bluuebunny/crossref_metadata_2025 huggingface dataset:
SHALLOW STRATIGRAPHIC VARIATIONS OVER GULF COAST STRUCTURES
Authors: Rosaire E. E.
Published: October 2002

Depth predictions for geophysical data obtained at the surface of the earth are usually made with the tacit assumption that lateral variations in the intervening strata are negligible. However, the evidence at hand indicates that, over Gulf Coast structures, local mineralization has resulted in appreciable stratigraphic variations which are not inverse functions of the depth nor direct functions of the relief of the underlying structures, but do appear to be excellent indices of the areal extent of deep seated, low relief, traps. More than a generation ago, evidence for this mineralization over and around Gulf Coast oil fields was recognized in the induration responsible for lowered drilling rates, and in the characteristic “halo” or “aureole” pattern of surface gas seeps. This mineralization results in increased acoustic speeds in the sediments overlying Gulf Coast traps, and so was responsible for the marginal refraction anomalies discovered from 1928–1930, and has been indicated indirectly by the exaggerated relief frequently predicted by the reflection surveys made in this area to date. This mineralization also modifies the electrical properties of the shallow sediments, and has been responsible for the shallow anomalies discovered by the recently introduced electrical transient method. Recent studies of the distribution of hydrocarbons and associated significant constituents in the surface soils have rediscovered and confirmed the “halo” pattern of surface gas seeps responsible for the discovery of the Goose Creek oil field in 1907. The recognition that these stratigraphic variations occur at shallow depths even over low relief deep seated structures should result in a material simplification in exploration procedure through affording more easily attainable objectives in the way of shallow but important clues to deep seated traps.
PRESENT STATUS AND FUTURE ASPECTS OF GEOPHYSICAL EXPLORATION IN POLAND
Authors: Mitera Z. A.
Published: October 2002

Geophysical methods of prospecting were introduced in Poland in 1923, or at about the same time as in the Gulf Coast. In the first stage local seismic refraction and electrical surveys were employed for private mining and oil companies. The Mintrop’s refraction method has shown success in locating extension of salt deposits near Wieliczka and in Wapno. The Sundberg electromagnetic method was not entirely successful in mapping some lead and copper ore deposits. The same refers to the use of electromagnetic methods for mapping of oil structures. Steep anticlines and complicated structures in which oil mostly occurs within the area of the Carpathian Range were not favorable to be worked out with these methods which were not entirely developed and proved at that time.
AN INTRODUCTION TO THE SECOND DERIVATIVE CONTOUR METHOD OF INTERPRETING TORSION BALANCE DATA
Authors: Klaus H.
Published: October 2002

After auspicious beginnings in the interpretation of torsion balance data, i.e., gradients and curvatures, the balance has been misused as a gravity instrument, the gradients being integrated into gravity, and the curvatures either neglected or not even observed in the field. Gravity was then made the sole basis of interpretation work, the regional effects being determined with more or less luck, subtracted from the total either before or after integration (regional gravity or regional gradient), and the residue held to be “local effect.” This method appears to be now in vogue for most torsion balance and gravity meter work. In contrast to this procedure, the method here described is based on the quantities measured directly by the torsion balance, the gradients and curvatures, or second derivatives, and constitutes a considerable amplification of the original methods of investigating these quantities. Gravity is simply a by‐product of this method, and is not needed at all for its functioning. The essential parts of this method are: 1) the re‐determination of all second derivative components with respect to a new system of rectangular coordinates, one axis of which has been made parallel to the direction of elongation of anomalous features; 2) the contouring of these second derivative components on four separate maps; and 3), the interpretation of the resulting contour patterns. The outstanding advantages of this method over the total gravity methods are the following: 1) full utilization of the two independent aspects of the gravitational field furnished by the gradients and curvatures; 2) virtual independence from regional effects; 3) much greater resolving power when compared to gravity; and 4), complete absence of assumptions, such as are involved in estimating the regional, and in computing gravity from the gradients.
THREE DIMENSIONAL REFLECTION CONTROL
Authors: Rock S. M.
Published: October 2002

A pattern is presented in which ΔT’s are obtained from intersecting lines of detectors. Assuming a) plane wave fronts at the detectors, and b) rectilinear wave propagation, formulas are presented for: (1) ψ, the angle of arrival of the reflected wave in the wave‐travel plane: i.e., the plane through the line of exploration and perpendicular to the reflecting plane; (2) θ the angle between the wave‐travel plane and a vertical plane through the line of exploration; (3) α, the dip component in the wave‐travel plane; (4) δ, the total dip; and (5) γ, the angle between direction of total dip and the line of exploration. Application of the pattern to field work is described.
NOMOGRAM FOR DIP COMPUTATIONS
Authors: Lawlor Reed
Published: October 2002

A universal nomogram to be used in seismographing with three‐dimensional control is presented. This chart can be used for computing the dip components in and perpendicular to the wave‐travel plane. By a simple change of scales it can be adapted to all shot distances, geophone spreads, and all wave velocities which are functions of wave travel time only. Principles underlying the construction of the chart are given.
A DELTA‐MODULATION RECORDING SYSTEM FOR SEISMIC USE
Authors: Thompson Noel J.
Published: October 2002

