bluuebunny/crossref_metadata_2025 huggingface dataset:
Geophysical exploration for gold—A review
Authors: Doyle Hugh A.
Published: October 2002

It is almost impossible to get a direct geophysical response from gold because of the low grades in deposits, except when electromagnetic detectors are used for individual shallow nuggets. However, indirect geophysical indications may occur through association of gold with particular host rocks, marker beds, or structures which are, for example, of unusual magnetization, density, electric polarization, or conductivity/resistivity. Useful markers may be magnetic dolerites, banded iron formations, shales with magnetite, conductive and/or polarizable pyrites, or other sulfide (detectable by IP methods) and silicified zones more resistive than surrounding rocks. Mapping of faults and shear zones, with which gold may be associated, is also valuable (e.g., by magnetic, EM surveys, etc.). Magnetite depletion is characteristic of some deposits, resulting in zones of low anomalies; for example, ferromagnetic minerals in mafic volcanics are destroyed by carbonatization. Resistivity patterns may indicate altered rocks which contain mineralization, thus the use of VLF-EM methods in Canada and MMR in Australia. Geophysical methods have been important in the search for gold in the Witwatersrand Basin of South Africa (magnetic, gravity, and even seismic reflection methods), various areas of Canada, e.g., the Abitibi greenstone belt (magnetic, EM, and IP surveys), and to a smaller extent in Australia (e.g., Tennant Creek, NT and Water Tank Hill, WA). However, the value of geophysical methods should not be exaggerated since physical contrasts are often low.
A transparent boundary for finite‐difference wave simulation
Authors: Long L. T. , Liow J. S.
Published: October 2002

Transparent (or absorbing) boundaries can be used in finite‐difference wave simulation to reduce the size of the computational grid and to eliminate reflections from the edges. An efficient and accurate transparent boundary can be formulated by decomposing the elastic waves into dilatational and rotational strains. The wave motions for the strains at the boundary can then be approximated by a one‐way wave equation. The direction of propagation is determined at each grid point by the gradient. This transparent boundary condition eliminates artificial reflections for a wave arriving at any angle of incidence and reduces the error to the level of precision of the finite‐difference approximation. Application of this transparent boundary condition is restricted to a medium that is homogeneous at the boundary to assure full separation of P waves from S waves. Also, interfering waves that generate phase velocities significantly greater than the assumed group velocity introduce errors. An example of the transparent boundary condition shows that it is a significant improvement over the A1 boundary condition of Clayton and Engquist.
North Sea reservoir description: Benefits of an elastic migration/inversion applied to multicomponent vertical seismic profile data
Authors: Beydoun W. B. , Mendes M. , Blanco J. , Tarantola A.
Published: October 2002

A linearized elastic migration/inversion (M/I) technique is applied to multicomponent offset VSP data collected in the North Sea. High‐resolution elastic images of the target zone (P- and S-wave velocities and density) are obtained from upgoing P-P and P-S wave fields. These M/I images are an approximate representation of rapid variations of subsurface parameters. Three image confidence criteria provide a measure of uncertainty in the interpretation. VSP-CDP transform maps assisted in the definition of the confidence region in M/I images. M/I was helpful in delineating the Brent reservoir, and detecting a twofault system. Preliminary interpretation indicates an increase in Poisson’s ratio at the top of the reservoir. Surface data were not able to image this target due to strong multiples at the Cretaceous base. This is the first time elastic depth images have been obtained from M/I of offset VSP field data. Generally, image artifacts depend on the following conditions: (1) good quality data with satisfactory coverage of the target zone; (2) accurate, amplitude‐preserving data preprocessing techniques for filtering out “unwanted” waves; and (3) an elastic background model which adequately represents the main subsurface features. Our M/I approach is flexible, since it can be used with single or multicomponent prestack data P-P, P-S, S-P, and S-S scattered waves simultaneously, an arbitrary acquisition geometry, and a two‐dimensional heterogeneous background model. Computational efficiency results from concentrating the inversion on a target zone and using the paraxial ray method to compute ray‐approximate Green’s functions.
On: “A least‐squares approach to depth determination from gravity data” by O.P. Gupta (GEOPHYSICS, 48, 357–360, March 1983)
Authors: Abdelrahman El‐Sayed M.
Published: October 2002

In the article by Gupta, the problem of depth determination of a buried structure from the residual gravity anomaly has been transformed into a problem of finding the solution of a nonlinear equation of the form f(z) = 0. Gupta begins his formulation of the problem with equation (1) from Mettleton (1942) Eq. (1) [Formula: see text]
Gaussian scaling noise model of seismic reflection sequences: Evidence from well logs
Authors: Todoeschuck J. P. , Jensen O. G. , Labonte S.
Published: October 2002

