bluuebunny/crossref_metadata_2025 huggingface dataset:
UPWARD CONTINUATION OF TOTAL INTENSITY MAGNETIC FIELDS
Authors: Robinson Edwin S.
Published: October 2002

Upward continuation of gravity and magnetic field data has been used in interpretation for over two decades. The additional perspective gained from viewing a field at different levels has been widely recognized as useful in interpreting subsurface anomaly sources.
ELECTROACOUSTIC CHARACTERISTICS OF MARINE SEISMIC STREAMERS
Authors: Bedenbender J. W. , Johnston R. C. , Neitzel E. B.
Published: October 2002

To specify intelligently an unambiguous tow‐noise level in marine seismic exploration, the electroacoustic characteristics of the streamer must be understood. In this paper, these characteristics are examined as a basis for an industry‐wide standard for specifying streamer tow‐noise level. Sources of tow noise—including electrical, ambient, flow, radiated, and mechanically induced—are examined and the important parameters that control their amplitude spectrum are presented. The theoretical bases for various means of reducing the components of tow noise are analyzed and compared with experimental results. With this background of tow‐noise sources and noise‐reduction schemes, the noise signal is traced from the hydrophone/seawater interface to the recording system. The hydrophone array’s amplitude response and phase response are determined from the transfer characteristics of the coupling circuit and the recording system. Finally, a method for specifying tow‐noise level is proposed. The specified tow‐noise level is referenced to the hydrophone input terminals and is monitored at the output of the recording system. Therefore, the standard requires that the important electroacoustic characteristics outlined in this paper be specified.
RECONSTRUCTION OF SUBGLACIAL RELIEF FROM RADIO ECHO SOUNDING RECORDS
Authors: Harrison C. H.
Published: October 2002

In 1967 a party from the Scott Polar Research Institute, Cambridge, England, carried out a radio echo sounding survey of the Antarctic ice sheet; their data have been used in the analyses discussed. The radio echo sounding apparatus, basically a pulse‐modulated radar operating at 35 Mhz, was installed in a U.S. Navy Constellation aircraft with the aerials attached to the tail, giving a fan‐shaped beam. The beam is broadest along the line of flight, causing problems of interpretation analogous with those encountered in marine echo sounding and seismic surveying. Also, bottom echoes are affected by refraction at the ice surface. Examples of hyperbolic variations of echo delay with horizontal movement may be seen at the edges of ice shelves, crevasses, or cracks at the bottom of the ice. Sometimes more complex surfaces can give roughly hyperbolic echo profiles. It is possible to distinguish specular from nonspecular reflections; we use a method of computing the position of specular reflecting points from the echo profile which allows the shape of the reflecting surface to be calculated. A computer program transforms the digitized film record into a real space profile. Errors may arise from faulty digitization, but these have been largely eliminated by checking the digitized points early in the program. Occasionally, because of their similar range and echo strength, strong side echoes may be confused with sub‐ice features. A cross‐section of Nimrod Glacier and the bottom of the ice sheet near Vostok have been plotted; in both cases there is a striking difference between the computed space profile and the echo profile.
ELECTROMAGNETIC AND ELECTRICAL MODELING BY THE FINITE ELEMENT METHOD
Authors: Coggon J. H.
Published: October 2002

Application of the finite element method to the solution of physical problems is based on minimization of energy; in the present case electromagnetic energy is minimized. Representation of a volume of space by a number of finite elements and description of field or potential distribution by a finite set of unknown values make it possible to replace the energy variational equation by matrix equations. It is shown that a solution for secondary rather than total field quantities can be obtained directly. Such a procedure has several advantages. Approximations are involved in using non‐infinitesimal elements and finite meshes of elements. It is usually necessary to pay more attention to mesh size than texture (element size). Examples of induced polarization anomalies over two‐dimensional models illustrate effects of topography and of a highly conducting layer above bodies of polarizable material. Computed electromagnetic anomalies of two‐dimensional structures, with line source excitation, include the effects of adjacent conductors and magnetic conductors set in a less conductive half‐space.
ELECTROMAGNETIC RESPONSE OF A CONDUCTIVE SPHERE SURROUNDED BY A CONDUCTIVE SHELL
Authors: Fuller B. D.
Published: October 2002

Using the formulation for a layered sphere in a plane wave field and specializing to the quasi‐static range, we have obtained a solution for the electromagnetic response of a conductive sphere surrounded by a shell which is also conductive. The mathematical model is intended to simulate the geological situation of a massive sulfide body enclosed by a halo of disseminated sulfides. The in‐phase and quadrature responses are presented as two‐dimensional functions of the sphere and shell induction numbers and as one‐dimensional functions of frequency for particular cases. It is demonstrated that, depending upon the respective induction numbers, the sphere and shell may respond separately in different ranges of frequency.
A PHOTOELASTIC STUDY OF STRESS WAVE PROPAGATION IN A QUARTER‐PLANE
Authors: Henzi A. N. , Dally J. W.
Published: October 2002

