bluuebunny/crossref_metadata_2025 huggingface dataset:
LIMITATIONS ON RESISTIVITY METHODS AS INFERRED FROM THE BURIED SPHERE PROBLEM
Authors: Van Nostrand Robert G.
Published: October 2002

The study of resistivity surveys over buried spheres is facilitated by the use of a bipolar coordinate system. Without solving the incident potential problem exactly, the author obtains an exact solution for the apparent resistivity as measured by the Wenner configuration of electrodes situated directly over a buried conducting sphere. From a study of depth profiles over a set of spheres buried at different depths, it is concluded that one cannot expect to find by direct current methods a sphere buried to a depth greater than the radius of the sphere. This result is generalized to include bodies of more arbitrary shape.
SIGNAL‐TO‐NOISE RATIO IMPROVEMENTS BY FILTERING AND MIXING
Authors: Frank Harold R. , Doty William E. N.
Published: October 2002

This paper is concerned with the signal‐to‐noise (interference) amplitude ratio of seismograms. The problem of improving the signal‐to‐noise ratio by filtering and mixing is approached quantitatively. Records of known signal‐to‐amplitude ratio and known frequency content were made employing multiple‐trace variable area equipment such that identical energy was reproduced and re‐recorded through various filtering and mixing schemes and comparisons made. Equipment effects on wavelet character and stepout times are illustrated.
GRAVITY CASE HISTORY: DAWN NO. 156 POOL, ONTARIO
Authors: Pohly Richard A.
Published: October 2002

The Dawn No. 156 pool in southwestern Ontario was discovered by the Union Gas Company of Canada Limited, in January, 1952, by drilling on a gravity maximum. The drilling supported the original interpretation that this maximum was produced by a Silurian reef. Initial production of 6.5 million cubic feet per day was obtained from the upper fourteen feet of the Guelph in highly porous reef dolomite at a depth from 1,720 to 1,734 feet below the surface. The geology of the area is discussed as a background for the gravity interpretation. Bouguer and residual gravity maps of the original survey are presented.
MUTUAL COUPLING OF LOOPS LYING ON THE GROUND
Authors: Wait James R.
Published: October 2002

The theory of a method to measure electrical ground constants by mutual coupling of loops is outlined. Curves are given in parametric form showing the dependence of the mutual impedance on conductivity, dielectric constant, frequency, and separation. An expression is derived in the appendix for the field of a circular wire loop laid on a homogeneous conducting ground.
SURFACE AND NEAR‐SURFACE WAVES IN THE DELAWARE BASIN
Authors: Dobrin Milton B. , Lawrence Philip L. , Sengbush Raymond L.
Published: October 2002

Seismic propagation studies made in the Delaware Basin of West Texas by the Field Research Laboratories of the Magnolia Petroleum Company have disclosed several unusual kinds of traveling waves. The near‐surface zone in this area is characterized by alternating high‐ and low‐velocity layers, with a thin high‐velocity cap. Physical characteristics of the recorded waves have been correlated with this layering. Five types of waves have been identified: 1) Waves refracted along the tops of high‐speed near‐surface markers which have been multiply reflected, at the critical angle, between the marker beds and interfaces nearer the surface. 2) Shear waves refracted at shear velocity along a competent bed several hundred feet deep. 3) Compressional waves propagated by normal‐mode transmission in the wave guide formed by a low‐speed layer situated between two high‐speed layers. 4) A single‐cycle, apparently non‐dispersive Rayleigh wave propagated in a thin limestone surface layer and in an underlying low‐speed layer of sand and gravel. 5) An inversely dispersive Rayleigh wave train in which the group velocity appears to decrease with increasing wave length; this type of dispersion, just the opposite of the kind ordinarily recorded, is attributed to the fact that the low‐speed surface layer is unusually thick compared with the wave length corresponding to the cut‐off frequency of the instrumental system.
COMPARISON OF BREAK‐POINT AND TIME‐INTERCEPT METHODS IN REFRACTION CALCULATIONS
Authors: Zirbel N. N.
Published: October 2002