A simple model for reflection series is desirable for two reasons: first, to have a means of generating model reflection sequences on which to test geophysical data processing techniques, specifically methods of predictive deconvolution, and second, to suggest a way of parameterizing real reflection sequences for classification and further geologic study. In this note we wish to discuss a model suggested by Hosken (1980); our discussion is founded upon the powerful ideas of self‐similarity and scaling developed by Mandelbrot (1983) and on the paucity of typical lengths in geology. Typical lengths are common in physics problems; for example, the length of an organ pipe governs the wavelengths of the notes played. When taking photographs, geologists include some manmade object to give scale to the photo. Otherwise it is very hard to tell if, say, folds are of centimeter size or form whole mountains.
An evaluation of Bristow’s method for the detection of subsurface cavities
Authors: Lowry Tony , Shive Peter N.
Published: October 2002

The Bristow method, an electrical resistivity technique employing a pole‐dipole measurement array in conjunction with a simple graphical method of interpretation, has proven an effective means of locating subsurface cavities. There have been questions, however, regarding the limits of the method and whether the Bristow method is indeed the most suitable of the various electrical resistivity techniques for cavity detection. In hopes of resolving some of the controversy surrounding Bristow’s method, resistivity traverses are numerically modeled over spherical and cylindrical cavities given a variety of circumstances. Using a slight variation of Bristow’s original interpretive technique on modeled data, the size and location of subsurface cavities can be determined with surprising accuracy. However, when the simulation is altered to incorporate geologic noise, the maximum depth at which a cavity can be detected is found to be far less than has been reported in field investigations. In this instance the presence of a cylindrical cavity cannot be discerned beyond a depth to the top approximately equal to the diameter of the cavity, and spherical cavities are indistinguishable at depths much greater than the radius. One should note that the noise field generated for this model may not be representative of what would normally be found in the real earth. In the field, the maximum achievable depth of detection will vary depending on the actual geologic conditions and whether some technique is employed to reduce the effects of noise. In any case, a comparison of traverses using various electrode arrays confirms that the Bristow method is the most satisfactory of the applicable electrical resistivity techniques.
Incremental pay thickness modeling of hydrocarbon reservoirs
Authors: Neff Dennis B.
Published: October 2002

The one-dimensional convolution model or synthetic seismogram provides more information about the seismic waveform expression of hydrocarbon reservoirs when petrophysical data (porosity, shale volume, water saturation, etc.) are systematically integrated into the seismogram generation process. Use of this modeling technique, herein called Incremental Pay Thickness (IPT) modeling, has provided valuable insights concerning the seismic response of several offshore Gulf of Mexico amplitude anomalies. Through integration of the petrophysical data, comparisons between seismic waveform response and expected reservoir pay thickness are extended to include estimates of gross pay thickness, net pay thickness, net porosity feet of pay, and hydrocarbons in place.

These 1-D synthetic data easily convert to 2-D displays that often show exceptional waveform correlations between the synthetic and actual seismic data. Anomalous observed waveform responses include complex tuning curves; diagnostic isochron measurements even in unresolved thin-bed reservoirs; and extreme variations in the seismic expression of hydro-carbon-fluid contacts. While IPT modeling examples illustrate both the variability and nonuniqueness of seismic responses to hydrocarbon reservoirs, they often show good seismic predictability of pay thickness if the appropriate choice of amplitude-isochron versus pay thickness is made (i.e., peak amplitude, trough amplitude, or average amplitude versus gross pay thickness, net pay thickness, net porosity feet of pay, or hydrocarbons in place).
Two‐dimensional automatic resistivity inversion technique using alpha centers
Authors: Shima Hiromasa
Published: October 2002

I propose a method that uses alpha centers for two‐dimensional automatic analysis of dc electrical exploration data. Past papers on the alpha centers method have been concerned with relatively simple applications, such as identification of the location of anomalies; but by making a number of improvements on the method, complex two‐dimensional structures can be analyzed. The appropriateness of using negative C parameters in the alpha centers method is demonstrated by considering the resistivity and electric potential distribution expressed by the alpha centers. In this way, the structural expression of alpha centers is improved. Using a network of fixed alpha centers both stabilizes the analysis and makes it possible to employ a large number of alpha centers (more than 100). The combination of these improvements, compensation for topography, and application of a nonlinear least‐squares method, which has good convergence throughout the iteration process, makes automatic analysis possible. Three numerical experiments demonstrate the effectiveness of this method. The first two simulate standard exploration from the ground surface. One is a simple prism model; the other, an inclined fault model. The final experiment simulates measurements between boreholes or adits. These experiments demonstrate that good results can be obtained by the alpha centers method, which has heretofore not been suitable for highly resistive structures or underground structures having sharp boundaries. Furthermore, the experiments show that this method has high resolution. Finally, the fact that one of the experiments shows the alpha centers method to be effective for cross‐borehole data analysis suggests its wide applicability. The applicability of the method is also demonstrated by two field experiments. The first identifies the form and distribution of lava in the ground, and the second investigates the distribution of intrusive rock and alteration zones in a steep, mountainous area.
Wave velocities in hydrocarbon‐saturated rocks: Experimental results
Authors: Wang Z. , Nur A.
Published: October 2002