A system of delta modulation with magnetic‐tape recording is proposed. It should achieve excellent signal‐to‐noise ratio without excessive equipment complexity. The system would be of use in crustal seismology where large signal‐to‐noise ratio is more important than minimum channel bandwidth. The recording of data in binary form as achieved by delta modulation eliminates tape transport wow and flutter and certain other noise components found in analog recording. Compared to FM recording, delta modulation provides similar complexity of recording and nearly identical “quick‐look” playback equipment. Delta modulation has the advantage, in common with other digital systems, that, with more complete playback equipment (either analog or computer), excellent removal of transport speed variations is possible. Delta modulation is less complex than other digital‐recording systems, and combines many of the best features of both the digital and analog techniques.
SOUND SPEED AND RELATED PHYSICAL PROPERTIES OF SEDIMENTS FROM EXPERIMENTAL MOHOLE (GUADALUPE SITE)
Authors: Hamilton Edwin L.
Published: October 2002

Sediment samples from the Guadalupe Site Mohole drilling program of 1961 have been tested for sound speed and related physical properties. Laboratory measurements of sound speed have been corrected to in‐situ values by making corrections for the temperature gradient within the sediment body, for hydrostatic water pressures within sediment pore spaces, and for a small reduction in porosity. Resulting in‐situ sound speeds vary from 1,484 m/sec at the sediment surface to 1,584 m/sec at a depth in the sediment of 136 m. The resulting sound‐velocity gradient is approximately [Formula: see text]; the interval velocity is about 1,565 m/sec. Sound speed in the surficial sediments is about 2.2 percent less than in the water just above the sea floor.
REFRACTION ALONG A LAYER
Authors: Spencer T. W.
Published: October 2002

High‐frequency geometric ray theory is used to investigate the refracted arrival from a high‐speed layer embedded in an infinite medium. The effect of changing the layer thickness to dominant wavelength ratio [Formula: see text] and the range to depth ratio (ρ/H) is analyzed for a point compressional source. The results approximate the exact solution when [Formula: see text]. The theory predicts shingling and shows that it is range‐limited. Factors which improve the resolution between reflected arrivals increase the range over which shingling occurs. As the range increases, the traveltime curves for all the multiply reflected rays which cross the layer the same number of times in the shear mode approach the same asymptote (regardless of the number of crossings in the compressional mode). When the layer is thick compared to the dominant wavelength, the refracted arrival may consist of a series of events separated by equal time intervals. Each event is produced by the superposition of reflected waves which cross the layer the same number of times in the shear mode. The amplitude of each event satisfies [Formula: see text], where H is the layer depth. Because the head waves decay like [Formula: see text], the reflected waves predominate at large ranges.
A MAGNETIC SURVEY OFF THE COAST OF PORTUGAL
Authors: Allan T. D.
Published: October 2002

A large anomaly in the total‐magnetic intensity was recorded by a towed proton‐precession magnetometer at the mouth of the River Tagus, Portugal. By comparing the observed anomaly with that of a dipole field, values for the intensity of magnetisation and the depth to the center of the disturbing body have been calculated. The ship’s magnetic compass suffered deflections with a range of 8 degrees in the region of the main anomaly. Calculations of the deflections produced by the theoretical dipole were also in this range.
THE PARTICLE STATISTICS OF RUBBLE PRODUCED BY UNDERGROUND NUCLEAR EXPLOSIONS
Authors: Rodean Howard C.
Published: October 2002

Underground nuclear explosions produce cavities into which the overburden may collapse, forming a rubble‐filled chimney. The particle statistics, especially mean surface‐volume diameter, are important to various potential applications of nuclear explosives. The available rubble statistics are rather limited, even with the best data obtained from post‐shot exploration of the Hardhat event. Two sets of Hardhat data are presented: one based on photographs of relatively undisturbed rubble, and another obtained from visual estimates of what would be obtained by screening. The method of obtaining particle statistics must be consistent with the intended use of the statistics, because the method of measurement, such as handling for sieving, affects the statistics. It has long been established that crushing and grinding (which occur during chimney collapse) force particle statistics toward the lognormal frequency distribution. Both sets of Hardhat data fit the lognormal distribution in a satisfactory manner. The volume‐surface mean particle diameter for the undisturbed Hardhat rubble is 1.33 ft, and the estimate for the volume‐surface mean particle diameter obtained by screening is 0.433 ft. Additional statistics from the field plus a better understanding of rock‐fracture phenomena are required to develop statistical relations for use in prediction and control of underground nuclear explosion effects.
QUANTITATIVE INTERPRETATION OF THE “MONTAGNE DU SORCIER” MAGNETIC ANOMALY, CHIBOUGAMAU, QUEBEC
Authors: Gaucher Edwin H. S.
Published: October 2002

