bluuebunny/crossref_metadata_2025 huggingface dataset:
Introduction to Special Issue on Engineering and Groundwater
Authors: Romig P. R.
Published: October 2002

This section of the issue of Geophysics is draw from a collection of papers originally submitted for a monograph on engineering and groundwater geophysics. It is a mixture of tutorial, new developments, and case histories focused on seismic methods. This introduction will review the evolution of this section and introduce each of the papers. These papers complement the overview presented by Dobecki and Romig (1985) in the Golden Anniversary Issue of Geophysics. That paper reviews the history, summarizes the state of the art, and attempts to predict possible future developments.
On: “The influence of pore pressure and confining pressure on dynamic elastic properties of Berea sandstone,” by N. I. Christensen and H. F. Wang (GEOPHYSICS, 50, 207–213, February 1985).
Authors: Gardner G. H. F.
Published: October 2002

The authors present their results as if Berea sandstone were an elastic material; that is, velocities are given as functions of confining and pore pressure. In fact, most rocks are inelastic and velocities depend on the history of the confining and pore pressure, and not just on the present values. Some measurements of hysteresis were reported by Gardner et al. (1965). The confining pressure was cycled between two pressures [Formula: see text] and [Formula: see text] for a fixed pore pressure [Formula: see text], following a fixed schedule of pressure changes, until repeatable values of velocity were obtained. (At any intermediate pressure the velocity measured for increasing pressure was different from the value for decreasing pressure, giving rise to a hysteresis cycle). When the same schedule of pressure changes for the differential pressure [Formula: see text] was followed by holding [Formula: see text] fixed and varying [Formula: see text], the measured velocities followed the same hysteresis curve within the limits of experimental accuracy. In brief, when hysteresis was taken into account, changes in pore and confining pressures were equally effective in changing velocity. In their article, Christensen and Wang do not refer to hysteresis; perhaps they would like to comment on its relevance.
A Fourier method for borehole electromagnetic problems
Authors: Howard A. Q.
Published: October 2002

The induction logging problem is solved for a two‐coil sonde in a borehole surrounded by a bedding sequence which varies arbitrarily in the vertical direction. The solution is based on parallel computation of 500–[Formula: see text] m of log data. Both the R and X components of the voltage log are modeled. The problem is solved in the spatial‐frequency domain in which a transfer function relates the anomalous borehole voltage for a two‐coil sonde to the conductivity spectrum of the formation, resulting in a natural extension of Doll’s geometric factor theory. This computationally compact formulation does not require matrices and is designed as a computer aid for log analysts. The mathematical method (with proper conditioning) can efficiently solve the inverse problem of transforming the voltage induced in the receiving coil to a formation conductivity log. This paper establishes the forward method. Numerical results are given for beds of finite thickness for both brine‐ and oil‐base muds. The Fourier method is in good agreement with both a finite‐element method and a semi‐analytic method.
Archaeological geophysics in Britain
Authors: Clark Anthony J.
Published: October 2002

I describe the approach followed by the Ancient Monuments Laboratory in adapting the instrumentation and techniques of resistivity and magnetic prospecting to the near‐surface problems of mapping buried archaeological sites. Such sites demand rapid and intensive ground coverage and the highest possible spatial resolution and instrument sensitivity. Resistivity is used largely for planning building foundations. Optimized resistivity results have required comparative studies of electrode configurations and the effect on resistivity of climatic variations in different lithologies. Magnetic prospecting is especially effective for detecting (1) fired structures such as kilns, and (2) excavated features such as ditches and pits filled with topsoil of relatively high susceptibility (which is further enhanced by human activities). Speed and resolution requirements have caused the proton magnetometer to be abandoned in favor of the fluxgate gradiometer. Valuable information about human activity can also be obtained from magnetic susceptibility measurements on topsoil.
A numerical investigation of head waves and leaky modes in fluid‐filled boreholes
Authors: Paillet Frederick L. , Cheng C. H.
Published: October 2002

Although synthetic borehole seismograms can be computed for a wide range of borehole conditions, the physical nature of shear and compressional head waves in fluid‐filled boreholes is poorly understood. This paper presents a series of numerical experiments designed to explain the physical mechanisms controlling head‐wave propagation in boreholes. These calculations demonstrate the existence of compressional normal modes equivalent to shear normal modes, or pseudo‐Rayleigh waves, with sequential cutoff frequencies spaced between the cutoff frequencies for the shear normal modes. Major contributions to head‐wave spectra occur in discrete peaks at frequencies just below mode cutoff for both compressional and shear modes. This result is confirmed by calculations with synthetic waveforms at frequencies corresponding to mode cutoff, and by branch‐cut integrals designed to yield independent spectra for the compressional mode. For soft formations where shear velocity falls below acoustic velocity in the borehole fluid, leaky compressional normal modes attain properties similar to those of shear normal modes in the case of hard rock. In the limit of vanishing S-wave velocity, this result is formally related to a fluid‐fluid waveguide with undamped compressional normal modes. Synthetic waveforms demonstrate that high‐ amplitude arrivals, traveling at velocities less than the acoustic velocity of the borehole fluid and at frequencies above a few kilohertz, represent the Airy phase of the compressional mode and are not a tube wave. Comparison of synthetic waveforms with waveforms obtained in soft sea sediments indicates that the predicted Airy phase arrivals are present in the experimental data.
On: “Well logging—A 25‐year perspective,” by D. D. Snyder and D. B. Fleming (GEOPHYSICS, 50, 2504–2529, December 1985)
Authors: Hardy Hugh W.
Published: October 2002