This paper contains laboratory measurements of the temperature dependence of velocities, determined by ultrasonic pulse transmission methods, in hydrocarbon liquids and rock samples saturated by the liquids. The samples discussed include 26 hydrocarbons of varying molecular weight, nine mixtures of these hydrocarbons, four heavy oils (tar), three saturated sandstones, and a saturated sand. The data provide encouragement that high‐frequency, high‐resolution seismic techniques may infer formation temperature changes, thereby detecting the progress of thermal enhanced oil recovery processes.
Crosshole seismics using vertical eigenstates
Authors: Pai David M.
Published: October 2002

This paper introduces the concept of vertical eigenstates to crosshole data analysis. In surface seismics, plane waves have played a fundamental role in many applications. This paper points out that for crosshole seismics, vertical eigenstates play a similar role. The vertical eigenstates separate the wave equation, and thus in terms of vertical eigenstate expansion the solution is a linear combination of modes, each mode traveling sideways with a distinct phase velocity. As a result, the vertical eigenstates form a natural set of basis functions for solution and data expansion, with potential applications to modeling, migration, and inversion.
An absolute wireline calibration to support a test of Newton’s inverse square law
Authors: Ander M. E. , Kerr W. , Aiken C. L. V. , Glover C. C. , Zumberge M. A.
Published: October 2002

As part of a Greenland ice cap experiment to measure possible scale length violations of Newton’s inverse square law over geophysical scales of 200 to 1500 m, it was necessary to locate the depth of a gravity meter attached to a wireline down a borehole to about 1 part in 10 000. In order to do this, the wireline and cable length measuring system had to be calibrated both before and after the Greenland expedition. The measuring system used a combination of a mechanical wheel measuring device and a magnetic mark counter. The calibration was conducted in a 1200 m vertical mine shaft at the Consolidated Silver Mine in Osborn, Idaho. Distances in the mine shaft were first calibrated to a precision of about 0.005 m using a geodetic laser system model 4L Geodimeter operating at 30 MHz. To calibrate the wireline, it was run up the mine shaft five times before and five times after the Greenland experiment. The calibration before the experiment was good to about 4 parts in 10 000 and the calibration after was accurate to about 1 part in 10 000. A total inelastic stretch of only 0.102 m occurred during the Greenland operation.
On: “Ground‐roll suppression by the stackarray” by P. F. Morse and G. F. Hildebrandt (GEOPHYSICS, 54, 290–301, March 1989).
Authors: Vermeer G. J. O.
Published: October 2002

The stackarray approach proposed in Anstey (1986a) is a large improvement over seismic acquisition techniques with shot intervals that are 3 or 4 times as large as group intervals. The stackarray criterion (Anstey, 1986b) prescribes “an even, continuous, uniform succession of geophones across the CMP gather,” and Morse and Hildebrandt (1989) illustrate the effectiveness of this regular spatial sampling, using some interesting experiments. However, the stackarray criterion does not explain the residual ground roll that remains even after a two‐trace mix or simulation of overlapping geophone arrays as shown in Figures 14b and 13b, respectively, of Morse and Hildebrandt. Morse and Hildebrandt state that these events are not suppressed due to “the unusually low fold of the stack.” In the following I will argue that Morse and Hildebrandt’s argument does not satisfactorily explain the presence of residual ground roll and that the residual ground roll most likely results from not using a shot array in data acquisition. I put forward a hypothesis as to the nature of residual ground roll, and this hypothesis can be easily tested using the basic data recorded by Morse and Hildebrandt.
Seismic amplitude anomalies and AVO analyses at Mestena Grande Field, Jim Hogg Co., Texas
Authors: Burnett R. C.
Published: October 2002

Mestena Grande field is located in northeast Jim Hogg Co., Texas. It produces gas and condensate, primarily from the middle member of the Middle Eocene Queen City formation. The producing zone is a deep, thin, high impedance sandstone which generates amplitude anomalies on the stacked data. AVO (amplitude versus offset) analyses were performed to investigate those anomalies and determine if they could aid in field development or exploration along the trend. Modeling the AVO response of a productive well has predicted an amplitude decrease with offset from a high impedance sandstone. However, amplitudes increase with offset on the crest of the field. At Mestena Grande field, three categories of seismic amplitudes correspond with production with only one exception. The first category exhibits strong amplitudes on the stacked data and amplitudes increase with offset. This amplitude category is seen around the best wells in the field. Second are the moderate amplitudes which do not increase with offset that surround the wells producing at moderate rates. The third category is characterized by very weak amplitudes which decrease with offset, occurring near all but one of the dry holes. The disagreement between the results of the modelling and the real data is attributed to the lack of accurate shear wave velocities and the presence of very thin beds.
The two‐dimensional elastodynamic distributed surface load problem
Authors: Stam H. J.
Published: October 2002

