bluuebunny/crossref_metadata_2025 huggingface dataset:
Remote monitoring of the steam‐flood enhanced oil recovery process
Authors: Laine E. F.
Published: October 2002

Cross‐borehole seismic velocity and high‐frequency electromagnetic (EM) attenuation data were obtained to construct tomographic images of heavy oil sands in a steam‐flood environment. First‐arrival seismic data were used to construct a tomographic color image of a 10 m by 8 m vertical plane between the two boreholes. Two high‐frequency (17 and 15 MHz) EM transmission tomographs were constructed of a 20 m by 8 m vertical plane. The velocity tomograph clearly shows a shale layer with oil sands above it and below it. The EM tomographs show a more complex geology of oil sands with shale inclusions. The deepest EM tomograph shows the upper part of an active steam zone and suggests steam chanelling just below the shale layer. These results show the detailed structure of the entire plane between boreholes and may provide a better means to understand the process for in situ heavy oil recovery in a steam‐flood environment.
Three‐dimensional traveltime equation for dipping layers
Authors: Diebold John B.
Published: October 2002

The traveltime equation for homogeneous layers with two‐dimensionally dipping interfaces is well known, but it has been proven only for a limited variety of profile geometries. Existing proofs do not necessarily lend themselves to use with other types of profiles, or to extension into three dimensions. When the distance that a ray travels and the elapsed time are decomposed into orthogonal components, and each interaction of the ray and the interface is examined separately, the time contributions for ray travel in the two orthogonal directions can be isolated for ray segments between horizontal planes of reference. The terms in which these contributions are expressed are independent of any part of the structure beyond the ray segment under consideration. Therefore, the total traveltime for a ray can be obtained by simple summation of such contributions. This process represents a new proof of the traveltime equation, but, more importantly, it provides a method for constructing the correct equation for virtually any profile geometry and arrival type, including direct, reflected, and refracted waves as well as converted phases. When the rays and interface normals are treated as three‐dimensional vectors, this proof and method can be extended to three dimensions. The result is a general expression describing point‐to‐point traveltimes for any source‐receiver geometry in structures comprising homogeneous layers with interfaces having arbitrary three‐dimensional dip. Since this equation accurately describes the incremental traveltime for every portion of any propagating wave, nearly any three‐dimensional geometry can be accommodated.
Attenuation of acoustic modes due to viscous drag at the borehole wall
Authors: Schoenberg Michael , Sen Pabitra N. , White J. E.
Published: October 2002

White (1983), among others, has shown that the propagation and attenuation of Stoneley waves in a fluid‐filled borehole contain information on the porosity and permeability of the surrounding formation. Prior to extracting permeability from attenuation, the correct viscous loss of Stoneley waves in a borehole in a nonporous medium must be completely understood. With this motivation in mind, the purpose of this paper is to derive the viscous loss due to drag at the wall of a borehole from the exactly solvable case of a perfectly cylindrical borehole in an elastic, nonporous formation filled with a fluid that is viscous in shear. Thus the fluid pressure and fluid motion in the small but critical region near the fluid‐solid interface can be calculated. We have obtained dispersion and attenuation of trapped waves and tube waves, but here we report only on tube waves.
A spectral method for the inverse electromagnetic logging problem
Authors: Howard A. Q.
Published: October 2002

This paper presents a new inverse method for electromagnetic logging. The analysis assumes a two‐coil sonde operated in a CW mode. The geometric model allows for a piecewise continuous, axially inhomogeneous formation intersected at a right angle by a circular borehole. The inversion uses both in‐phase and quadrature‐phase log data. These data are Fourier transformed with respect to the axial coordinate z. An optimized fast Fourier transform technique, which selects the numerical transform parameters for both the sampling interval and window size, provides an accurate parallel computation of the theory’s integrals and yields the entire log segment in one computation. The inversion of the borehole voltage data to determine the formation conductivity profile requires no matrices and is ideally suitable for combinations on a minicomputer array processor. Central to the method is a transfer function relationship between the spectra of the anomalous borehole voltage and the formation conductivity. Typically, 1 024 point transforms are adequate. When the complex transfer function sequence is stored on disc, log data can be inverted in real time. Numerical simulations show that the constrained inversion procedure tolerates the zeros in the transfer function and is robust in the presence of additive noise. Both formation conductivity and dielectric constant estimates are computed. Results are given for a piecewise constant formation model.
Archaeomagnetism and magnetic anomalies in the American Southwest
Authors: Sternberg Robert S.
Published: October 2002

Magnetic surveying is probably the most widely used geophysical technique at archaeological sites (Weymouth and Huggins, 1985). It is useful for locating features in which the magnetic susceptibility contrasts with the magnetic susceptibility of the surroundings because of induced and/or remanent magnetization. A contrast in magnetic induction can arise, for example, when the disturbed soil fill inside a formerly occupied room has a different magnetic susceptibility from the subsoil outside the room (Tite and Mullins, 1971). A contrast in magnetic remanence most commonly occurs when a baked clay artifact or feature such as a hearth has acquired a thermoremanent magnetization after being heated to several hundred degrees Celsius.
The tube wave as a Biot slow wave
Authors: Norris Andrew
Published: October 2002