In reading the review by Snyder and Fleming in which they emphasized “those developments which we deem significant or which we have learned are held significant by our colleagues,” I was dumbfounded by the total absence of any mention of Humble Oil & Refining Co.’s leadership in the development of the two‐receiver, continuous velocity logging tool. That tool employed the techniques contained in W. D. Mounce’s patent number 2 200 476 on “Measurements of acoustical properties of materials” which was applied for in December, 1935, and approved in May, 1940. The Humble Seismic Velocity Logger was used to log a shallow core hole near Ozona, TX, on October 18, 1949, and the first deep hole velocity logging operation was achieved on March 5, 1951, in the Ida Rosenbaum no. 1 well in Washington County, Texas. The formal announcement of Humble’s success was in the paper “Seismic velocity logging” by W. D. Mounce, C. L. Hubbard, C. J. Charske, and H. P. Kuppers of Humble’s Geophysics Research Section, presented on November 20, 1951, at the Fifth Annual Midwestern Meeting of the SEG in Dallas, TX.
Scalar audiomagnetotellurics applied to base‐metal exploration in Finland
Authors: Lakanen Ensio
Published: October 2002

The potential of scalar audiomagnetotellurics (AMT) for base‐metal exploration in Finland has been used on a trial basis since 1976. The behavior of the source fields changes depending upon the season and the weather. Techniques similar to reference stations have been applied. In the search for elongated conductors, an azimuth of the minimum apparent resistivity is taken at every station to utilize the E-polarized field. Nevertheless, there are severe restrictions as to where AMT can be applied reliably. The target should be highly conductive; it should be large or elongated; and there should be no other good conductors at shallow depth or manmade sources nearby. A practical depth limit seems to be 2 000 m when frequencies between 8 and 3 700 Hz are used. A one‐dimensional model is convenient for modeling sounding data, and the model can be theoretically extended to more complex structures. It is important to have a dense array of stations. Two reconnaissance surveys and three detailed cases demonstrate the unique character of the AMT method. Difficult though it has been to squeeze reliable facts out of the data, at times they have provided fascinating new information. The next step is to improve observation techniques with a more sophisticated measuring device.
Efficient traveltime calculations for Kirchhoff migration
Authors: Gray Samuel H.
Published: October 2002

Acoustic Kirchhoff migration is performed for each point (x, z) in an output depth section by an integrating or weigthed summing procedure. Each trace of the input time section contained in an aperture about x contributes to the sum, and the contribution from a trace at x + Δx consists of the element whose time is equal to the total time required for a seismic wave to travel from the source to the point (x, z) to (x+Δx,0). That is, Kirchhoff migration sees every subsurface location as a possible point scatterer and explicitly reverses the operation of scattering the diffracted seismic energy from such an obstacle onto an array of receivers as a function of time. Continuous reflectors are considered ensembles of point scatterers, and they are imaged by summing over traces for each point on the reflector. If the array of [Formula: see text] output depth points for a two‐dimensional (2-D) poststack migration contains [Formula: see text] points, and on the average [Formula: see text] points are included in each summation aperture, Kirchhoff summation requires [Formula: see text] add‐multiply operations.
Automated stratigraphic interpretation of well‐log data
Authors: Wu Xuanzhi , Nyland Edo
Published: October 2002

A computerized well‐log stratigraphic interpretation system based on artificial intelligence can be seen as two steps, contact recognition and interval identification. Unlike previous approaches to mathematical zonation which are essentially statistical, consideration of the geologic environment is included for effective interpretation. Following the logic of human experts, the system first determines the log signature of the contacts. An algorithm incorporating linear regression with variable breakpoints is used to describe the structure of log curves. Log features are mapped to the assigned signature of contacts. Multiple logs are taken into account to set up the final set of contacts, which divides the test borehole into a string of intervals with attributes. A pattern string of intervals is based on the integrated geologic column for the section where the borehole is located. A string‐to‐string matching program then determines the optimal map of the test borehole as the pattern to generate a geologic column based on the computer suggestion. The cost formulas for matching represent knowledge about the stratigraphy of the area under study, and the string‐to‐string matching algorithm includes this knowledge of stratigraphy. In test boreholes in an oil‐sand deposit, 86 percent of the computer identifications of intervals were consistent with those determined by geologists from core descriptions, suggesting that the design concept and computer algorithm are consistent with the characteristics of the interpretation problem.
Tomographic inversion via the conjugate gradient method
Authors: Scales John A.
Published: October 2002

