bluuebunny/crossref_metadata_2025 huggingface dataset:
THE DIGITAL PROCESSING OF SEISMIC DATA
Authors: Silverman Daniel
Published: October 2002

The paper discusses the background of the problem of signal and noise in the seismic process, and the application of the principles of communication theory to this problem. The limitations of the seismic process are discussed along with the types of noises involved, the methods of rejecting noise, the use of filters to reduce noise, characteristics of filters, and the relationships between frequency domain, time domain, mathematical, and digital filters. In the discussion of the electronic data processing of seismic information, the characteristics of an ideal seismic digital computer system are developed in relation to the characteristics of seismic data. The choice between digital and analog field recording is discussed in relation to the needs of the seismic process and the quality of the seismic data. Among the mathematical processes discussed are velocity filtering and a number of types of Wiener filtering, including horizontal stacking, deghosting, deconvolution, and multitrace digital filtering.
MEASUREMENT OF TELLURIC “RELATIVE ELLIPSE AREA” BY MEANS OF “VECTOGRAMS”
Authors: Yungul S. H.
Published: October 2002

The subject of this note is a method of acquisition and reduction of data in connection with what is commonly known as the “telluric method,” which primarily deals with the parameter known as “relative ellipse area.” Let us suppose that at an instant t the orthogonal components of the telluric (electric) field at a point B, called Base Station, are [Formula: see text] and [Formula: see text]. At the same instant, the components at another point F, called Field Station, measured in another orthogonal coordinate system, are [Formula: see text] and [Formula: see text]. It is assumed that these components are related to each other as follows: [Formula: see text] (1) where a, b, c, and d are real numbers, called the “correlation coefficients,” which depend only on the directions of the measuring axes and on the electrical properties of the subsurface. It follows that the Jacobian of the transformation from the x‐y system to the u‐v system, [Formula: see text] depends only on the electrical properties of the subsurface; it is called the relative ellipse area at F with respect to B. For information concerning the validity of equation (1), and the geologic meaning of J, the reader may refer to a book by Berdichevskii (1960).
ELECTROMAGNETIC FIELDS OF AN OSCILLATING MAGNETIC DIPOLE OVER AN ANISOTROPIC EARTH
Authors: Sinha Ajit K.
Published: October 2002

The problem of electromagnetic wave propagation from an oscillating magnetic dipole placed over a uniaxially anisotropic earth has been considered. Formal expressions for the vector potentials inside the earth have been derived. It has been shown that for a vertical magnetic dipole, the field components are identical to those in the case of an isotropic medium in which the conductivity is the “horizontal or longitudinal conductivity.” For a horizontal dipole, directed along the x axis, it has been shown that the vector potential inside the earth will have a y component as well as x and z components. Formal expressions for the vector potentials in air have been obtained for the case of a horizontal magnetic dipole. However when the conductivity of air is considered to be negligibly small, the field components are not affected by the anisotropy.
SHORT‐PERIOD SIGNAL PROCESSING RESULTS FOR THE LARGE APERTURE SEISMIC ARRAY
Authors: Capon J. , Greenfield R. J. , Kolker R. J. , Lacoss R. T.
Published: October 2002

Processing of seismic data from LASA can be accomplished by simple delay and sum or beam forming, weighted delay and sum or by maximum‐likelihood filtering. It is found that band‐pass prefiltering the data has a considerable effect on the results. The extra gain produced by maximum likelihood filtering does not justify use of this process on‐line, but could justify off‐line processing in special cases. The spurious precursor introduced by maximum‐likehood processing, and shown to be caused primarily by the signal amplitude scatter within a subarray, can be effectively reduced by using amplitude equalization. The signal amplitude scatter within a subarray, and between subarrays, does not degrade seriously the performance of the signal processing. The effectiveness of the processing depends not only on the array spatial filtering ability, but on differences in absolute level of the noises in the various sensors. Our experience in processing a weak event whose apparent magnitude, as averaged over the LASA was 3.5, indicated that the maximum likelihood filter was capable of extracting the event from the background noise well enough that the event could be identified as an earthquake and not a nuclear explosion.
REMANENT MAGNETISM AND THE ANOMALY AT COTTONER MOUNTAIN, MADISON COUNTY, MISSOURI
Authors: Park Frederick B.
Published: October 2002

A 2000‐gamma anomaly at Cottoner Mountain, Madison County, Missouri is caused by thermoremanently magnetized black Precambrian rhyolite contained in a volcanic neck. The black color of the rhyolite is imparted by tiny needles of magnetite which may approximate single‐domain grains. The direction of remanent polarization in the rhyolite is S30°W, inclined 35 degrees, and the remanent component is approximately 40 times greater than the natural susceptibility magnetization. A total intensity curve calculated on the assumption that the black rhyolite body is an inclined dike of limited depth extent is a good fit to the observed anomaly. Irregularities in the surface magnetic properties were probably caused by lightning and were acquired after the thermoremanent magnetization.
EFFECTS OF MEDIUM VARIATIONS ON THE MEASUREMENT OF IN‐SITU HYDROGEN CONTENT
Authors: Hearst Joseph R.
Published: October 2002