The tube wave speed in a simple fluid‐filled circular bore reduces to [Formula: see text] as the frequency goes to zero, where [Formula: see text] is the acoustic sound speed in the fluid, [Formula: see text] is the fluid density, [Formula: see text] is the fluid bulk modulus, and μ is the formation shear modulus. Biot (1952) deduced this simple relation by considering the low‐frequency asymptotic expansion of the exact dispersion relation. In 1956, Biot proposed a theory (Biot, 1956a, b) that predicts a new type of compressional bulk wave in fluid‐saturated porous media. This “slow wave” is associated mainly with the motion of pore fluids. It appears that Biot never related this theory to his previous work on the bore problem, although the connection is apparent if the bore is considered as a pore. Typically, the bore radius is about 10 cm, while the relevant acoustic logging frequency is on the order of 1 kHz. With water as the bore fluid, the viscous skin depth is on the order of 100 μm. Therefore, if the bore is to be considered as a pore, the relevant form of Biot’s theory is the limit in which the pore radius is large relative to the viscous skin depth of the fluid. This form is the high‐frequency limit, in which the effects of the fluid viscosity are negligible and the slow‐wave dissipation is relatively low.
Transient electromagnetic field computations for polygonal loops on layered earths
Authors: Raiche A. P.
Published: October 2002

The transient electromagnetic response (vertical and horizontal components of dB/dt) of a large polygonal transmitting loop on a layered earth is calculated using a nested interpolation scheme based on the dipole‐dipole response function. The frequency‐domain field of a vertical magnetic dipole is inverse Laplace transformed into the time domain, for selected values of the Hankel transform variable, using the Gaver‐Stehfest method. After interpolation, the result is inverse Hankel transformed (for selected values of distance) using digital filters. Interpolating over distance allows integration of the dipole response over the area of one or more transmitting loops. An interpolation over time gives the step‐current response, which in turn is convolved with the transmitter‐receiver characteristics to yield the system response. This method allows robust calculation of several transmitting loops (with different signal parameters) and several receiver positions in little more time than that required for one loop with one receiver. The computation time for the single transmitter‐receiver response can be decreased by analytical integration of the Bessel function over the transmitter loop area before performing the inverse Hankel transform. Since this procedure precludes the use of standard digital filters for the inverse Hankel transform, it is not efficient for multireceiver computations.
Vector attenuation: Some implications for plane waves in anelastic layered media
Authors: Winterstein D. F.
Published: October 2002

Plane waves in layered anelastic media where Q varies from one layer to the next in general will have maximum attenuation in a direction different from the direction of propagation. Lockett (1962) was first to recognize this, and several others—notably Buchen (1971), Borcherdt (1971, 1973, 1977, 1982), Krebes (1983, 1984), Richards (1984), and Wennerberg (1985)—explored and expanded the theory.
On: “Euler’s differential equation and the identification of the magnetic point‐pole and point‐dipole sources” by J. O. Barongo (GEOPHYSICS, 49, 1549–1553, September 1984).
Authors: Green Ronald
Published: October 2002

In the article by Barongo, the problem of distinguishing between the anomaly patterns arising from an isolated pole and a point dipole was examined. The method the author recommended concentrated on a detailed analysis of the shape of the principal meridional profile. I suggest that a better method is to use the isogam map, rather than the profile. From the relevant expressions for contour maps of an isolated pole and a point dipole, the salient differences between the two cases become apparent.
Surface emittance, temperature, and thermal inertia derived from Thermal Infrared Multispectral Scanner (TIMS) data for Death Valley, California
Authors: Kahle Anne B.
Published: October 2002

The NASA airborne Thermal Infrared Multispectral Scanner (TIMS) was flown over Death Valley, California, on both a daytime flight and a nighttime flight within a two‐day period in July 1983. This Daedulus scanner has six channels in the thermal infrared, between 8 and 12 μm. Calibrated digital spectral radiance data from these flights, along with Landsat Thematic Mapper (TM) reflectance data, permit the calculation of both spectral emittance and thermal inertia. Spectral emittance images were derived for the test area for data sets from both the day and night tests, and they show good qualitative agreement. Comparison of the numerical values of emittance derived from these day and night images shows a decrease in spectral contrast at night. This is probably due primarily to an increased atmospheric contribution to the radiance reaching the sensor at night when the ground is cold, rather than to a change in spectral characteristics of the surface at night. These spectral emittance data contribute to an understanding of the physical basis for the discrimination of differences in surface materials afforded by TIMS data. These emittance data show good qualitative agreement with field emittance data taken in the same areas and with laboratory spectral reflectance data for samples from the Death Valley area. The strongest spectral feature commonly seen lies between 8 and 10 μm and is ascribed to the fundamental silicon‐oxygen stretching vibration of quartz and other silicate minerals. This feature allows identification of quartzite and discrimination of other silicate rocks in images produced from the TIMS data. Spectral features of other minerals, such as the ≈ 11.3 μm band of carbonates, are also detectable in laboratory spectra and field spectra. Using the day and night surface temperature data and Landsat TM reflectance data, an apparent thermal inertia image has been produced. This image allows separation of some bedrock units and separation of bedrock from alluvium. The temperature images allow inferences about the soil moisture and/or soil conditions on some of the alluvial fans.
Analysis of absorption and dispersion effects in synthetic τ-p seismograms
Authors: Martinez Ruben D. , McMechan George A.
Published: October 2002

