bluuebunny/crossref_metadata_2025 huggingface dataset:
THE ELECTROMAGNETIC RESPONSE OF A LOW‐LOSS, 2‐LAYER, DIELECTRIC EARTH FOR HORIZONTAL ELECTRIC DIPOLE EXCITATION
Authors: Annan A. P. , Waller W. M. , Strangway D. W. , Rossiter J. R. , Redman J. D. , Watts R. D.
Published: October 2002

The use of the radio interferometry method requires a detailed understanding of the nature of electromagnetic wave propagation in structures composed of materials with low electrical loss. This paper contains the results of a detailed experimental and theoretical study into the response of a 2-layer, plane‐stratified, low‐ loss dielectric earth. The technique used to construct a scale model with microwave equipment to experimentally simulate the 2-layer structure response is discussed. The wave nature of the response derived from the theoretical investigations is used to interpret the features of the experimental results. The experimental results are useful in turn in demonstrating the reliability of approximate theoretical solutions for the electromagnetic fields about the dipole obtained by normal mode and geometrical optics methods. Such features as (1) the modification of the dipole radiation pattern seen when the antenna is placed at the interface between media of differing electrical properties, and (2) the development of guided and leaky modes in the layered structure are examined in detail.
RECOMMENDED STANDARDS FOR DIGITAL TAPE FORMATS
Authors: Barry K. M. , Cavers D. A. , Kneale C. W.
Published: October 2002

Recently a new demand for demultiplexed formats has arisen in the seismic industry due to the utilization of minicomputers in digital field recording systems and because of a growing need to standardize an acceptable data exchange format.
THE DIPOLE MAPPING METHOD
Authors: Keller George V. , Furgerson Robert , Lee C. Y. , Harthill Norman , Jacobson J. J.
Published: October 2002

In the bipole‐dipole mapping method, current field is set up by the use of a bipole current source. The current field is then studied by making measurements of electric field intensity with dipole receivers at many locations around the bipole source. The values for electric field intensity may be used to compute apparent resistivities if we assume that the earth is uniform or to compute apparent conductance if we assume that the earth resembles a conducting sheet. Maps of apparent resistivity varies or apparent conductance values may be interpreted by comparing them with similar maps computed analytically for various simplified earth models. The bipole‐dipole mapping method is useful mainly in locating areas where ground resistivity varies rapidly in the horizontal direction. It has found application mainly in exploration for geothermal reservoirs but also has been used for mining exploration and engineering studies, and an example of each is described.
URANIUM MEASUREMENT BY AIRBORNE GAMMA‐RAY SPECTROMETRY
Authors: Grasty R. L.
Published: October 2002

In the airborne measurement of uranium, window type gamma‐ray spectrometers are used and it is necessary to correct for scattered high energy radiation from thallium 208 in the thorium decay series. This radiation can be scattered in the crystal, in the ground, and in the air. A theory, analogous to the theory of radioactive decay, is developed; it can adequately explain the spectrum buildup in the uranium window for a point source of thorium oxide immersed to different depths in water and for a detector above the water. The theory is extended to predict the buildup as a function of altitude for detectors of different sizes and shows that errors in the airborne measurement of uranium can be, significant if no allowance is made for radiation scattered in the ground and in the air.
MAGNETIC ANOMALY EFFECTS DUE TO APPROXIMATING INFINITE RECTANGULAR PRISM SOURCES BY INFINITE LINE OR INFINITE SHEET SOURCES
Authors: Shapiro J. N. , Weynand G. , Gangi A. F.
Published: October 2002

The inversion of oceanic magnetic anomaly data to obtain the crustal magnetization requires some assumptions regarding the magnetization distribution (Parker and Huestis, 1974). Two simplifying assumptions typically made for data near ridge systems are that the source magnetization is lineated parallel to the ridge system and that it is unidirectional. These assumptions follow directly from the theory of plate tectonics and have been used near the major ridge systems of the world. The presence of transform faulting can offset the lineations usually observed and render the first assumption invalid. In this note, only those cases in which both assumptions are valid will be considered.
A SEISMIC MODEL USING SOUND WAVES IN AIR
Authors: Woods John P.
Published: October 2002

For seismic reflections, for example in exploration seismology, the energy returned to the surface must come from a subsurface area of some ten “square wavelengths.” Energy is not returned by a “reflection point,” and it is not returned by a “fault line.” Ray geometry is a poor approximation, and its uncritical use in seismic interpretation causes important error. The seismic interpreter must think always that reflections come from a large area, and such thinking will surely affect the way he maps subsurface topography and faulting. This paper describes various experiments to show that reflected energy must come from a relatively large area. The experiments were made with a laboratory model using sound waves in air. A spark source, a receiver consisting of a three‐inch speaker, and plywood reflectors were used. Wavelengths in air of the order of three inches were used to model typical exploration dimensions. Reflection amplitudes are shown for models of horizontal and dipping layers terminated at a fault, disc‐ and ring‐shaped reflectors, a reflector with a gap, anticlines, and synclines.
REVISION OF THE IGRF: A Summary Report on the Zmuda Memorial Conference on Geomagnetic Field Models
Authors: Regan Robert D. , Cain Joseph C.
Published: October 2002