A sill of serpentinized dunite, called magnetite serpentinite, outcrops on the south flank of the “Montagne du Sorcier” in Chibougamau, Province of Quebec. This sill is associated with an oval‐shaped mass of anorthosite in the Precambrian rocks of that locality. The sill contains about 20 percent of magnetite by volume (24 percent Fe) with one percent of [Formula: see text]. It extends for two miles in an east‐west direction, dips 80° N, and is about 500 ft thick. The magnetic anomaly associated with this sill is unusual as it shows a magnetic low south of the magnetic high. Measurements of the susceptibility and of the remanent magnetization of 73 oriented cores (281 specimens) indicate that the remanent magnetization (0.034 emu) is in the same direction as the present earth’s magnetic field and therefore in the same direction as the induced magnetization (0.053 emu), and thus suggest that the shape of the anomaly could be explained by the topography of the mountain. Additional evidence such as the direction of magnetization of an adjoining talc rock layer suggests that the alignment of the remanent magnetization and of the present earth’s field is due to a viscous realignment of the remanent magnetization. In the rocks studied, the relation between the susceptibility K and the proportion of the magnetite by volume X can be expressed as follows: [Formula: see text]. The sill presents a certain economic interest as a possible iron ore deposit.
SOME RESULTS OF AEROMAGNETIC SURVEYING WITH A DIGITAL CESIUM‐VAPOR MAGNETOMETER
Authors: Giret Raoul I.
Published: October 2002

This magnetometer has been developed by C.S.F. (Compagnie Générale de Télégraphie Sans Fil) and C.G.G. (Compagnie Générale de Géophysique). The magnetic field is measured by an optical pumping sensor giving a frequency proportional to the field. A digital frequency‐meter delivers a reading each second with a sensitivity of ± one period (31.5 periods for one gamma). These readings are recorded on a magnetic tape recorder and on an analog TEXAS recorder. The digital data are processed on a 1620 IBM. During the flight a second magnetometer records the variation of the earth’s field in a ground station located in the center of the area surveyed. The ground record data are subtracted from the airborne data. The following data obtained with this equipment are presented: Analog record with a sensitivity of one gamma for two mm. Restitution with 1620 IBM and CALCOMP of the magnetic record to an analog record with a sensitivity of one gamma for one cm. Isogam maps with a contour interval of one gamma and some intermediate contours at 0.5 gamma.
VELOCITY AND FREQUENCY FILTERING OF SEISMIC DATA USING LASER LIGHT
Authors: Dobrin Milton B. , Ingalls Arthur L. , Long James A.
Published: October 2002

When coherent light from a laser beam is passed through a transparent reduction of a variable‐density or variable‐area record section, the seismic signals act as an optical grating to produce a diffraction pattern which is the two‐dimensional Fourier transform of the section itself. With suitable lenses the diffraction pattern can be converted back into an image of the original section. By obstructing portions of the pattern corresponding to particular frequencies or dips on the section one can remove such frequencies or dips from the reconstructed image. The equipment developed for this processing incorporates special design features to combine high optical resolution, precise discrimination of moveouts and frequencies, limitation in the length of the overall optical path to permit the use of a short optical bench, and visual monitoring by use of a microscope or a closed‐circuit TV system. Filter elements consist of wedges mounted on a rotary stand for velocity rejection, wires of various diameters for band stop frequency rejection, and plates bounded by knife edges for low‐pass filtering. The technique is applicable to most problems encountered in seismic prospecting where spurious events obscure desired reflections. The most frequent application so far has been the removal of multiple reflections. The method has turned out to be highly useful for eliminating noise, regardless of origin, which interferes with reflections whenever the noise consists of traveling events, even though fragmental, which have different apparent velocities from the reflections. The method has also been effective in solving structural problems in tectonic areas by removing diffractions or, alternatively, by enhancing them at the expense of the reflections to delineate faults and other sources of diffraction. Ringing or reverberation can often be attenuated or eliminated in marine shooting by passing reflection frequencies that are less than the lowest observed harmonic of the fundamental reverberation frequency. Examples are shown of transforms and/or filtered sections illustrating these applications. A particularly valuable feature of this optical processing system is the ease of monitoring the results. The facility with which this can be done gives the technique distinct advantages over digital or analog methods, where the geophysicist loses contact with his results while processing is under way. Optical filtering also offers an intrinsically more economical approach to seismic data processing because hundreds of information channels can be handled n a single photographic operation.
DOWNWARD CONTINUATION AND ITS APPLICATION TO ELECTROMAGNETIC DATA INTERPRETATION
Authors: Roy Amalendu
Published: October 2002

The existing methods of downward continuation were developed primarily for the interpretation of gravity and magnetic data, and, therefore, assume the validity of Laplace’s equation. The data collected in electromagnetic surveys, strictly speaking, obey Maxwell’s wave equation. However, in practice, two approximations are frequently made. First, the effects of displacement currents are neglected in view of the fact that the frequencies used are low. Second, the contribution of the country rock to the measured field is assumed to be nil, as its electrical conductivity is usually much smaller than that of the target body. Experience seems to indicate that these two approximations are permissible in general. It will be seen that, under these two approximations, Maxwell’s wave equation degenerates to that of Laplace, and the existing techniques of continuation become applicable to electromagnetic field data as well. This hypothesis has been tested by using continuation to interpret 20 electromagnetic profiles, involving a variety of theoretical, model, and field data. It has been found that the depths to the tops of the anomaly‐causing bodies can be determined with acceptable accuracy by this method.
CORRELATING SEA‐SURFACE AND AERIAL GRAVITY MEASUREMENTS
Authors: Thyssen‐Bornemisza Stephen
Published: October 2002