Analysis of absorption and dispersion effects may be done in intercept time‐ray parameter (τ-p) synthetic seismograms calculated using the slowness formulation of the reflectivity method. Seismograms initially computed in the frequency‐ray parameter (ω-p) domain to incorporate absorption and dispersion effects are then Fourier transformed to the (τ-p) domain. Absorption and dispersion are functions of p. Modeling both simple and more realistic stratigraphic sequences shows the interaction of only velocity and density for infinite Q and the complicated effects added when Q is finite. The observed null reflection at p = 0 for infinite Q is no longer null when Q is finite. For p ≠ 0, the inclusion of absorption and dispersion effects complicates the amplitude and phase of the seismic response. Reflectivity due to Q alone (i.e., at an interface with no impedance contrast), as a function of Q contrast and p, contains interesting variations of amplitude and phase. The responses of three geologically realistic models (a brine sand, a partially saturated gas sand, and an ocean‐sediment interface) demonstrate the cumulative nature of the attenuation effect and how the Q contributions become dominant when the acoustic impedance contrast is small. For large acoustic impedance contrasts, the attenuation effect occurs as an amplitude decay and phase rotation for some (especially high) frequencies. The modeling results suggest that absorption and dispersion effects should be taken into account in seismic inversion. Q estimations (in addition to velocity and density) are particularly desirable in exploration for hydrocarbons because of the sensitivity of Q to lithology and fluid content. Q contributes to the reflectivity information inherent in the seismic data.
Numerical evaluation of Green’s functions for axisymmetric boreholes using leaking modes
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 boreholes are expressed completely as sums of modes. The integrations with respect to k are performed exactly using Cauchy residue theory. The remaining integrations with respect to ω are then carried out using the fast Fourier transform (FFT). The complete solution, including all possible body waves, is expressed simply as a superposition of modes without any contributions from branch line integrals. There are no spurious arrivals and, provided that the number of points in the FFT can be taken sufficiently large, no restrictions on distance. The method is fast, accurate, and easy to apply.
Downward continuation of refracted arrivals to determine shallow structure
Authors: Hill N. Ross
Published: October 2002

Information contained in refracted arrivals often can determine shallow, complex structure within the earth. An established way of interpreting refraction arrivals employs graphical construction of wavefronts. Here I extend this graphical method by using numerical downward continuation techniques. For examples of synthetic data and field data, seismic images of irregular interfaces are formed by downward continuing refracted arrivals. For a field‐data example, the image formed from the refraction arrivals is used to correct time‐delay anomalies caused by irregular near‐surface structure. Incorporation of downward continuation into refraction interpretation has several advantages. The method reduces complications due to raypath effects, diffractions, and shadow zones in the refracted arrivals. In addition, this interpretation method reduces the labor and ambiguities associated with identifying first breaks. p‐tau decomposition of the wave field provides a wide‐angle method for downward continuation of refracted arrivals. This method of downward continuation is well suited for refracted arrivals for several reasons. The method allows convenient evaluation of the wave field beneath an irregular recording datum, and it helps overcome spatial aliasing. Also, the calculations can easily be limited to the region of p‐tau space that contains the refracted arrival.
Air‐gun source instabilities
Authors: Dragoset Bill , Hargreaves Neil , Larner Ken
Published: October 2002

The signature of an air‐gun array can change over a period of time or even from one shot to the next. If the signature variations are large, then deterministic deconvolution, with an operator designed from a single signature or from an average signature, could produce errors significant enough to affect data interpretation. Possible sources of air‐gun instability include changes in gun positions, firing times, and pressures, gun failures, and scattering from the fluctuating rough ocean surface. If an air‐gun array were perfectly stable, after application of signature deconvolution the residual signatures for a sequence of shots would be identically shaped, broadband, zero‐phase wavelets. In practice, air‐gun instabilities lead to two major defects in band‐ limited residual signatures: the central portion of the wavelet can become asymmetrical, and unsuppressed energy can occur in the residual bubble region. Processing experiments done with synthesized air‐gun array signatures show that of all types of air‐gun instabilities likely to occur, only gun dropouts cause signature variations severe enough to affect data interpretation. Gun dropouts produce unsuppressed residual bubble energy that can show up as phantom events on a stacked section or that can obscure small‐amplitude events following large‐amplitude events. Neither gun dropouts nor any other kind of air‐gun instability has a significant effect on resolution within the seismic band. Since gun dropouts do not happen on a shot‐to‐shot basis and other instabilities are unimportant, there is no practical benefit to be gained by deriving and applying individual signature deconvolution operators for each shot. The influence of gun dropouts can be minimized through other actions taken in acquisition and processing.
Effects of in situ permeability on the propagation of Stoneley (tube) waves in a borehole
Authors: Cheng C. H. , Jinzhong Zhang , Burns Daniel R.
Published: October 2002

