superimposed to simulate a dipole‐dipole array. This superposition of the data was done to enhance the anomaly and facilitate interpretation. A numerical modeling study was performed in conjunction with the field experiment in order to interprete the results. A three‐dimensional modeling program was used to simulate the geological setting of the field experiment and the salt water injection. This modeling revealed that an asymmetric displacement of the salt water slug results in asymmetric current channeling which is observable as a 25 to 40 percent difference between preinjection and postinjection borehole‐to‐surface resistivity. In addition to demonstrating the sensitivity of subsurface arrays, this experiment demonstrated that the measurement of bulk resistivity can identify a groundwater flow pattern not detected by hydrological measurements.
Field measurements of azimuthal anisotropy: First 60 meters, San Francisco Bay area, CA, and estimation of the horizontal stresses’ ratio from Vs1/Vs2
Authors: Lynn Heloise Bloxsom
Published: October 2002

[Formula: see text], Q) of near‐surface strata. The azimuthal anisotropy revealed in these field experiments is the result of a combination of circumstances: unequal horizontal stresses (the data were collected near the center of the San Andreas and associated fault systems), fabric anisotropy introduced by the depositional agent, and stress‐aligned fluid‐filled microcracks, cracks, or pore spaces.
Seismic sources in open and cased boreholes
Authors: Winbow G. A.
Published: October 2002

Theoretical modeling, based on the equations of linear elasticity, is used to analyze the performance of downhole seismic sources, specifically, symmetrical radial sources which exert outward pressure on a length of the borehole wall and vertically driven sources clamped to the borehole wall. Torsional sources radiate pure S-waves whose form is independent of the existence of the borehole, and therefore requires no special discussion. The radial sources discussed are airguns, waterguns, and implosive sources. Such sources are in widespread use and development at present. From the calculations we find that radial sources emit almost all (>99 percent) their energy as tube‐waves which travel along the borehole and not out into the formation. Vertical sources radiate almost all their energy into the formation as P- and S-waves. However, if the casing bond slips, the vertical sources lose efficiency and radiate energy as extensional waves along the casing. The radial sources are little affected by the quality of cement bonding. Radial sources may be used at or near the surface to direct tube‐waves down the hole. Conversion of the tube‐waves to P-waves and S-waves either from the end of the hole or from a specially configured converter such as a constriction placed in the hole theoretically represents a viable downhole seismic source for reversed VSP (RVSP) and crosshole imaging. Little energy is lost in tube wave transmission and high acoustic power may be injected into a well.
Magnetic effects of a finite, arbitrarily oriented line of dipoles
Authors: Jain Birendra K.
Published: October 2002

Lines of dipoles are an important class of 3-D theoretical bodies and are often used to model the effects due to narrow, elongated magnetic bodies of limited depth extents in the interpretation of magnetic data. The application includes geologic bodies such as mineralized veins and pipes, as well as nongeologic bodies such as well casings. Though closed‐form expressions for the magnetic effects of such a finite line of dipoles are available in the published literature, the cases treated are quite restricted, i.e., the expressions are given for the line either parallel or perpendicular to the plane of observation (e.g., Nettleton, 1942, Hall, 1959, Singh and Sabina, 1978) or for the line inductively magnetized and dipping in the direction of the earth’s magnetic field (e.g., Henderson and Zeitz, 1967).
On: “An analytical study of a two-layer transient thermal conduction problem as applied to soil temperature surveys” by T. H. Larson and A. T. Hsui (February 1992 GEOPHYSICS, p. 306–312)
Authors: Lunardini Virgil J.
Published: October 2002

This paper presents a solution to the two‐layer conduction problem, a problem which has already been solved exactly and published (Lachenbruch, 1959; Lunardini, 1981). In fact the three‐layer solution is also available, (Lachenbruch, 1959). Unfortunately the Larson and Hsui paper uses an incorrect boundary condition for the energy flow continuity between the layers; this refers to the last equation after equation (3), or equation (A-20) of their paper. The temperature gradient must be multiplied by the thermal conductivity, not the thermal diffusivity as the paper has done. The results obtained in the paper are only valid if the heat capacities ρc of the two layers are equal. However, few soils have the same heat capacity, as can be seen from the thermal properties of materials often encountered in geotechnical situations, given in Table 1. Thus, a second parameter is required for interpreting ground temperatures: the ratio of the thermal conductivities of the two layers. The interpretation given to the effects of only the thermal diffusivity ratios is then open to question. Calculations of the same ground conditions as used by Larsen and Hsui, with the same diffusivity ratios but with realistic thermal conductivity ratios, give significantly different temperature predictions. Of course, the general conclusions of the paper on the profound effect of layering on the thermal response of soils to periodic surface temperatures are still true.
Elastic wave propagation using fully vectorized high order finite‐difference algorithms
Authors: Vafidis A. , Abramovici F. , Kanasewich E. R.
Published: October 2002