Monte Carlo calculations have been made in an attempt to evaluate the effects of such perturbations as “reactor poisons,” washouts in the borehole or holes in the medium, and large variations in the chemistry and density of the medium on several techniques for the measurement of in‐situ hydrogen content [Formula: see text] with epithermal neutrons. A density ρ range of 1.0 to [Formula: see text] was examined. In a series of parameter studies, the epithermal neutron flux in three detectors ([Formula: see text], [Formula: see text], and [Formula: see text]) was calculated for a disk geometry, and compared to a few more realistic calculations in a cylindrical geometry. Several combinations of calculated fluxes give reasonable fits to analytical expressions. With air in the borehole, the best fits were obtained to [Formula: see text]and[Formula: see text]With water, the best fits were to [Formula: see text]and[Formula: see text]The results were compared to those from Tittman’s sidewall epithermal‐neutron log. No serious disagreement was found.
THE RIDS: A DENSITY LOGGER FOR ROUGH HOLES
Authors: Hearst J. R. , Carlson R. C.
Published: October 2002

The RIDS (acronym for rugosity‐insensitive density system) is a gamma‐gamma system that can operate in very rough boreholes by virtue of special sensors that make contact with the walls of the hole. It has been tested over a density range of 0.6 to [Formula: see text] in the laboratory, and from 1.3 to [Formula: see text] in the field. Error is believed to be less than ±2.5 percent over much of the range. Tight collimation and energy discrimination limit detection to singly Compton‐scattered gamma energy, with the data obeying [Formula: see text], where I is detected intensity, ρ is density, and [Formula: see text] and [Formula: see text] are constants. Output spectra of counts versus energy over the density range appear to confirm this equation. It is shown that substantial errors in density measurements can occur in other logging systems, in particular where the hole is rough enough to cause gaps between the sonde and the wall. For example, such errors can range from 4 to 25 percent in other systems for only a 1‐cm gap between sonde and wall, whereas the RIDS does not permit such a gap. RIDS results agree within a few percent with those of other logging systems where the latter are not perturbed by gaps between sonde and wall, and with core data when available.
SOURCE DISTANCE DEPENDENCE OF THE SURFACE‐IMPEDANCE CONDUCTIVITY MEASUREMENT TECHNIQUE
Authors: Bannister Peter R.
Published: October 2002

The surface impedance conductivity measurement technique, commonly called the magnetotelluric method (Wait, 1962a), has been employed for many years at frequencies below 1 Hz. Recently, frequencies in the extremely low frequency (ELF) and very low frequency (VLF) bands have been employed (Watt et al, 1963).
ON THE EFFECT OF OVERBURDEN ON ELECTROMAGNETIC ANOMALIES—A REVIEW
Authors: Roy Amalendu
Published: October 2002

The effect of a conducting overburden in EM prospecting intuitively is considered to be one of degeneration of anomalies, in the sense that the detection of a target and the determination of its unknown parameters become more difficult or ambiguous when an overburden is present than when it is absent. Recently, however, Negi (1967) and Negi and Raval (1969) have suggested on the basis of theoretical work that, if a certain combination of the parameters involved occurs, a conducting overburden can make a target more detectable than it would be without any overburden. These theoretical results and the existing experimental evidence are examined in this paper for the possible existence of “negative screening,” as this effect has been called. Due to a number of incorrect assumptions made in the theoretical analyses by the authors who predicted negative screening, their conclusions do not seem to be valid. A fundamental objection in the case of the stratified sphere, for instance, pertains to the assumption that, in presence of the annulus, the contribution of the inner sphere alone to the total external measurable magnetic field can be obtained by simply subtracting the response of the larger uniform isolated sphere from that of the double sphere. Another major objection concerns the notion in the theoretical analyses that detectability is determined by the contribution from the target alone—a quantity which one can never measure—without regard to the simultaneous contribution from the conducting overburden. Defined on the basis of the measurable total response from the system as a whole, detectability falls progressively with overburden conductivity. Although the existing results of model EM experiments are generally indicative of the absence of a phenomenon like negative screening, no clearcut and indisputable conclusion can be arrived at on the basis of model studies. In some recently published experimental results, there is one solitary instance, unnoticed by the experimenters themselves, which could suggest the existence of negative screening. We believe, however, that, due to the many inherent uncertainties in model EM work, conclusions based on theoretical investigations have to be accepted as more reliable until carefully planned model work is carried out with this specific problem in view.
On: “Dynamic Photoelastic Studies on P and S Wave Propagation in Prestressed Media,” by Ker C. Thomson, T. J. Ahrens, and M. N. Toksöz (GEOPHYSICS, October 1969, p. 696–712)
Authors: Grover A.
Published: October 2002

The authors mention in the section, “P and S waves in a stress‐free plate,” that they could observe propagating in an unstressed plate the P and S waves which were produced by an explosive source. Furthermore, they have related the observed S wave isoclinics (Figure 7) to the theoretical isoclinics.
RF ELECTRICAL PROPERTIES OF BITUMINOUS COAL SAMPLES
Authors: Cook John C.
Published: October 2002