An experimental investigation was conducted on a quarter‐plane loaded with an explosive charge at one point on the boundary. Experimental methods of dynamic photoelasticity and interferometry were used to obtain full‐field data of the dynamic event. Results of the study were obtained for both the P and R‐wave interaction with the corner. Energy reflection coefficients were found for the P‐wave interaction. For the Rayleigh wave, where the response was more significant, transmission and reflection coefficients based on both stress and surface energy were established. Subsurface stress distributions for the transmitted and reflected Rayleigh waves were obtained in the region near the corner.
SEISMIC STUDY ALONG A PORTION OF THE DEVONIAN SALT FRONT IN NORTH DAKOTA
Authors: Langstroth Ward T.
Published: October 2002

A study which covers portions of Bottineau, McHenry, Ward, and Renville Counties, North Dakota is described. Conventional seismic data and deep well control combine to form an ideal model for study of the Devonian Prairie salt and its effects on overlying reservoir beds. The trapping mechanism in one oil field and reasons why several structures are dry can be explained with the results of this review.
AEROMAGNETIC FIELD TEST OF TOTAL INTENSITY UPWARD CONTINUATION
Authors: Kontis Angelo L.
Published: October 2002

The theoretical basis for applying the upward‐continuation integral, [Formula: see text]z⩽0, (1) to total magnetic intensity data T(α, β) measured on the plane z=0 has been recently reviewed by Henderson (1970). To perform upward continuation in the spatial domain, weights or coefficients obtained by numerical evaluation of equation (1) (Peters, 1949; Henderson, 1960; Fuller, 1967) may be convolved with the total intensity anomaly T(α, β) to produce T(x, y, z) at heights z<0 (for z positive downward). The accuracy of upward continuation is, therefore, dependent on the validity of the numerical coefficients and of the assumptions required to show that T(α, β) satisfies the conditions of the Dirichlet problem for a plane. These assumptions are that the quantity sensed by a total‐intensity magnetometer is in the direction of the earth’s normal field and that this direction is invariant over the area of interest.
APPARENT VELOCITY FROM DIPPING INTERFACE REFLECTIONS
Authors: Levin F. K.
Published: October 2002

When a seismic reflector is a dipping plane and the subsurface can be approximated by a single bed, the velocity needed to stack CDP data is higher than the velocity with which the energy travels in the subsurface. The ratio of the stacking velocity to the energy‐travel velocity increases from unity for strike lines to the secant of the dip angle for dip lines. Although the reflection points are not common, reflections stack even for steep dips. Stacking velocities for multiple reflections increase with the order of the multiple, but multiple reflections may also be stacked. To convert from reflection time to depth requires knowledge not given by the stacking velocity. In theory, a split‐spread procedure can furnish the needed information.
DENSITIES OF METAMORPHIC ROCKS
Authors: Smithson Scott B.
Published: October 2002

Although metamorphic rocks comprise a large part of the crystalline crust, relatively few data concerning metamorphic rock densities are available. In this paper, we present rock densities from seven different metamorphic terrains. Mean densities for rock types range from [Formula: see text] for biotite granite gneiss to [Formula: see text] for diopside granofels. Mean rock densities for metamorphic terrains range from 2.70 to [Formula: see text]. Rock density may decrease in the lower part of the upper crust. Most mean rock densities for metamorphic terrains fall between 2.70 and [Formula: see text]; the mean density of [Formula: see text] commonly used for the upper crystalline crust is too low.
MAGNETOTELLURIC MEASUREMENTS IN CENTRAL ALBERTA
Authors: Reddy I. K. , Rankin D.
Published: October 2002

Magnetotelluric measurements were made in the period range 1–1000 sec at sixteen sites in central Alberta. Tensor impedances and tensor apparent resistivities were computed with digital spectral techniques. The results indicate that the magnetotelluric fields in the plains of central Alberta are controlled by the strike of the contacts of undeformed sedimentary rocks of the plains with the crystalline rocks of the underlying Precambrian basement on the one hand, and with the deformed sedimentary rocks of the Rocky Mountains on the other. One‐dimensional models derived from the apparent resistivities for E‐polarization are consistent with the known sedimentary section of the area.
On: “A Predicted Pedestal Effect for Pulse Propagation in Constant-Q Solids” by E. Strick (GEOPHYSICS, June 1970, p. 387–403)
Authors: Thyssen‐Bornemisza Stephen
Published: October 2002

In his very interesting paper, Dr. Strick has shown the pedestal effect to be real and applicable to seismic exploration in general. However, he seems to require the assumption that the attenuation function vary linearly with frequency.
INVESTIGATION OF MAGNETIZATION AND DENSITY OF A NORTH ATLANTIC SEAMOUNT USING POISSON’S THEOREM
Authors: Cordell Lindrith , Taylor Patrick T.
Published: October 2002

