bluuebunny/crossref_metadata_2025 huggingface dataset:
An eigenstate formulation of the magnetotelluric impedance tensor
Authors: Eggers Dwight E.
Published: October 2002

An important step in the interpretation of magnetotelluric (MT) data is the extraction of scalar parameters from the impedance tensor Z, the transfer function which relates the observed horizontal magnetic and electric fields. The conventional approach defines parameters in terms of elements of a coordinate‐rotated tensor. The rotation angle is chosen such that Z′(θ) approximates in some sense the form for a two‐dimensional (2-D) subsurface conductivity distribution, with zero elements on the diagonal. There are two major problems with this approach. (1) Apparent resistivities, defined from the off‐diagonal elements of the rotated tensor, are independent of the trace of Z. It is problematic that apparent resistivities, the parameters for which we have physical analogs and which are most heavily used in interpretation, are insensitive to the addition of an arbitrary constant on the diagonal of Z. (2) The conventional parameter set is incomplete; there are two degrees of freedom in Z which are transparent to all parameters. Through a variation of the classical eigenstate formulation of a matrix, it is shown that in general there exist two, and only two, polarization states for which the electric and magnetic fields have the same polarization at perpendicular orientations. For each eigenstate the magnetic and electric fields are related by a scalar, the eigenvalue for that state. This scalar relationship between fields is of identical form to the solution for transverse electromagnetic (TEM) waves in a homogeneous medium and thus provides a physically more satisfactory basis for defining apparent resistivity than the conventional approach using the off‐diagonal elements of the coordinate‐rotated impedance tensor. The eigenstate and coordinate‐rotation methods yield identical results in the limited cases of 1-D and 2-D subsurface conductivity distributions. The eigenstates provide the basis for new definitions of parameters as concise, closed expressions which are complete and more amenable to interpretational insight. The polarization ellipses defined by the eigenstates provide a concise display in real space of all the information contained in the impedance tensor.
Wide‐angle diffracted multiple reflections
Authors: Estevez Raul , Claerbout Jon F.
Published: October 2002

Riley and Claerbout (1976) gave a method for the modeling and suppression of diffracted multiple reflections which employs the two‐dimensional (2-D) wave equation. Their theory applies, however, to vertical incidence plane waves. In practice, this requires a good knowledge of the near‐zero‐offset traces which are not usually recorded. Later, in connection with a velocity analysis study, Schultz and Claerbout (1977) developed a theory for angular incidence plane waves. Taking advantage of this concept, this paper extends Riley and Claerbout’s method to the range of operational offsets. One of the features of this modification is that it takes into account the different traveltimes of peg‐leg multiples of equal order in a nonhorizontally layered earth. Synthetic seismograms are presented which illustrate this point. Our experiences with marine data and synthetic data show that routine application will require accurate shot‐waveform estimation. The same imaging algorithms used in the general theory indicate the possibility of estimating the signature directly from the data.
The deconvolution of phase‐shifted wavelets
Authors: Levy S. , Oldenburg D. W.
Published: October 2002

The assumption that a seismogram can be represented as a convolution of a source wavelet with a set of real impulses breaks down when the wavelet is phase shifted upon reflection from a boundary. For plane waves and plane layers, this effect occurs only for wide‐angle supercritical reflections, but it may also occur in normal incidence seismograms when either the impinging wavefront or the reflective boundary is curved. We show that seismograms containing time‐displaced, phase‐shifted replications of the source wavelet can be deconvolved to recover both the amplitude and phase of the reflectivity coefficients. The method begins by writing the analytic seismogram as the convolution of a complex reflectivity function with an analytic source wavelet; linear inverse theory is then used to carry out the deconvolution.
Effect of surface polarization on resistivity modeling
Authors: Guptasarma D.
Published: October 2002

Spectral measurements of apparent resistivity show that serious errors may occur in scale modeling of resistivity with an electronic conductor immersed in an electrolyte unless the frequency of measurement is high enough to prevent the target‐electrolyte interface impedance from affecting the observations. At lower frequencies the observed apparent resistivity is complex, and its magnitude neither represents the correct value nor obeys the scaling laws. Experiments demonstrate that the minimum safe frequency of measurement for acceptable resistivity modeling depends not only on the target and electrolyte material, but also on the shape, size, and relative disposition of the target and the measuring array. In some cases an operating frequency of 200 Hz may be adequately high; in others, 5000 Hz may be too low. The same considerations apply to the determination of “dilution factor” of a target by measuring apparent resistivities with two slightly different resistivity contrasts. With a solid conductor target the change in the observed apparent resistivity spectrum with a change of scale does not seem to follow any uniform scaling rule. This makes it difficult to model the apparent polarizability of a buried massive conductor target on a small scale.
Integration of geologic, geochemical, and geophysical data of the Cement oil field, Oklahoma, using spatial array processing
Authors: Eliason Patricia Termain , Donovan Terrence J. , Chavez Pat S.
Published: October 2002