In order to estimate the transparency of coal in place to radio waves in the VHF range, some preliminary measurements were made on freshly collected samples of two bituminous coals. A capacitance test cell and an RF impedance bridge were used. The dielectric constant was found to be fairly uniform, ranging between about 2 and 4 for all frequencies. The RF losses were equivalent to a resistivity in the range of 500 to 2000 ohm‐m at 100 Mhz. Equivalent resistivities at lower frequencies were greater, roughly inversely proportional to frequency. These coals would carry 100 Mhz waves over distances of the order of 50 m with 60 db of attenuation, and 1 Mhz waves, for 1 to 10 km. However, the clay and pyrite veins occurring in the Pittsburgh coal bed, where they are large and numerous, may well obstruct radio waves (especially of horizontal polarization).
REFLECTION COEFFICIENTS AND BOTTOM LOSSES AT NORMAL INCIDENCE COMPUTED FROM PACIFIC SEDIMENT PROPERTIES
Authors: Hamilton Edwin L.
Published: October 2002

Rayleigh reflection coefficients and bottom losses of compressional waves at normal incidence on the water‐sediment interface are computed with values of density and velocity measured in sea‐floor sediment samples; main sediment types in three major environments of the Pacific and adjacent areas are included. Some typical average computed values of acoustic bottom loss at normal incidence in db are (1) continental shelf: sands, 8; silty sand, 10; sandy silt, 14; silty clay, 16; (2) abyssal plain: clayey silt, 17; silty clay and clay, 21; and (3) abyssal hill: silty clay and clay, 17. Comparisons with actual measurements at sea by several investigators demonstrate the validity of the approach.
ELECTROMAGNETIC SCATTERING BY CONDUCTORS IN THE EARTH NEAR A LINE SOURCE OF CURRENT
Authors: Hohmann Gerald W.
Published: October 2002

A theoretical solution is developed for the electromagnetic response of a two‐dimensional inhomogeneity in a conductive half‐space, in the field of a line source of current. The solution is in the form of an integral equation, which is reduced to a matrix equation, and solved numerically for the electric field in the body. The electric and magnetic fields at the surface of the half‐space are found by integrating the half‐space Green’s functions over the scattering currents. One advantage of this particular numerical technique is that it is necessary to solve for scattering currents only in the conductor and not throughout the half‐space. The response of a thin, vertical conductor is studied in some detail. Because the only interpretational aids available previously were scale model results for conductors in free space, the results presented here should be useful in interpreting data and in designing new EM systems. As expected, anomalies decay rapidly as depth of burial is increased, due to attenuation in the conductive half‐space. Depth of exploration appears to be greatest for measurements of horizontal magnetic field phase, while vertical field phase is diagnostic of conductivity. Horizontal location and depth of burial are best determined through measurements of vertical or horizontal magnetic field amplitude.
AUTOMATIC DETERMINATION OF DEPTH ON AEROMAGNETIC PROFILES
Authors: Naudy Henri
Published: October 2002

In our method of determining depths from aeromagnetic profile data, several sampling intervals adapted to the possible ranges of depths are used for a preliminary analysis of the profiles. This step detects and locates all the anomalies. We then study each anomaly in detail, using several structural hypotheses such as bottomless prisms (dikes) and thin plates. In this way, we produce a series of theoretical structures for each anomaly; only the best‐fitting ones are kept. Our results are shown as a distance‐depth representation, with the symbols plotted so as to indicate the probable degree of realiability. The complete processing is done automatically by a computer and a plotter.
EM RESPONSE OF A RECTANGULAR THIN PLATE
Authors: Lamontagne Yves , West G. F.
Published: October 2002

By formulating the fundamental electromagnetic (EM) equations in terms of the stream potential of the surface current density, we can express the EM response of a rectangular thin plate as the solution of a single equation subject to simple boundary conditions. A finite difference approximation of this equation reduces the problem to that of solving a large set of linear algebraic equations. The solution of these equations by a modified Gauss‐Seidel iterative method yields the stream potential and thus permits visualization of the eddy currents circulating inside the plate conductors. The secondary field calculated from the stream potential compares well with that given by scale model measurements provided that the grid spacings used in the finite differences are small enough. Using a further approximation, we can also simulate inductively thick conductors if the conductors are not geometrically thick.
PULSED NEUTRON METHOD FOR URANIUM WELL LOGGING
Authors: Czubek Jan A.
Published: October 2002

The physical principles of a new method for uranium detection based on use of a pulsed source of neutrons are presented. The method employs measurement of the neutron time distributions in rock media following a burst of fast neutrons. A time‐ and energy‐dependent (but space‐independent) theory is developed for the four principal time distributions that follow production of fission neutrons by each of the two possible reactions: 1. Prompt thermal fission of [Formula: see text]: a. Epithermal neutron time distribution, b. Thermal neutron time distribution. 2. Delayed fission of uranium: a. Neutron time distribution due to delayed fission in [Formula: see text], b. Neutron time distribution due to delayed fast fission in [Formula: see text]. Theoretical analysis of these phenomena leads to the conclusion that the detection of uranium in rocks is possible using either the epithermal neutron time distribution from the prompt fission of [Formula: see text] by thermal neutrons or the delayed neutron distribution from fast neutron fission of [Formula: see text] or thermal neutron fission of [Formula: see text]. Experiments performed on laboratory models of uranium ore show the feasibility of detecting fission neutrons and the validity of the theory presented in the paper. For normal borehole‐logging measurements, a pulsed neutron source with an average output of about [Formula: see text] n/sec is needed.
THE RELATIONSHIP BETWEEN MAXIMUM ENTROPY SPECTRA AND MAXIMUM LIKELIHOOD SPECTRA
Authors: Burg John Parker
Published: October 2002