On March 24 and 25, 1975, the Zmuda Memorial Field Model Conference cosponsored by the Society of Exploration Geophysicists (SEG) was held at the Broadmoor Hotel, in Colorado Springs, Colorado. This meeting was designed to provide an opportunity for dialogue between those who derive geomagnetic field models and the various users and to serve as an opportunity to discuss the plans for the proposed revision of the International Geomagnetic Reference Field Model (IGRF). The conference included an objective appraisal of the use of field models in magnetic surveys and documented the need for a more accurate reference field. This report summarizes the conference results as they pertain to the members of SEG. A complete summary, including abstracts, is published in the July (1975) issue of EOS, Transactions of the American Geophysical Union.
SPECTRAL ANALYSIS OF GRAVITY AND MAGNETIC ANOMALIES DUE TO TWO‐DIMENSIONAL STRUCTURES
Authors: Bhattacharyya B. K. , Leu Lei‐Kuang
Published: October 2002

The expressions for the spectra of both gravity and magnetic anomalies due to a two‐dimensional structure consist of (except for a factor) sums of exponentials. The exponents of these exponentials are functions of frequency and the locations of the corners of the polygonal cross‐section of the structure. Two computationally feasible methods for determining the exponents from a given spectrum are described in this paper; they are essentially based on the generation of a system of linear equations. The unknown coefficients in this system of equations are functions of the corner locations. The first method requires expansion of the exponentials in the expressions for the spectra in the form of a series and works reliably when the amplitudes of low frequencies are analyzed. The unknown parameters are determined fairly accurately with this method by suitable combinations of the spectra of the observed anomaly and its moments. The second method utilizes an exponential approximation technique for producing the system of linear equations. If only the spectrum of the anomaly is used, the system of equations becomes ill‐conditioned in most cases resulting in grossly inaccurate solutions. However, particular combinations of the spectra of the anomaly and its first and second order moments are found to improve significantly the behavior of the system of equations and thus the quality of results. It has also been found that the mean values of corner locations can be calculated fairly accurately by taking the ratios of the spectra of the anomaly and its moments. Once the corner locations are found, computation of the density contrast in the case of a gravity anomaly and the magnetization contrast for a magnetic anomaly is straightforward.
CENSUS OF NEW GEOPHYSICAL EMPLOYEES AND GEOPHYSICS GRADUATES
Authors: Sheriff Robert E.
Published: October 2002

During the 1975 SEG Academic‐Industry Seminar held in Houston, June 22–26, participants asked for statistical data regarding the employment of university graduates as geophysics. Accordingly, a questionnaire was sent to about 75 companies which employ geophysicist: oil companies, geophysical contractors, etc. The following tabulation details the result of the 31 questionnaire returned by December 20, 1975. Three questions were asked one regarding employees hired directly from universities, a second regarding summer employees, and third regarding requirements for foreign languages. The last question was designed to obtain information for a study being conducted for the AAAS.
URANIUM ASSAY LOGGING USING A PULSED 14‐MEV NEUTRON SOURCE AND DETECTION OF DELAYED FISSION NEUTRONS
Authors: Givens W. W. , Mills W. R. , Dennis C. L. , Caldwell R. L.
Published: October 2002

An in‐situ uranium assay logging system has been developed that measures directly and quantitatively the uranium concentration in the formation surrounding a drill hole. System operation is based on the DFN (delayed fission neutron) method which involves (1) bombarding a formation with short duration bursts of neutrons from a pulsed‐neutron generator to induced fission in any uranium present; (2) separating delayed fission neutrons, from source and prompt fission neutrons, by waiting a few milliseconds after each neutron burst before activating the neutron counter system; (3) deactivating the neutron counter system before the beginning of the next neutron burst; and (4) repeating this bombard‐wait‐count cycle a sufficient number of times to accumulate a statistically acceptable number of the delayed neutron counts. The DFN logging method has been used routinely in our field operations for several years, and it has successfully overcome one of the most perplexing problems encountered in uranium exploration and production—that of radioactive disequilibrium. The need for coring and chemical assaying is virtually eliminated. The information provided by chemical assay of cores is made available by DFN assays on site and in minutes after the data are accumulated. The disequilibrium ratio is available to the field geologist immediately by comparing DFN assay ore grade to ore grade from a calibrated natural gamma‐ray log. A DFN uranium assay logging system is capable of 8 ft/minute continuous semiquantitative logs and stationary quantitative assays. Calibration procedures and a method for correcting DFN assays for variable formation parameters, such as porosity and macroscopic absorption cross‐section, have been developed. DFN logs, assays, and chemical assays of cores are in excellent agreement.
On: “EM Coupling, Its Intrinsic Value, Its Removal, and the Cultural Coupling Problem”, by Jeffrey C. Wynn and Kenneth L. Zonge (GEOPHYSICS, October 1975, p. 831–850)
Authors: Swift Charles M. , Hohmann Gerald W.
Published: October 2002