Geologic, geochemical, and geophysical measurements were made at the Cement oil field, Oklahoma, test site using airborne and spaceborne sensors coupled with ground‐based data collection. The data collected include (1) airborne gamma‐ray spectrometry ([Formula: see text], [Formula: see text], [Formula: see text], and total intensity channels), (2) low‐altitude aeromagnetic profiles, (3) precision gravity measurements, (4) images from the Landsat multispectral scanner (MSS) systems and U-2 photography, and (5) geologic and topographic maps. In order to reduce, analyze, display, and correlate the information, it was necessary to transform the data from vector space to raster space (a two‐dimensional image array) with fixed resolution and array dimension. With the data in array form, spatial array processing techniques were applied to (1) correct geometrically the data for proper registration, (2) perform areal interpolation and smoothing, (3) display the data as images, and (4) perform integration and correlation studies. Each data set was transformed into a rectangular array covering approximately 0.3 degrees of latitude and longitude, with each picture element encompassing [Formula: see text]. Because most variables only sparsely populate the raw image array (i.e., flight line data), the data were interpolated and smoothed using spatial filtering techniques to construct continuous images. The individual data sets were displayed as black and white continuous tone images, color coded to form color contour maps, or manipulated to generate shaded‐relief models. Methods for correlation and data interpretation were systematically investigated by using all available sources. Predetermined factual information (“prior knowledge” correlation statistics) was used to establish grounds for correlation and better define the limits of the data. This kind of data manipulation provided an enhanced pictorial representation of the geologic, geochemical, and geophysical anomalies previously documented at Cement.
Discovery of a mineralized breccia pipe using gradient array induced polarization
Authors: West R. E. , Wieduwilt W. G. , Hall D. K.
Published: October 2002

The Promontorio breccia pipe located in southern Sonora, Mexico, has been mined intermittently for silver since the turn of the century. Mineralization occurs as sulfides in the matrix of the breccia. Two to five percent pyrite also occurs in the premineral andesite that the breccia pipe has intruded. Geologic mapping led to the discovery of a small, unmineralized breccia in andesite 200 m west of Promontorio. The discovery of this breccia encouraged the search for other hidden mineralized breccias in areas obscured by postmineral alluvium. Exploration of the covered areas was carried out with an induced polarization (IP) and resistivity survey after a test line indicated that a distinct IP anomaly occurs over the Promontorio breccia pipe. The gradient array was chosen as the most effective electrode configuration to explore the covered areas because it can provide rapid detailed IP coverage necessary to locate small breccia pipes. Several anomalous IP trends were located. The most interesting one has an apparent response of 60 to 90 msec in low‐resistivity rocks similar to Promontorio. Additional information provided by dipole‐dipole profiles indicates that this anomalous zone is close to surface and about 20 m wide with a vertical to steep dip. Trenching and drilling confirmed the presence of a 20-m wide elongate breccia with sulfide mineralization similar to Promontorio. Silver grades were marginal for making ore in 1977.
Gamma‐ray logging
Authors: Wahl J. S.
Published: October 2002

The response of a gamma‐ray sonde to formation radioactivity is derived as a function of the density of the formation, the borehole diameter, and the density of the borehole fluid. Also the response to radioactive material in the borehole, as affected by the borehole diameter and fluid density, is examined. Two important geometrical parameters of a logging sonde are its radial depth of investigation and its vertical resolution. Values are obtained for typical operating conditions, and it is shown how they vary with source and detection energies, with formation density, and with borehole geometry. The influence of high atomic number materials, either in the formation or the borehole, is elucidated, both for the total gamma‐ray counting rate and for specific detector energy intervals.
Magnetic models of crystalline terrane: Accounting for the effect of topography
Authors: Blakely Richard J. , Grauch V. J. S.
Published: October 2002

Igneous rocks commonly have large magnetic susceptibilities so that high topographic relief in crystalline terrane can produce significant anomalies in aeromagnetic surveys. Topographic anomalies are particularly significant in relatively undeformed volcanic terrane because young volcanic rocks generally have large natural remanent magnetizations as well as large susceptibilities. These anomalies commonly appear in aeromagnetic surveys as a complex pattern of high‐amplitude, short‐wavelength magnetic features that tend to obscure anomalies caused by deeper geologic sources. We have facilitated geologic interpretation of an aeromagnetic survey of the Oregon Cascade Range by calculating the magnetic field caused by a three‐dimensional (3-D) topographic model. Maps of the calculated field are compared with observed aeromagnetic data both visually and with a numerical technique that produces a contour map of correlation coefficients for the model. These comparisons allow quick recognition of anomalies caused by normally or reversely magnetized topographic features and, more importantly, identification of anomalies caused by geologic features not obviously caused by the topography.
Automatic P and S velocity determination from full waveform digital acoustic logs
Authors: Willis Mark E. , Toksöz M. Nafi
Published: October 2002