Tomographic inversion of seismic traveltime residuals is now an established and widely used technique for imaging the Earth’s interior. This inversion procedure results in large, but sparse, rectangular systems of linear algebraic equations; in practice there may be tens or even hundreds of thousands of simultaneous equations. This paper applies the classic conjugate gradient algorithm of Hestenes and Stiefel to the least‐squares solution of large, sparse systems of traveltime equations. The conjugate gradient method is fast, accurate, and easily adapted to take advantage of the sparsity of the matrix. The techniques necessary for manipulating sparse matrices are outlined in the Appendix. In addition, the results of the conjugate gradient algorithm are compared to results from two of the more widely used tomographic inversion algorithms.
Cascaded migrations: Improving the accuracy of finite‐difference migration
Authors: Larner Ken , Beasley Craig
Published: October 2002

The accuracy of time migrations done with finite‐difference schemes deteriorates with increasing reflector dip. Some properties of migration in general, and of finite‐difference approaches in particular, suggest a way of improving the accuracy of finite‐difference schemes for migrating steep dips. First, although data will be undermigrated when too low a velocity is used in migration, a correctly migrated result can be obtained by migrating again, this time with the previously undermigrated result as input. In fact, a sequence of undermigrations will yield the correct result as long as the sum of the squares of the migration velocities used in the different migration stages equals the square of the correct migration velocity. A second property is that the apparent spatial dip of a reflector perceived by the migration process is a function of not only the time dip of the unmigrated reflection, but also the velocity used in the migration. In a sequence of low‐velocity migrations, the apparent spatial dip perceived at each migration stage can be considerably less than the true dip. Thus, because finite‐difference migration is accurate for small spatial dips, the cascaded migrations yield a more accurate result than that of single‐stage migration. Also, because each migration stage is done with low velocity, the depth step can be large; hence, the computational effort need not be. The accuracy of the method is not compromised (in fact, it improves) in media in which velocity increases with depth. Moreover, the cascaded approach suffers no more than other methods of time migration where velocity varies mildly in the lateral direction. In applications of the method to stacked data from the Gulf of Mexico, reflections from near‐vertical flanks of salt domes were migrated with accuracy comparable to that achieved by frequency‐wavenumber domain migration.
Three‐component recordings: Interest for land seismic source study
Authors: Cliet C. , Dubesset M.
Published: October 2002

Three‐component recordings provide access to the particle trajectory due to seismic events. This three‐dimensional picture shows successions of complex phenomena. Nevertheless, this trajectory has predominant directions of polarization. A method of spatial directional filtering is described to calculate the polarization direction of parts of the trajectory. The application of this method provides means to reject trajectory parts for which the angular gap between their polarization axis and a reference direction, for example X, Y, or Z, is greater than a given threshold angle. Real data are submitted to this method, leading to a color‐coded display showing the direction of polarization of each wave train. The same noise spread, used in conjunction with two different seismic sources, was recorded with triphone groups. The two sources were an SH‐wave vibrator and Marthor®. Using spatial directional filterings with threshold angles varying from 0 to 90 degrees, the corresponding quantities of energy emitted in each principal direction X, Y, and Z were evaluated. A set of diagrams was plotted for comparing the behavior of both sources and to calculate the part of the received energies along X, Y, and Z directions as a function of (1) the emitted energies, and (2) the local propagation effects. ® SH‐wave weight‐drop source, IFP trademark.
Correction for the static shift in magnetotellurics using transient electromagnetic soundings
Authors: Sternberg Ben K. , Washburne James C. , Pellerin Louise
Published: October 2002