Two finite‐difference schemes for solving the elastic wave equation in heterogeneous two‐dimensional media are implemented on a vector computer. A modified Lax‐Wendroff scheme that is second‐order accurate both in time and space and is a version of the MacCormack scheme that is second‐order accurate in time and fourth‐order in space. The algorithms are based on the matrix times vector by diagonals technique that is fully vectorized and is described using a novel notation for vector supercomputer operations. The technique described can be implemented on a vector processor of modest dimensions and increase the applicability of finite differences. The two difference operators are compared and the programs are tested for a simple case of standing sinusoidal waves for which the exact solution is known and also for a two‐layer model with a line source. A comparison of the results for an actual well‐to‐well experiment verifies the usefulness of the two‐dimensional approach in modeling the results.
Channel waves in cross‐borehole data
Authors: Lines Larry R. , Kelly Kenneth R. , Queen John
Published: October 2002

Layered geological formations with large seismic velocity contrasts can effectively create channel waves in cross‐borehole seismic data. The existence of channel waves for such waveguides can be confirmed by ray tracing, wave equation modeling, and modal analysis. Channel wave arrivals are identified in cross‐borehole data recorded at Conoco’s Newkirk test facility. For these data, where velocity contrasts are about 2 to 1, tomography based on first arrival traveltimes, is limited due to problems with extreme ray bending and seismic shadow zones. However, it may be possible to extract geological information using channel wave information. The seismometer differencing method appears to be a promising approach for detecting waveguide boundaries by use of cross‐borehole data.
Characterizing three‐dimensional wavefields with high‐resolution spectra
Authors: Meek R. A. , Vassiliou A. A.
Published: October 2002

Three‐dimensional spectra (frequency‐x‐wavenumber‐y‐wavenumber or [Formula: see text] spectra) can be used to determine the frequency content, velocity, and direction of waves entering an array of receivers. This information is important in detecting aliasing problems, understanding coherent noise, designing arrays, and determining parameters for coherent noise filters. Because of the limited spatial dimensions of most arrays the discrete Fourier transform produces an estimate of the three‐dimensional (3-D) spectrum with severe wavenumber distortion. We extend a 2-D hybrid spectral estimation method to three dimensions by combining a temporal Fourier transform with a spatial 2-D maximum entropy spectral estimation technique. The method produces [Formula: see text] spectra with higher wavenumber resolution and less spectral distortion than corresponding 3-D Fourier spectra. The 2-D maximum entropy spectral estimation algorithm uses a sequence of Fourier transforms to extrapolate the estimated autocorrelation function of the data. We assume the wavenumber spectrum of the data comprises a sum of a few poles. Field and synthetic data are used to demonstrate how 3-D wavefields can be characterized with this method of spectral analysis. From these results we conclude that the method gives excellent wavenumber resolution but performs poorly in detecting small signals in the presence of high amplitude signals. We feel this limitation is not serious for characterizing strong amplitude coherent energy recorded by an array of receivers.
Transient electromagnetic modeling of shallow A‐type sections with 3-D inhomogeneities
Authors: Poddar M. , Anderson Walter L.
Published: October 2002