Automatic methods of determining P and S velocities from full waveform acoustic logs are studied and compared. The suggested P‐wave method is an event detector which is based on threshold detection in a window near the previous picks and fine adjustment by a semblance correlation. The moveouts found by the correlation process are used to find common source P velocities as well as effective borehole compensated (BHC) P velocities. Compensated velocities are derived from waveforms from complementing tool positions which compare favorably to velocities from standard BHC sonic logs. We call the most reliable method for S waves the P‐correlated S‐method. It consists of correlating the P waveform with the rest of the record to find the S arrival. The S waveform is then correlated with the next record to determine the S velocity. This method is compared with published techniques and proves more stable and reliable. The method does not necessitate a distinct S‐wave arrival and can utilize the existence of the reflected conical wave whose phase velocity is controlled by the S velocity of the formation.
Nonlinear magnetic inversion using a random search method
Authors: Silva Joao B.C. , Hohmann Gerald W.
Published: October 2002

A robust magnetic interpretation technique is described. It is based on the minimization of an objective function by a random search algorithm. Instead of just one estimate, this algorithm yields several points in parameter space, all producing objective function values below an assumed noise level. The centroid of the search points is in general a better estimate than each individual search point, especially when data are corrupted by noise. Since no derivatives are employed, even nondifferentiable objective functions, such as the [Formula: see text] norm of residuals, can be used. Computation of parameter covariance matrices and confidence ellipses is possible using simple formulas. The usefulness of R‐ and Q‐mode factor analysis applied to the correlation matrix is demonstrated. The R‐mode factor analysis can be used in parameter variance control, and the Q‐mode factor analysis provides means to find models which are extreme in some sense. The technique was applied to theoretical data and to field data from three different geologic settings. The results show that the algorithm produces accurate estimates of source parameters under different types of geologic constraints. The proposed technique has three main advantages. First, it attacks the multiple minima problem, which is always present in nonlinear inversion. A possible range of acceptable models consistent with a priori geologic information is computed. Second, when the multiple minima are not caused by the inherent ambiguity in potential field data, for example when presence of noise in data is significant or the wrong interpretation model is used, the technique provides parameter estimates which can be more significant than the estimates obtained by most optimization algorithms. Third, it can easily incorporate a priori geologic information, which is always available and should be used to reduce ambiguity. The technique can be applied at any stage of magnetic interpretation. However, its efficiency will depend upon the amount of a priori geologic and geophysical information available.
Addendum to: Quantifying resolving power of seismic systems
Authors: Widess M. B.
Published: October 2002

The subject paper (Widess, 1982) presents a quantitative formulation of the vertical resolving power of seismic exploration systems, and this formulation is offered as a proposed characteristic, or standard, resolving power identified with individual systems. The present Addendum supplements the theoretical approach in the subject paper by covering some closely related practical aspects.
Modeling of self‐potential anomalies near vertical dikes
Authors: Fitterman David V.
Published: October 2002

The self‐potential (SP) Green’s function for an outcropping vertical dike is derived from solutions for the dc resistivity problem for the same geometry. The Green’s functions are numerically integrated over rectangular source regions on the contacts between the dike and the surrounding material to obtain the SP anomaly. The analysis is valid for thermoelectrical source mechanisms. Two types of anomalies can be produced by this geometry. When the two source planes are polarized in opposite directions, a monopolar anomaly is produced. This corresponds to the thermoelectrical properties of the dike being in contrast with the surrounding material. When the thermoelectric coefficients change monotonically across the dike, a dipolar anomaly is produced. In either case positive and negative anomalies are possible, and the greatest variation in potential will occur in the most resistive regions. Examples of the effect of changing different model parameters are given for sources that have constant intensity throughout the rectangular source regions. For these patch models the depth to the top of the source region is approximately equal to the distance between the minimum (or maximum) of the anomaly outside of the dike and the edge of the dike. Field data collected over a hot intrusive fissure are presented which have been modeled by the technique described.
Analysis of error due to presence of gaps in the measurement of rock samples in a coaxial line
Authors: Huang Frank S. C. , Shen Liang C.
Published: October 2002

The theory of coaxial-line-scattering-matrix method is reviewed and the theory of gap correction is developed. Discussion includes the influence due to the presence of gaps on the apparent rock property. Correction charts to correct measured complex permittivity of rock sample to its actual value are produced. These charts are useful in the interpretation of the measured data of rock properties using the coaxial-line-scattering-matrix method.
DC telluric fields in three dimensions: A refined finite‐difference simulation using local integral forms
Authors: Hermance John F.
Published: October 2002