Shallow inhomogeneities can lead to severe problems in the interpretation of magnetotelluric (MT) data by shifting the MT apparent resistivity sounding curve by a scale factor, which is independent of frequency on the standard log‐apparent‐resistivity versus log‐frequency display. The amount of parallel shift, commonly referred to as the MT static shift, can not be determined directly from conventionally recorded MT data at a single site. One method for measuring the static shift is a controlled‐source measurement of the magnetic field. Unlike the electric field, the magnetic field is relatively unaffected by surface inhomogeneities. The controlled‐source sounding (which may be a relatively shallow sounding made with lightweight equipment) can be combined with a deep MT sounding to obtain a complete, undistorted model of the earth. Inversions of the static shift‐corrected MT data provide a much closer match to well‐log resistivities than do inversions of the uncorrected data. The particular controlled‐source magnetic‐field sounding which we used was a central‐induction Transient ElectroMagnetic (TEM) sounding. Correction for the static shift in the MT data was made by jointly inverting the MT data and the TEM data. A parameter which allowed vertical shifts in the MT apparent resistivity curves was included in the computer inversion to account for static shifts. A simple graphical comparison between the MT apparent resistivities and the TEM apparent resistivities produced essentially the same estimate of the static shift (within 0.1 decade) as the joint computer inversion. Central‐induction TEM measurements were made adjacent to over 100 MT sites in central Oregon. The complete data base of over 100 sites showed an average static shift between 0 and 0.2 decade. However, in the rougher topography and more complex structure of the Cascade Mountain Range, the majority of the sites had static shifts of the order of 0.3 to 0.4 decade. The static shifts in this area are probably due to a combination of topography and surficial inhomogeneities. The TEM apparent resistivity (which is used to estimate the unshifted MT apparent resistivity) does not necessarily agree with either the transverse electric (TE) or the transverse magnetic (TM) MT polarization. TEM apparent resistivity may occur between the two, or may agree with one of the two polarizations, or may lie outside the MT polarizations.
Dispersive noise removal in t-x space: Application to Arctic data
Authors: Beresford‐Smith Greg , Rango Rolf N.
Published: October 2002

Strongly dispersive noise from surface waves can be attenuated on seismic records by Flexfil, a new prestack process which uses wavelet spreading rather than velocity as the criterion for noise discrimination. The process comprises three steps: trace‐by‐trace compression to collapse the noise to a narrow fan in time‐offset (t-x) space; muting of the noise in this narrow fan; and inverse compression to recompress the reflection signals. The process will work on spatially undersampled data. The compression is accomplished by a frequency‐domain, linear operator which is independent of trace offset. This operator is the basis of a robust method of dispersion estimation. A flexural ice wave occurs on data recorded on floating ice in the near offshore of the North Slope of Alaska. It is both highly dispersed and of broad frequency bandwidth. Application of Flexfil to these data can increase the signal‐to‐noise ratio up to 20 dB. A noise analysis obtained from a microspread record is ideal to use for dispersion estimation. Production seismic records can also be used for dispersion estimation, with less accurate results. The method applied to field data examples from Alaska demonstrates significant improvement in data quality, especially in the shallow section.
Exploration beneath volcanics: Snake River plain, Idaho
Authors: Young Roger A. , Lucas James E.
Published: October 2002

Coincident gravity, magnetotelluric (MT), and seismic reflection‐refraction surveys were conducted along a line traversing the boundary between the volcanic‐covered Eastern Snake River plain, Idaho, and the sediment‐covered Western overthrust belt, One‐dimensional MT and seismic refraction analysis away from the immediate boundary between sediments and volcanics successfully maps the depths to units of igneous and sedimentary—or metasedimentary—rock. Substantially different models of the volcanic‐covered terrain and of the sediment‐covered terrain are linked by a gravity anomaly and by an elongation and rotation of MT anisotropy ellipses which indicate a fault. Fault dip, depth extent, and motion are constrained by marked changes in the first‐arrival apparent velocities and by an electrical marker layer. The analysis shows that metasediments extend laterally at least 16 km beneath the volcanic cover. The two terrains are connected by a boundary zone which is a major normal fault.
International Geomagnetic Reference Field Revision 1987
Authors: Barraclough D. R.
Published: October 2002

The International Geomagnetic Reference Field (IGRF) is a series of mathematical models of the main geomagnetic field and its secular variation, the models consisting of sets of spherical harmonic (or Gauss) coefficients. The IGRF has become a widely used means of deriving values of geomagnetic field components in, for example, studies of magnetic anomalies and investigations of charged particle motions in the ionosphere and the magnetosphere.
Noise‐adaptive filtering of seismic shot records
Authors: Anderson Richard G. , McMechan George A.
Published: October 2002

Ambient noise can obscure reflections on deep crustal seismic data. We use a spectral subtraction method to attenuate stationary noise. Our procedure, called noise‐adaptive filtering, is to Fourier transform the noise before the first arrivals, subtract the amplitude spectrum of the noise from the amplitude spectrum of the noisy data, and inverse Fourier transform. The phase spectrum is not corrected, but the method attenuates noise if the phase shift between the signal and noise is random. The algorithm can be implemented as a frequency filter, as a frequency‐wavenumber filter, or as two separate frequency and wavenumber filters. Noise‐adaptive filtering is often superior to conventional frequency or frequency‐wavenumber filtering because it adapts to spatial variations in the noise without parameter testing. Noise‐adaptive filters can achieve noise rejection ratios of up to 45 dB; their dynamic range is about 25 dB. These filters work best when the input signal‐to‐noise ratio is on the order of 0 dB and there are significant differences between the frequency‐wavenumber amplitude spectra of the signal and noise. Application of the method to field data can enhance events that are not visible in the input data.
On: “Results of a controlled‐source audiofrequency magnetotelluric survey at the Puhimau thermal area, Kilauea Volcano, Hawaii, by L. C. Bartel and R. D. Jacobson (GEOPHYSICS, 52, 665–677, May 1987)
Authors: Szarka Lásaló
Published: October 2002