The relationship between the gravitational and magnetic potentials caused by a uniform distribution of mass and magnetization may be used to obtain independent information about these physical properties. The general relationship in the frequency domain between the Fourier transforms of the gravity and magnetic anomaly fields is established through the Poisson theorem. The discrete Fourier transforms of the sampled continuous functions are used in an analysis which leads to a system of linear equations involving terms in density, magnetization, and calculated finite Fourier‐series coefficients. A least squares solution of the system yields the three components of the total magnetization vector divided by the density. From these results, the direction of total magnetization and the minimum of the Koenigsberger ratio Q can be determined uniquely. The remanent magnetization direction and certain other information can be derived for special cases in which the value of one or more of the physical property terms can be assigned. Accurate results were obtained in the analysis of data from a theoretical model. Analysis of gravity and magnetic data from the North Atlantic Gilliss seamount indicates the presence of a significant component of remanent magnetization and leads to derived physical properties which are in fairly close agreement with dredged sample data. The calculated direction of remanent magnetization indicates a paleomagnetic pole position in eastern Siberia, in general agreement with the predicted position for a Cretaceous source in the North Atlantic. The seamount example illustrates certain contingent problems to be considered in practical application of the method.
On: “The Direct Approach to Magnetic Interpretation and Its Practical Application,” by L. J. Peters (GEOPHYSICS, July 1949, p. 290–320)
Authors: Choudhury S. K. , Amaravadi R.
Published: October 2002

Even two decades after its publication, Peters’ method of continuing the potential fields remains as one of the main tools for gravity‐magnetic interpretation. Recently, while calculating the downward continued field for the second level, i.e., for h=2, we suspected that there were errors in the coefficients given by Peters in his paper; hence, the coefficients were recalculated.
SEISMIC SIGNATURES OF SEDIMENTATION MODELS
Authors: Harms J. C. , Tackenberg P.
Published: October 2002

Seismic techniques have been used mainly for structural interpretation, but mounting interest in stratigraphic applications is evident. Estimation of sand‐shale ratios from seismically derived average velocities is a recent example of a stratigraphic application. Except in the case of tall pinnacle reefs, today direct location of stratigraphic traps by reflection methods is restricted, at best, to areas of very high quality data and abundant well control. However, it may be possible to interpret some useful stratigraphic characteristics from seismic reflections, the interpretation being based upon the concept of sedimentation models. Most stratigraphic sequences are not random stacks of various lithologies. Commonly, they are well organized and have units with characteristic contacts, thicknesses, lateral extents, lateral facies changes, and vertical sequence. These orderly characteristics are summarized in sedimentation models, where the control of lithologic distribution by dominant depositional processes is emphasized. Three sedimentation models for sandstone and shale sequences are presented. For each, one example is described and converted to a synthetic reflection seismic cross‐section. These cross‐sections are each distinct in terms of reflection polarities, areal changes in reflection amplitudes, continuity of events, and lateral interval velocity changes. The simplified models, although limited in their scope, suggest that additional stratigraphic information can be gleaned from reflection seismic data. To exploit this promise, record processing techniques that emphasize recognition of reflection polarities, amplitudes, continuity, and interval velocities must be developed or improved. It is also necessary to improve our knowledge of seismic boundaries in a variety of stratigraphic sequences. Though difficult, these valuable goals appear attainable.
COMPUTER MAPPING OF SEISMIC AND VELOCITY DATA IN AUSTRALIA
Authors: Shetrone Harry A.
Published: October 2002

The time‐depth curves to the main mapping horizons “C” and “P” in the Cooper basin of Australia are expressed as two distinctly different functions of the form [Formula: see text]. The velocity variations throughout the basin are depicted by a contoured interpretation of the deviations (Delta Z’s of well values from average formula values. The Delta Z interpretations are largely independent of local structure. Consistent isopach and structural maps, computed and contoured by machine using data from the Delta Z (velocity) interpretations and seismic times from 11,450 shotpoints, fit the well information but could not practicably be kept up to date with new velocity and seismic control because of computer program limitations. The work involved in manually updating the velocity adjusted maps in subsequent years was reduced by the tabulated data lists made by the computer. The computer method for reviewing the interpretations was revived in 1970 when exploration activity mushroomed and improved contouring programs became available in Australia.
PROPOSED ATTENUATION‐DISPERSION PAIR FOR SEISMIC WAVES
Authors: White J. E. , Walsh D. J.
Published: October 2002

Several papers in recent years have dealt with the causality‐imposed relation between attenuation and dispersion for waves in lossy solids, with emphasis on seismic waves. While the published formulas for dispersion within a particular frequency band are supported by experimental evidence within that band, the mathematical behavior of these expressions outside the band, particularly at low frequencies, is physically unacceptable. In the present paper, one‐dimensional seismic waves are modeled as propagation along a simple lumped‐element transmission line, leading to expressions for attenuation and velocity as functions of frequency which not only satisfy the experimental data available, but exhibit no objectionable behavior outside the range of available data. This is achieved by introducing a resistive element whose value is inversely proportional to frequency. Numerical application of the Hilbert transform shows the condition of causality to be satisfied by this model quite accurately.
VELOCITY VARIATIONS AROUND LEDUC REEFS, ALBERTA
Authors: Davis Thomas L.
Published: October 2002