In a long needed paper, R. T. Lacoss (1971) has presented many examples of spectra obtained by the maximum likelihood method and by the maximum entropy method and has shown that these newer techniques are in general superior to the more conventional spectral analysis methods. This short note shows that there exists a simple, exact relationship between maximum entropy spectra and maximum likelihood spectra when the correlation function is known at uniform intervals of lag. The data are of this form in almost all practical cases of time series analysis as well as in the special case of wavenumber spectral analysis of wave propagation as seen by a linear array of equally spaced sensors. The wavenumber case will be explicitly considered in this note since it requires the complex variable form of the theory.
WELL‐TO‐WELL SEISMIC MEASUREMENTS
Authors: Bois P. , La Porte M. , Lavergne M. , Thomas G.
Published: October 2002

Records have been obtained by well‐to‐well seismic propagation, shooting primacord charges into one well, and locking geophones into other wells several hundred meters away. Seismic arrivals containing a large amount of information have been recorded and analyzed by an iterative ray‐tracing technique. If the conditions are favorable, significant wave speed transition zones and small structures can be detected between wells.
INTERPRETATION OF DIPOLE‐DIPOLE RESISTIVITY SURVEYS USING A HEMISPHEROIDAL MODEL
Authors: Bibby H. M. , Risk G. F.
Published: October 2002

The solutions for the potential due to a current source near a hemispheroidal body of finite resistivity, which have been given by Cook and Van Nostrand, are adapted for use with dipole‐dipole resistivity arrays. In the limiting case where the current dipole is at a large distance from the hemispheroid a simple expression is obtained for the apparent resistivity measured at the center of the hemispheroid. For an arbitrary placement of current electrodes, apparent resistivities are obtained by summing a double series involving the associated Legendre functions. Theoretical results using differently shaped hemispheroids are compared with field data obtained from several dipole‐dipole resistivity surveys of the hot water reservoir of the Broadlands Geothermal Field, New Zealand. The comparison enables an estimate to be made of the depth of the geothermal reservoir.
THE EFFECT OF LATERAL INHOMOGENEITY ON VELOCITY DETERMINATION
Authors: Levin Franklyn K.
Published: October 2002

In recent papers, Taner et al. (1970) and Schneider (1971) pointed out that when the subsurface varies laterally at a rate such that all rays collected for velocity analysis do not penetrate the same earth column, the velocity found from the analysis will be erroneous. The conclusions of Taner were based on ray‐tracing computations; Schneider did not state the basis for his illustrations. If we are willing to assume the Dix (1955) approximation, i.e., that a time‐distance curve for a reflection from a plane is the hyperbola [Formula: see text] we can predict, at least roughly, the magnitude of the errors discussed by Taner and Schneider. In equation (1), t is traveltime for a reflection detected by a geophone a distance X from the source. [Formula: see text] is the traveltime for X = 0 and V is the rms velocity (see Dix).
COMPLEX DIELECTRIC PERMITTIVITY IN ROCKS: A METHOD FOR ITS MEASUREMENT AND ANALYSIS
Authors: Alvarez Román
Published: October 2002

The interfacial effects arising in electrical measurements of rocks, when a two‐electrode system is used, are analyzed via the Maxwell‐Wagner effect. The similarity in electrical behavior between rock samples and heterogeneous dielectrics with nonnegligible ohmic conductivities leads to a general analysis of the Maxwell‐Wagner effect. We conclude that, in general, it is necessary to have volume charge accumulations at interfaces within the sample and at the electrode‐sample interface in order to have electrical steady‐state conditions in rock samples. The expression for the charge accumulation at the interface as a function of frequency is obtained as well as expressions for the effective dielectric permittivity in a composite material; the relaxation time associated with the charge accumulation and that of the effective dielectric permittivity are the same. Given a sample consisting of two different materials, it is shown that whenever both components have nonnegligible ohmic conductivities, the imaginary component of the effective dielectric permittivity tends to infinity when the frequency tends to zero; such behavior is not observed when one of the two conductivities is negligible. This fact constitutes the basis of the method of measurement for heterogeneous dielectrics with nonnegligible ohmic conductivities proposed herein. The method consists essentially of utilizing a dielectric with negligible ohmic and dielectric conductivities as one of the two components of the sample, which isolates the other one from the electrodes. Theoretical examples of the behavior of such a composite material for various cases are analyzed and three experimental examples are presented. The method presents the advantage of permitting application of theory developed for dielectrics in general.
AN ALGORITHM FOR THE BEST FIT SOLUTION OF A SYSTEM OF LINEAR REGRESSION EQUATIONS FOR SEISMIC REFRACTION DATA
Authors: Smith Thomas A. , Sendlein Lyle V. A.
Published: October 2002