When sea‐surface gravity observations were supplemented by corresponding values from the airborne meter, average vertical gradients of gravity could be computed. In a borehole these gradients are observable by moving the borehole gravity meter up and down to another level (Thyssen‐Bornemisza, 1963, 1964, 1965a), but measurements taken on two horizontal profiles separated by the constant vertical interval h could furnish only relative gradient values or variations in the profile direction. Of course, gravity profiles on the ground surface or the sea floor can be likewise supplemented by aerial observations.
REPORT ON THE LUNAR GEOLOGICAL FIELD CONFERENCE, BEND, OREGON, AUGUST 22–28, 1965
Authors: Hafner W.
Published: October 2002

A majority of the membership of the Society of Exploration Geophysicists is engaged, directly or indirectly, in the exploration for oil and gas. Since the habitat of these deposits is confined almost conclusively to the sedimentary portion of the earth’s crust, it is only natural that this portion constitutes their primary field of interest. Their concern with volcanic terrain is often but a negative one, being directed towards the determination of their occurrences and outlines for the purpose of eliminating them as potential oil hunting grounds. This, at least, has been the experience of the writer during a lifetime of oil exploration work. It will, therefore, be of interest to this group of geophysicists to take cognizance of another field of application of the geophysical methods: their use in the investigation of volcanic and impact structures. While this activity is presently of limited extent and confined to the earth, it may in the foreseeable future be extended to the exploration of the moon. The writer had the good fortune to become more fully aware of this field of activity as the representative of the SEG at the recent Lunar Geological Field Conference.
NUMERICAL SIMULATION OF SEISMIC DISTURBANCES
Authors: Cherry J. T. , Hurdlow W. R.
Published: October 2002

This paper presents the results from a Lagrangian numerical scheme of calculating cylindrically symmetric, elastic‐plastic, transient disturbances. The numerical technique involves transforming the Eulerian equations of motion containing a cylindrically symmetric stress tensor into Lagrangian coordinates. These transformed equations are then differenced in the Lagrangian coordinate system. Therefore, a given instantaneous stress field determines the accelerations of the points in the Lagrangian mesh. These accelerations are allowed to act over a small time‐step so that the mesh becomes distorted. This distortion determines the change in strain at a point; the strain change is related to a stress change by Hooke’s law. This new stress is used to determine new accelerations. If the material behaves plastically, then the stresses are adjusted so that a von Mises yield condition is satisfied. Five numerical solutions of various elastic‐plastic wave propagation problems are presented; these solutions agree extremely well with their corresponding analytic and experimental solutions. A shear and tensile failure mechanism is presented that is consistent with the continuum hypothesis. This mechanism gives good results when applied to NTS alluvium. Data for hard rock are not yet available.
DEVELOPMENTS IN EXPLORATION GEOPHYSICS, 1962–1965
Authors: Levin F. K. , Bayhi J. F. , Grieves W. A. , Watson R. J. , Webster G. M.
Published: October 2002

The geophysical industry is in the midst of a digital revolution. Digital seismic recorders are replacing analog field systems; digital data processing techniques are now widely used; and many publications have appeared on the principles of digital filtering and the design of optimum filters, particularly multichannel filters. Far less material has appeared on parameter and spectrum estimation and on time‐varying procedures. Analysis using optical diffraction has found considerable favor. Although the dynamic range reported is only about 40 db, the ability to handle entire record sections has resulted in close scrutiny of this nondigital analysis method. Several nondynamite seismic sources have been developed for land use; while in marine exploration, more powerful sources and multidetector cables have led to penetrations reported to be up to eight thousand ft. Common reflection point record stacking methods are being used on about 75 percent of seismic held crews. Strong interest continues in the development of mobile magnetometers with increased sensitivity. A growing use of shipborne gravity meters has resulted from precisions of two or three mgal at a ship speed of 10 knots. Digital computers are being used to apply terrain and elevation corrections and to aid in separating the gravity field into regional trends and anomalies. Digital recorders for use with dipmeters and other well logging tools are available in many areas. A system for recording on a single tape up to eleven logging curves from separate surveys is under active development. Analog computers at the well produce for visual interpretation computed curves that emphasize hydrocarbon anomalies. A new logging system, neutron lifetime logging, promises to locate oil behind pipe. Improved design of acoustic, gamma‐gamma density, and dipmeter tools minimize borehole effects.
ANALYSIS FOR MAGNETITE UTILIZING MAGNETIC SUSCEPTIBILITY
Authors: Shandley P. D. , Bacon L. O.
Published: October 2002