We agree with the authors that an understanding of inductive coupling is important in the proper interpretation of induced polarization data. We also agree that “the ultimate value and purpose of studying coupling is to effect its removal from induced polarization data and thereby contribute to the value and usefulness of these data.”
2-D MULTIPLE REFLECTIONS
Authors: Riley Don C. , Claerbout Jon F.
Published: October 2002

Starting with a 1-D subsurface model, a method is developed for modeling and inverting the class of multiple reflections involving the near‐perfect reflector at the free surface. A solution to the practical problem of estimating the source waveform is discussed, and application of the 1-D algorithm to field data illustrates the successful elimination of seafloor and peg‐leg multiples. Extending the analysis to waves in two dimensions, we make the approximation that the subsurface behaves as an acoustic medium. Based on several numerical and theoretical considerations, the scalar wave equation is split into two separate partial differential equations: one governing propagation of upcoming waves and a second describing downgoing waves. The result is a pair of propagation equations which are coupled where reflectors exist. Finite difference approximations to the initial boundary value problem are developed to integrate numerically the surface reflection seismogram. Use of the 2‐D algorithm for modeling free‐surface multiple reflections is illustrated by several reflector models. The 2-D inverse problem of simultaneously migrating primary reflections and inverting diffracted multiples consists of reversing the forward calculation with the data as boundary conditions. Causal directions of propagation are related to downward continuation of surface data. Reflector mapping principles are used to develop a general reflection coefficient estimator. The inverse algorithm is illustrated using the results of the 2-D forward calculation as the boundary conditions.
AUTOREGRESSION ORDER SELECTION
Authors: Jones Richard H.
Published: October 2002

In the recent paper by Ulrych and Bishop (1975), Akaike’s (1969) final prediction error criterion (FPE) was used to select the order of autoregressions calculated using Burg’s (1967) maximum entropy algorithm, and compared to the Yule‐Walker estimates for which this criterion was developed (Jones, 1974). For the Burg estimates, the FPE criterion tended to choose too large an order.
On: “Experimental determination of effective demagnetizing factors for a short cylinder” by Knut Åm and Rune Otto Stemland (GEOPHYSICS, v. 40, p. 527–29)
Authors: Hjelt Sven‐Erik
Published: October 2002

The authors have introduced the concept of effective demagnetizing factor to be used when measuring susceptibilities of highly magnetized samples. They have used one experiment to show that the factor thus defined differs considerably from factors computed theoretically. From its definition, it seems obvious that the numerical value of the effective demagnetizing factor will depend on the measuring geometry and, more so, on the shape of the sample. Furthermore, the relation between the new demagnetizing factor and susceptibilities calculated using it on the one hand, and true magnetization on the other hand, is far from trivial.
IMPROVED NORMAL MOVEOUT DIFFERENCE CURVE
Authors: Pan Poh‐Hsi
Published: October 2002

The normal moveout difference, or the residual normal moveout between the primary and the multiple reflections, are used in studying multiple reflection cancellation and designing effective geophone cables. Its definition was clearly described in Mayne’s paper (1962, p. 932) and referred to by Marr and Zagst (1967, p. 213).
DEMAGNETIZING FACTORS IN NONELLIPSOIDAL SAMPLES—A REVIEW
Authors: Joseph R. I.
Published: October 2002

It is well known that the surface divergence of the magnetization vector gives rise to a magnetic field, usually termed the “demagnetizing field.” In ellipsoidal bodies which are uniformly magnetized, the demagnetizing field is in turn also uniform. In nonellipsoidal bodies, however, the demagnetizing field is in general nonuniform. Experimental data taken on nonellipsoidal samples are, therefore, extremely difficult to interpret. For this reason, experiments on magnetic materials are best performed on samples of ellipsoidal shape. Quite frequently, however, other considerations make it advantageous to use samples that have a nonellipsoidal shape; for instance, rectangular prisms, or cylinders with flat endfaces.
SHEAR‐WAVE VELOCITY VERSUS DEPTH IN MARINE SEDIMENTS: A REVIEW
Authors: Hamilton Edwin L.
Published: October 2002