A statistical method of best fitting several line segments has been successfully applied to shallow refraction data. Adjacent regression line segments either may be joined at a distance which is between two data points or may be constrained to join at a distance corresponding to one data point which is shared by two regression lines. The calculation of wave slowness, time intercept, and residual sum of squares for each line segment may be found for either join point case. Several tests have been found which eliminate the calculation of many combinations of data in the second join point case. The over‐all residual sum of squares is found by adding the local residual sum of squares for each connected line segment, and the best fit is the combination of line segments which has the best total residual sum of squares. An algorithm has been found which reduces the number of possible combinations of data for three or more line segments to a manageable number by successively searching for one join point on sections of the data, rather than calculating regression fits for all possible combinations of the data.
SYNTHETIC MICROSEISMOGRAMS: LOGGING IN POROUS FORMATIONS
Authors: Rosenbaum J. H.
Published: October 2002

Biot’s theory of wave propagation in a fluid‐filled porous elastic solid takes account of energy dissipation due to relative motion between viscous pore fluid and solid matrix. This theory has been applied to a numerical study of sound pulses propagating along a cylindrical borehole and along a plane interface. It is found that properties such as permeability affect the attenuation of the signal only at high frequencies. For the plane interface, the effect on the P-arrival is small; on the S-arrival it is moderate; and on the Stoneley‐wave it is large, but only if source and detector are close to the interface and the flow of fluid across the interface is relatively unrestricted. With a wide‐band signal, the low‐frequency pseudo‐Rayleigh wave can partially mask the S-arrival. Similar conclusions hold for the logging tool centered in the borehole, and arrivals other than the first P may be difficult to pick, especially for narrow‐band signals. The amplitude of the wave train arriving with approximately the tube‐wave velocity is particularly sensitive to the fluid‐transfer conditions at the borehole wall. The results suggest that acoustic permeability logging tools require high‐frequency signals: their performance could depend critically on the acoustic characteristics of mud cake in situ. Narrow‐band signals are not suitable for the identification of phases other than the first P-arrival; attenuation measurements probably must be based on the energies observed in gated sections of the pressure response. For signals in the seismic range, inelastic effects predicted by Biot’s theory are too small for the detection of formation properties, especially when thin impermeable beds are also present.
TWO‐DIMENSIONAL AND THREE‐DIMENSIONAL MIGRATION OF MODEL‐EXPERIMENT REFLECTION PROFILES
Authors: French William S.
Published: October 2002

A reflection profile represents an unfocused picture of the subsurface. In areas of rapid structural change, this unfocused picture may not reveal directly the true geometry of subsurface structures. Computer processing techniques, collectively called migration, have been used by many companies to focus 2-D reflection data. A description of the migration process can be given which allows immediate generalization to three‐dimensions with arbitrary source and receiver positions. Reflection profiles digitally recorded in the laboratory over known acoustically semitransparent structural models establish the effectiveness of migration. Processed reflection data over 3-D models demonstrate that 3-D migration eliminates many of the lateral correlation ambiguities caused by “sideswipes” and “blind structures.” Structure maps developed from the results of 3-D migration of reflection data give a true and precise picture of 3-D models. When the same data are processed using 2-D migration, the mapped structures are distorted. In structurally complex areas it is desirable to collect 3-D reflection data. Single profiles cannot, and conventional grids may not, reveal adequate cross‐dip information.
ESTIMATION AND CORRECTION OF NEAR‐SURFACE TIME ANOMALIES
Authors: Taner M. Turhan , Koehler F. , Alhilali K. A.
Published: October 2002

The problem of computing static corrections for CDP seismic reflection data is discussed. A new approach is presented and it is related to various existing approaches. The approach consists of using crosscorrelation computations to find time shifts which appear to align the traces of each common‐depth‐point. These shifts are expressed in terms of surface corrections, one for each source and receiver position; a residual NMO correction for each common‐depth‐point; and a fixed correction for each common‐depth‐point. These simultaneous equations form an overdetermined set which can be solved for the unknown static and NMO corrections. The least‐square‐error solution to these equations has an important indeterminancy which is discussed. Methods for its resolution are proposed. Application of the technique to real data is illustrated by several examples. Validity of the corrections is demonstrated by velocity analyses before and after correction of the traces.
A REACTION TYPE SHAKING TABLE
Authors: Kelly Dunford
Published: October 2002

A steady state shaking table utilizing the principle of reaction has been constructed for the measurement of the response of seismometers and the over‐all response of seismograph apparatus. A mass of 500 pounds is driven at amplitudes of the order of [Formula: see text] inches by a small eccentric mass which is rotated at the center of percussion of a bar, the upper end of which is pivoted to the bottom of the heavy mass. The amplitude of motion of the table top is independent of frequency between 10 cycles per second and an undetermined upper limit higher than 150 cycles per second.
DIP REFLECTIONS ON TWO FAULTS IN THE GULF COAST
Authors: Campbell F. F.
Published: October 2002

Two examples of dip reflection work on faulted structures in the Gulf Coast are presented. In one example a large fault is clearly shown by geophysical data alone, and in the other example the assistance of geological data is required to prove the existence of a small fault which the geophysical data alone did not show. The problem of whether misclosure in dip reflection traverses is due to observational errors or faulting is not resolved finally, but suggestions are offered which may lead toward a solution in any particular case.
INTERPRETATION OF INPUT AEM MEASUREMENTS IN AREAS OF CONDUCTIVE OVERBURDEN
Authors: Palacky G. J.
Published: October 2002