The analysis for magnetite or “magnetite equivalent” by means of magnetic susceptibility is more rapid, lower in cost, and in general, as precise as other methods of analysis. “Magnetite equivalent” analysis is affected by grain size if the individual particles are less than 40 microns in diameter, by the remnant magnetization if greater than about 12 percent of saturation magnetization, and by the presence of other magnetic minerals. The presence of other magnetic minerals does not reflect as great an error in the final result using the susceptibility method as in other methods. Davis tube separation as a method for magnetite analysis is affected by other magnetic minerals but is mainly dependent upon degree of mineral liberation by grinding. Chemical analysis with the subsequent calculation of magnetite content is in error due to presence of other iron‐bearing minerals whether magnetic or nonmagnetic and is particularly affected by the presence of metallic iron from sample preparation. Metallic iron apparently enters solution by the following reaction: [Formula: see text] Thus, one unit of metallic iron would be counted as approximately 12 units of magnetite in the calculation of magnetite content. Analysis for “magnetite equivalent” and chemical analysis for total soluble iron can provide an accurate evaluation of the amount of nonmagnetic iron in any particular ore, especially in mixed magnetite‐hematite ores. Equipment for direct readout of grams magnetite per 100 cc was designed using the differential transformer principle. Calibration is linear below 20 grams magnetite per 100 cc, and measurement of apparent density of sample readily converts the readings to percent magnetite by weight.
OUTLINING OF SHALE MASSES BY GEOPHYSICAL METHODS
Authors: Musgrave A. W. , Hicks W. G.
Published: October 2002

Shale masses are here defined as large bodies of shale at least several hundred feet in thickness. These may be formed either as diapiric masses or as depositional masses. The shale masses are like salt masses and the two are many times combined to form domal masses; they both may form the updip seal for stratigraphic accumulation of oil. The shale masses exhibit the following properties by comparison to the normal section: (1) low velocities—in the range of 6,500 to 8,500 ft/sec with very little increase of velocity with depth, (2) low densities—estimated to be in the range [Formula: see text] to [Formula: see text], (3) low resistivities—approximately 0.5 ohm‐m, and (4) high fluid pressures—about 0.9 overburden pressure. These properties all seem to be caused by the high porosity and low permeability of these large shale masses. Maps and cross sections of an example area block 113, Ship Shoal Area are shown. The low shale velocities were measured by acoustic logs and verified by refraction shooting. The low densities were deduced from gravity maps. The low resistivities are shown on electric logs, and high pressure is evidenced by the drilling difficulties with heaving shales. These physical properties allow the outlining of the shale mass by one or more of the following ways: the gravity method is used to outline the low density material, the seismic reflection method is used to outline the lack of reflection contrast and in some cases map the velocity configuration, the seismic refraction method is used to indicate the velocity of the anomalous mass, thereby differentiating between shale and salt.
FAMILIES OF SALT DOMES IN THE GULF COASTAL PROVINCE
Authors: Selig Franz , Wermund E. G.
Published: October 2002

If two fluids of different densities are superposed one over the other, the plane interface between the two fluids becomes unstable if the heavy fluid overlays the lighter one. This type of hydrodynamic instability is called Rayleigh‐Taylor instability. The theory of Rayleigh‐Taylor instability is a useful tool to study the distribution of salt domes in the coastal region of the Gulf Coastal Province. In spite of a drastic simplification of the geologic situation, the model shows: a) that the spacing of salt domes about an initial disturtbance depends upon the thickness of the mother salt and viscosity ratio of overlying sediment to salt; b) that domes not only grow upward from the initial disturbance, but domes are also triggered in the vicinity of the primary disturbance, forming a family of incipient domes with a regular pattern; c) that the family of incipient domes develops out of the initial disturbance starting at the location of maximal instability and spreading radially. Several numerical examples provide a framework for examining the disturbance of Gulf Coastal salt domes.
On: “Vertical gradient of gravity on axis for hollow and solid cylinders” by T. A. Elkins (GEOPHYSICS, August, 1966, pp. 816–820)
Authors: Thyssen‐Bornemisza Stephen
Published: October 2002

In the above captioned short note the case of a vertical gradient of gravity generated from a vertical cylinder along its axis is briefly discussed, formulas are given, and several conclusions presented. However, the author does not seem to be aware of previous papers in Geophysics providing the same formulas and conclusions in a somewhat different version (Hammer, 1950, Smith, 1950; Thyssen‐Bornemisza, 1963, 1965, 1966).
ELASTIC WAVES RADIATED BY A SMALL SOURCE
Authors: Case K. M. , Colwell J. F.
Published: October 2002

The Green’s function for elastic waves is developed. The elastic waves generated by a source contained in a cavity in an infinite elastic medium are written as an integral over the surface of the cavity; the integrand contains the product of the Green’s function and the stresses and displacements on the cavity surface. A multipole expansion of this integral is made for sources small compared with the wavelengths radiated. In this case, the first two terms in the expansion accurately describe the radiation. Thus, the explicit time‐dependent solutions of the propagation equations are obtained in terms of a few parameters that are calculated by taking prescribed averages of the stresses and displacements on the source surface. The procedure is illustrated by finding the radiation from a simple spherical source. Finally, a variational principle is developed by means of which the displacement on the source surface can be found if only the applied stresses are known.
DUAL‐SPACED NEUTRON LOGGING FOR POROSITY
Authors: Allen L. S. , Tittle C. W. , Mills W. R. , Caldwell R. L.
Published: October 2002