This paper compares two methods of calculating depths from refraction data where the subsurface consists of a series of horizontal layers with velocities [Formula: see text], etc. Equations are developed for calculating depths from the horizontal distances to the intersections of the various segments of the time‐distance graph (the break‐points). Examples are worked out to show that the error in depth due to an uncorrected change in thickness of the weathered layer is much less when the depths are calculated from break‐points than when they are calculated from time‐intercepts. In many areas weathering corrections should be unnecessary if the depths can be calculated from the break‐points by the method outlined here.
APPARENT RESISTIVITY FOR DIPPING BEDS
Authors: Maeda Katsuro
Published: October 2002

The potential field of a point source of electric current, located on the surface of the earth above a dipping bed, is determined exactly by solving the appropriate differential equation. It is concluded that image theory is useful only in the two cases in which the reflection coefficient is plus one and the angle of dip is [Formula: see text] and in which the reflection coefficient is minus one and the dip angle is [Formula: see text] m being an integer. Computing methods are also developed for the cases in which the image theory is not applicable. Some numerical tables necessary for computation and several apparent resistivity curves are presented.
THE FIRST VERTICAL DERIVATIVE OF GRAVITY
Authors: Ackerman Herman A. , Dix C. Hewitt
Published: October 2002

A formula is given which permits the relatively convenient calculation of the first vertical derivative of gravity in terms of the distribution of the second vertical derivative of gravity. The use of the formula is illustrated in a simple case. The formula is compared to Evjen’s with respect to convenience of use.
SIMILARITY OF OUTPUT TRACES AS A SEISMIC OPERATOR CRITERION
Authors: Simpson Stephen M.
Published: October 2002

The problem of emphasizing signals on multiple trace seismograms is approached by considering a relationship between the input and output records. It is proposed that the transformation to output record be one which causes the output traces to be most “similar” or “in phase” according to a certain definition of this property. If the noise and signal are “properly behaved,” it may be demonstrated that a linear transformation chosen by this criterion must have a response emphasizing frequency ranges of high input signal‐to‐noise ratio. The determination of such a transformation from the input data alone is carried out for discrete linear operators. The numerical work involved in computing such operators is formidable. As an example the computations were carried out for a mixture of an artificial signal introduced into a noise record. The results are about as good as those obtained with conventional filtering techniques depending on prior knowledge of input signal‐to‐noise ratios.
CHANNEL WAVES IN THE EARTH’S CRUST
Authors: Gutenberg B.
Published: October 2002

Three major low‐velocity layers seem to exist in the earth’s crust, of which two, the lithosphere channels, are found respectively in the “granitic” and “basaltic” (“gabbro”) layers of the continents; a third channel extends from the Mohorovičič discontinuity downward into the asthenosphere. Several types of waves are guided by these channels, especially Pa and Sa by the asthenosphere channel, Lg1, Lg2 and Rg by the lithosphere channels; waves guided by low‐velocity layers in sediments must also be expected. Many records of the Southern California earthquake of July 21, 1952 show channel waves with periods and velocities as reported for other paths. The regular microseisms with periods of 4 to 10 sec have properties similar to those of the Lg—Rg group in earthquake records and are probably propagated by the same mechanism. Most of these interpretations and conclusions are tentative; pertinent observations are scanty; and complications have thus far prevented development of adequate equations to calculate the amplitudes of waves guided by a given channel.
MAGNETIC DELAY LINE FILTERING TECHNIQUES
Authors: Jones Hal J. , Morrison John A. , Sarrafian G. P. , Spieker L. J.
Published: October 2002

Modern communications theory and information theory have placed considerable emphasis upon the problem of detection and extraction of signals in noise backgrounds. Some of the methods and equipment developed have possible geophysical applications. This paper presents a discussion of the delay line method of filter approximation. Magnetic recording delay line equipment which permits approximation of a wide variety of linear filtering and correlation operations is described. The fundamental theory of linear operator filtering by delay line techniques is outlined and some possible applications to processing seismic data are briefly discussed.
AN AMPLITUDE STUDY ON A SEISMIC MODEL
Authors: Clay C. S. , McNeil Halcyon
Published: October 2002