During the past six years, the INPUT system has established itself as the world’s most widely used airborne electromagnetic system. From experience with the currently available aids for interpretation it became obvious that more research was needed to understand the effects of conductor size, dip, and depth and also the effect of conductance (σt) of dikes and overburden. An extensive program of time‐domain model measurements with a new Mk VI receiver was initiated to provide the needed answers. In the modeling approach, dike‐like conductors and overburden were simulated by thin aluminum sheets, and the effect of host rock was neglected. The results obtained agree well with previous experiments which were conducted mostly in the frequency domain. Because this project surpassed others in its extent and in the variety of models, new conclusions about the nature of certain anomaly types could be made. In the case of dipping dikes under a horizontal layer of infinite extent, the anomaly shapes were found, to change considerably as a function of the conductances of the bodies involved. For certain combinations of dip, depth, and conductance the peaks on channel one changed into troughs. As the anomaly patterns obtained over horizontal strips and some dipping dikes are similar, a recognition problem may arise in this case, The resulting ambiguity in interpretation is best resolved by measuring with an additional horizontal‐coil receiver, and profiles were obtained with this configuration over a number of models. Detailed quantitative interpretation of data recorded in areas of Western Australia, which are covered by conductive overburden, was made possible by their comparison with the Atlas of INPUT Model Profiles.
ELECTRIC FIELD RESPONSE OF TWO‐DIMENSIONAL INHOMOGENEITIES TO UNIPOLAR AND BIPOLAR ELECTRODE CONFIGURATIONS
Authors: Dey Abhijit , Meyer Wallace H. , Morrison H. Frank , Dolan William M.
Published: October 2002

A quantitative comparative analysis of some bipolar and unipolar resistivity arrays is made in terms of perturbation effects due to buried lateral inhomogeneities. The Schlumberger, the collinear dipole‐dipole, the pole‐dipole, and the unipole electrode configurations of approximately the same dimensions are employed. A “focusing effect” in the vertical component of the current density distribution is demonstrated in the unipole configuration. Characteristic diagrams illustrating the configuration responses to lateral inhomogeneities located at different depths and under various overburden layer conductivities indicate that, for comparable geometric spreads, the unipole and the pole‐dipole configurations are significantly more effective.
EMPIRICAL OBSERVATIONS IN LOCATING PRESENT AND PRIOR STREAMBEDS WITH A MAGNETOMETER
Authors: Stanley John M.
Published: October 2002

The correlation between anomalous high magnetic field intensity values in the proximity of present and prior river flats has been observed across the western plains of New South Wales (Australia). This short note describes the observations, assumes an explanation, and suggests significant applications.
APPARENT RESISTIVITY CURVES FOR AN INFINITE LINE SOURCE PARALLEL TO AN INCLINED CONTACT
Authors: Niwas Sri , Upadhyay S. K.
Published: October 2002

Investigations of apparent resistivity due to a point source over an inclined contact have been reported by Aldredge (1937), Unz (1953), Maeda (1955), and Chastenet de Gery and Kunetz (1956). In these investigations either the image or the harmonic method has been utilized. In this note, we propose to solve the same problem as follows: (1) Transform point‐source potential data into line‐source apparent resistivity data. (2) Interpret transformed apparent resistivities by the master curves provided.
COMPUTER DATA PROCESSING AND QUANTITATIVE INTERPRETATION OF AIRBORNE RESISTIVITY SURVEYS
Authors: Palacky G. J. , Jagodits F. L.
Published: October 2002

The recently constructed airborne electromagnetic system called E-Phase measures the intensity of the vertical and horizontal electric fields. Standard broadcasting, VLF, and LF navigation aid transmitters are used as sources of the primary EM field. A system of this kind responds best to horizontal layers of large extent and therefore is suitable for geologic mapping and for the detection of resistive materials such as gravel and permafrost. A successful application of the system would not have been possible without digital recording of the data and subsequent computer processing. An efficient algorithm consisting of three processing steps assures low processing cost and provides for two intermediate data checks. Final outputs are printer plots of apparent resistivity for all flight lines and maps of stacked profiles or contours. Quantitative interpretation was made possible by the simultaneous recording of the data at three transmitter frequencies and by the availability of theoretical solutions for layered media. Instead of generating an atlas of type curves, an interactive program was written which enables the geophysicist to rapidly obtain apparent resistivities assuming a three‐layer model. A close match with the measured data is easy to achieve when a reasonable estimate of two of the parameters (resistivities, thicknesses) can be made initially. The interpretation procedure is demonstrated on a case history, a 1973 survey conducted near Wadena, Saskatchewan.
THE DUAL LOOP CONFIGURATION OF THE TRANSIENT ELECTROMAGNETIC METHOD
Authors: Spies Brian R.
Published: October 2002