The objectives of this paper are to review and study selected measurements of the velocity of shear waves at various depths in some principal types of unlithified, water‐saturated sediments, and to discuss probable variations of shear velocity as a function of pressure and depth in the sea floor. Because of the lack of data for the full range of marine sediments, data from measurements on land were used, and the study was confined to the two “end‐member” sediment types (sand and silt‐clays) and turbidites. The shear velocity data in sands included 29 selected in‐situ measurements at depths to 12 m. The regression equation for these data is: [Formula: see text], where [Formula: see text] is shear‐wave velocity in m/sec, and D is depth in meters. The data from field and laboratory studies indicate that shear‐wave velocity is proportional to the 1/3 to 1/6 power of pressure or depth in sands; that the 1/6 power is not reached until very high pressures are applied; and that in most sand bodies the velocity of shear waves is proportional to the 3/10 to 1/4 power of depth or pressure. The use of a depth exponent of 0.25 is recommended for prediction of shear velocity versus depth in sands. The shear velocity data in silt‐clays and turbidites include 47 selected in‐situ measurements at depths to 650 m. Three linear equations are used to characterize the data. The equation for the 0 to 40 m interval [Formula: see text] indicates the gradient [Formula: see text] to be 4 to 5 times greater than is the compressional velocity gradient in this interval in comparable sediments. At deeper depths, shear velocity gradients are [Formula: see text] from 40 to 120 m, and [Formula: see text] from 120 to 650 m. These deeper gradients are comparable to those of compressional wave velocities. These shear velocity gradients can be used as a basis for predicting shear velocity versus depth.
AUTOMATIC CONTOURING OF FAULTED SUBSURFACES
Authors: Bolondi G. , Rocca F. , Zanoletti S.
Published: October 2002

The problem of contouring a faulted surface known at randomly spaced points is analyzed and different types of solutions are proposed. The data may in fact be from a field which satisfies an elliptic partial differential equation; if the equation is harmonic, the surface corresponds to the displacement of a membrane properly raised from a horizontal plane in correspondence to the data points and cut along the faults. If the equation is biharmonic, the surface corresponds to the displacement of an elastic plate, properly riveted in correspondence to the data points and again cut along the faults. A third method analyzed, that corresponds to a family of interpolation methods, is that of two‐dimensional estimation. The technique used is that of modeling the autocovariance of the data as a function of the distance between the points only. The surface will depend upon the particular function chosen and it will tend to be peaked at the data points, if the function is peaked at the origin, and smoother if the autocovariance is smoother. When faults are present, the distance between two points is defined to be the length of the shortest linking path, not cutting a fault. In the latter case, it is shown that the set of functions eligible to be chosen as autocovariances is very limited. The first method has the useful property that maxima and minima of the surface are data points. The second method generates smoother surfaces that sometimes may overshoot. Both methods are implemented by iteratively smoothing the interpolated lattice (except in the neighborhood of data points), and therefore are rather expensive in terms of computer time. The third method is not iterative and is less expensive; since the surfaces that it generates are noisy, it may be used to supply a tentative solution to be refined with an iterated smoothing. These different techniques arc discussed in detail and some examples of their application are shown.
NET DOWNGOING ENERGY AND THE RESULTING MINIMUM‐PHASE PROPERTY OF DOWNGOING WAVES
Authors: Robinson Enders A. , Treitel Sven
Published: October 2002

It has been known for some time that the downgoing normal‐incidence wave train emanating from a horizontally stratified nonabsorptive medium into a homogeneous half‐space below it has the minimum‐phase property whenever the surface source pulse is a unit spike, and whenever the downgoing wave motion is measured relative to its first arrival at the deepest interface. We demonstrate that this behavior is quite general, namely that the minimum‐phase property is associated with the downgoing wave in each subsurface layer, provided that each downgoing wave be measured relative to its first arrival time.
MEASUREMENTS OF THE LONGITUDINAL MODULUS OF PIERRE CLAY SHALE AT VARYING STRAIN RATES
Authors: Bless Stephan J. , Ahrens Thomas J.
Published: October 2002

The propagation speed of finite‐amplitude compressional waves of strain‐rate [Formula: see text] in Pierre clay shale have been measured using shock wave techniques. The inferred longitudinal modulus perpendicular to the bedding is approximately a factor of four higher than that measured by others in uniaxial strain tests at strain rates of [Formula: see text]. The present shock wave speeds measured in the laboratory and in‐situ finite‐amplitude explosive experiments measured by others correlate more closely with field seismic velocities than with laboratory uniaxial strain measurements. Elastic properties of this highly saturated friable material measured parallel to the bedding are less sensitive to sampling disturbances than those measured in the perpendicular direction.
THE EFFECT OF SONDE POSITION IN THE HOLE ON RESPONSES OF RESISTIVITY LOGGING TOOLS
Authors: Gianzero S. C. , Rau R.
Published: October 2002