We investigated the theoretical relationship between propagation characteristics of Stoneley (tube) waves in a borehole and in situ permeability by using a modified formulation of a borehole model with a formation that behaves as a Biot porous medium. We found that Stoneley‐wave attenuation and phase‐velocity dispersion increased with increasing permeability and porosity, and decreased with increasing frequency. In rocks with low to medium permeabilities (less than 100 mD), variations in formation velocity and attentuation were major contributors to variations in Stoneley‐wave properties at normal logging frequencies. However, in high‐permeability rocks (greater than 100 mD), coupling between the borehole and pore fluids associated with in situ permeability was more important than lithological changes in controlling Stoneley‐wave properties. Pore‐fluid viscosity had an effect on Stoneley‐wave propagation equal but opposite to permeability, and hence must he taken into account. We compared our theoretical results with published data on core permeability and Stoneley‐wave phase velocities and amplitudes. The Stoneley‐wave amplitude was more sensitive to the permeability of the formation than Stoneley‐wave phase velocity. By assuming an appropriate average value of intrinsic attenuation, we obtained reasonable agreements between theory and the published data. We conclude that relative permeability within a formation can be determined quite well using Stoneley‐wave amplitude and phase velocity, but absolute permeability determination requires accurate measurements of parameters such as the intrinsic attenuation of the formation and the viscosity and compressibility of the pore fluid.
Evaluation of two‐pass three‐dimensional migration
Authors: Dickinson John A.
Published: October 2002

The theoretically correct way to perform a three‐dimensional (3-D) migration of seismic data requires large amounts of data manipulation on the computer. In order to alleviate this problem, a true, one‐pass 3-D migration is commonly replaced with an approximate technique in which a series of two‐dimensional (2-D) migrations is performed in orthogonal directions. This two‐pass algorithm produces the correct answer when the velocity is constant, both horizontally and vertically. Here I analyze the error due to this algorithm when the velocities vary vertically. The analysis has two parts: first, a theoretical analysis is performed in which a formula for the error is derived; and second, a field data comparison between one‐pass and two‐pass migrations is shown. My conclusion is that two‐pass 3-D migration is, in general, a very good approximation. Its errors are usually small, the exceptions being when both the reflector dip is large (in practice this typically means greater than about 25 to 40 degrees) and the orientation of the reflector is in neither the inline nor the crossline direction. Even then the error is the same order of magnitude as that due to the uncertainty in the migration velocities. These conclusions are still valid when there is lateral velocity variation, as long as this variation is accounted for by trace stretching. The analysis presented here deals with time migration; no claims are made regarding depth migration.
Cooperative inversion of geophysical data
Authors: Lines Larry R. , Schultz Alton K. , Treitel Sven
Published: October 2002

Geophysical inversion by iterative modeling involves fitting observations by adjusting model parameters. Both seismic and potential‐field model responses can be influenced by the adjustment of the parameters of the rock properties. The objective of this “cooperative inversion” is to obtain a model which is consistent with all available surface and borehole geophysical data. Although inversion of geophysical data is generally non‐unique and ambiguous, we can lessen the ambiguities by inverting all available surface and borehole data. This paper illustrates this concept with a case history in which surface seismic data, sonic logs, surface gravity data, and borehole gravity meter (BHGM) data are adequately modeled by using least‐squares inversion and a series of forward modeling steps.
Nonlinear reverse‐time inversion of elastic offset vertical seismic profile data
Authors: Sun Robert , McMechan George A.
Published: October 2002

An iterative nonlinear inversion algorithm for two‐dimensional elastic media gives estimates of P-velocity and S-velocity distributions from synthetic offset vertical seismic profiles. The algorithm is a hybrid of inversion and principles borrowed from reverse‐time migration. Gradients of the misfit function are dynamically determined by crosscorrelations of the computed incident wave fields with the scattered compressional and shear wave fields. Model perturbations are defined in the steepest descent direction. In order to optimize the sensitivity of the inversion to both compressional and shear velocity distributions, two data collection experiments are required, one with a compressional wave source and the other with a shear wave source. Inversion iterations alternate between the compressional and shear source data sets. In test examples, the new algorithm converges successfully to the correct solution when the starting model’s compressional and shear velocities deviated by as much as 20 percent from the correct solution.
Fourth‐order finite‐difference P-SV seismograms
Authors: Levander Alan R.
Published: October 2002