The transient electromagnetic method offers considerable advantages over conventional electromagnetic systems; these advantages make the method attractive under typical Australian conditions of conductive overburden. A common transient electromagnetic ground system used in Australia is a combined loop version in which the same loop is used for transmitting and receiving. This system employs a horizontal loop which is strongly coupled to horizontal conductors. By means of scale model studies and a field example it is shown that an alternative loop configuration is a system in which two adjacent loops are connected in parallel, the “dual loop” configuration. It provides improved detectability of a vertical or steeply dipping conductor. In addition, the dual loop configuration reduces electrical interferences. The main disadvantage of this configuration is the extra time required to lay out a dual loop.
INTERVAL VELOCITIES FROM SURFACE MEASUREMENTS IN THE THREE‐DIMENSIONAL PLANE LAYER CASE
Authors: Hubral Peter
Published: October 2002

The basic requirements to recover plane layers of constant interval velocity, arbitrary dip and strike from common depth point (CDP) recordings are the following four quantities related to the primary event of each reflector at the common midpoint of a CDP profile: a) Two‐way normal time b) Normal moveout velocity within one arbitrary CDP profile c) Time slope of normally reflected rays within the profile d) Time slope of normally reflected rays in some other direction. The solution of the inverse problem is obtained directly. The moveout velocity is expressed in terms of seismic parameters along the normal incidence path in three dimensions and the direction of the profile within the free surface. A formula connecting dip and strike of the emerging normal ray with the measured time gradients is given and discussed. The method includes, as a special case, the Dix formulas for plane parallel layers.
On: “Special Report of the Subcommittee on Polarity Standards” by Ben B. Thigpen, A. E. Dalby, and Ralph Landrum (GEOPHYSICS, August 1975, p. 694–699)
Authors: Valenta W. T.
Published: October 2002

In their report, the authors recommend that the arrival of a compressional wave at a geophone or hydrophone shall ultimately produce a trough (downward or leftward kick) on a seismogram; but they don’t say why. On sonic logs and displays of velocity versus time, velocity increases are up; likewise for density increases on density logs; likewise for positive reflection coefficients on reflection coefficient logs computed from the previously mentioned logs. Should not seismograms, which ideally are bandlimited reflection coefficient logs, conform to the same convention? The committee succeeded in the objectives of establishing the relationship between impulse‐source systems and vibratory‐source systems and of establishing tests for the determination of the polarity of a given system. I feel strongly that the committee should recommend that a compressional arrival shall produce a peak on a seismogram.
On: “Gamma‐Ray Spectometry in Central Morocco,” by A. Demnati and H. Naudy (GEOPHYSICS, April 1975, p. 331–343)
Authors: Sampaio Edson S.
Published: October 2002

In their paper, Demnati and Naudy give the following formula for altitude correction of the total radiometric count:[Formula: see text][Formula: see text]where Y′ is the corrected value, Y is the total count corrected for the background, h is the recorded altitude (in meters).
THE DETECTION AND DEVELOPMENT OF SILURIAN REEFS IN NORTHERN MICHIGAN
Authors: Caughlin W. G. , Lucia F. J. , McIver N. L.
Published: October 2002

The reflection seismic method has proven to be an excellent technique for locating and mapping Silurian reef buildups in northern Michigan. Current industry estimates of total discovery volumes are 400 to 600 million barrels of oil and 3 to 5 trillion cu ft of gas. Production is from carbonate buildups (reef pinnacles) whose areal extents range from 40 to 600 acres at depths of 3000 to 7000 ft. Extending northeast‐southwest across the northern part of Michigan, the reef trend is 10 to 20 miles wide. Reef heights increase basinward from the shelf margin, growing to a maximum of 600+ ft. Reef porosity throughout the trend is variable, ranging from 5 to 30 percent, depending upon a combination of diagenetic processes. Reefs grew on a ramp from the Niagaran shelf edge to a flat basinal plain. A series of salt and carbonate beds were deposited around and over these carbonate buildups. Anomalies are recognizable from seismic data because the inter‐reef section produces a strong characteristic seismic event which becomes weak and disrupted in the presence of a buildup. The Niagaran shelf margin can be mapped because it produces a seismic response similar to that of a pinnacle reef. In some areas, seismic interpretation is difficult due to high noise levels and distortion of seismic data by the irregular distribution of Pleistocene glacial deposits. Data quality has been improved through a combination of field techniques, common‐depth‐point stacking, static correction refinement, and filtering.
FLUID SATURATION EFFECTS ON DYNAMIC ELASTIC PROPERTIES OF SEDIMENTARY ROCKS
Authors: Gregory A. R.
Published: October 2002