The measured amplitudes of two seismic events which have traveled through a two layer seismic model are compared with the amplitudes calculated from plane wave reflection and transmission theory. The relative amplitudes of the events, one a dilatational‐dilatational and the other a dilatational‐to‐shear conversion event, are found to be in agreement with calculations based on reflection theory for plane waves, after correction for [Formula: see text] divergence.
A MARINE SEISMIC MODEL
Authors: Sarrafian George P.
Published: October 2002

A model for the study of marine seismic phenomena is described. Study of multiple‐reflection phenomena forms the basis for the course of experiments. It is shown that the multiple‐reflection phenomenon of a disturbance with slowly decaying amplitude may be duplicated in the model. Multiple‐reflection problems are studied in which the bottom of the water layer is tilted or thin. A mass of air bubbles is shown to be of use in attenuating multiple reflections. The possible application of the marine model in a search for information about certain problems in field prospecting is suggested.
A TRI‐POTENTIAL METHOD OF RESISTIVITY PROSPECTING
Authors: Carpenter E. W. , Habberjam G. M.
Published: October 2002

This paper describes a method whereby three resistances are measured for a four electrode configuration, and relations between these three resistances and their corresponding apparent resistivities are derived. The practical application of the resistance relation as a means of detecting observational and instrumental errors is indicated. The concept of apparent resistivity is examined and by means of examples it is shown that the apparent resistivity can take negative values. Finally, the possibility of using the triple resistivity method as a means of distinguishing between the effects of lateral and vertical resistivity changes on depth probes is discussed.
A PRELIMINARY REPORT ON MODEL STUDIES OF MAGNETIC ANOMALIES OF THREE‐DIMENSIONAL BODIES
Authors: Zietz Isidore , Henderson Roland G.
Published: October 2002

Model experiments were made to devise a rapid method for calculating magnetic anomalies of three‐dimensional structures. The magnetic fields of the models were determined using the equipment at the Naval Ordnance Laboratory, White Oaks, Md. An irregularly shaped mass was approximated by an array of prismatic rectangular slabs of constant thickness and varying horizontal dimensions. Contoured maps are being prepared for these magnetic models at different depths and for several magnetic inclinations. The fields of these three‐dimensional structures are obtained by super‐imposing the appropriate contoured maps and adding numerically the effects at each point. The equipment and laboratory methods are described. Theoretical and practical examples are given.
BOUGUER CORRECTIONS WITH VARYING SURFACE DENSITY
Authors: Vajk Raoul
Published: October 2002

The Bouguer correction is made, as a rule, to sea level and by using an average density value of the rocks, determined by the profile method. However, if the density of the rocks forming the topography varies from place to place, then the proper density value should be used for every topographical feature. The Bouguer correction with varying density should be made only to a surface drawn through the low points of the topography. Below this surface, and to sea level, a constant density value should be used all over in the area; otherwise, nonexisting gravity anomalies may be introduced into the results of the survey, or existing gravity anomalies may be distorted. A few examples are given for computing Bouguer corrections with varying surface density, and to illustrate the errors introduced by using improper density values.
SOUND PROPAGATION IN COMPACTED OCEAN SEDIMENTS
Authors: Laughton A. S.
Published: October 2002

Experiments were made in the laboratory to measure the velocity of propagation of compressional and transverse elastic waves in ocean sediments while under varying conditions of compacting pressure. Various attempts to formulate expressions relating velocity with composition are discussed and are found to be not applicable to compacted sediments. The concept of a structure bulk modulus is introduced to account for the observed velocity changes. Shear waves were observed at compacting pressures exceeding 500 kg/sq cm. The effects of hydrostatic pressure and the development of anisotropy are also studied.
POLARITY COINCIDENCE SCHEME FOR REVEALING SIGNAL COHERENCE
Authors: Melton Ben S. , Karr Philip R.
Published: October 2002