Theoretical relations exist in the literature for calculating the responses of electrode‐type resistivity logging tools when they are centered in the wellbore and the formations are thick and homogeneous (Fok, 1933; Stefanesco et al., 1929–32). These analyses are usually restricted to devices using dc or low‐frequency surveying currents, and they generally make use of the approximation of point electrodes. An analysis has also been made comparing the responses of such tools in anisotropic and isotropic formations (Kunz and Moran, 1958). In this paper, relations are derived for calculating the responses of electrode‐type logging tools when the sonde is not centered in the wellbore. Although similar relations could be applied to tilted sondes, the discussion in this paper is mainly restricted to the simple eccentered case in which the axis of the sonde is parallel to the axis of the hole. As will be shown, eccentering has a comparatively small effect on the responses of a normal device. On the other hand, certain types of focused‐log devices, in which a large proportion of the focusing current flows parallel to the hole, may exhibit an appreciable eccentering effect; this is particularly true when the contrast between formation resistivity and hole‐fluid resistivity is large. Spherically‐Focused‐Log (SFL*) resistivity devices (Schuster et al., 1971) have this type of current distribution, and, depending on electrode spacings, they may be appreciably influenced by eccentering. The applications shown in this paper will largely be done from SFL computations and measurements. The theoretical relations have been used in the development of a spherically focused tool less affected by eccentering. Tool responses predicted on the basis of the theoretical relations were found to be consistent with the results of test‐tank measurements made for one of the devices studied. Some field experiments have been made with centered and eccentered spherically focused tools, and more are to be done.
CALCULATION OF THE GRAVITY ANOMALY AT TSUMEB MINE
Authors: Fuller B. D.
Published: October 2002

The Tsumeb occurrence is an irregular pipelike body of metallized pseudo‐aplite within a sequence of dolomites. The metal distribution, and hence the density, is quite irregular and mining has taken place, off and on, for about 70 years. From recorded metal distribution data, the densities are computed and a Fourier transform approach used to compute the gravity anomaly of the variable density, three‐dimensional body, as it would have appeared prior to mining. The result is useful in defining a target anomaly for continuing exploration for similar bodies in that area.
PEG‐LEG MULTIPLES AND DIPPING REFLECTORS
Authors: Levin F. K. , Shah P. M.
Published: October 2002

When seismic reflections, whether primaries or multiples, are collected in CDP gathers, their curves are hyperbolas with the smallest value of time corresponding to coincident source and detector (SGD = 0). We have discovered an exception to this rule. If one of the reflecting interfaces dips, some of the peg‐leg multiples have hyperbolic time‐distance curves with heads at SGD’s other than zero. Furthermore, sets of peg‐legs which have the same traveltime when all interfaces are horizontal split into distinct events when their raypaths involve at least one dipping interface. The members of a set have different apparent stacking velocities—some higher and some lower than the stacking velocities for multiples that are not peg‐legs. Raypaths for a peg‐leg generally leave a source and arrive at a detector at angles which are not the same. We derive equations expressing the time‐distance behavior of peg‐legs for systems made up of beds differing in density but not interval velocity. We also consider models with varying interval velocities and show CDP data generated with a computer and stacked subsequently. Peg‐leg reduction by stacking suffers because of the several velocities for each peg‐leg set. It appears that when a subsurface includes dipping reflectors, peg‐legs may be an important source of noise.
MONOFREQUENCY BOREHOLE TRAVELTIME SURVEY
Authors: Ward R. W. , Hewitt M. R.
Published: October 2002

A novel technique was successfully tested to measure compressional‐wave traveltime from a vibrator surface source, using monofrequency pilot signals, to a borehole seismometer. The difference between traveltime measurements by a direct‐phase‐measurement system and those obtained by computer processing of the digitally recorded monofrequency signals was a fraction (0.2 − 0.3) of a millisecond. Absolute monofrequency traveltime measurements obtained by adding integral numbers of signal cycles to phase measurements were not consistent for two different frequencies at the same depth. The suspected cause of this inconsistency is a frequency‐dependent phase shift, due to vibrator‐earth coupling, between the baseplate accelerometer and the borehole seismometer. However, adjustment of the two monofrequency (35 Hz and 55 Hz) traveltime surveys to match the check‐shot traveltime to the 600 ft depth produced excellent agreement between the two monofrequency traveltime surveys and the check‐shot traveltimes. After adjustment, the rms traveltime difference between the 35 Hz survey and the check‐shot survey was 2.5 msec and between the 55 Hz survey and the check‐shot survey was 1.3 msec. Several possible advantages of this technique include the use of a source which is more acceptable in some environments, ability to obtain a digital traveltime reading in the field, and more efficient operation than conventional surveys.
MAGNETIC SURVEYING USING HORIZONTAL GRADIENT VECTORS
Authors: Thyssen‐Bornemisza S.
Published: October 2002

It was pointed out some time ago (Bhattacharyya, 1965) that the total intensity anomaly of a magnetic field ΔT in the direction of the normal magnetic field of earth is expressed by the equation, [Formula: see text]Here ΔV denotes the anomaly of the magnetic potential and t the unit vector in the direction of earth’s undisturbed total field. Horizontal and vertical gradients observed along the tracks of airborne magnetic surveys were discussed by several authors (Wickerham, 1954; Glicken, 1955; Hood, 1965; Langan, 1966). These gradients are obtained from the formulas [Formula: see text] [Formula: see text]where the magnetic intensity differences are observed over horizontal and vertical intervals Δx and Δz between two sensors. However, this approach is only valid when the depth h of the causative body or structure is relatively large compared to Δx and Δz; thus in cases of shallow anomalies, the nonlinear characteristics of the anomalous magnetic field would distort the observed gradients and render interpretation of data very difficult.
A METHOD FOR THE DIRECT INTERPRETATION OF MAGNETIC ANOMALIES CAUSED BY TWO‐DIMENSIONAL VERTICAL FAULTS
Authors: Qureshi I. R. , Nalaye A. M.
Published: October 2002