Two‐group neutron diffusion theory has been applied to two concentric cylindrical regions representing borehole and formation to show that the ratio of thermal neutron flux at two appropriately large distances along the axis is virtually identical to the epithermal flux ratio, and is a measure of a single epithermal parameter of the formation, viz., the slowing down length. The slowing down length is most strongly dependent on hydrogen content. It is shown that variations in salinity of the liquid in the formation or the borehole and uncertainties in borehole size have relatively little effect on the flux ratio. Experiments in a borehole model showed that the thermal flux ratio does not depend on eccentricity of location of the logging tool in the borehole. Fits of theory to experiment allowed determination of slowing down lengths for Pu‐Be neutrons in salt water filled sandstone at two porosities for which experimental values have not previously been available. These investigations suggest that an improved porosity log can be obtained by use of 70‐ and 90‐cm source‐detector spacings with a neutron source of [Formula: see text] neutrons/sec or greater. Proportional counters containing [Formula: see text] are recommended as detectors. Such a system employing flux ratios is expected to be insensitive to casing and cement in cased holes.
ELECTROMAGNETIC SCREENING DUE TO A DISSEMINATED SPHERICAL ZONE OVER A CONDUCTING SPHERE
Authors: Negi Janardan G.
Published: October 2002

The electromagnetic field response under the influence of an alternating magnetic field is investigated for an isometric ore body covered by a disseminated spherical zone. The embedding medium is homogeneous and a relatively poor electrical conductor. The formal solution of the diffraction problem is obtained when the overlying non‐uniform conducting shell has a general spherical symmetry. More complete analysis is made for three important cases where the screening shell is assumed to have: 1. a power law conductivity distribution, 2. propagation constant decreasing linearly outside, and 3. a homogeneous electrical behaviour. The computational procedure for the evaluation of the in‐phase and quadrature components of the screening factor for the last case is presented along with some relevant curves for parametric values suitable in the exploration of massive sulfide deposits. The computed results establish a paradoxical phenomenon of negative screening, meaning that under favorable conditions an ore body with a conducting cover will show up better than an uncovered one. Though these results are contrary to the present philosophy of prospecting, they are able to explain some observations recorded in the field and model experiments. Because of the generalized treatment the analysis with suitable modifications can find applications in many other diffraction problems.
EXPLORATION HORIZONS FROM NEW SEISMIC CONCEPTS OF CDP AND DIGITAL PROCESSING
Authors: Marr John D. , Zagst Edward F.
Published: October 2002

The more recent developments in common‐depth‐point techniques to attenuate multiple reflections have resulted in an exploration capability comparable to the development of the seismic reflection method. The combination of new concepts in digital seismic data processing with CDP techniques is creating unforeseen exploration horizons with vastly improved seismic data. Major improvements in multiple reflection and reverberation attenuation are now attainable with appropriate CDP geometry and special CDP stacking procedures. Further major improvements are clearly evident in the very near future with the use of multichannel digital filtering‐stacking techniques and the application of deconvolution as the first step in seismic data processing. CDP techniques are briefly reviewed and evaluated with real and experimental data. Synthetic data are used to illustrate that all seismic reflection data should be deconvolved as the first processing step.
AEROMAGNETIC STUDY OF COYANOSA
Authors: Steenland Nelson C.
Published: October 2002

The Coyanosa field is situated seventeen miles northwest of Fort Stockton within the eastern part of the Delaware basin adjacent to the southwestern corner of the Central Basin Platform. Deep production, discovered by Mobil Oil Corporation in May, 1963, now covers an area of 30 sq mi of Devonian and Ellenburger formations. Devonian products have a market value currently estimated at almost $500 million and Ellenburger at $400 million.
MOVEOUT AVERAGING EXPERIMENTS
Authors: Simpson S. M. , Fink D. , Treitel S.
Published: October 2002

The experiments described here utilized the variable stepout averaging procedure, discussed in some detail in Section 4 of GAG Report 10b. This was done with a Whirlwind I computer program which, in effect, converts time scales of seismic traces to a depth scale by removing normal moveout, and then finds a direction across the modified record which maximizes correlation averages. Actually, the process is performed in the reverse order.
STATISTICAL TEST OF THE HYPOTHESIS THAT A TIME SERIES IS STATIONARY
Authors: Bryan J. G.
Published: October 2002

In order that a time series be stationary, a sufficient condition is the time invariance of all statistical parameters. Hence, a necessary condition is the time invariance of an arbitrary selection of statistical parameters. The object of this section is the derivation of a significance test of invariance; and in line with general principles governing the construction of statistical tests, the aim is to find nontrivial functions of the variates having two properties: 1. Sample values of the test function can be determined uniquely from observations of the initial variates; that is, no unknown constants are involved in the test. 2. The probability distribution of the test variate is fully determinate; that is, the mathematical derivation can actually be carried out, and the result is free from unknown constants. The test here presented assumes knowledge of the autocorrelation function, whereas it is usually necessary to estimate this function empirically. Hence, to this extent it has been necessary to compromise on the second property.
THE APPLICATION OF REMOTE SENSING INSTRUMENTS IN EARTH RESOURCE SURVEYS
Authors: Badgley Peter C. , Childs Leo , Vest William L.
Published: October 2002