A new finite‐difference form is developed for simulating the distortion of telluric fields by 3-D azimuthally symmetric structures. The technique involves a sequence of local integrations of the electric current density crossing closed surfaces surrounding each mesh node. The resulting expressions, which are accurate to second degree everywhere, correctly describe first‐order discontinuities in the electric field normal to electrical discontinuities in the interior of the model. Moreover, the new form (a nine‐point finite‐difference operator) accounts for cross‐derivative (e.g., [Formula: see text]) effects in the region about each node, which can lead to significantly improved accuracy near sharp, localized discontinuities where the anomalous field decays as [Formula: see text] or [Formula: see text] with distance. Numerical simulations are compared with analytical solutions for two simple models: (1) a circular disk‐shaped heterogeneity in a thin sheet; and (2) a sphere imbedded in a homogeneous, infinite medium. The comparison between the analytical and numerical results for both of these models indicates that an accuracy of better than a few percent is not exceptional.
Rock density from borehole gravity surveys
Authors: LaFehr T. R.
Published: October 2002

The borehole gravity meter (BHGM) is recognized as an important logging tool for obtaining formation bulk density. In general, however, the difference between two gravity observations vertically separated in a well leads to an apparent and not the actual bulk density. BHGM‐derived apparent densities are equal to the formation bulk densities when the instrument passes through beds which are horizontal, infinitely extended laterally, uniformly thick, and constant in density. For many applications, departures from these assumed conditions are so slight that their effects can be ignored, and the BHGM essentially yields bulk density with a large radius of investigation. In the presence of anomalous masses, significant distortion in formation bulk density is possible. The apparent density anomaly produced in the well by an elongated, offset density contrast is proportional to the angle subtended by the density‐change interface. For a density‐change boundary having circular symmetry with respect to the well, the apparent density anomaly at the center of the bed is proportional to the sine of the subtended angle. Because the distortion in bulk density is the same above a horizontal boundary as it is just below (in the limit, at the boundary, for a normally incident well), an abrupt change in apparent density is equal to the real density change at the boundary. This change in density, termed “the Poisson jump,” is independent of geometry; our ability to measure it, however, is a function of station location with respect to the geologic bodies. Two methods are suggested for obtaining bulk densities from BHGM apparent densities: (1) by obtaining two stations just outside as well as just within the zone of interest, the Poisson jump can be approximated and added to an independent density source (e.g., the gamma‐gamma log), and (2) the apparent density anomaly within the formation of interest can be derived by modeling (perhaps based on seismic or well data) and added to the BHGM‐determined densities. Thinner beds can be studied with the BHGM than generally believed, even with much greater station spacing.
Microwave properties of saturated reservoirs
Authors: Lange James N.
Published: October 2002

Dielectric properties of saturated, porous geologic materials reflect the large difference in dielectric constant ε of typical saturating fluids such as water (ε = 78) or oil and gas (ε = 1–3). The deconvolution of in‐situ dielectric properties of saturated porous materials into the component parts requires a detailed model of the composite material. Defining aspects of this model in the microwave frequency regime is the primary purpose of this investigation. A model is examined in which the dielectric constant of the composite is equal to the sum of the dielectric constants of the components weighted by the volume fraction occupied by each. That model is compared to measurements at microwave frequencies made on systems consisting of glass beads, quartz, or sand saturated with chlorobenzene, 1,2‐dichloroethane, methanol, or air, and find satisfactory agreement. When water is the saturant an interaction between water and the solid matrix has an important effect on the composite dielectric constant. This interaction is observed to be particularly large for quartz and water and suppresses the composite dielectric constant quite considerably. This interaction is dependent upon the relative surface area per unit volume. An empirical relationship between the surface area and composite dielectric constant is obtained for clean, saturated, unconsolidated reservoirs. The inverse process of determining surface area from in‐situ measurements of the composite dielectric constant may be possible for clean reservoirs of known lithology. In sandstones from cores the dielectric constant is also below the volume fraction model and corrections are needed to evaluate water content.
Vertical seismic profiling: Separation of upgoing and downgoing acoustic waves in a stratified medium
Authors: Seeman B. , Horowicz L.
Published: October 2002

One of the essential steps in the processing of a vertical seismic profile is the separation of upgoing and downgoing signals. With this perspective in mind, seismic data recorded in a borehole are modeled in terms of these waves and a mathematically optimal “least‐squares” technique for extracting them is derived. The method imposes practically no constraints on the spacing between recording levels and allows almost perfect rejection of a coherent downgoing signal. The exact formulation of the one‐dimensional model requires that acoustic impedance information be included, but a reasonable and realistic approximation can neglect impedance. We derive frequency‐wavenumber response plots for the two limiting cases of even and randomly spaced levels and compare these to the response of a “conventional” velocity filtering technique. By a careful study of available logs, recording levels can be chosen to optimize geophone coupling rather than insisting on uniform spatial sampling. Data editing, normalization, and true amplitude recovery (TAR) are required prior to application of the technique. The TAR correction can be computed from sonic log data, which emphasizes the possibility of more complete synergism between the seismic and logging worlds.
Anomalous seismic character—Bering Sea Shelf
Authors: Hammond Roger D. , Gaither John R.
Published: October 2002