The magnetic anomaly profile across a two‐dimensional block faulted at right angles to the ground surface looks very similar to the anomaly profile across a dike and can be decomposed into a symmetric part and an antisymmetric part. Following Koulomzine et al (1970), these parts are separately analyzed and expressions for diagnostic parameters derived. The values of these parameters, obtained from the components of a field profile, help in distinguishing the source (dike or faulted block) and in determining the geometry of the block analytically or with the aid of six master curves. Some features of the application and sensitivity of the method are discussed. Field examples are given from the western margin of the Perth basin in Western Australia and from Lachlan foldbelt in New South Wales.
SELECTION OF A SUITABLE MODEL FOR QUANTITATIVE INTERPRETATION OF TOWED‐BIRD AEM MEASUREMENTS
Authors: Palacky G. J.
Published: October 2002

Many steeply dipping massive sulfide ore bodies have a dike‐like shape, and this has led to wide acceptance of the vertical half‐plane model in the interpretation of electromagnetic data. This model assumes that the conductor is thin, but the restriction has not been considered critical and in practice has frequently been disregarded. Conductance and conductor depth estimates based on the results of towed‐bird AEM surveys have been observed to be lower and less accurate than those obtained from helicopter EM and ground EM measurements. In order to explain the low reliability of the towed‐bird estimates, AEM responses over 17 Canadian ore bodies were analyzed. In the study, field results obtained by the time‐domain Input system and two dual‐frequency quadrature systems were interpreted. Error in conductance and depth estimates results from the frequency‐dependent, diffusive behavior of thick geologic conductors. This dependence makes invalid the basic assumption made in the interpretation of dual‐frequency quadrature EM data, that of the equivalence of response parameters at two frequencies. The estimated conductance and depth are too small when applying the current interpretation procedure based on amplitude ratios at two high and widely separated frequencies. The error is smaller in the case of Input time‐domain measurements, because the delay times are relatively long and the channels narrowly spaced. The vertical half‐plane model has been found to hold for ore bodies less than 10 m wide. In the case of wide mineralized zones, which are more important economically, the vertical half‐plane model could be successfully applied only at long delay times. Applying the vertical half‐plane nomogram at short delay times, the conductance and depth were underestimated, and better values could only be achieved by fitting the field data to a horizontal ribbon model. The consistently low conductance values interpreted from towed‐bird measurements for wide conductive zones have probably resulted in not selecting many potential massive sulfide targets for ground followup.
A PRACTICAL APPROACH TO FINITE‐DIFFERENCE RESISTIVITY MODELING
Authors: Mufti Irshad R.
Published: October 2002

Highly efficient finite‐difference resistivity modeling algorithms which yield accurate results are put forward. The given medium is discretized and divided into rectangular blocks by using a very coarse system of vertical and horizontal grid lines, whose distance from the source(s) increases logarithmically. Expressions are derived to compute the longitudinal conductance and transverse resistance associated with each of these blocks for a parallel‐layer medium followed by a generalized treatment to accommodate arbitrarily shaped structures. Since the values of Dar Zarrouk parameters are derived from the exact resistivity distribution of the given medium, fine features such as a thin but anomalously resistive bed which ordinarily would be missed entirely in coarse discretization can be taken into account. Further reduction in the size of the model is achieved by making use of a symmetry wherever possible. In most cases the computation of the potential field which involves the inversion of a small sparse matrix requires about 0.5 sec of computer time. Moreover, changes in geology affect neither the size nor the zero structure of the matrix. Therefore, when more than one model is to be computed, the factorization of the matrix can be done symbolically only once for all models, followed by numeric factorization for each individual model. The coarse grid algorithm was applied to a number of horizontally layered models involving a point source. The results obtained for each model were in excellent agreement with the corresponding analytical data. Finite‐difference investigation of the potential field for two‐dimensional structures and a line source dipole indicates that as long as one is interested only in the evaluation of the Schlumberger‐type apparent resistivity curves, the line‐source results may be a much better approximation to the corresponding point‐source data than is commonly believed.
On the theory of magnetometric resistivity (MMR) methods
Authors: Edwards R. N. , Lee H. , Nabighian M. N.
Published: October 2002