A growing number of papers being published on the CSAMT-MT curve transformation, which — as the authors state — allows a simpler magnetotelluric interpretation of the corrected CSAMT curves. The concept of near‐field corrections is based on electromagnetic relations over a homogeneous earth, and the effects of subsurface layers or lateral inhomogeneities are usually neglected. Bartel and Jacobson (1987) especially suppress the bounds of the near‐field correction: After presenting several near‐field correction curves over a homogeneous earth in their Figure 2 (which includes an idealistic demarcation line instead of a gradual change between near‐field and far‐field regions), they simply add that “…for a layered earth a similar demarcation occurs between the far‐ and near‐field regimes.” Further, the problem of lateral inhomogeneities is not mentioned in the paper. Such a description might lead to an oversimplification. I should like here to underline both limitations.
A diffusion model for pulsed neutron logging
Authors: Dunn Keh‐Jim
Published: October 2002

Based on Polyachenko’s formulation, the complete solution of the boundary value problem for pulsed neutron logging is derived for a centralized tool model. The solution is more general than Polyachenko et al.’s earlier solution in that the unrealistic assumption of zero thermal neutron lifetime in the fluid‐filled borehole is eliminated and that, in addition to the formation signal, the borehole signal is also considered. For formations of weak absorption, the theoretical solution predicts features in the gamma‐ray die‐away curve similar to those observed in real logging cases. For formations of strong absorption, the asymptotic behavior of the gamma‐ray decay curve is influenced by the borehole size. This theoretical solution, even though not directly applicable to the complex real logging environment, offers us a means for understanding qualitatively the interrelationship among all thermal neutron parameters of the borehole and the formation.
A heat flow profile across the Sverdrup Basin, Canadian Arctic Islands
Authors: Jones F. W. , Majorowicz J. A. , Embry A. F.
Published: October 2002

An average geothermal gradient of 25 ± 5 mK/m and an average heat flow of [Formula: see text] have been determined for 16 out of 20 analyzed wells along a profile across the Sverdrup Basin in the Canadian Arctic. These estimates, based on deep bottom‐hole temperature (BHT) data from exploration wells and the permafrost base boundary temperature, together with assumed heat conductivities from net rock analysis, are surprisingly low and disagree with previously published results based on shallow data. The differences may be due to the dramatic changes in boundary temperature conditions from moderate subsea conditions to ground‐surface low temperatures as a result of marine regression. Because of these effects, it appears that deep BHT temperature data are valuable in providing information about the deep heat flow. The heat flows thus determined indicate that the basin has approached thermal equilibrium.
A new method for the automatic interpretation of Schlumberger and Wenner sounding curves
Authors: Zohdy A. A. R.
Published: October 2002

A fast iterative method for the automatic interpretation of Schlumberger and Wenner sounding curves is based on obtaining interpreted depths and resistivities from shifted electrode spacings and adjusted apparent resistivities, respectively. The method is fully automatic. It does not require an initial guess of the number of layers, their thicknesses, or their resistivities; and it does not require extrapolation of incomplete sounding curves. The number of layers in the interpreted model equals the number of digitized points on the sounding curve. The resulting multilayer model is always well‐behaved with no thin layers of unusually high or unusually low resistivities. For noisy data, interpretation is done in two sets of iterations (two passes). Anomalous layers, created because of noise in the first pass, are eliminated in the second pass. Such layers are eliminated by considering the best‐fitting curve from the first pass to be a smoothed version of the observed curve and automatically reinterpreting it (second pass). The application of the method is illustrated by several examples.
On the prediction of low‐frequency geomagnetic pulsations for geophysical prospecting
Authors: Lam Hing‐Lan
Published: October 2002

Pulsations of the earth’s magnetic field due to solar‐terrestrial interaction are used for geomagnetic soundings to study conductive structures in the earth. However, the pulsations are also the unwanted background noise for magnetic exploration surveys. It is, therefore, desirable to conduct the sounding or survey work during geomagnetic conditions optimum for that particular undertaking. This paper relates the pulsational activity to a parameter (the DRX index) which characterizes the magnetic activity for each individual day and which is routinely forecast in Canada. By using the relations established in this study, it is possible to infer the future levels of pulsations from the forecast DRX, and a period of desired pulsational activity can thus be selected for the field work. In the course of a day, the afternoon is appropriate for conducting magnetic surveys because of a generally lower level of pulsational activity and a virtual absence of intense short‐period pulsations, while morning is optimal for carrying out induction soundings because of more intense pulsational activity and a tendency of enhanced long‐period pulsations to recur during that time of day.
Application of multioffset vertical seismic profiling in fracture mapping
Authors: Carswell Allan , Moon Wooil M.
Published: October 2002