Seismic data collected within basins along the outer Bering Sea Shelf often exhibit a distinct change in seismic character between 1.0 and 2.0 sec two‐way time. This change appears on seismic sections as a reflector or as an increase or decrease in amplitude. The feature is of regional extent. This change in seismic character is a manifestation of what has been called in other basins a bottom simulating reflector (BSR). BSRs are reflectors that (1) are subparallel with sea floor topography, (2) are discordant with stratigraphy where the sea floor dictates, and (3) do not demonstrate all the characteristics of a multiple. Two causes of BSRs are generally accepted. One involves an ice‐like mixture of water and gas, called “gas hydrate,” in which gas molecules are trapped within a framework of water molecules. The other cause involves the diagenetic alteration of biogenic opal‐A to opal‐CT in diatomaceous sediments. BSRs were penetrated at three locations in the Bering Sea in water depths greater than 1800 m on leg 19 of the Deep Sea Drilling Program (DSDP). The BSRs at these locations were attributed to the diagenetic alteration of opal‐A. This same diagenesis of opal‐A to opal‐CT is interpreted to be the cause of seismic character changes noted in basins on the Bering Sea Shelf. Pitfalls in seismic interpretation may be encountered where this reflector intersects other reflectors at an observable angle. The BSR may look like a sequence boundary or a direct hydrocarbon indicator. Recognition of the presence of this seismic character change is of two‐fold importance to explorationists: (1) it aids the understanding of the geology of the Bering Sea Shelf, and (2) it helps avoid seismic interpretational pitfalls.
LaCoste and Romberg straight‐line gravity meter
Authors: LaCoste Lucien
Published: October 2002

The LaCoste and Romberg straight‐line gravity meter uses a new suspension in which the movable element moves vertically in a straight line rather than in an arc of a circle (LaCoste, 1973a). It was designed primarily for shipboard operation to avoid effects from cross coupling between various ship accelerations, thereby making it unnecessary to correct for such effects. The straight‐line suspension is a modification of the zero length spring suspension used in all LaCoste and Romberg gravity meters. The new model also uses silicone fluid damping rather than the air damping used in earlier models. Its main advantages over the older models appear to be: it is (1) free of cross‐coupling effects, (2) easier to build and adjust, (3) less subject to slight degradation in performance from rough handling, and (4) less sensitive to ship vibrations. In spite of the above advantages it is doubtful whether the new model will give substantially better accuracy than the previous models, if the previous models are kept in good operating condition by making occasional crosscorrelation analyses (LaCoste, 1973b). Valliant (1983, this issue) describes sea tests of the new instrument.
Field trials with the LaCoste and Romberg straight‐line gravimeter
Authors: Valliant H. D.
Published: October 2002

During field tests in 1980, the prototype of the LaCoste and Romberg straight‐line gravimeter (ser. no. SL-1) generally produced gravity values intermediate between those of two comparison “S” meters (ser. nos. S-56 and S-41). Correlation analysis shows that the data from SL-1 with no cross‐coupling correction are 15 percent smoother even in rough weather than the corrected data from either S meter. When corrections to the cross‐coupling effects are applied through postcruise correlation techniques, the curvature (degree of smoothness) of the respective data agrees to within 2 percent for each meter. These results verify that cross‐coupling errors have been virtually eliminated in the new gravimeter.
Case history of the exploration of the Grand Isle 95 Field in the Gulf of Mexico
Authors: Pan Poh‐Hsi
Published: October 2002

Grand Isle 95 Field is located 40 miles offshore Louisiana in 200 ft of water. First stage seismic exploration work was completed by Mobil in 1972. High‐amplitude events were observed on the seismic sections which were believed to be related to the existence of hydrocarbons. Reprocessed seismic lines showed some amplitude change near two questionable flat spots which were thought to be the gas‐oil and oil‐water sand contacts. Careful examination of the original seismic record proved that one of the flat events was caused by noise or distortions under the fault. Subsequent seismic modeling studies led to the success of exploration and exploitation of the field.
The harmonic and transient electromagnetic response of a thin dipping dike
Authors: Weidelt P.
Published: October 2002

An exact solution is given for the electromagnetic induction in a dipping dike of finite conductivity, represented as a thin half‐sheet in a nonconducting surrounding. The problem is formulated for arbitrary dipole or circular loop [Formula: see text] configurations. The formal solution obtained by the Wiener‐Hopf technique is cast into a rapidly convergent triple integral suitable for an effective numerical treatment. A good agreement is found between numerical results and analog measurements available for harmonic excitation. The transient response is obtained as a superposition of the half‐sheet free‐decay modes and is illustrated by some numerical examples for coincident loops, including a diagram for the approximate determination of conductance and depth of a vertical dike.
Geothermal prospecting with Shallo‐Temp surveys
Authors: LeSchack L. A. , Lewis J. E.
Published: October 2002