Subsurface and surface velocity determination methods in south‐central Alberta have revealed a significant velocity increase in the geologic section adjacent to and above Upper Devonian Leduc reefs. Through the use of well‐velocity survey data, velocity anomalies have been measured within individual stratigraphic units over Leduc reefs including Strachan, Chedderville, Rimbey‐Homeglen, Westerose, Bonnie Glen, and Innisfail. These anomalies indicate an above‐reef average interval velocity increase of 9 percent for the Upper Devonian, 3 percent for the Mississippian, 5 percent within the basal Mesozoic and comprise a total Blairmore‐to‐base‐of‐Ireton interval average change of about 7 percent. Velocity determinations through seismic‐reflection velocity analyses in the Strachan area are consistent with these results, indicating that the use of seismic reflection velocity analysis may prove to be an effective tool for delineating and confirming reef presence in problem areas. Geophysical and geological studies concerning the nature of the velocity variations reveal the existence of density contrasts above a Leduc reef similar in distribution to the observed velocity anomalies. These contrasts appear to be sufficient to account for the gravity anomalies observed above Leduc reefs. Within at least part of the Upper Devonian section above‐reef, these anomalies are believed to be due to primary lithologic facies changes that localized post‐depositional processes including dolomitization. In addition, these post‐depositional processes may have involved the Upper Devonian to Lower Cretaceous interval above Leduc reefs; however, no complete geologic model can be offered at this time.
MAGNETIC UPWARD CONTINUATION FROM AN UNEVEN TRACK
Authors: Parker Robert L. , Klitgord Kim D.
Published: October 2002

A new method for continuing two‐dimensional potential data upward from an uneven track is developed with special emphasis on solving a particular practical problem, that of magnetic data taken near the bottom of the ocean. The method is based on the use of the Schwarz‐Christoffel transformation, which maps the original, irregular track into a horizontal straight line. It has been found to be very fast computationally and to suffer none of the restrictions found in some earlier two‐dimensional algorithms.
RADAR LOGGING OF A SALT DOME
Authors: Holser W. T. , Brown R. J. S. , Roberts F. A. , Fredriksson O. A. , Unterberger R. R.
Published: October 2002

Encouraging results of laboratory and field tests of radio‐frequency transmission in piercement‐dome salt led to the construction of a 230 Mhz pulse‐radar well‐logging system. Reflection data gathered during tests of the system in a well drilled into the Pine Prairie salt dome in Evangeline Parish, Louisiana, were reduced using techniques developed to pick out the best reflections from those recorded. An interpretive cross‐section and a contour map of the near flank of the dome were constructed from the reduced data. The upper flank position of the dome was confirmed by surface‐gravity data; the lower flank position was confirmed by slant drilling which intercepted the salt‐sediment interface at 8112 ft.
A LABORATORY INVESTIGATION OF ELECTRICAL ANISOTROPY IN PRECAMBRIAN ROCKS
Authors: Hill D. G.
Published: October 2002

Laboratory electrical anisotropy measurements were made on selected dry metasediments and metavolcanics from the Precambrian complex of northern Michigan. Directional ac conductivity and dielectric‐constant values were obtained in six or more directions for each sample. These directional values were used to obtain a least‐square determination of the six independent coefficients needed to completely define the symmetric second‐rank, conductivity and dielectric constant tensors. Tensor principal values and directions were obtained from these coefficients. The results of this investigation indicate that metamorphic rocks may be characterized by strongly anisotropic electrical properties. The tensor representation surface symmetries reflect the symmetry of the rock fabric. Electrical anisotropy tends to increase, and the symmetry of the representation surfaces tends to decrease, at lower signal frequencies. The frequency spectra for all samples follow a relaxation‐type model, with critical frequencies occurring between 200 and 600 hz, indicating interfacial polarization as the dominant polarization mechanism.
DETERMINISTIC DECONVOLUTION OPERATORS—3 POINT or 4 POINT?
Authors: Neidell N. S.
Published: October 2002

Peacock and Treitel (1969) in their excellent discussion of predictive deconvolution opened up once again a matter which is worthy of some attention: how do the Backus 2- and 3-point operators (Backus, 1959) relate to other dereverberation operators? This note attempts to clarify such relationships. The most general two‐layer ringing system requires a 4-point operator (Koehler, 1964) for complete dereverberation. More complex layered systems correspondingly would require dereverberation operators with greater numbers of terms; however, the approach presented here again would be of value in putting them into a general context.
AN ELECTRICAL MODEL FOR THE SUB‐ICELANDIC CRUST
Authors: Hermance John F.
Published: October 2002

Active resistivity and magnetotelluric experiments in southwest Iceland suggest typical resistivities for the crust of 10 to 100 ohm‐m. These low crustal resistivities are compared with calculations on the expected resistivity of fluid‐saturated crustal rocks for plausible ranges of temperature, pore‐pressure, and water chemistry. The comparison of the synthetic models with actual field data suggests: (1) The suppression of resistivity at shallow depth is caused by regional hydrothermal activity. (2) Appreciable effects from water are obtained from depths to 8 or 10 km. (3) Below 10 km the effects from conduction along electrolytic paths are probably dominated by conduction in the solid rock itself.
COMPLEX RESISTIVITY SPECTRA OF PORPHYRY COPPER MINERALIZATION
Authors: Van Voorhis G. D. , Nelson P. H. , Drake T. L.
Published: October 2002