Multioffset vertical seismic profiling (VSP) combines the improved vertical resolution of VSP with the lateral resolution of the conventional seismic method. In this study, the multioffset VSP technique was employed to map the fracture zones in a granite batholith in which the Atomic Energy of Canada Ltd.’s Underground Research Laboratory (URL) is located. With shotpoints along a vertical shaft and receiver arrays on the surface (cemented to outcrops), six 2-D seismic sections were obtained. The upcoming and downgoing events were separated using a Radon transform wave‐field separation method. For the given multioffset experimental configuration, the VSP-CDP transformation converted the VSP section into conventional type seismic sections. The results indicate that the multioffset configuration is an effective method for mapping deep fracture zones, in this case with respect to the URL shaft. However, the VSP-CDP transformation method used in this study tends to stretch the shallow reflection events, resulting in reduced resolution.
Exploring for dolomitized Slave Point carbonates in northeastern British Columbia
Authors: Phipps Graeme G.
Published: October 2002

In northeastern British Columbia, a number of significant gas fields have been discovered in dolomitized Slave Point carbonates located along the edge of the mid‐Devonian barrier‐reef system. Reflection seismic techniques have been used extensively in exploring for these fields. A common exploration practice has been to detect the reef edge on seismic data and then to drill on isochron thins and structural highs close to this edge. The remaining reserves, however, are proving more difficult to find, and this exploration technique does not always guarantee that the Slave Point formation is dolomitized at this location. As with many other complex play types, combining geologic concepts with geophysical exploration techniques can improve the explorationist’s success ratio. In this example, a geologic model (the Dorag dolomitization model) provides a possible explanation for the localization of the Slave Point dolomites. In this model dolomitizing solutions are created by mixing fresh, phreatic water with salt water. This condition could occur if porous reefal Slave Point carbonates were exposed to fresh‐water infiltrations adjacent to open‐marine conditions. The edges of the mid‐Devonian barrier reef and the edge of the Keg River embayments were areas that could have met these requirements. Thus the optimum condition for dolomitization should occur slightly basinward of the reef edge in the zone of possible mixing of fresh and salt waters. To increase the chance of encountering a potentially narrow band of dolomitized Slave Point reservoirs, exploration wells should be positioned on the edge of the carbonate reef slightly basinward of the isochron thin. An exploration example from a Keg River embayment area illustrates and supports this exploration concept.
Depth of investigation of collinear symmetrical four‐electrode arrays
Authors: Barker R. D.
Published: October 2002

A study of collinear symmetrical four‐electrode arrays and their tripotential variations indicates the existence of an electrode array for which all the tripotential arrangements have the same depth of investigation. Examination of computer‐generated sounding curves confirms this result only when depth of investigation is defined as the median of the depth of investigation characteristic curve. The results lend support to this being the most practically useful definition of depth of investigation.
A grain conductivity approach to shaly sandstones
Authors: de Lima Olivar A. L. , Sharma Mukul M.
Published: October 2002

The effects of clay conduction on the electrical behavior of shaly sandstones at low frequencies are analyzed by considering self‐similar mixtures of conductive grains, as representative of these porous rocks. An essential step in this model is the representation of the surface conductivities of the clay platelets by an equivalent volume conductivity. Based on their pattern of occurrence, the clays are treated either as a continuous coating over the sand grains or as individual detrital grains mixed with a nonconductive mineral matrix. In the first case, the solids are modeled as layered conductive particles, taken as spheres to a first approximation. In the second case, we generalize from the results of a three‐component self‐similar mixture of spherical particles. For linear approximations, the models are described by expressions quite similar to the frequently used Waxman‐Smits and dual‐ water equations, but dependent upon the clay type, concentration, and its distribution in the rock. The models are written in terms of geometric parameters readily available from well‐log measurements. The derived analytical expressions are successfully applied to describe a set of available experimental core and geophysical log data. The results clearly demonstrate the applicability of the equations to well‐log interpretation, as a practical procedure for computing formation factors and shaliness of argillaceous sandstones.
An accurate and efficient scheme for wave propagation in linear viscoelastic media
Authors: Tal‐Ezer H. , Carcione J. M. , Kosloff D.
Published: October 2002