The Shallo‐Temp® survey is an inexpensive and rapid “first look” geophysical technique that is useful in planning the more traditional and costly reconnaissance drilling geothermal exploration programs. The technique is based on making many soil temperature measurements at 2-m depths over a given exploration area and correcting these measurements for the effects of elevation and surface geologic and meteorologic conditions. Corrections for surface conditions are made with an “annual wave correction model.” The output from the model is the normally expected 2-m temperature for the given site at the date for which input data were provided. The difference between the measured and computed 2-m temperature data represents effects of geothermal heat flow. A Shallo‐Temp residual map is compared both to a 2-m temperature map for a specific date (September, 1977) and to a mean annual 2-m temperature map for the Coso known geothermal resource area producing the same anomaly pattern in each case. Additional case history studies at Upsal Hogback in Nevada, and Animus Valley in New Mexico provide evidence to support the applicability of the Shallo‐Temp technique throughout the Basin and Range Province. The technique developed is not designed to replace reconnaissance drilling but rather help focus standard reconnaissance programs. The two potentially most reliable applications of the technique are in extending trends where standard reconnaissance holes have been drilled or filling in detail between widely spaced holes, and in surveying for near‐surface anomalies that might be developed for direct heating applications. ®Registered trademark of LeSchack Associates, Ltd.
The current status of the IGRF and its relation to magnetic surveys
Authors: Regan Robert D.
Published: October 2002

As aeromagnetic surveys covered increasingly larger areas and as the need arose for the compilation of adjacent surveys with significant time differences, there has been an increase in the use of geomagnetic field models for regional field definition. Indeed, few aeromagnetic contracts exist today without some provision for removal of the International Geomagnetic Reference Field (IGRF). Yet, as publications indicate (Regan and Cain, 1975a, 1975b; Reford et al, 1976; Regan, 1977, 1978), the IGRF is far from perfect for this application despite the considered attention of the IAGA (International Association of Geomagnetism and Aeronomy) working group charged with its upkeep and maintenance. One reason for this is that the model is meant to serve all areas of geomagnetism and, as such, is not specifically tailored to the needs of the exploration community. The recent summary report of the IAGA committee (Peddie, 1982) could lead to some confusion because there are seven IGRF models published. A rational question for those involved in aeromagnetic surveys is “Which model should be used for regional field definition?”
Butterworth dip filters
Authors: Hale Dave , Claerbout Jon F.
Published: October 2002

Dip filters enable a geophysicist to discriminate between various seismic events on the basis of apparent dip. The frequency‐wavenumber (ω, k) domain seems an attractive domain to perform dip filtering because it permits the application of an arbitrary transfer function of dip. Seismic applications of dip filtering, however, seldom require the flexibility offered by the (ω, k) domain; one may often be willing to sacrifice this flexibility to obtain features not possible with (ω, k) domain filters, but readily available with timespace (t, x) domain filters. Examples are (1) time and space variability, (2) flexible treatment of computational grid boundaries, and (3) an efficient, recursive implementation. We describe a (t, x) domain dip filtering method with these features. In the derivation of a (t, x) domain filter, we first discuss (t, k) and (ω, x) domain dip filters. While not fully possessing the advantages of a (t, x) domain filter, these filters are an attractive combination of two very efficient and commonly available processes: (1) one‐dimensional Butterworth filtering and (2) one‐dimensional Fourier transforms. We then derive (t, x) domain approximations to these filters which have the features noted above.
The resonant acoustic pulser—A continuous‐frequency marine seismic source
Authors: Hardee H. C. , Hills R. G.
Published: October 2002

A marine seismic source is described which produces a continuous low‐frequency (10–100 Hz) harmonic signal. High acoustic power levels (∼20 kW) are reached by using a resonant system. The seismic source has been successfully tested in lake and ocean environments. Geophysical applications are discussed and modifications are described which would allow the source to be operated in a swept‐frequency mode.
A short and straightforward derivation of two equations from Hubral’s paper “Computing true amplitude reflections in a laterally inhomogeneous earth”
Authors: Krey Th.
Published: October 2002

In his important paper “Computing true amplitude reflections in a laterally inhomogeneous earth” (Hubral, 1983, this issue) the author derives the fundamental equation [Formula: see text] [Hubral (25)] Here [Formula: see text] is a factor compensating the energy of a zero‐offset reflection for geometrical spreading and [Formula: see text] and [Formula: see text] are certain wavefront curvature matrices which will be explained later.
Attenuation of seismograms obtained by the Cagniard‐Pekeris method
Authors: Kelamis P. G. , Kanasewich E. R. , Abramovici F.
Published: October 2002

Attenuation and dispersion are included in synthetic seismograms obtained by a Cagniard‐Pekeris formulation for the problem of a point source in a layer over a half‐space. The solution is decompose into generalized rays, and the effects of attenuation and dispersion are incorporated in an ad hoc manner in the frequency domain. The effects of the viscoelastic interfaces are taken into account by corrections to the reflection coefficient for an elastic medium. The results are illustrated with synthetics for a model simulating a weathered layer over a halt‐space. Both the SH and P‐SV cases are treated.
A mise‐à‐la‐masse study of the Cavendish Geophysical Test Site
Authors: Mansinha L. , Mwenifumbo C. J.
Published: October 2002