The Magnetometric Resistivity (MMR) method is based on the measurement of the low‐level, low‐frequency magnetic fields associated with noninductive current flow in the ground. A component of the magnetic field is measured in the vicinity of one or more grounded electrodes. Recently, the method was tested successfully in the field. The present paper presents the theoretical basis of the method in a unified format. Part of the material is derived from valuable published papers which are difficult to obtain. The remainder of the paper contains original unpublished theoretical results. It is shown that a horizontally layered earth yields no MMR anomaly. The characteristic anomalies for an anisotropic earth, vertical and dipping contacts, thin and thick dikes, and semicylindrical and hemispherical depressions, as well as alpha media are presented in detail. There are two factors which influence the MMR anomaly; geometry and conductivity contrast. For many models, it is possible to separate these two effects. Type curves are presented for very large conductivity contrasts to illustrate the effect of geometry only. Ancillary curves enable finite conductivity contrasts to be deduced from field data.
On: “A modified pseudosection for resistivity and IP” by L. S. Edwards (GEOPHYSICS, August 1977, p. 1020–1036).
Authors: Roy Amalendu , Apparao A.
Published: October 2002

1) Edwards’ demonstration of the physical validity and practical use of our earlier concepts (references listed by Edwards) is most welcome. For a standard definition of spacing that would apply consistently to all arrays in relation to modified pseudosections or in general, the distance between the two outermost active electrodes seemed to us to be the answer. In this definition, the passive infinity electrodes, where they exist, do not count. This was not a matter of some sly “convenience” on our part but had the backing of what we believe are good reasons.
Choice of operator length for maximum entropy spectral analysis
Authors: Berryman James G.
Published: October 2002

Empirical evidence based on maximum entropy spectra of real seismic data suggests that M = 2N/ln 2N is a reasonable a priori choice of the operator length M for discrete time series of length N. Various examples support this conclusion.
Attenuation and dispersion of compressional waves in fluid‐filled porous rocks with partial gas saturation (White model)—Part II: Results
Authors: Dutta N. C. , Odé H.
Published: October 2002

In this investigation, Biot’s (1962) theory for wave propagation in porous solids is applied to study the velocity and attenuation of compressional seismic waves in partially gas‐saturated porous rocks. The Physical model, proposed by White (1975), is solved rigorously by using Biot’s equations which describe the coupled solid‐fluid motion of a porous medium in a systematic way. The quantitative results presented here are based on the theory described in Dutta and Odé (1979, this issue). We removed several of White’s questioned approximations and examined their effects on the quantitative results. We studied the variation of the attenuation coefficient with frequency, gas saturation, and size of gas inclusions in an otherwise brine‐filled rock. Anomalously large absorption (as large as 8 dB/cycle) at the exploration seismic frequency band is predicted by this model for young, unconsolidated sandstones. For a given size of the gas pockets and their spacing, the attenuation coefficient (in dB/cycle) increases almost linearly with frequency f to a maximum value and then decreases approximately as 1/√f. A sizable velocity dispersion (of the order of 30 percent) is also predicted by this model. A low gas saturation (4–6 percent) is found to yield high absorption and dispersion. An analysis of all of the field variables (stresses and displacements) is presented in terms of Biot’s type I (the classical compressional) wave and type II (the diffusion) wave. It is pointed out that the dissipation of energy in this model is mainly due to the relative fluid flow from the type II wave. From our formulation, many of White’s equations can be derived as suitable approximations, and it is shown that the discontinuity in fluid pressure assumed by White at the gas‐water interface is the discontinuity in the fluid pressure contribution by the type II wave. Our quantitative results are in reasonably good agreement with White’s (1975) approximate theory. However, the phase velocities computed by White’s approximate treatment do not approach the correct zerofrequency limit (Gassmann‐Wood) when compared to the present theory. Most of these disagreements disappear if the corrections to White’s theory as suggested by Dutta and Seriff (1979, this issue) are incorporated.
On White’s model of attenuation in rocks with partial gas saturation
Authors: Dutta N. C. , Seriff A. J.
Published: October 2002

In two important papers, J. E. White and coauthors (White, 1975; White et al, 1976) have given an approximate theory for the calculation of attenuation and dispersion of compressional seismic waves in porous rocks filled mostly with brine but containing gas‐filled regions. Modifications of White’s formulas for [Formula: see text] and Q in the case of gas‐filled spheres brings the results into good agreement with the more exact calculations of Dutta and Odé (1979a, b, this issue), who used Biot’s theory for porous solids. In particular, the modified formulas give the expected Gassmann‐Wood velocity at very low frequencies. Inclusion of the finite gas compressibility in numerical calculations for gas‐filled spheres shows an interesting maximum of the attenuation at low gas saturations which is not seen if the gas is ignored. A comparison of the attenuation calculated for the same rock and fluids but for three different geometries of the gas‐filled regions suggests that the configuration of the gas‐filled zones does not have an important effect on the magnitude of the attenuation.
Crustal electrical conductivities along the eastern flank of the Sierra Nevadas
Authors: Lienert Barry R.
Published: October 2002