Complex resistivity spectra, in the frequency range 0.001 to 10 hz, have been obtained through computer analysis of waveforms tape recorded at porphyry copper deposits. When care is taken to avoid distortions caused by the geometric effects of electrical inhomogeneities, the spectra of typical porphyry copper mineralization are remarkably uniform in character. The geometric effects of veins, which generally complicate the response of hand samples, can be reduced through in‐situ measurements using electrode separations of a few meters. In the frequency range of interest for inducedpolarization (IP) exploration, the observed spectra are accurately described by [Formula: see text] where K is a constant and b is a positive fraction. The fraction b has a value less than 0.1 and is a complete measure of IP. In the frequency range of interest, Laplace transformation gives the step response to be approximated by [Formula: see text] Typical current waveforms used in IP prospecting can be synthesized by superposition of steps. Using these frequency and time response functions, percent frequency effect, phase, and pulse‐transient parameters are compared as measures of IP. By using a volume distribution function, it is shown that a distribution of lossy capacitors will explain the observed response. Physically, this might correspond to the double‐layer capacitance of metallic particles in mineralized rock.
AN AUTOMATIC LEAST‐SQUARES MULTIMODEL METHOD FOR MAGNETIC INTERPRETATION
Authors: McGrath Peter H. , Hood Peter J.
Published: October 2002

The magnetic anomalies caused by such diverse model shapes as the finite strike length thick dike, the vertical prism, thes loping step, the parallelepiped body, etc., may be obtained through an appropriate numerical integration of the expression for the magnetic effect produced by a finite thin plate. Using models generated in this manner, an automatic computer method has been developed at the Geological Survey of Canada for the interpretation of magnetic data. Because the magnetic anomalies produced by the various model shapes are nonlinear in parameters of shape and position, it is necessary to use an iterative procedure to obtain the values for the various model parameters which yield a least‐squares best‐fit anomaly curve to a set of discrete observed data. The interpretation method described in this paper uses the Powell algorithm for this purpose. The procedure can sometimes be made more efficient using a Marquardt modification to the Powell algorithm. Examples of the use of the method are presented for an elongated anomaly in the Moose River basin of the Hudson Bay lowlands in northern Ontario, and for an areally large elliptical anomaly in the Sverdrup basin of the Canadian Arctic Islands.
On: “Proposed Attenuation‐Dispersion Pair for Seismic Waves” by J. E. White and D. J. Walsh (GEOPHYSICS, June 1972, p. 456–461)
Authors: Strick Ellis
Published: October 2002

In their paper, White and Walsh claim dispersion relations based upon an assumed minimum‐phase property of rock lead to such drastic low‐frequency behavior that any complex propagation function based upon dispersion relations is unacceptable. Since my power‐law model (Strick, 1967, 1970a) was placed in such a category, my response is as follows.
GRAVITY ANALYSIS USING AN EXPONENTIAL DENSITY‐DEPTH FUNCTION—SAN JACINTO GRABEN, CALIFORNIA
Authors: Cordell Lindrith
Published: October 2002

In the analysis of gravity data over thick sedimentary basins, density contrast can sometimes be approximated by a continuous function decreasing exponentially with depth. Typical values of the exponential decrement are in the order of 0.3 to [Formula: see text]. The gravity effect of an infinite (Bouguer) slab in this system tends to a finite limit as the slab becomes infinitely thick, leading to quantitative, and sometimes stringent, limitations on the resolving power of gravity analysis over deep structures. The exponential density term acts as an integrating factor in deriving the gravity effect of a prismatic building element, leading to a simple expression involving both the gravity field and its vertical derivative. A recursive computer algorithm giving the inverse solution automatically in terms of assigned density parameters is applied to a gravity profile over a graben structure at San Jacinto, California. The computed basin configuration is consistent with the seismically determined basement depth of 2.4 km and the estimated density layering of the graben fill.
ELECTROMAGNETIC COUPLING BETWEEN GROUNDED WIRES AT THE SURFACE OF A TWO‐LAYER EARTH
Authors: Hohmann Gerald W.
Published: October 2002

A technique has been developed for computing the effects of electromagnetic coupling in induced polarization surveys, when the earth can be approximated by a two‐layer model. Results are given for the dipole‐dipole array, but the numerical solution described can be applied to any electrode configuration. No unusual effects were observed for the models computed. As the thickness of the upper layer is increased, there is a smooth transition between the coupling response of a homogeneous earth having the resistivity of the upper layer and that of a homogeneous earth having a resistivity equal to that of the lower layer.
On: “Radar Logging of a Salt Dome,” by W. T. Holser, R. J. S. Brown, F. A. Roberts, O. A. Fredriksson, and R. R. Unterberger (GEOPHYSICS, October 1972, p. 889–906)
Authors: Steenland Nelson C.
Published: October 2002

It doesn’t matter but, with reference to last paragraph on p. 891, the survey was made with a gravity meter not a torsion balance.
On: “An Automated Method for the Measurement of Cation Exchange Capacity of Rocks” by A. E. Worthington (GEOPHYSICS, Feb. 1973, p. 140–153).
Authors: Guyod Hubert
Published: October 2002

In the paper introduction the author points out that cation exchange capacity (CEC) is an imporant rock property, especially for shales, that is seldom utilized at present in current exploration work.
MEETING OUR CURRENT CHALLENGES
Authors: Allen Samuel J.
Published: October 2002