The problem of wave propagation in a linear viscoelastic medium can be described mathematically as an exponential evolution operator of the form [Formula: see text] acting on a vector representing the initial conditions, where [Formula: see text] is a spatial operator matrix and t is the time variable. Techniques like finite difference, for instance, are based on a Taylor expansion of this evolution operator. We propose an optimal polynomial approximation of [Formula: see text] based on the powerful method of interpolation in the complex plane, in a domain which includes the eigenvalues of the matrix [Formula: see text]. The new time‐integration technique is implemented to solve the isotropic viscoacoustic equation of motion. The algorithm is tested for the problem of wave propagation in a homogeneous medium and compared with second‐order temporal differencing and the spectral Chebychev method. The algorithm solves efficiently, and with machine accuracy, the problem of seismic wave propagation with a dissipation mechanism in the sonic band, a characteristic of sedimentary rocks. The computational effort in forward modeling based on the new technique is half compared to that of temporal differencing when two‐digit precision is required. Accuracy is very important, particularly for propagating distances of several wavelengths, since the anelastic effects should not be confused with nonphysical phenomena, such as numerical dispersion in time‐stepping methods. Finally, we compute the seismic response to a single shot in a realistic geologic model which includes a gas‐cap zone in an anticlinal fold. The results clearly show the importance of the attenuation and dispersion effects for an appropriate interpretation of the seismic data. In particular, the gas‐cap response (a bright spot) suffers significant variations in amplitude, phase, and arrival time compared to the purely acoustic case.
Velocity‐depth ambiguity of reflection traveltimes
Authors: Bickel Samuel H.
Published: October 2002

The conversion of time horizons to depth is fundamental to exploration geophysics. The interval velocity used in the conversion is often estimated from the stacking velocity, assuming that each layer’s interval velocity is homogeneous. However, even for one laterally inhomogeneous layer above a flat reflector the stacking velocity can swing violently about its average and conventional methods of velocity estimation fail. I show that violent swings in the stacking velocity are a symptom of a long‐wavelength ambiguity between the burial depth to an interface and interval velocity. Lateral variations in seismic velocity with a spatial wavelength of about 2.7 D, where D is the depth to the reflecting horizon, cannot be unambiguously resolved from traveltime measurements. The spatial wavelength of this ambiguous component varies from 2.57 D, for very small source‐receiver separations, to 2.86 D for source‐receiver separations equal to D. Spectral components of the stacking velocity at wavelengths shorter than this ambiguous value are amplified in size and reversed in polarity relative to the interval velocity. A practical inverse filter that corrects for these distortions produces an interval velocity that is almost totally lacking in low‐frequency components, giving a very distorted picture of the interval velocity. Since the wavelength of total ambiguity changes with offset, a complete description of the velocity and depth fields can, in theory, be extracted from a combination of multiple‐offset traveltime measurements. However, the wavelength of total ambiguity is such a weak function of source‐receiver separation that multiple offset processing, in practice, does little to resolve the ambiguity. In fact, the Rayleigh resolution limit implies that three or more offset measurements are more effective than two only if the seismic‐line length is at least 20 D. In a series of numerical experiments with the line set to 100 D and a spatial noise level of .01% in each channel I used a two‐channel Wiener filter to successfully extract the full‐band response for a simultaneous step change in velocity and in depth. The method fails for lines shorter than 20 D because of the transients that arise when the data are shorter than the filter. Stability was achieved by increasing the noise level to 1% in the design of the Wiener filter, but low spatial frequencies were lost and the estimated velocity‐depth model was distorted. If the results of this single flat‐layer analysis apply to practical situations, the velocity‐depth ambiguity may continue to plague exploration seismologists for some time to come.
Velocity anisotropy terminology for geophysicists
Authors: Winterstein D. F.
Published: October 2002

Interest in velocity anisotropy increased sharply among exploration seismologists following publication in 1986 of evidence for azimuthal anisotropy in sedimentary rocks. This increased level of interest generated a need for a glossary of anisotropy terminology in the language of geophysicists. This glossary is to: • help geophysicists find proper words • promote correct usage. The introduction preceding the alphabetically organized glossary focuses on concepts that have special importance for anisotropy but may be unfamiliar to most geophysicists. Symmetry concepts receive special emphasis because of their fundamental importance.
Ground roll: Rejection using polarization filters
Authors: Shieh Chiou‐Fen , Herrmann Robert B.
Published: October 2002

Ground roll noise on land data sets overwhelms the desired reflection seismic signal unless special steps are taken in data acquisition and processing to control it. This is usually done in the field by the design of group arrays for data acquisition. On the other hand, if multicomponent data are acquired, it is possible to remove ground roll during processing using polarization analysis. Even though this processing is computation‐intensive, the potential exists for obtaining results similar to conventional data acquisition, but with deployment of fewer sensors in the field with minimal group array effects. It also has potential for deriving new information. We describe a two‐dimensional polarization‐filter analysis for use with vertical and in‐line sensors. A time‐domain spectral matrix technique is developed since the recorded seismic signal is the superposition of multiple signals in the time domain, each with different frequency content and time‐varying polarization. This technique is implemented by decomposing the signal into individual frequency components using narrow band‐pass filters and defining the polarization characteristic using sliding time windows. We show that both incoherent noise and specific linearly polarized constituents can be successfully filtered.
Moving‐window Poisson analysis of gravity and magnetic data from the Penokean orogen, east‐central Minnesota
Authors: Chandler Val W. , Malek Kelley Carlson
Published: October 2002