The mise‐à‐la‐masse method of electrical prospecting is used when a conductive, mineralized zone has already been located. A current electrode is placed directly in the mineralized zone, and the resulting potential field is mapped on the surface or in the subsurface via drill holes or underground openings. The potential field reflects the shape, size, and orientation of the mineral deposit. A field study of this method was carried out at the Cavendish Geophysical Test Site in Ontario, Canada. Two vein‐type mineralized bodies, zone A and zone B, are known to exist at the site. Drill holes were used to implant a current electrode in each of the veins and surface potentials were measured. The extension of the two zones in the strike direction was established with the resulting mise‐à‐la‐masse potential profiles. However, no electrical continuity between zones A and B could be discerned, indicating that the veins are not connected at depth. The dip of zone A appears to be toward the west, contrary to that inferred from the drill hole logs. The measured potentials are influenced by the mutual proximity of the far current and potential electrodes as well as by the location of the electrodes with reference to the local geologic structure.
Migration velocity analysis by wave‐field extrapolation
Authors: Yilmaz Oz , Chambers Ron
Published: October 2002

Velocity information is essential to both common midpoint (CMP) stacking and migration. CMP stacking provides the basis for conventional velocity estimation techniques in that, for a number of trial velocities, the stack response of a CMP gather is computed and displayed in the form of a velocity table. An alternative approach to velocity estimation makes use of the basic ingredients of migration—downward extrapolation and imaging of seismic wave fields. The procedure involves migration of a CMP gather with a number of trial velocities and collection of the zero‐offset information, again in the form of a velocity table. Operating on a CMP gather, the migration‐based approach produces results similar to those of the conventional method. Analyses of synthetic CMP gathers using both methods show essentially equivalent treatments of seismic signal, and similar dependence of accuracy and resolving power on recording geometry. We have extended the migration‐based approach to include more than one CMP gather in each analysis. This extension allows proper treatment of dipping events and yields velocity information that is more appropriate for use in migration. By using the intermediate wave field at each step of downward extrapolation, we need only do a single constant‐velocity migration of the unstacked data followed by a simple mapping procedure in order to recover the velocity information.
Seismic reciprocity field tests from the Italian Peninsula
Authors: Fenati D. , Rocca F.
Published: October 2002

The conditions for applicability of the reciprocity theorem, commonly invoked in seismic data gathering, are rarely met in usual practice. A field test was conducted in an area characterized by remarkable inhomogeneities of the surface layers to assess quantitatively the discrepancies between direct and reciprocal traces; the energy sources used were both of vibratory and impulsive type. The results show that the coherence between the two traces is good, except for the case of short offsets and early times with explosive sources. The vibratory source, even if theoretically “more reciprocal” than the explosive one, yields coherencies in the same range, and is less uniform along the frequency axis.
The principle of complex frequency scaling—Applicability in inclined continuation of potential fields
Authors: Nagendra R. , Laxminarayana N.
Published: October 2002

A remarkable property of Fourier transforms, especially applicable to inclined continuation of potential geophysical fields, is the principle of “complex frequency scaling.” Briefly stated, let [Formula: see text] be the (gravity/magnetic) field due to a two‐dimensional structure along a principal profile and F(ω) be its Fourier transform. The field along a profile passing through the same reference origin and tilted by an angle ϕ in the counter‐clockwise direction (+X to −Z) is obtained by inverse Fourier transforming F[ωexp(−iϕ)] for positive ω. The complex scaling property and proof of the resulting space‐frequency domain relationship are presented, introducing the total field as the analytic signal of the horizontal component. The applicability of the complex scaling principle is illustrated by considering selected geometric models. This principle can be advantageously applied for continuation of two‐dimensional potential fields onto inclined planes.
On modeling a well casing for resistivity and induced polarization
Authors: Wait James R.
Published: October 2002

In a previous communication I proposed an analytical model to simulate the electromagnetic (EM) and induced polarization (IP) response of a metal well casing (Wait, 1983). To facilitate the analysis, the earth was idealized as a homogeneous conducting half‐space of electrical properties (σ, ε, μ). The well casing was represented as a filamental vertical conductor of semiinfinite length that was characterized by a series axial impedance to account for eddy currents and interfacial polarization. A further basic simplification was to neglect displacement currents in the air; this was justified when all significant distances were small compared with the free‐space wavelength. Initially, the source was taken to be a horizontal electric dipole or current element I ds on the air‐earth interface. By integration of the results, the mutual impedance between two grounded circuits could be ascertained. In the absence of the vertical conductor (i.e., the well casing) the results reduced to those given by Sunde (1968) and Ward (1967).
On: “Does draping aeromagnetic data reduce terrain‐induced effects?”, by V. J. S. Grauch and David L. Campbell (GEOPHYSICS, 49, 75–80, January, 1984).
Authors: Reford M. S.
Published: October 2002