A hard rock area underlain by granitic/gneissic or basaltic rocks often has an A‐type three‐layer geoelectric section in which resistivity increases with depth. The middle layer of moderate resistivity caused by fracturing/fissuring that lies between the surface‐weathered layer and the substratum of unfractured rock is not a good target for a direct current (DC) resistivity sounding since it is generally suppressed in the observations. Moreover, its definition requires expanding the electrode spacing to a length several times the depth of the target layer, and this may be a drawback if the target layer is either laterally variable or limited in its horizontal extent. We first studied the transient electric field of a horizontal electric dipole (HED) source excited by a step turn‐off current for a 1-D model of an A‐type geoelectric section. The results of this theoretical study are presented as graphs of normalized apparent resistivity versus a time‐related dimensionless parameter. Irrespective of the separation between the transmitter and receiver dipoles, these transient sounding curves become similar to the corresponding Schlumberger sounding curves at late time. Hence the transient electric field measurement offers the possibility of sounding at a fixed transmitter‐receiver spacing that may be shorter than the target depth. Also, at early times, for a certain ratio of the dipole separation to the target depth, there is a dramatic increase in the resolution of the response. Thus, it is possible to resolve suppressed layers of an A‐type section in this type of sounding. A study of the effects of transmitter ramp time and receiver bandwidth on the transient apparent resistivity curves shows that a very fast current shut‐off and wideband measurement are required to realize all the possibilities suggested by this modeling. Some 3-D transient electromagnetic (TEM) modeling was also done to simulate (1) a lateral variation in the resistivity of the middle layer of an A‐type section and (2) a weak zone of limited horizontal extent in the substratum of a two‐layer section. We observed that the 3-D inclusion has less effect at late time but is more pronounced at early time. In view of the above results, we conclude that the transient E‐field sounding with a grounded wire source can be used in place of a conventional DC resistivity sounding to overcome the problem of poor resolution due to the suppression of the intermediate layer in a geoelectric section where the resistivity increases with depth. As such, it has a potential application in groundwater as well as geotechnical surveys, because together with the overlying weathered layer, the fractured rock constitutes the aquifer in hard rocks.
On: “An exact solution for the gravity curvature (Bullard B) correction” by T. R. LaFehr (GEOPHYSICS, 56, 1179–1184, August 1991)
Authors: Hensel E. Gerald
Published: October 2002

Dr. LaFehr has devoted quite a bit of effort to championing the use of the curvature correction, and this published paper is welcome because it contains an explicit expression for the curvature correction and a good analysis of conditions under which this correction may be important. I would like to make comments on parts of this paper.
Interpretation of 3-D effects in long‐offset transient electromagnetic (LOTEM) soundings in the Münsterland area/Germany
Authors: Hördt Andreas , Druskin Vladimir L. , Knizhnerman Leonid A. , Strack Kurt‐Martin
Published: October 2002

The interpretation of long‐offset transient electromagnetic (LOTEM) data is usually based on layered earth models. Effects of lateral conductivity variations are commonly explained qualitatively, because three‐dimensional (3-D) numerical modeling is not readily available for complex geology. One of the first quantitative 3-D interpretations of LOTEM data is carried out using measurements from the Münsterland basin in northern Germany. In this survey area, four data sets show effects of lateral variations including a sign reversal in the measured voltage curve at one site. This sign reversal is a clear indicator of two‐dimensional (2-D) or 3-D conductivity structure, and can be caused by current channeling in a near‐surface conductive body. Our interpretation strategy involves three different 3-D forward modeling programs. A thin‐sheet integral equation modeling routine used with inversion gives a first guess about the location and strike of the anomaly. A volume integral equation program allows models that may be considered possible geological explanations for the conductivity anomaly. A new finite‐difference algorithm permits modeling of much more complex conductivity structures for simulating a realistic geological situation. The final model has the zone of anomalous conductivity aligned below a creek system at the surface. Since the creeks flow along weak zones in this area, the interpretation seems geologically reasonable. The interpreted model also yields a good fit to the data.
Seismic detection of overpressuring and fracturing: An example from the Qaidam Basin, People’s Republic of China
Authors: Paul Robert J.
Published: October 2002

Shallow hydrocarbon reserves were discovered in 1959 in the Nan Yi Shan structure located near the western corner of the Qaidam Basin. The first successful deep well encountered an overpressured zone at 3000 m that resulted in a well blowout. To improve the structural definition of the field and delineate the overpressured layer a 3-D seismic survey was conducted. A region of anomalous seismic time sag associated with fracturing and small quantities of oil and gas was identified on the northwest plunging nose of the Nan Yi Shan anticline. The distribution of stacking (NMO) velocities in this region was regarded as abnormal; stacking velocities derived on the steeply dipping flanks adjacent to the sag anomaly were found to be slower than those on the shallower crest. Ray‐trace modeling of a buried low‐velocity anomaly provided a possible geometric solution to explain both the time variant nature of the sag and the unusual stacking velocity signature associated with it. A significant difference in seismic and sonic traveltimes was also observed for wells that penetrated the sag region and was attributed to localized fracturing. In a deeper interval, seismic amplitudes were used to identify gas‐saturated fracture porosity and to describe the spatial limits of overpressuring within a thin‐bed reservoir. Wells drilled through high‐amplitude anomalies encountered overpressuring, those drilled in a region of moderate seismic amplitude tested significant quantities of gas, and wells located outside the region of good coherent signal encountered poor or no hydrocarbon shows. These results demonstrate that with good quality seismic data and sufficient lateral and vertical resolution, thin fractured hydrocarbon‐bearing reservoirs can be delineated and overpressure zones identified.