Analytical correlation of gravity and magnetic data through moving‐window application of Poisson's theorem is useful in studying the complex Precambrian geology of central Minnesota. Linear regression between the two data sets at each window position yields correlation, intercept, and slope parameters that quantitatively describe the relationship between the gravity and magnetic data and, in the case of the slope parameter, are often accurate estimates of magnetizatons‐to‐density ratios (MDR) of anomalous sources. In this study, gridded gravity and magnetic data from a 217.6 × 217.6 km area in central Minnesota were analyzed using a 8.5 × 8.5 km window. The study area includes part of the Early Proterozoic Penokean orogen and an Archean greenstone‐granite terrane of the Superior Province. The parameters derived by the moving‐window analysis show striking relationships to many geologic features, and many of the MDR estimates agree with rock property data. Inversely related gravity and magnetic anomalies are a characteristic trait of the Superior Province, but moving‐window analysis reveals that direct relationships occur locally. In the Penokean fold‐and‐thrust belt, gravity and magnetic highs over the Cuyuna range produce a prominent belt of large MDR estimates, which reflect highly deformed troughs of iron‐formation and other supracrustal rocks. This belt can be traced northeastward to sources that are buried by 3–5 km of Early Proterozoic strata in the Animikie basin. This configuration, in conjunction with recent geologic studies, indicates that the Animikie strata, which may represent foreland basin deposits associated with the Penokean orogen, unconformably overlie parts of the fold‐and‐thrust belt, and that earlier stratigraphic correlations between Cuyuna and Animikie strata are wrong. The results of this study indicate that moving‐window Poisson analysis is useful in the study of Precambrian terranes.
Investigations of electrical properties of weathered layers in the Yala area, western Kenya, using resistivity soundings
Authors: Barongo J. O. , Palacky G. J.
Published: October 2002

Investigation in many parts of the world have indicated that resistivity of saprolite (a chloritization zone at the base of the weathering profile) depends uniquely on the underlying rock type. A study of electrical properties of the weathered layer was undertaken near Yala in the Nyanzian‐Kavirondian greenstone belt in western Kenya. Resistivity soundings were systematically carried out in a 24 × 40 km area. Saprolite formed over basalt had the lowest resistivity (14 Ω•m), followed by andesite (27 Ω•m) and rhyolite (67 Ω•m). Statistically meaningful data sets conclusively show, for the first time, that saprolite resistivity increases with the silica content of parent volcanic rocks. Plutonic and sedimentary rocks had higher average resistivities (granite 135 Ω•m, mudstone 79 Ω•m, grit 213 Ω•m). Thickness of the weathered layer varied between 10 and 40 m, with the largest values observed over grit (sandstone). Compared with other tropical regions, the saprolite layer in western Kenya appears relatively thin, possibly because of the relatively rapid tectonic uplift of the area which leads to fast erosion. As observed in earlier studies, resistivity and electromagnetic surveys can increase the speed and reliability of geologic mapping in tropical terrains.
Electromagnetic wave propagation in disrupted coal seams
Authors: Greenfield R. J. , Wu S. T.
Published: October 2002

It is of economic importance to locate zones of geological disturbance in longwall coal panels prior to the start of mining operations. This can be done with electromagnetic (EM) longwall attenuation tomography in which the amplitude of a single frequency wave in the 300 kHz range is measured over a number of raypaths. The main component of the signal is a fundamental waveguide mode. Zones with high attenuation rates have been observed to be zones of geological disturbance. A two‐dimensional TM (magnetic field transverse to the plane of propagation) mode, time domain finite difference solution was developed and used to determine the excess attenuation (EA) that occurs in disturbed zones. EA is defined as the actual attenuation going through a disturbed zone less the attenuation for an equal length of the normal, undisturbed waveguide. A typical model has a coal seam bordered above and below by rock with a disturbed zone; various types of disturbances such as sand channels and faults were modeled. Synthetic magnetograms are computed at points along the seam and are Fourier transformed to get spectral amplitudes. Model results show that high attenuation can occur in disturbed zones. However the EA that occurs going through a zone is highly dependent on the properties of the zone and waveguide. Models with high conductivity (.05 S/m) roof and floor rock gave larger EA than low conductivity models (.001 S/m). However, low conductivity roof and floor models can still give 5 to 10 db of attenuation, which is consistent with observed values. With high conductivity, simple roof and floor faults can cause significant attenuation.