The two‐dimensional elastodynamic P- and SV-wave motion generated by an arbitrary distributed impulsive surface load on the boundary of a semiinfinite perfectly elastic solid is investigated theoretically. With the aid of the Cagniard‐de Hoop method, closed‐form expressions are obtained for the particle velocity and the stress of the generated wave motion. The cases of a uniformly distributed strip load of the normal‐load and the shear‐load types are worked out in detail. Synthetic seismograms are presented for an array of receivers close to the strip source and for an array of receivers at depth. The results are important as test cases for accuracy in the computational modeling of elastodynamic wave problems, by time‐domain finite‐difference or finite‐element methods.
New approach to seismic‐reflection event detection and velocity determination
Authors: Key Scott C. , Smithson Scott B.
Published: October 2002

Recently a number of high‐resolution techniques for event detection and parameter estimation (velocity) have emerged in the field of multichannel sonar array processing. We find that a practical and computationally efficient implementation of these methods is possible for seismic reflection data. This implementation results in development of the covariance measure for event detection and velocity estimation in seismic reflection data. The measure is based on the eigenstructure of the sampled data covariance matrix. This decomposition is carried out within hyperbolic windows that are moved through common‐depth‐point (CDP) data. Eigenvalues of the data covariance matrix allow simultaneous estimation of the noise and signal energy present within each window, resulting in the development of a coherency measure. Computer implementation of the covariance measure provides a high resolution method for determination of velocity spectra relative to currently used techniques. The method applied to synthetic data resolves reflections closely spaced in time and velocity as well as those with low signal‐to‐noise ratio (S/N). These results are also achieved when the method is applied to real data. The results in each case are directly compared to the widely used semblance measure. The covariance measure provides results superior to the crosscorrelation‐based semblance measure and offers a degree of resolution not previously attainable. These results can be achieved with a reduction in computational cost relative to semblance for typical analysis parameters. We find that the ability to continuously estimate noise energy, as well as signal energy, is critical to event resolution and noise rejection.
Geothermal gradients and terrestrial heat flow along a south‐north profile in the Sverdrup Basin, Canadian Arctic Archipelago
Authors: Jones F. W. , Majorowicz J. A. , Embry A. F. , Jessop A. M.
Published: October 2002

Data from eleven petroleum exploration wells along a south‐north profile in the Sverdrup Basin of the Canadian Arctic Islands indicate large variations in temperature gradients(18 ± 2 to 39 ± 2 mK/m) and heat‐flow values [Formula: see text]. High values occur near the axis of the basin and values decrease systematically toward the southern and northern flanks of the basin. The basin axis in this area is the zone of maximum crustal attenuation and Lower Cretaceous dike and sill intrusion, but any thermal anomaly associated with these events will have dissipated by now. The present heat‐flow pattern is likely the result of thermal refraction or fluid flow in the basin.
Generation and interpretation of geophysical images with examples from the Rae Province, northwestern Canada shield
Authors: Broome H. John
Published: October 2002

Different types of images generated from gravity, magnetic, and gamma ray spectrometry data from the Rae Province of the Canadian shield were compared with each other and geologic maps to evaluate their effectiveness for displaying the geologically relevant content of the data sets. Shading methods were useful for enhancing weak directional anomalies in the aeromagnetic data. Multi‐directional, shaded‐relief images produced by overlaying three colored, shaded‐relief images are useful for analysis of anomalies associated with structure. Vertical gravity derivative images display a continuous gravity feature linking the Wager Bay and Amer Lake shear zones that is obscured on the Bouguer gravity intensity image. Detailed vertical magnetic derivative images of the shear zone clearly displayed anomalies associated with the internal structure. Composite images generated using three different geophysical parameters show correlations between the magnetic, gravity, and radiometric data which can be related to the geology. Subtle variations in uranium, thorium, and potassium concentrations determined by gamma ray spectrometry can be effectively displayed using ternary radioelement images
Vertical resolution of thick beds, thin beds, and thin‐bed cyclothems
Authors: Knapp Ralph W.
Published: October 2002

Resolution has two common definitions. The first of these relates to determination of the position of a reflector in time or space. Resolution by this definition is inversely related to pulse width or sharpness of the wavelet and, potentially, directly related to bandwidth of the frequency spectrum. The second definition of resolution relates to determination of the spacing of close features such as thin beds. This type of resolution is determined by the Rayleigh criterion which inversely relates resolution to wavelength. Resolution of thin beds improves with shorter wavelength, or higher frequency. In comparison to the Rayleigh criterion, the Ricker and Widess criteria improve the resolution potential of thin beds, but they rely on models of the reflectors and require that the beds be isolated. It is a long established and widely accepted axiom that the best wavelets contain bandwidths of at least a couple of octaves. This results in wavelets with reduced side lobes and ringiness, a more easily interpreted waveform. The concept of octaves does not figure into the equation of resolution according to either of the above definitions, but that is not to disagree with the axiom. For wavelets with bandwidths of at least a few octaves, the two definitions of resolution find a consensus in that resolution quality becomes dependent on the value of the highest frequency. That is, resolution is proportional to the highest frequency of the data when it contains several octaves of information.
The effect of transverse isotropy on isotropic traveltime inversion of vertical seismic profile data—A modeling study
Authors: Douma Jan
Published: October 2002