A unique controlled‐source experiment utilizing a high‐voltage dc transmission line and an array of three‐component variometers was used in an extensive low‐frequency electromagnetic (EM) survey along the eastern flank of the Sierra Nevada batholith. Complex image theory was used to interpret magnetic field data, obtained at over 90 stations, in terms of one‐dimensional (1-D) electrical conductivity models of the lower and upper crust. Electrical conductivities in the upper crust were found to be very high in the area between Mono Lake and the Coso geothermal area. The zones of high conductivity are narrow linear features which may be associated with conductive fluids within highly permeable shatter zones. These shatter zones are caused by extensive north‐south trending faults defining the eastern margin of the Sierra Nevada batholith. A profile line extending from the Sierra Nevada Mountains into Indian Wells Valley suggests that the electrical conductivity of the crust increases sharply on the eastern side of the Sierra Nevada fault zone. This may be indicative of processes in the deep crust and upper mantle associated with extension of the Basin and Range province westward to this point. South of the Garlock fault zone, the overall crustal conductivity is low. However, high‐conductivity anomalies of shallow origin are seen in many places. These are interpreted as the effects of conductive sediments such as those filling the San Joaquin Valley, combined with complicated current systems flowing within major fault zones.
Determination of depths to an irregular interface in shallow seismic refraction surveys using a pocket calculator
Authors: Cummings David
Published: October 2002

Engineering geologic problems commonly involve the determination of depth to bedrock where it is overlain by residual soil or dry and saturated alluvium. The buried bedrock surface is commonly irregular and the nature of such irregularities must be known, for example, in (1) foundation studies where irregularities on the bedrock surface could indicate faults, or (2) groundwater studies where low areas in the bedrock may indicate channels for groundwater flow which may be targets for drilling. Seismic refraction surveys have proven to be a simple and accurate means of providing adequate data for describing irregularities on the bedrock surface, especially when the data are analyzed by the “plus‐minus” method of the wavefront approach. The method can be programmed readily for a Hewlett‐Packard HP-45 hand calculator, enabling rapid reduction of the data in the field. On‐site data reduction and interpretation result in greater efficiency by allowing immediate recognition of those areas where more detailed information may be needed, thereby precluding repeat surveys and reducing costs. For this purpose, suitable computer programs for the two‐ and three‐layer cases are developed, as well as programs for elevation and weathered zone corrections commonly needed in such shallow seismic surveys.
On: “A quadripole resistivity survey of the Imperial Valley,” by Norman Harthill (GEOPHYSICS, v. 43, December 1978, p. 1485–1500).
Authors: Meidav Tsvi
Published: October 2002

An unfortunately large number of misstatements and misconceptions regarding dipole resistivity and the Imperial Valley characterize this paper, resulting in a simplistic and, in my opinion, erroneous interpretation of the resistivity data gathered by the author, which may have already led to less‐than‐satisfactory results when the ultimate test—deep exploration drilling—was completed. The paper represents a specific school of thought regarding interpretation of dipole data which is not commonly accepted by other dc resistivity practitioners, for technical reasons enumerated below.
Synthetic seismograms using the state‐space approach
Authors: Mendel J. M. , Nahi N. E. , Chan M.
Published: October 2002

We develop time‐domain state‐space models for lossless layered media which are described by the wave equation and boundary conditions. We develop state‐space models for two cases: (1) source and sensor at the surface, and (2) source and sensor in the first layer. Our models are for nonequal one‐way traveltimes; hence, they are more general than most existing models of layered media which are usually for layers of equal one‐way traveltimes. A notable exception to this is the work of Wuenschel (1960); however, most of the useful results even in his paper are developed only for the uniform traveltime case. Our state‐space model treat all of the equations that describe a layered‐media system together in the time domain. Earlier approaches (e.g., Wuenschel, 1960; Robinson, 1968) recursively connect adjacent layers by means of frequency‐domain relationships. We refer to our state equations as “causal functional equations.” They actually represent a new class of equations. Why are we interested in a different class of models for what appears to be a well‐studied system? As is well known, there is a vast literature associated with systems which are described by time‐domain state‐space models. Most recent results in estimation and identification theories, for example, require a state‐space model. These time‐domain techniques have proven very beneficial outside of the geophysics field and we feel should also be beneficial in the geophysics field. In fact, our ultimate objective is to apply those theories to the layered‐media problem; but, to do so, of course, requires state‐space models—hence, this paper.
Velocity inversion procedure for acoustic waves
Authors: Cohen Jack K. , Bleistein Norman
Published: October 2002

An approximate solution is presented to the seismic inverse problem for two‐dimensional (2-D) velocity variations. The solution is given as a multiple integral over the reflection data observed at the upper surface. An acoustic model is used, and the reflections are assumed to be sufficiently weak to allow a “linearization” procedure in the otherwise nonlinear inverse problem. Synthetic examples are presented demonstrating the accuracy of the method with dipping planes at angles up to 45 degrees and with velocity variations up to 20 percent. The method was also tested under automatic gain control, in which case velocity estimates were lost but the method nonetheless successfully migrated the data.