I describe the properties of a fourth‐order accurate space, second‐order accurate time, two‐dimensional P-SV finite‐difference scheme based on the Madariaga‐Virieux staggered‐grid formulation. The numerical scheme is developed from the first‐order system of hyperbolic elastic equations of motion and constitutive laws expressed in particle velocities and stresses. The Madariaga‐Virieux staggered‐grid scheme has the desirable quality that it can correctly model any variation in material properties, including both large and small Poisson’s ratio materials, with minimal numerical dispersion and numerical anisotropy. Dispersion analysis indicates that the shortest wavelengths in the model need to be sampled at 5 gridpoints/wavelength. The scheme can be used to accurately simulate wave propagation in mixed acoustic‐elastic media, making it ideal for modeling marine problems. Explicitly calculating both velocities and stresses makes it relatively simple to initiate a source at the free‐surface or within a layer and to satisfy free‐surface boundary conditions. Benchmark comparisons of finite‐difference and analytical solutions to Lamb’s problem are almost identical, as are comparisons of finite‐difference and reflectivity solutions for elastic‐elastic and acoustic‐elastic layered models.
Electrical response of a leak in a geomembrane liner
Authors: Parra J. O.
Published: October 2002

A leak in a geomembrane lined impoundment or landfill has a characteristic electrical response. I simulate the waste material, the liner, and the soil under the liner by infinite horizontal layers and express the secondary potential for a leak in the geomembrane liner in terms of a three‐layer medium Green’s function and the unknown current density distribution at the leak. The area of the leak is sufficiently small for the leak current density to be essentially uniform. I add the primary potential associated with a leak‐free liner to the secondary potential to form an integral equation and derive a general expression for the current density at the boundary between the waste material and the liner. From the expression for the current density, I determine the current flow through the leak by assuming that the total current distribution flows vertically across a finite region of the infinite, thin liner layer. This finite region has the same surface area as does the waste disposal site or landfill. My analysis implies that the current density is the most sensitive variable affecting the magnitude of current flow through the leak and the amplitude of the leak anomaly response. Multiple circular leaks in the thin resistive liner are included in the analysis. The potential anomaly of a leak is a localized response which is capable of providing a useful means for detecting and locating such leaks accurately in large waste disposal sites or landfills. Excellent agreement between experimental and model data shows my general solution is accurate in predicting leak signatures and suggests the solution may be useful to model field data acquired in geomembrane lined impoundments or landfills.
Magnetotelluric inversion for minimum structure
Authors: Smith J. Torquil , Booker John R.
Published: October 2002

Structure can be measured in terms of a norm of the derivative of a model with respect to a function of depth f(z), where the model m(z) is either the conductivity σ or log σ. An iterative linearized algorithm can find models that minimize norms of this form for chosen levels of chi‐squared misfit. The models found may very well be global minima of these norms, since they are not observed to depend on the starting model. Overfitting data causes extraneous structure. Some choices of the depth function result in systematic overfitting of high frequencies, a “blue” fit, and extraneous shallow structure. Others result in systematic overfitting of low frequencies, a “red” fit, and extraneous deep structure. A robust statistic is used to test for whiteness; the fit can be made acceptably white by varying the depth function f(z) which defines the norm. An optimum norm produces an inversion which does not introduce false structure and which approaches the true structure in a reasonable way as data errors decrease. Linearization errors are often so small that models of σ (but not log σ) may be reasonably interpreted as the true conductivity averaged through known resolution functions.
A generalized Haskell matrix method for borehole electromagnetics: Theory and applications
Authors: Pai David M. , Huang Ming
Published: October 2002

In borehole electromagnetics, both cylindrical and planar interfaces are present, leading to nonseparable field equations. The problem is two‐dimensional (2-D), and the finite‐element method is usually employed for solution. In this paper, the Generalized Haskell Matrix/Layer Eigenstate Propagator method is introduced to this class of problems. In the method, the solution problem is decomposed into a set of one‐dimensional (1-D) problems, and then the 1-D solutions are combined to form the final solution. The method employs no approximation, other than discretization of a continuous system as in all computer methods. Induction logs are calculated for the 6FF40 tool and a number of models. Results agree well with those of the finite‐element method. An important case in induction‐log interpretation is studied; namely, a three‐layer formation traversed by a borehole, the center layer being an oil‐bearing (resistive) layer sandwiched between two conductive shoulder layers. Simulation shows that conventional correction methods ignoring borehole‐bed coupling can lead to resistivities that differ from the true resistivities by a factor of 2 or even higher.
Directional spectral analysis and filtering of geophysical maps
Authors: Thorarinsson Freyr , Magnusson Stefan G. , Bjornsson Axel
Published: October 2002