Traveltime inversion of multioffset VSP data can be used to determine the depths of the interfaces in layered media. Many inversion schemes, however, assume isotropy and consequently may introduce erroneous structures for anisotropic media. Synthetic traveltime data are computed for layered anisotropic media and inverted assuming isotropic layers. Only the interfaces between these layers are inverted. For a medium consisting of a horizontal isotropic low‐velocity layer on top of a transversely isotropic layer with a horizontal axis of symmetry (e.g., anisotropy due to aligned vertical cracks), 2-D isotropic inversion results in an anticline. For a given axis of symmetry the form of this anticline depends on the azimuth of the source‐borehole direction. The inversion result is a syncline (in 3-D a “bowl” structure), regardless of the azimuth of the source‐borehole direction for a vertical axis of symmetry of the transversely isotropic layer (e.g., anisotropy due to horizontal bedding).
Three‐dimensional migration of swath surveys
Authors: Krebs Jerome R.
Published: October 2002

A three‐dimensional (3-D) wave‐equation migration program is used to migrate swath data (swath data are here defined as a very narrow 3-D survey consisting of approximately ten seismic lines and having a width of about 500 m). Three‐dimensionally migrated swath data give an accurate 3-D image of the subsurface and have a higher signal‐to‐noise ratio than 2-D data. These advantages are gained at the expense of lateral resolution in the crossline direction and less extensive 3-D subsurface coverage. Since swaths are on the order of one‐tenth the size of a normal 3-D survey, the costs of gathering and processing swath data are about 10 percent those of a conventional 3-D survey. Thus 3-D migrated swath surveys are a practical means of producing 3-D images in areas where the expense of conventional 3-D surveys is not justified. Following 3-D wave‐equation migration, inlines from the swath data are geologically interpretable. However, crosslines from the swath data are dominated by migration artifacts (often called migration smiles), making them uninterpretable. Relatively high‐amplitude portions of these migration smiles indicate where the data were accurately migrated. These facts lead to a two‐step method of interpreting 3-D migrated swath data. First the inlines are interpreted geologically, then the accuracy of that interpretation is verified by tying it to the crossline data.
On: “Analysis of the Rayleigh pulse” by P. Hubral and M. Tygel (GEOPHYSICS, 54, 654–658, May 1989)
Authors: Aldridge David F.
Published: October 2002

Recently, Professors Hubral and Tygel presented an interesting analysis of the Rayleigh wavelet, emphasizing its twin attributes of simplicity and flexibility. My comments here are concerned mainly with the final section of their paper, where the length properties of the Rayleigh pulse are described. In particular, I demonstrate that the class of Rayleigh wavelets is characterized by a simple length formula, provided that a proper definition of waveform length is adopted. Although there are various useful definitions for signal length, width, breadth, or duration, the specific form that I employ here is commonly used by exploration seismologists. It has been examined in detail by Berkhout (1984, p. 16–28) and others.
Three‐dimensional true‐amplitude zero‐offset migration
Authors: Hubral Peter , Tygel Martin , Zien Holger
Published: October 2002

The primary zero‐offset reflection of a point source from a smooth reflector within a laterally inhomogeneous velocity earth model is (within the framework of ray theory) defined by parameters pertaining to the normal‐incidence ray. The geometrical‐spreading factor—usually computed along the ray by dynamic‐ray tracing in a forward‐modeling approach—can, in this case, be recovered from traveltime measurements at the surface. As a consequence, zero‐offset reflections can be time migrated such that the geometrical‐spreading factor for the normal‐incidence ray is removed. This leads to a so‐called “true‐amplitude time migration.” In this work, true‐amplitude time‐migrated reflections are obtained by nothing more than a simple diffraction stack essentially followed by a time derivative of the diffraction‐stack traces. For small transmission losses of primary zero‐offset reflections through intermediate‐layer boundaries, the true‐amplitude time‐migrated reflection provides a direct measure of the reflection coefficient at the reflecting lower end of the normal‐incidence ray. The time‐migrated field can be easily transformed into a depth‐migrated field with the help of image rays.
A review of some experience with the induced‐polarization/resistivity method for hydrocarbon surveys: Successes and limitations
Authors: Sternberg Ben K.
Published: October 2002