The influence of saturation by water, oil, gas, and mixtures of these fluids on the densities, velocities, reflection coefficients, and elastic moduli of consolidated sedimentary rocks was determined in the laboratory by ultrasonic wave propagation methods. Twenty rock samples varying in age from Pliocene to early Devonian and in porosity from 4 to 41 percent were tested under uniform pressures equivalent to subsurface depths of 0 to 18,690 ft. Fluid saturation effects on compressional‐wave velocity are much larger in low‐porosity than in high‐porosity rocks; this correlation is strengthened by elevated pressures but is absent at atmospheric pressure. At a frequency of 1 MHz, the shear‐wave velocities do not always decrease when liquid pore saturants are added to rocks as theorized by Biot; agreement with theory is dependent upon pressure, porosity, fluid‐mineral chemical interactions, and the presence of microcracks in the cementing material. Experimental results support the belief that lower compressional‐wave velocities and higher reflection coefficients are obtained in sedimentary rocks that contain gas. Replacing pore liquids with gas markedly reduces the elastic moduli of rocks, and the effect is enhanced by decreasing pressure. As a rule, the moduli decrease as the porosity increases; Poisson’s ratio is an exception to the rule. Liquid and gas saturation in consolidated rocks can also be distinguished by the ratio of compressional and shear wave velocities [Formula: see text]. The potential diagnostic value of elastic moduli in seismic exploration may stimulate interest in the use of shear‐wave reflection methods in the field.
THEORETICAL MODELING OF THE MAGNETIC AND GRAVITATIONAL FIELDS OF AN ARBITRARILY SHAPED THREE‐DIMENSIONAL BODY
Authors: Barnett C. T.
Published: October 2002

An analytical method is developed for modeling the magnetic field or the gravitational attraction of a homogenous, arbitrarily shaped, three‐dimensional body. In this method, the body to be modeled is represented by a polyhedron composed of triangular facets. A convenient method for inputting the apices and assembling the facets of a polyhedron is developed. As the solution is analytic, the polyhedron facets can be any size, and the number of input points depends only upon the number of apices required to outline the body.
GRAVITY GRADIENTS AND THE INTERPRETATION OF THE TRUNCATED PLATE
Authors: Stanley John M. , Green Ronald
Published: October 2002

The truncated plate and geologic contact are commercially important structures which can be located by the gravity method. The interpretation can be improved if both the horizontal and vertical gradients are known. Vertical gradients are difficult to measure precisely, but with modern gravimeters the horizontal gradient can be measured conveniently and accurately. This paper shows how the vertical gradient can be obtained from the horizontal gradient by the use of a Hilbert transform. A procedure is then presented which easily enables the position, dip angle, depth, thickness, and density contrast of a postulated plate to be precisely and unambiguously derived from a plot of the horizontal gradient against the vertical gradient at each point measured. The procedure is demonstrated using theoretical data.
On: “Recent Advances and Applications in Complex Resistivity Measurements,” by K. L. Zonge and J. C. Wynn (GEOPHYSICS, Oct. 1975, p. 851–864)
Authors: Nelson P. H. , Van Voorhis G. D.
Published: October 2002

In presenting a variety of induced polarization spectral data, Zonge and Wynn refer to a paper published earlier by us (Van Voorhis et al., 1973) which deals with the same topic. We feel Zonge and Wynn have misrepresented our measuring techniques, data, and conclusions in their references to our paper. Our principal objections center on three statements by the authors.
A SIMPLE UPGRADE FOR SOME BRIGHT SPOTS
Authors: Ware Jerry A.
Published: October 2002

Confirmation that a bright spot zone in question is low velocity can sometimes be made by looking at constant velocity stacks or the common‐depth‐point gathers. When this confirmation does exist, then it is usually possible to do simple ray theory to get a reasonable estimate of the pay thickness, especially if the water‐sand velocity and the gas‐sand velocity are either known or can be predicted for the area. The confirmation referred to can take the form of under‐removal of the primary events or be exhibited by multiple reflections from the bright spot zone. Such under‐removals or multiple reflections will not be seen on the stacked sections but are sometimes obvious on the raw data, such as the common‐depth‐point gathers, or can be implied by looking at constant velocity stacks of the zone in question at different stacking velocities.
A PROPAGATING ALGORITHM FOR DETERMINING NTH-ORDER POLYNOMIAL, LEAST‐SQUARES FITS
Authors: Gangi Anthony F. , Shapiro James N.
Published: October 2002

An algorithm is described which iteratively solves for the coefficients of successively higher‐order, least‐squares polynomial fits in terms of the results for the previous, lower‐order polynomial fit. The technique takes advantage of the special properties of the least‐squares or Hankel matrix, for which [Formula: see text]. Only the first and last column vectors of the inverse matrix are needed at each stage to continue the iteration to the next higher stage. An analogous procedure may be used to determine the inverse of such least‐squares type matrices. The inverse of each square submatrix is determined from the inverse of the previous, lower‐order submatrix. The results using this algorithm are compared with the method of fitting orthogonal polynomials to data points. While the latter method gives higher accuracy when high‐order polynomials are fitted to the data, it requires many more computations. The increased accuracy of the orthogonal‐polynomial fit is valuable when high precision of fitting is required; however, for experimental data with inherent inaccuracies, the added computations outweigh the possible benefit derived from the more accurate fitting. A Fortran listing of the algorithm is given.
ELASTIC PROPERTIES OF UNCONSOLIDATED POROUS SAND RESERVOIRS
Authors: Domenico S. N.
Published: October 2002

The effect of (1) compressive stress and (2) pore fluid properties on elastic properties of unconsolidated sand reservoirs was determined by laboratory velocity and pore volume measurements on two specimens. The latter consisted of a naturally occurring very fine‐grained sand and glass beads, each with a porosity of approximately 38 percent. Constituent compressibilities and densities of the two reservoir specimens are similar; thus, differences in measured elastic properties likely are attributable to differences in grain shape and pore size.