The detection of linear anomalies in map data is facilitated by studying the two‐dimensional power spectrum, because the directivity of the energy in the map is preserved in the Fourier transform. The lineaments associated with individual peaks in the spectrum are then separated from the map data by directional filtering and studied independently of other map features. Gravity and magnetic maps from an active rift area in southwestern Iceland are analyzed in this manner. The agreement between the filtered maps is good and they fit the observed tectonic features quite well.
Differential reduction‐to‐the‐pole of regional magnetic anomalies
Authors: Arkani‐Hamed J.
Published: October 2002

I present a differential reduction‐to‐the‐pole technique that reduces regional scale magnetic anomalies to the geomagnetic pole, while taking into account the variations in the direction of the geomagnetic field and that of the magnetization of the crust over the region. The technique is developed in the spectral frequency domain as an inverse problem solved iteratively using a perturbation method. I regard the variations in the directions as finite perturbations about the mean values of the directions over the region and evaluate the nonlinear coupling terms due to these perturbations in the space domain at each iteration before transforming them into the spectral frequency domain. The technique is applied to the magnetic anomalies of three semiinfinite prisms, which are inductively magnetized and located at high, mid, and low latitudes in a region where the inclination of the ambient field changes from 10 to 90 degrees and its declination changes from −30 to +30 degrees. The differential reduction to the pole shifts the positive anomaly of the low‐latitude prism toward the north more than it shifts that of the mid‐latitude prism, which in turn is shifted northward more than that of the high‐latitude prism. The reduction also suppresses the negative lobes to the north of the mid and low‐latitude prisms and moves the positive anomalies to points directly over the prisms. I also apply the technique to the marine magnetic anomalies off the east coast of Canada. The positive magnetic anomalies in the southern part of the area are displaced northward by about 30 km, whereas those in the northern part are not moved significantly.
Thermal measurement of soils using a multineedle probe with a pulsed‐point source
Authors: Larson T. H.
Published: October 2002

The shallow temperature survey has long been recognized as a promising method for rapid reconnaissance exploration for shallow aquifers (Cartwright, 1968). The problem with the method has been the need for additional thermal property measurements to interpret the temperature anomalies correctly. The controlling property for this work is the thermal diffusivity [Formula: see text], defined as the ratio of conductivity [Formula: see text] to heat capacity [Formula: see text]. I have developed a multineedle thermal probe that can be used to make rapid, direct measurements of the thermal diffusivity of soil, as well as its conductivity. This is an improvement over previous methods which either required a long monitoring time (Birman, 1969) or which computed diffusivity indirectly from single‐needle measurements of conductivity (LeSchack et al., 1979).
Three‐dimensional modeling of a hole‐to‐hole electrical method: Application to the interpretation of a field survey
Authors: Poirmeur C. , Vasseur G.
Published: October 2002

Hole‐to‐hole electrical measurements can be used to localize or define the extension of conductive bodies whether or not they are penetrated by the holes. Two promising applications of this electrical method are the optimization of mining boreholes and the delineation of fractures. Although no particular problem arises in the instrumentation of a hole‐to‐hole electrical array, the interpretation of the results has proven to be difficult. A program has been developed to compute the electrical potential caused by direct current injection in an inhomogeneous half‐space (three‐dimensional bodies embedded within a layered earth). This program allows computation of the responses of most classical electrical arrays. It is based on calculation of a disturbance potential and on resolution of a Fredholm integral equation. The modeling program is used to define the apparent‐resistivity type responses of a hole‐to‐hole pole‐pole electrical configuration for different shapes and locations of bodies in the vicinity of the boreholes. To obtain a synthetic representation, the results are plotted on a plane where the X-axis corresponds to the depth in the measuring hole and the Y-axis corresponds to the depth in the injection hole. This representation seems to be particularly convenient for the electrical configuration examined here. A detailed study of simple geometrical models has yielded interpretation rules which have been applied to the qualitative interpretation of a field survey carried out at the mining prospect of SALAU (France). The program is then applied to this interpretation to check and confirm it, by direct calculation of the response corresponding to the model.
On: “A Kalman filter approach to susceptibility mapping” by Mark Pilkington and D. J. Crossley (GEOPHYSICS, 52, 655–664, May 1987).
Authors: Steenland Nelson C.
Published: October 2002

A single anomaly may be exactly satisfied by a single, constant, magnetization contrast. (“Magnetization” proves to be a much more effective unit to use than “susceptibility.”) These contrasting bodies have discrete boundaries, so the distribution of magnetization shows abrupt discontinuities between adjacent bodies with uniform, single values. Field studies confirm this habit. So to show “apparent susceptibilities” (i.e., Figure 9) as a continuously varying “potential” field may be correct, depending upon the definition of “apparent,” but it is not an accurate nor even helpful exercise. (Try to imagine this process operated over a gravity field in a salt dome province with densities varying continuously within salt which, as is well known, stay remarkably constant at 2.2 g/cc.)
On: P. Maliphant’s Discussion (GEOPHYSICS, 52, p. 816, June 1987) of “Alternative processing techniques and data improvement provided by single‐sweep recording,” by S. W. Belcher, T. L. Pratt, J. K. Costain, and C. Coruh (GEOPHYSICS, 51, 1736–1742, September 1986).
Authors: Heath R. G.
Published: October 2002