The National Aeronautics and Space Administration is supporting research in those areas of air‐ and spaceborne remote sensing which are related to the study of natural and cultural resources. Resources which can be studied in this manner include mineral districts, uncharted petroleum basins, little known coastal areas, soils and soil moisture, crops, timber, land and land use in both agricultural and metropolitan areas, watershed areas, transportation networks, sea state, marine near‐surface life, and so forth. Remote sensing instruments in earth‐orbital spacecraft possess a number of unique advantages, among which are: rapidity and continuity of observations, synoptic imagery for regional analyses, reduced data acquisition time, relative freedom from weather disturbances, reduced costs for those features requiring periodic measurements, and better quality data of some types. The progress being made by NASA in these areas is discussed.
ELECTRIC DIPOLE OVER AN ANISOTROPIC AND INHOMOGENEOUS EARTH
Authors: Sinha Ajit Kumar , Bhattacharya Prabhat K.
Published: October 2002

The electromagnetic fields of a low‐frequency horizontal electric dipole placed over a two‐layer earth have been derived. The overburden is considered to be transversely isotropic with respect to the conductivity. The unequal principal axis of the conductivity tensor is normal to the layers. The substratum is taken as isotropic. The vector potentials at the surface of the earth have been evaluated and expressed in such a way that the fields may be easily calculated. We have considered the special cases of a perfectly conducting, a perfectly insulating, and a substratum whose conductivity is very close to the longitudinal conductivity of the overburden. Asymptotic expressions for the fields have also been calculated. All the results are given in terms of known functions, and some numerical results have been included.
ON THE THERMAL INSTABILITY OF LARGE DIAMETER WELLS—AN OBSERVATIONAL REPORT
Authors: Gretener Peter E.
Published: October 2002

A fluid in a tube subjected to a thermal gradient will be stable as long as this gradient does not exceed a certain critical value. Observational evidence is presented that the fluid in large diameter wells, such as oil wells, subjected to the normal geothermal gradient is, indeed, unstable as predicted by the theory. Our independent observations support the evidence presented by Diment (1967) in a companion paper.
AN ELECTROMAGNETIC ANALOG METHOD FOR COMPUTING GRAVITY AND MAGNETIC EFFECTS OF TWO‐DIMENSIONAL BODIES
Authors: Saha S. N.
Published: October 2002

An analog method can provide quick and accurate computation of gravity and magnetic anomalies arising from bodies in the form of horizontal cylinders of any arbitrary cross section and polarization. It has been possible to simulate the gravitational and magnetic fields of such a body by a current system flowing along a set of parallel wires. The underlying principle is that the magnetic field due to current in a long straight wire is mathematically similar to the gravitational field due to a line mass of uniform density. This similarity has been extended to the magnetic case, since the currents in the opposite directions reproduce the fields due to positive and negative line poles. Comparison of the simulated field for some simple bodies has been made with theoretically computed values. Generally, these agreed within less than 3 percent error. In the case of the magnetic analog it has been possible to reproduce the field for any direction of polarization perpendicular to the strike. It is also possible to simulate the effects of density changes or changes in susceptibility contrast and in permanent magnetism. The method may be extended to the measurement of magnetic field components for any direction of strike of the body. Torsion balance quantities may be simulated as well as simple field quantities.
CONVERGENCE AND DIVERGENCE IN DOWNWARD CONTINUATION
Authors: Negi Janardan G.
Published: October 2002

The paper reports solutions for some unanswered questions regarding the properties of the downward continued field for some simple source configurations. The radii of convergence, the conditions for predicting presence or absence of singularities on the boundary of the region of convergence and the equations for the boundary curves for the regions of convergence and divergence for arbitrary values of x are obtained from Taylor series representations of the fields of line and strip sources. The results provide useful connections in the discussion of some pertinent convergence and divergence characteristics of downward continuation.
A SPINNER MAGNETOMETER WITH A MAGNETICALLY SHIELDED COIL
Authors: Bruce C. R.
Published: October 2002

This magnetometer uses magnetic shielding instead of a compensating coil to reduce ambient magnetic noise. Advantages of shielding include greater coupling to the specimen field, reduced stationary field at the specimen, lessened coherent noise from rotor magnetism, and a smaller coil size. The minimum detectable moment is [Formula: see text] emu. Specimen shape is either a 1×1 inch cylinder or a solid formed by the intersection of three mutually perpendicular cylinders. The functional principle is conventional but the electronic circuit is greatly simplified through mechanical design. Direction of magnetization is indicated directly on a scale on the sensing head after the sensing coil has been oriented to attain a null output on the phase sensitive detector. Then, by switching to a second optical band on the rotor, the reference phase is shifted 90 degrees and intensity is read on the phase‐detector output meter. Operating frequency is 155 Hz. The instrument is calibrated by spinning a current loop of known characteristics in the specimen position. Three separate units—a sensing head, a control unit, and a power supply—compose the total system.
ON THE APPLICATION OF THE RECIPROCITY THEOREM IN RESISTIVITY PROSPECTING
Authors: Habberjam G. M.
Published: October 2002