Each group in every age is faced with seemingly insurmountable challenges which appear to be different from and more formidable than those faced previously by others. History, however, records innumerable achievements in all fields of endeavor which have resulted from meeting such challenges successfully. The history of science describes periods of rapid advancements followed by more stable periods when the advances become accepted as the state‐of‐the‐art. Depending upon the suddenness of the advancements, they are frequently referred to as explosions or revolutions. Many occur in the normal course of development, while others result from meeting unusual challenges.
Correction to letter “On Electromagnetic Induction in Elongated Ore Bodies,” GEOPHYSICS, v. 38, no. 5, p. 984–985
Authors: Wait James R.
Published: October 2002

If galley proofs of this letter had been sent to author, I would have indicated that the effect of the truncation of the infinite cylinder would be to “de‐emphasize” rather than to “wipe out” the monopole or m=0 term. Quantitative results to illustrate this effect for a finite cylinder are included in an article accepted for Geofisica Internacional (Mexico). Also it should be noted that the erratum to the 1952 reference was incorrectly printed in the cited letter.
COMBINED GEOELECTRICAL AND DRILL‐HOLE INVESTIGATIONS FOR DETECTING FRESH‐WATER AQUIFERS IN NORTHWESTERN MISSOURI
Authors: Frohlich Reinhard K.
Published: October 2002

A geoelectrical investigation was conducted in an area of northwestern Missouri which was previously explored by drill holes. The purpose of the survey was to evaluate the effectiveness of the method in locating potable groundwater. It was found that resistivity depth soundings using the Schlumberger arrangement can partially localize and determine the thickness and depth of both near‐surface and basal fresh‐water‐bearing gravel bodies in glacial deposits. The basal gravel is usually confined to preglacial stream channels and has a particular importance as a fresh‐water aquifer. Only in a few cases does the resistivity increase at the depth of the bedrock. More often the depth soundings show an appreciable increase of resistivity at a depth well within the bedrock. High permeability and porosity at a lower water conductivity in the gravel is compensated by lower permeability and porosity at a higher water conductivity in the bedrock. This can explain why the formation resistivities found from depth soundings are basically the same. An increase of resistivity in the deeper bedrock occurs due to a tightening of fissures containing salt water. In one particular well, which penetrates a basal gravel, it can be shown that the salt content of the well water originates from salt water encroachment, most likely from the bedrock.
On: “Time‐Dependent Electromagnetic Fields of an Infinite, Conducting Cylinder Excited by a Long Current‐Carrying Cable” by S. K. Verma (GEOPHYSICS, April 1973, p. 369–379)
Authors: Singh Shri Krishna
Published: October 2002

In this paper Verma obtains a time‐domain solution by inverting the frequency‐domain solution given by Wait (1952). However, it has been recently pointed out by Singh (1973a) (see also Wait, 1973) that there is an error in the quasi‐static solution of Wait. Wait neglected the axially symmetric inducted electric current in the cylinder giving rise to a secondary transverse magnetic field outside (the n=0 term in the scattered wavefield). Singh (1973a) has shown that this term dominates. [It should be noted that Wait in his other works on the cylinder retains this term (e.g., Wait, 1959).] It is clear that this term would be dominant in the time‐domain also. This has been shown by Singh (1972, 1973b). Since the theoretical solution given by Verma in the paper under discussion is incomplete, his interpretation schemes are meaningless.
FILTERING OF WELL‐LOG CURVES
Authors: Branisa F.
Published: October 2002

In well logging the signal‐to‐noise ratio is usually not a great problem. However, modification of the recorded signal can be very beneficial in well‐logging applications by making the different recorded curves more compatible with each other. The design and use of filters for this purpose has been discussed before, e.g., George et al. (1964), Foster et al. (1962), and Lindseth (1966). The techniques described in these articles (and in this paper) apply to digitized well logs. The purpose of this note is to present further examples of the use of filters to equalize various logs.
MULTISPECTRAL ELECTROMAGNETIC EXPLORATION FOR SULFIDES
Authors: Ward S. H. , Pridmore D. F. , Rijo L. , Glenn W. E.
Published: October 2002

A 14 frequency electromagnetic system has been developed and applied to continuous sounding‐profiling over the two sulfide deposits of the Cavendish Test Site in Ontario, Canada. The tilt of the major axis, tilt angle, and the ratio of minor to major axis, ellipticity, of the ellipse of magnetic field polarization are measured over the frequency range 10.5 hz to 86,016 hz. A dual coil receiver employing a phase‐sensitive detector has been used to measure tilt angle and ellipticity with three transmitter configurations along a single traverse line of the Cavendish test site. These different transmitting coil configurations are referred to as horizontal coil, vertical axial coil, and vertical rotating coil. In the horizontal coil configuration, tilt angle and ellipticity are measured in the vertical plane passing through the axis of a horizontal transmitting coil. In the vertical axial coil configuration, tilt angle and ellipticity are measured in the vertical plane which passes through the axis of a vertical transmitting coil; the axis of the transmitting coil is oriented normal to the strike of the subsurface conductivity inhomogeneity. Finally, in the vertical rotating coil configuration, the transmitting coil is located along strike from the traverse over the conductivity inhomogeneity and rotated to contain each point of observation in turn; measurements of tilt angle and ellipticity are made in a vertical plane normal to the plane of the transmitting coil. The limitations of single‐body interpretation schemes are illustrated by the data. Recourse to calculation of scattering by two‐ and three‐dimensional complex environments will be required to provide any sort of full interpretation of these data from one simple Precambrian geologic setting. A comparison of the results for the three transmitting coil configurations indicates that the vertical axial coil configuration is least sensitive to overburden and host rock, while adequately responsive to concentrated and disseminated sulfides.
EFFECT OF WATER SATURATION ON SEISMIC REFLECTIVITY OF SAND RESERVOIRS ENCASED IN SHALE
Authors: Domenico S. N.
Published: October 2002