It is clear that the field cost of recording single sweep Vibroseis data or any other form of nonsite‐processed data increases at least in proportion to the tape cost. Picking the example of 9-track 1600 bpi (phase encoding) drives may not be of direct relevance in Europe, where most recording is on 6250 bpi (GCR) decks, but the hardware cost of adding another 6250 bpi drive, plus possibly some small additional necessary pieces of equipment, is of order of $100,000. Costs would be less in purchasing a 1600 bpi drive but savings made over the cost of the 6250 bpi system would quickly be lost by extra tape cost. Most real Vibroseis crews nowadays have a higher data rate than that suggested by the 48 traces mentioned in Maliphant’s discussion. Additional daily cost would, therefore, be much larger than $1300/day.
III‐posedness of absorbing boundary conditions for migration
Authors: Howell Louis H. , Trefethen Lloyd N.
Published: October 2002

Absorbing boundary conditions for wave‐equation migration were introduced by Clayton and Engquist. We show that one of these boundary conditions, the B2 (second‐order) condition applied with the 45° (third‐order) migration equation, is ill‐posed. In fact, this boundary condition is subject to two distinct mechanisms of ill‐posedness: a Kreiss mode with finite speed at one boundary and another mode of a new kind involving wave propagation at unbounded speed back and forth between two boundaries. Unlike B2, the third‐order Clayton‐Engquist boundary condition B3 is well‐posed. However, we show that it is impossible for any boundary condition of Clayton‐Engquist type of order higher than one to be well‐posed with a migration equation whose order is higher than three.
Viscoacoustic wave propagation simulation in the earth
Authors: Carcione Jose M. , Kosloff Dan , Kosloff Ronnie
Published: October 2002

Anelasticity of earth materials produces significant changes in the amplitude and phase spectra of seismic waves. The anelastic properties of real materials, particularly of porous rocks, are described using the theory of linear viscoelasticity based on Boltzmann’s superposition principle. Wave‐propagation simulation with this model requires implementing the convolutional relation in the equation of motion. The choice of a viscoacoustic constitutive relation based on a spectrum of relaxation mechanisms allows a realistic description of the anelastic effects, and the introduction of memory variables obviates storing the entire strain history required by the time convolution. A pseudospectral time‐integration technique is used to solve the equation of motion. Applications of viscoacoustic modeling suggest the need for considering the correct attenuation‐dispersion effects for various fundamental seismic problems in anelastic earth models. Comparison of acoustic and viscoacoustic synthetic seismograms shows differences in the amplitudes and arrival times of the wave fields which are enhanced for particular combinations of anelastic and geometrical effects.
Full‐wave synthetic acoustic logs in radially semiinfinite saturated porous media
Authors: Schmitt Denis P. , Bouchon Michel , Bonnet Guy
Published: October 2002

The wave field generated by a point source in an axisymmetric fluid‐filled borehole embedded in a saturated porous formation is studied in both the spectral domain and time domain. The formation is modeled following Biot theory modified in accordance with homogenization theory. When the borehole wall is permeable, guided waves can be significantly affected by the permeability of the formation. Whatever the formation, fast or slow, Stoneley‐wave phase velocity and energy decrease and attenuation (in the sense of [Formula: see text]) increases with increasing permeability. These effects are more important in the very low‐frequency range, where Darcy’s law governs the fluid motion and the wave energy at the interface is maximum, than at higher frequencies. The effects increase and persist over a larger frequency range with decreasing viscosity and increasing compressibility of the saturant fluid, with increasing pore‐fluid volume, and with decreasing borehole radius. In contrast, the effects decrease with decreasing stiffness of the formation because of more efficient coupling of the interface wave to the surrounding medium. When present, the first pseudo‐Rayleigh mode also carries useful information. Fluid flow affects only the attenuation of the pseudo‐Rayleigh mode’s Airy phase; an increase in attenuation may be used to detect permeable zones and to infer the saturant fluid properties. However, the most reliable types of information are the formation shear‐wave velocity and attenuation from the low‐frequency part of the mode. In the time domain, all the modes overlap. Any signal processing should then be performed in the frequency domain, where mode spectra are more easily separable. The frequency band of the actual logging tool has to be large enough to ensure significant amplitude for each mode. Finally, the larger the number of receivers and the offset range, the better.
Gravity inversion of an interface above which the density contrast varies exponentially with depth
Authors: Chai Yufu , Hinze William J.
Published: October 2002