In a recent publication, Van Nostrand and Cook (1966) draw attention to the reciprocity theorem of electrical conduction, whereby if current is supplied between two points of an electrical conductor and the potential difference is measured between a second pair of points, this potential difference is the same as would arise between the first pair if the same current were supplied between the second pair. In particular, they cite limitations to the application of this theorem put forward by Wenner and claim that when adapting this theorem to actual field conditions “the assumptions for its validity often do not hold” (p. 39). This appears to be at variance with a later statement (p. 46) that it is safe to apply the reciprocity theorem.
On: “The Gravitational Attraction of a Right Rectangular Prism” by Dezsö Nagy (GEOPHYSICS, April 1966, p. 362–371)
Authors: Sharma P. Vallabh
Published: October 2002

I would like to offer the following comment on the paper “The Gravitational Attraction of a Right Rectangular Prism” by Nagy in the April 1966 issue of Geophysics. It might be of interest to note that besides the papers (Sorokin, 1951; Haáz, 1953) which have been cited by the author, an earlier publication by Ansel (1936) also gives a solution in closed form for the gravitational attraction of a rectangular prism. Furthermore, Müller (1963) has given a solution which is valid for all possible positions of the prism with vertical sides.
AN INVERSE SLOPE METHOD OF DETERMINING ABSOLUTE RESISTIVITY
Authors: Narayan P. V. Sanker , Ramanujachary K. R.
Published: October 2002

In conventional interpretation of electrical resistivity data, either one uses an empirical method (Moore, 1945) or an analytical method (Tagg, 1934; Roman, 1934; or Mooney and Wetzel, 1956). While the Tagg, Roman, and Mooney and Wetzel systems are based on theoretical considerations, they are tedious and time‐consuming. The Moore method, while it is very rapid, has the disadvantage of not yielding absolute values of resistivity. Sarma (1963) suggested a modification of the Moore method using Hummel’s (1931) principle, whereby one could obtain absolute values of resistivity. However it is seen that, under certain conditions, Sarma’s method yields resistivity values with a negative sign which cannot be interpreted as having any real physical significance in geological situations.
AN OCEAN BOTTOM, D COMPONENT MAGNETOMETER
Authors: Filloux J. H.
Published: October 2002

The distribution of electric conductivity in the crustal and upper mantle materials beneath the ocean may be estimated from measurements of the relationship between the magnetic fluctuations and the induced electric field at the ocean bottom. Techniques for the measurement of the electric field have been available for a few years. The horizontal magnetic fluctuations to the magnetic east, usually called D, can be recorded with a simple instrument placed on the sea floor at any depth. This instrument uses a magnet pair which orients itself among the main horizontal field H. The coupling of the magnets to the mirror of a sensitive optical lever is delayed until the instrument has reached the bottom. There is no need to perform any orientation in situ. The instrument resolves 1 γ or less and has a dynamic range of at least 2500 γ. It is capable of recording for approximately 40 days at the rate of 30 readings per hour on self‐contained dry cells. It is lowered to the sea floor and recovered by means of a mooring line connected to a surface float. The low‐profile supporting tripod is effectively decoupled from the mooring tackle as evidenced by the lack of motion of the magnetometer during 26 days of recording. A sample of the observed fluctuations on the floor of the North Pacific Ocean, 600 km offshore, is given.
OPTIMUM DIRECTION OF SURVEY LINES
Authors: Lilley F. E. M.
Published: October 2002

The pattern of a magnetic anomaly varies with the dip of the inducing field. For an aeromagnetic survey intended to discover induced dipole sources, the best flight direction is defined as the one offered maximum target width by an anomaly. This direction may depend on the amplitude of the anomaly. A method is proposed for estimating efficiency of direction for a range of anomaly amplitudes. The efficiencies of lines bearing north, northeast, and east are compared. At the equator, lines bearing north are twice as efficient as lines bearing east. With increasing inclination, the advantage of lines bearing north disappears at dips of about 20 degrees. Lines bearing northeast are chosen for dips between 20 and 70 degrees. Flight direction is unimportant for dips greater than 70 degrees.
SPECTRUM OF THE POTENTIAL FIELD DUE TO RANDOMLY DISTRIBUTED SOURCES
Authors: Naidu Prabakar
Published: October 2002

Covariance and spectral density functions of all three components of random potential fields are mutually related. The spectra of any one component on two different levels are related through an exponential function of the separation of the levels. If, starting from the earth’s surface, we continue the observed random field downward, we find that at a certain level the field becomes unstable. Such a limiting depth can be estimated from the spectrum of the observed field. The spectrum of the random field is related to the spectrum of random density or intensity of magnetization. The random sources are assumed to be confined to a thin sheet or a thick, infinite sheet, or to a semi‐infinite medium. In all these cases, a relation connecting the spectrum of the random density or the intensity of magnetization and the spectrum of the random field has been established.