Recent discoveries of the correspondence between seismic reflectivity and the presence of hydrocarbons in reservoirs have had a profound effect on petroleum exploration. This correspondence appears most pronounced in poorly consolidated sand reservoirs encased in shale. Reflectivity, defined as the absolute value of the reflection coefficient at the shale/sand‐reservoir interface, is determined by the product of the longitudinal velocity and bulk density of the shale and, separately, of the sand reservoir. The formula for velocity taken from Geertsma shows that the square of the velocity in a sand reservoir varies inversely with bulk density and also inversely with fluid compressibility. As water saturation (fractional value of pore space occupied by water) increases in an oil sand, decrease in velocity caused by increasing bulk density is more than compensated by increase in velocity due to decreasing fluid compressibility. On the contrary, in a gas sand, the decrease in velocity due to increasing bulk density with increasing water saturation is not completely compensated by the increase in velocity due to decreasing fluid compressibility. Appropriate values for shale density and velocity, for reservoir porosity, and for reservoir constituent densities and compressibilities are used to determine the reflectivity of a shale/oil‐sand and, separately, a shale/gas‐sand interface as a function of water saturation at depths of 2000, 6000, and 10,000 ft. At each depth, the reflectivity of a shale/oil‐sand interface decreases moderately with increasing water saturation; whereas, the reflectivity of a shale/gas‐sand interface decreases moderately from a completely gas‐saturated reservoir to a water saturation of approximately 0.95, after which the reflectivity decreases abruptly and appreciably to the reflectivity of a completely water‐saturated sand reservoir. Thus, a small quantity of gas (5 percent or less) increases reflection amplitude significantly, and reflection amplitude is not a simple linear measure of the amount of gas in the reservoir.
RESOLUTION COMPARISON OF MINIMUM‐PHASE AND ZERO‐PHASE SIGNALS
Authors: Schoenberger M.
Published: October 2002

Despite their intuitive appeal, minimum‐phase wavelets are not the shortest wavelets achievable on a seismic section. For several amplitude spectra typical of processed seismic sections, both minimum‐phase and zero‐phase wavelets are presented. In each case, several measures of length reveal that the zero‐phase wavelet is shorter than the minimum‐phase wavelet corresponding to the same amplitude spectrum. Furthermore, the zero‐phase wavelet has smaller side lobes than the corresponding minimum‐phase wavelet. Synthetic seismograms were generated using both the zero‐phase and minimum‐phase signals as inputs. In each case, the seismogram generated with the zero‐phase input signal had better resolution. This relation is demonstrated quantitatively and is also visually obvious on the seismograms. In addition to comparing the wavelets’ resolution capabilities, the accuracies permitted in estimating reflection times were compared. The zero‐phase wavelets resulted in more accurate estimates of both reflection times and spacings.
GRAVITY PROFILE INTERPRETATION USING THE FOURIER TRANSFORM
Authors: Collins S. J. , Dodds A. R. , Johnson B. D.
Published: October 2002

A number of attempts have been made to perform direct interpretation of gravity profiles using the Fourier transform of the profile. Of these, the methods of Odegard and Berg (1965) and Sharma et al. (1970) appear to be most applicable. The purpose of this study was to take one of the proposed models (Odegard and Berg’s horizontal cylinder) and determine the applicability of the interpretation method in terms of the number and lateral extent of the data points. The relative accuracies of the estimates of the depth and mass of a cylinder were determined as criteria for estimating the effects of data length and number of data points. In addition, the interpretation was extended to include the separation of two cylinders.
ON THE COMPUTATION OF POLARIZATION ELLIPSE PARAMETERS
Authors: Smith Bruce D. , Ward S. H.
Published: October 2002

The purpose of this note is to review the computation of ellipse parameters for polarized electromagnetic waves. Though most of the comments and data given here pertain to the horizontal magnetic dipole (HMD) source or the vertical magnetic dipole (VMD) source, applications clearly extend to general EM exploration problems. This note was stimulated by constructive criticism offered by N. R. Paterson and F. S. Grant (personal communication) concerning the definitions of ellipticity given earlier by Ward (1967). Further, it brings attention to an error in the article by Dey and Ward (1970).
MAXIMUM ELECTROMAGNETIC DAMPING OF A RELUCTANCE SEISMOMETER
Authors: Sparks Neil R. , Hawley Paul F.
Published: October 2002