Our experience with the induced polarization (IP) and resistivity method for hydrocarbon exploration has shown both successful surveys and limitations of the method. Four examples demonstrate a close correlation between shallow IP and resistivity anomalies and deeper hydrocarbon production. In each of these examples, anomalies occurred over the producing fields which have significantly greater amplitudes than the variations in the surrounding background response. Another important result of our research is the development of a geological/geochemical model for the formation of IP and resistivity anomalies over hydrocarbon reservoirs. The two main requirements for formation of IP and resistivity anomalies, according to this model, are: (1) absence of any thick impermeable seals, such as evaporites, above the reservoir and (2) presence of porous, iron‐rich, near‐surface host rocks, such as clastic rock sequences. The IP and resistivity method can be more successfully applied by selecting those areas for surveys in which these two requirements hold. We have also found that the IP/resistivity method for hydrocarbon exploration has significant limitations. Many areas do not appear to have the required geological and geochemical conditions for the formation of IP or resistivity anomalies. IP and resistivity anomalies may also need to be tested with shallow drill holes to separate anomalies caused by hydrocarbon seepage from false anomalies due to other causes.
Dip and depth extent of density boundaries using horizontal derivatives of upward‐continued gravity data
Authors: McGrath Peter H.
Published: October 2002

A new computer method was developed to derive values of dip and vertical extent, as well as location, for the truncated face of a horizontal‐plate model. The method is based on lateral offsets of the zero‐crossover point of the second horizontal derivative of an upward‐continued gravity profile. Application requires that the distance to the upper surface of the model be known. Numerical tests indicate useful results are obtained when both the length of the gravity profile, and the finite‐strike length of a model are at least three times the maximum depth extent of the model. Gaussian noise with a standard deviation of 1 mGal can be tolerated given an anomaly amplitude of 70 mGal. An example from the Trans‐Hudson Orogen of central Manitoba, Canada, indicates that the Kisseynew metasedimentary gneisses extend over a vertical distance of 16–18 km, and are separated from the Baldock batholith to the north by a near‐vertical boundary.
A method for obtaining depth estimates from the geometry of Slingram profiles
Authors: Duckworth K. , Calvert H. T. , Juigalli J.
Published: October 2002

An approximate method for deriving conductor depth estimates from the geometry of horizontal coplanar coil Slingram type electromagnetic profiles is based on a transformation of the recorded profile to a form which is free of the influence of coil separation. Depth estimates are derived from an empirically determined linear relationship between the spatial separation of maxima on the transformed profile and the depth of the conductor. Field examples and electromagnetic scale modelling indicate that the method works well on both thin and thick conductors with dips ranging from 90 to 60 degrees. Field examples and model results also show that the method works well when a conductive overburden is present, even if the overburden causes phase inversion. Examples of the application of the method show that while knowledge of the conductance of a target will aid in depth estimation, good depth estimates can be made even if the conductance of the target cannot be determined. Comparative scale model studies of conductors located first in free‐space and then in a conductive host suggest that the method will also give good results for targets located in conductive host rocks.
Petrophysical characteristics of shales from the Scotian shelf
Authors: Katsube T. J. , Mudford B. S. , Best M. E.
Published: October 2002

Permeability, porosity, formation factor, mercury porosimetry, and stress‐strain measurements were made on 10 shale samples taken at depths between 4500 m and 5600 m in three wells on the Scotian shelf. The purpose was to obtain shale permeability values for quantitative sedimentary basin modeling and to investigate the reasons for the very low permeabilities, less than [Formula: see text] (10 nD), exhibited by many tight shales. Permeabilities of [Formula: see text] ([Formula: see text]) and porosities of 0.9–9.2 percent were measured. The results suggest that the extremely low permeabilities occur because the flow path consists of a network of very tortuous pores (true tortuosity = 3.3) with small diameters, of the order of 8–16 nm. Presence of calcite and dolomite apparently is associated with reduced porosity, possibly a result of blocking of the pores, while kaolinite shows the reverse trend.
Simulated annealing statics computation using an order‐based energy function
Authors: Vasudevan Kris , Wilson William G. , Laidlaw William G.
Published: October 2002

The residual statics problem in seismic data analysis is treated by introducing an optimization function that emphasizes the coherence of neighboring common depth point (CDP) gathers within a nonlinear simulated annealing technique. This optimization criterion contrasts with stack power optimization which only considers the coherence between traces within a single CDP. Emphasizing coherence between CDPs removes many of the phase space degeneracies that result from stack‐power based optimization techniques. We have applied the method to both synthetic and real data sets, and initial results display significant improvement over the input data in the coherence of reflections, even in structurally complex areas.
Assessing a fractured landfill cover using electrical resistivity and seismic refraction techniques
Authors: Carpenter P. J. , Calkin S. F. , Kaufmann R. S.
Published: October 2002