Grauch and Campbell showed the danger of a simplistic approach in aeromagnetic surveying over rough and magnetized terrain. It is impossible to remove the magnetic effects of the terrain simply by drape flying at a constant height above the terrain, or by an equivalent mathematical continuation. Perhaps their emphasis is unkind to the purpose of Bhattacharyya and Wynn (1977) who wished to reduce the high‐frequency magnetic effects observed over mountain peaks when flying at a constant level, and were quite successful in so doing.
COCORP: Utah area, Part II; Death Valley area, California; and Coalinga area, California
Authors: Kaufman S.
Published: October 2002

The Consortium for Continental Reflection Profiling (COCORP) announces the availability of the data packages and digital tapes for three areas: Utah area, Part II, lines 3 and 4 covering 126.5 line‐km; Death Valley area, California, lines 8–12 covering 256.9 line‐km; and Coalinga area, California, lines 1 and 3 covering 68.8 line‐km. The costs are for reproduction and shipping, only. The COCORP operation is part of the U.S. Geodynamics Program sponsored by the National Academy of Sciences and funded by the National Science Foundation. The executive group of the consortium consists of representatives from Cornell University, Dartmouth College, Princeton University, Rice University, and the University of Wisconsin. Cornell University is the operating institution.
A two‐way nonreflecting wave equation
Authors: Baysal Edip , Kosloff Dan D. , Sherwood J. W. C.
Published: October 2002

In seismic modeling and in migration it is often desirable to use a wave equation (with varying velocity but constant density) which does not produce interlayer reverberations. The conventional approach has been to use a one‐way wave equation which allows energy to propagate in one dominant direction only, typically this direction being either upward or downward (Claerbout, 1972). We introduce a two‐way wave equation which gives highly reduced reflection coefficients for transmission across material boundaries. For homogeneous regions of space, however, this wave equation becomes identical to the full acoustic wave equation. Possible applications of this wave equation for forward modeling and for migration are illustrated with simple models.
Positivity of the coincident loop transient electromagnetic response
Authors: Guptasarma D.
Published: October 2002

Field observations with a coincident loop transient EM frequently show that the measured decaying voltage changes its sign during the decay, reaches a maximum value with this changed sign, and then decays to zero. This change of sign has been ascribed to special distributions of magnetic permeability or conductivity (Spies, 1980), as well as to the presence of electrochemical polarizability (Lee, 1975, 1981). Gubatyenko and Tikshayev (1979) showed, however, that for any frequency‐independent linear medium the induced voltage caused by a step current excitation is always of one sign. Weidelt (1982) extended the results of Gubatyenko and Tikshayev (1979), and established further constraints on the slope and curvature of the decay curve. It is thus quite clear that with a coincident loop system a change of the polarity of the decaying voltage cannot be caused by any distribution of conductivity or permeability in the ground.
Evaluation of an observational method for estimation of remanent magnetization
Authors: Schnetzler C. C. , Taylor P. T.
Published: October 2002

This paper evaluates an observational method proposed by Zietz and Andreasen (1967) for estimating the remanent magnetization of a body from aeromagnetic data. Their method uses the position and relative intensity of the maximum and minimum produced by the causative body to infer the declination δ and inclination i of the total magnetization vector of the body. A series of plots which summarize over 800 modeled fields is presented which illustrate the utility and limitations at main field inclinations of 0, 30, 60, 75, and 90 degrees. As a general rule, the method is relatively insensitive for the estimation of δ and i near the geomagnetic equator, but it becomes increasingly sensitive toward the geomagnetic poles. However, the sensitivity is a complicated function of both δ and i at any magnetic latitude. Examination of the applicability of this method indicates that under limited circumstances magnetization declination δ can be estimated, but due to the zero‐level ambiguity inclination of the magnetization i is indeterminate.
Induced polarization time‐domain equipment and some model studies over thin dikes of finite strike extent
Authors: Majumdar R. K. , Dutta S.
Published: October 2002

An induced polarization unit designed for model studies with multiple choices of current pulse widths and constant current is described. The percentage polarizibility is estimated from the recorded decay potential. Model results with different arrays are presented and discussed over a plate‐like body for various depths and inclinations. Field cases are presented for supporting the model results.
Seismic geohistory analysis—A case history from the Canning Basin, Western Australia
Authors: Middleton M. F.
Published: October 2002

Seismic geohistory analysis is a method of reconstructing an interpreted seismic section, assuming generalized porosity‐traveltime and depth‐traveltime relations. The method can indicate timing of fault movements, juxtaposition of units across faults in the past, and depth or traveltime to specific reflection horizons throughout geologic time. The method applied to the Lennard shelf of the Canning Basin shows that the major movements of the Pinnacle fault system occurred during the Ordovician and Middle Carboniferous. The movements reflect subsidence of the Fitzroy trough, which is basinward of the Lennard shelf. The analysis also shows a prograding sediment wedge on the margin of the Lennard shelf to have had temperatures in the range 110°–116°C since Permian times. Being within the thermal oil window for the past 280 Ma, the sediment wedge may be a good hydrocarbon prospect.