Mapping of an interface above which the density contrast varies exponentially with depth, as is common at the basement surface of sedimentary basins, is efficiently achieved by a theoretically precise gravity method which can be applied to either profile data or twodimensional data. The contrast in mass above the interface is modeled by an array of vertical rectangular prisms with density contrasts varying exponentially with depth. Gravity anomalies due to the prisms are calculated in the wavenumber domain and then converted to the space domain. The precision of the inverse numerical Fourier transform in this procedure is significantly increased by a shift‐sampling technique based on the discrete Fourier deviation equation. Depth to the interface is determined by iterative adjustment of the vertical extent of the prisms in accordance with observed gravity anomaly data. The basement surface of the Los Angeles basin, California, calculated by this method, closely duplicates the published configuration based on drillhole data and seismic data.
On: “Phase‐field imaging: The electromagnetic equivalent of seismic migration” by S. Lee, G. A. McMechan, and C. L. V. Aiken (GEOPHYSICS, 52, 678–693, May 1987)
Authors: Lee Seunghee , McMechan George , Aiken Carlos
Published: October 2002

We were very happy to see the paper by Lee et al. which contains many interesting applications of electromagnetic migration to the solution of geoelectric problems. However, we were very suprised the authors were unaware of our previous papers published in both Eastern and Western international journals concerning the same subject (cf., bibliography). We proposed the generalization of seismic migration for electromagnetic data for the first time in 1982 during the Sixth Workship on EM-induction in the Earth and Moon (Zhdanov and Frenkel, 1982). Dr. John Booker from The University of Washington was the first to suggest calling our method “electromagnetic migration”; a detailed description of our method was given in Zhdanov and Frenkel (1983a and b). Work on electromagnetic migration was published by Zhdanov and Frenkel (1983c) in a special issue of the Proceedings of Oulu University. In 1985 we presented an invited paper (Velikhov et al., 1987) on this topic at the Prague General Assembly of IAGA.
Seismic stratigraphy study of algal mound reservoirs, Patterson and Nancy fields, Paradox basin, San Juan County, Utah
Authors: Davis Thomas L. , Jackson Geoffrey M.
Published: October 2002

Seismic data from the Paradox basin, southeast Utah, exhibit amplitude anomalies associated with the presence of Pennsylvanian algal mound reservoirs. An experimental line over the Patterson and Nancy fields shows amplitude reductions corresponding to reservoir mound facies. Amplitude reduction is due to the lateral facies change from intermound evaporite‐dominated facies to the carbonate algal mound facies. Porosity in the mound facies accentuates the amplitude reduction but the influence of porosity is minor compared to the effect of the lateral lithologic change. Recurrent movement on basement faults during the Pennsylvanian may have controlled the location of these mounds.
Random rays and seismic amplitude anomalies
Authors: White Benjamin , Nair Balan , Bayliss Alvin
Published: October 2002

We give an explanation of the phenomenon, sometimes observed in exploration seismology, of anomalously large amplitudes which seem inconsistent with the traveltime curves when the data are interpreted as resulting from reflections from smooth interfaces of piece‐wise homogeneous media. Monte Carlo simulations illustrate how this phenomenon can occur when the homogeneous media have small, smooth, random velocity fluctuations which vary on a length scale which is large compared with a wavelength but small compared with the propagation distance. Synthetic gathers of reflections from a single plane‐stratified layer with and without the random lateral inhomogeneities produce an amplitude anomaly which is related to the random occurrence of a caustic; limit theorems for stochastic differential equations provide a theory. Theoretical curves, giving the probability of first occurrence of this phenomenon along a ray as a function of propagation distance (for plane waves and for point and line sources in two and three dimensions) are qualitatively similar: they have an initial flat portion where amplitude anomalies are very unlikely, rise to a peak at the distance most likely for first occurrence, and decay exponentially to zero, thus predicting that the phenomenon will occur at some finite distance with probability one.
The sign filter for seismic event detection
Authors: Hansen Katherine M. , Roy‐Chowdhury Kabir , Phinney Robert A.
Published: October 2002

The theory of statistical hypothesis testing is used to develop and apply a seismic signal detection filter. The filter, herein named the sign filter, scans a stacked section and designates a linear segment as “signal” or “noise” based on the value of the sign test statistic evaluated over the amplitudes within the segment; only the signals are passed. The sign test statistic is nonparametric, so that probabilistic calculations related to the filtering process do not require rigid assumptions regarding the noise distribution. Consequently, it is possible to calculate both the probability that the filter will pass a segment containing only noise, and the expected number of noise‐only segments to be passed. These numbers may be adjusted by changing the tunable parameters of the filter. The detector was tested on both synthetic and field data. For synthetic data, all of the signals present in the data were identified, and the output did not contain any spurious signals, even for a signal‐to‐noise ratio smaller than 1. For field data, the events chosen by the filter, for the most part, agree closely with those visible in the input section; and much of the spatially incoherent energy is suppressed. A few of the passed segments were not visually coherent in the input stack; we suggest a method by which such segments might be identified and removed. The method is fairly general and may be modified for different definitions of signal. The case of linear alignments is the easiest to implement, and the detector promises to be useful in both the processing (automatic picking of first arrivals in source gathers) and interpretation (identification of primary reflections in stacked sections) phases of seismic data analysis.