Fracturing or erosion of landfill cover materials may lead to escape of landfill gases and creation of leachate through infiltration of surface water. Periodic geophysical surveys could be used to assess landfill covers noninvasively and identify fractured or thinned areas. Models derived from electrical resistivity and seismic refraction measurements were evaluated over various portions of a closed municipal landfill in west surburban Chicago. Resistivity soundings could not delineate cover thickness over areas of fractured or new cover where resistivity contrasts with the underlying refuse were minor. Cover thickness estimates accurate to within 0.7 m, however, were obtained over unfractured mature cover where resistivities were 10–20 ohm‐m higher than in the refuse. Azimuthal resistivity variations as large as 16 ohm‐m indicate opening of cover fractures during dry weather; these same areas showed little or no azimuthal resistivity variation under moist conditions. Strong absorption of seismic energy and a velocity inversion in the refuse prevented calculation of cover thickness from seismic refraction profiles using a sledgehammer source. However, higher direct P-wave velocities (740 m/s) characterized areas of intact cover and lower velocities (370 m/s) characterized highly fractured cover. Qp for cover materials ranged from 3 to 9 and did not differ between fractured and unfractured areas.
Sounding sea ice thickness using a portable electromagnetic induction instrument
Authors: Kovacs Austin , Morey Rexford M.
Published: October 2002

Field trials using a man‐portable, commercially available, electromagnetic induction (EMI) sounding instrument, with a plug‐in data processing module for the remote measurement of sea ice thickness, are discussed. The processing module was made to allow for the direct determination of sea ice thickness and to show the result in a numerical display. The processing module system was capable of estimating ice thickness within 10 percent of the the true ice value for ice from about 0.7 to 3.5 m thick, the thickest of undeformed ice in our study area. However, since seawater under the Arctic pack ice has relatively uniform conductivity (2.55 ± 0.05 S/m), a simplified method can be used for estimating sea ice thickness using just an EMI instrument. This technique uses only the EMI conductivity measurement, is easy to put into use, and does not rely on theoretically derived look‐up tables or phasor diagrams, which may not be accurate for the conditions of the area.
Born theory of wave‐equation dip moveout
Authors: Liner Christopher L.
Published: October 2002

Wave‐equation dip moveout (DMO) addresses the DMO amplitude problem of finding an algorithm which faithfully preserves angular reflectivity while processing data to zero offset. Only three fundamentally different theoretical approaches to the DMO amplitude problem have been proposed: (1) mathematical decomposition of a prestack migration operator; (2) intuitively accounting for specific amplitude factors; and (3) cascading operators for prestack migration (or inversion) and zero‐offset forward modeling. Pursuing the cascaded operator method, wave‐equation DMO for shot profiles has been developed. In this approach, a prestack common‐shot inversion operator is combined with a zero‐offset modeling operator. Both integral operators are theoretically based on the Born asymptotic solution to the point‐source, scalar wave equation. This total process, termed Born DMO, simultaneously accomplishes geometric spreading corrections, NMO, and DMO in an amplitude‐preserving manner. The theory is for constant velocity and density, but variable velocity can be approximately incorporated. Common‐shot Born DMO can be analytically verified by using Kirchhoff scattering data for a horizontal plane. In this analytic test, Born DMO yields the correct zero‐offset reflector with amplitude proportional to the angular reflection coefficient. Numerical tests of common‐shot Born DMO on synthetic data suggest that angular reflectivity is successfully preserved. In those situations where amplitude preservation is important, Born DMO is an alternative to conventional NMO + DMO processing.
Absorbing boundary conditions for elastic waves
Authors: Higdon R. L.
Published: October 2002

Absorbing boundary conditions are needed for computing numerical models of wave motions in unbounded spatial domains. The boundary conditions developed here for elastic waves are generalizations of ones developed earlier for acoustic waves. These conditions are based on compositions of simple first‐order differential operators. The formulas can be applied without modification to problems in both two and three dimensions. The boundary conditions are stable for all values of the ratio of P‐wave velocity to S‐wave velocity, and they are effective near a free surface and in a horizontally stratified medium. The boundary conditions are approximated with simple finite‐difference equations that use values of the solution only along grid lines perpendicular to the boundary. This property facilitates implementation, especially near a free surface and at other corners of the computational domain.
Long‐wave anisotropy in stratified media: A numerical test
Authors: Carcione J. M. , Kosloff D. , Behle A.
Published: October 2002

When a seismic signal propagates in a stratified earth, there is anisotropy if the dominant wavelength is long enough compared to the layer thickness. In this situation, the layered medium can be replaced by an equivalent nondispersive transversely isotropic medium. Theoretical and experimental analyses of the required minimum ratio of seismic wavelength to layer spacing based on kinematic considerations yield different results, with a much higher value in the experimental test. The present work investigates the effects of layering by wave simulation and attempts to establish quantitatively the minimum ratio for which the long‐wave approximation starts to be valid. We consider two‐constituent periodically layered media and analyze the long‐wave approximation for different material compositions and different material proportions in 1-D and 2-D media. The evaluation of the minimum ratio compares snapshots and synthetic seismograms visually and through a measure of coherence. Layering induces scattering with wave dispersion or anisotropy depending upon the wavelength‐to‐layer thickness ratio. The modeling confirms the dispersive characteristics of the wave field, the scattering effects in the form of coda waves at short wavelengths, and the smoothed transversely isotropic behavior at long wavelengths. 1-D numerical tests for different media indicate that the minimum ratio is highest for the midrange of compositions, i.e., equal amount of each material, and for stronger reflection coefficients between the constituents. For epoxy‐glass, the value is around R = 8, while for sandstone‐limestone, it is between R = 5 and R = 6. Recent wave‐propagation experiments done in epoxy‐glass also imply a highest minimum ratio for midrange of composition; however, the 1-D numerical tests confirm the long‐wave approximation at shorter wavelengths than experimentally. The 2-D case shows that the more anisotropic the equivalent medium, the higher the minimum ratio, and that the approximation depends upon the propagation angle with longer wavelengths required in the direction of the layering.
Migration velocity analysis with the Kirchhoff integral
Authors: Kim Y. C. , Gonzalez R.
Published: October 2002