An explicit solution of the third order differential equation of motion of an electromagnetically damped reluctance seismometer is obtained for the case of greatest interest, i.e., when the terminating resistor is adjusted to give greatest damping. This solution shows that definite inter‐relationships among the instrument constants are necessary for appreciable damping. The theoretical limitations on the maximum damping which can be obtained are discussed.
MIGRATION VELOCITY DETERMINATION: PART II. APPLICATIONS
Authors: Dohr Gerhard P. , Stiller P. K.
Published: October 2002

The Migration Velocity Determination (MVD) program was run on field data to prove the effectiveness of the program and the accuracy of the results. The examples presented show good interpretability of MVD displays, consistency of MVD velocities along lines, and meaningful agreement of MVD velocities with velocities derived from well surveys. In addition, it is demonstrated that MVD velocities are suitable for migration before stack in order to enhance the quality of the migrated section.
EXPLORING FOR STRATIGRAPHIC TRAPS WITH GRAVITY GRADIENTS
Authors: Hammer Sigmund , Anzoleaga Rodolfo
Published: October 2002

The vast and growing literature on the search for stratigraphic traps for petroleum ignores gravity gradients, for which the theory has been available since the heyday of the Eötvös torsion balance decades ago. These are discussed in this paper. The horizontal and vertical gradients can be measured with available gravimeters and (to a limited extent) with the Eötvös torsion balance. A major advantage of the gradient method is that surveying to determine position and elevation of the station is not required. Both theory and practice have been reported in the geophysical literature, but the important application to stratigraphic traps has not been mentioned. We evaluate here the method for locating “pinchouts”, a term which embraces “stratigraphic” and “unconformity” traps in Halbouty’s (1972) classification. Both position and depth of the assumed pinchout are determined by the gradient anomaly. The magnitudes of anomalies of horizontal and vertical gradients are about equal. However, pending new instrumentation, only the horizontal gradient is practically useful for field surveys. The gradient method is quantitatively promising and, used in conjunction with other methods, should significantly advance the search for stratigraphic traps for petroleum.
On: “Resistivity, self‐potential, and induced‐polarization surveys of a vapor‐dominated geothermal system by A. A. R. Zohdy, L. A. Anderson, and L. J. P. Muffler (GEOPHYSICS, December 1973, p. 1130–1144).
Authors: Roy Amalendu
Published: October 2002

The paper deals chiefly with the quantitative interpretation, based on horizontally layered models, of sixteen vertical, electrical resistivity soundings (except for no. 11). If the section reproduced in Figure 2 is any indication, the requirement in such interpretation of the lateral continuity of layers does not seem to be even approximately satisfied. This is especially true over the central section involving the target area, where thirteen of these sixteen soundings with maximum spacings of the order of AB = 2000 ft are distributed over a linear distance of about 6000 ft.
INVERSION OF GRAVITY PROFILES BY USE OF A BACKUS‐GILBERT APPROACH
Authors: Green William R.
Published: October 2002

Geophysical inversion methods are most effective when applied to linear functionals: it is therefore advantageous to employ linear models for geophysical data. A two‐dimensional linear model consisting of many horizontal prisms has been developed for interpretation of gravity profiles. A Backus‐Gilbert inversion which finds the acceptable model “nearest” to an initial estimate can be rapidly computed; iterative application of the technique allows a single‐density model to be developed at a modest expense in computer time. Gravity data from the Guichon Creek batholith were inverted as a test of the method, with results comparable to those from a standard polygon model.
RESISTIVITY INVERSION WITH RIDGE REGRESSION
Authors: Inman Joseph Robert
Published: October 2002

The problem of direct interpretation of apparent resistivity curves from horizontally layered earth models is solved by using the ridge regression estimator. This ridge regression estimator is more stable than the generalized linear inverse estimator that was advocated in a previous paper. The generalized linear inverse method is unstable if the problem is nearly singular. The problem of estimating the standard deviations of the estimated parameters is analyzed. In some layered models the covariance matrix is an accurate estimate of the standard deviations. However, in problems where there is high correlation between parameters, the covariance matrix is not always an accurate estimate. Confidence regions can be contoured in selected parameter spaces to give an accurate estimate of the range of possible layered models that fit the data. Five soundings were chosen to test the inversion scheme. Two of the soundings were theoretical, and the remaining three were field cases. The method works well since it is possible to find models that fit the data, indicate the accuracy of fit relative to the noise in the data, and predict the accuracy with which each parameter is estimated.
RADAR TRANSPARENCIES OF MINE AND TUNNEL ROCKS
Authors: Cook John C.
Published: October 2002

Laboratory measurements of RF complex permittivity have been made on a variety of “rocks” encountered in mining, tunneling, and engineering works. An RF impedance bridge and a parallel‐plate capacitance test cell were employed at frequencies of 1, 5, 25, and 100 Mhz. The results predict that low‐loss propagation will be possible in certain granites, limestones, coals, and dry concretes. Existing VHF mining radar equipment should be capable of exploring into such rocks to distances of up to hundreds of feet. Useful but shorter probing distances are predicted for other coals, gypsums, oil shales, dry sandstones, high‐grade tar sands, and schists. Radar probing distances of less than 10 ft are predicted for most shales, clays, and fine‐grained soils. Uncombined moisture content is evidently the, governing factor. Efforts were made throughout the experiments to preserve or simulate the original moisture content of the “rocks” in place.