To obtain accurate migration velocities, we must estimate the velocity at migrated depth points. Wavefront focusing analysis with downward continuation yields the rms velocity at migrated depth points; however, the large amount of computation required for downward continuation limits use of this approach for routine processing. The purpose of this paper is to present an implementation of the Kirchhoff integral which makes the wavefront focusing analysis practical for time‐migration velocity analysis. Downward continuation focuses the wavefront to the zero offset at the depth controlled by the velocity used for the continuation. The migration velocity is then determined from the depth where the focused wavefront attains the maximum amplitude. The flexibility of the Kirchhoff integral allows us to compute only the zero‐offset trace at each depth point and lets us avoid most of the computation for the downward continuation of unstacked data. Furthermore, since the velocity is obtained from the location where the focused wavefront shows the maximum amplitude, prestack time migration with the velocity from this technique produces the maximum amplitude for the subsurface reflector.
Time‐domain behavior of wide‐angle one‐way wave equations
Authors: Kamel A. H.
Published: October 2002

The constant‐coefficient inhomogeneous wave equation reads [Formula: see text], Eq. (1) where t is the time; x, z are Cartesian coordinates; c is the sound speed; and δ(.) is the Dirac delta source function located at the origin. The solution to the wave equation could be synthesized in terms of plane waves traveling in all directions. In several applications it is desirable to replace equation (1) by a one‐way wave equation, an equation that allows wave processes in a 180‐degree range of angles only. This idea has become a standard tool in geophysics (Berkhout, 1981; Claerbout, 1985). A “wide‐angle” one‐way wave equation is designed to be accurate over nearly the whole 180‐degree range of permitted angles. Such formulas can be systematically constructed by drawing upon the connection with the mathematical field of approximation theory (Halpern and Trefethen, 1988).
Bounding seismic velocities using a tomographic method
Authors: Vasco D. W.
Published: October 2002

Traveltime data are used to determine upper and lower bounds on velocity variations in the earth by an iterative method. In effect, the range of models is found which is consistent with the data, rather than a single model which is “best fitting” in some sense. The algorithm used, a variant of the row‐action algorithms commonly applied to tomographic inversions, requires little in‐core memory and has proven feasible for data sets of the order of hundreds of thousands to a million traveltimes. Any inequality constraint, such as that the velocity is always positive, may be incorporated into the formulation. Data errors can be included both locally (as strict constraints on each datum) and globally (as a constraint on a total error measure). The method may also be used to derive the velocity structure which results in the minimum 𝓁1 norm of the residual misfit. Data from the Grimsel crosshole experiment are used to map confidence bounds of 0.02 ms on 1521 traveltime residuals into upper and lower bounds on seismic velocities. There is great variation in the widths of these bounds as a function of subsurface position from 0.1 to 0.9 km/s. The distributions of the bounds agree with the parameter resolution values found from a singular value decomposition (SVD) and suggest that a low‐velocity mylonitic zone, seen in tomographic inversions of the traveltime data, is adequately imaged. Though the data were corrected for seismic anisotropy, significant alternating positive and negative velocity perturbations in poorly constrained quadrants of the crosshole region suggest that some residual anisotropy effects are still present.
Surface gravity and magnetic responses of mineralization, Mt. Isa, northwest Queensland, Australia
Authors: Leaman D. E.
Published: October 2002

Surface gravity and magnetic surveys around the giant copper‐silver‐lead‐zinc mine at Mt. Isa in northwest Queensland have shown that each type of mineralization presents characteristic responses which are anomalous within the perspective of the host rocks. The responses reflect the entire mineralization‐alteration package and not simply the economic ore. Copper mineralization and associated alteration products, including “silica dolomite,” are not significantly magnetized, but the mineralization across a depth range of 1000 m produces a positive gravity anomaly of more than 2.5 mGal. The related alteration products are associated with large negative anomalies of up to 8 mGal in both hanging wall and footwall host rocks. Silver‐lead‐zinc mineralization is moderately magnetic (about 0.001 cgs). It is also denser and shallower and generates larger gravity anomalies. Negative secondary alteration effects, while significant, are equivalent in magnitude to the mineralization effect. Sites mineralized comparably to Mt. Isa would be located by semiregional surveys with a station spacing of 200 to 400 m and survey precision of better than 0.4 mGal, although there are limits to resolution of individual parts of the mineralization.