A simple statistical procedure, based on multiple coincidence of field polarities as indicated by several receptors, offers a criterion of the existence of a signal below the general noise level. As an example, with six receptors in a field where there is a sine wave signal ten decibels below the noise level, there will be twice the number of coincidences that would be obtained for the noise alone. A scheme is shown to illustrate how an electrical system can perform the analysis and present the results continuously.
DESCRIPTIVE GEOMETRY AND THE OFFSET SEISMIC PROFILE
Authors: Gates John P.
Published: October 2002

An illusion is created in attempting to portray steeply dipping seismic data by standardized geometric procedures. The offset seismic profile, although resembling the vertical geologic cross‐section in appearance, may have an inherent tilt which is a function of cross‐dip. Unless this tilt is accounted for during the solution of critical structural problems, serious errors can enter into the interpretation of the seismic data. Descriptive geometry procedures are applied to establish the tilt and determine its effects on fault traces, structural axes, and well ties.
CHART TO CHECK ELEVATION FACTOR EFFECTS ON GRAVITY ANOMALIES
Authors: Ivanhoe L. F.
Published: October 2002

The effect of varying surface densities on gravity anomalies is a more common problem in areas of topographical relief than is generally recognized. There is an unjustified tendency to assume that gravity maps are unique and final, even though one basic assumption (density of surface rocks) is inherent in all gravity maps. The use of incorrect elevation factors will produce gravity anomalies over any topographic feature. Both positive and negative gravity anomalies can be produced by either a topographic hill or valley depending on the degree of error in the elevation factor. These “elevation factor anomalies” are especially troublesome on residual gravity maps. The interpretation of gravity data should always include an analysis of the elevation factor effect as well as a study of the surface geology.
THE MOVEOUT FILTER
Authors: Savit Carl H. , Brustad John T. , Sider Joseph
Published: October 2002

The response of a seismometer array or “pattern” is a function of the wave length (also of the moveout or phase velocity) of incident energy. Patterns are thus “moveout filters.” The present paper develops the general theory of arrays and gives an explicit solution for the design of an array having a predetermined moveout response. It is further shown that the antenna theory approach to pattern design is inadequate in the seismic case by reason of the non‐uniqueness of the velocity of incident energy. An example of a pattern designed to meet specific requirements is exhibited. The case presented effects a calculated improvement of at least four to one in signal‐to‐noise ratio over conventional patterns. The output of the designed system for a horizontally traveling, transient pulse input is shown to be greatly reduced in peak amplitude. Results of experimental and production use of the moveout filter are presented and discussed.
SEISMIC REFLECTION RECORDS OBTAINED BY DROPPING A WEIGHT
Authors: Neitzel Edwin B.
Published: October 2002

Measurements of the wave shape of the seismic transient generated by dropping a weight indicate that the weight drop has a much higher efficiency than shooting dynamite. The surface wave which is generated by the weight drop is very large, but it is minimized by the use of large seismometer groups, many drops per trace, and large offset distances. For experimental studies, a special recording instrument adaptable for either shooting or weight dropping is used. The weight truck is hydraulically operated. Weight dropping versus shooting studies indicate that the quality of weight drop records is limited by the large surface wave generated by the weight impact on the ground.
TRANSMISSION AND REFLECTION OF RAYLEIGH WAVES AT CORNERS
Authors: de Bremaecker J. Cl.
Published: October 2002

The methods of two dimensional model seismology were used to investigate the phenomena occurring when a Rayleigh wave is incident upon a corner whose angle is comprised between 0° and 180°. The wave bends its path only for angles between 130° and 180°. For smaller angles large and abrupt variations in reflection and transmission occur; the wave travels to the extremity of the corner and never “cuts corners”; only about 50 percent of the energy of the indicent surface wave is preserved as such, the rest goes into body waves; for a 90° corner the proportion is about 23 percent in P and 26 percent in S, with sharply preferential angles of incidence. The percentages given were found for a “plate Poisson’s ratio” of 0.17.
SOME THEORETICAL CONSIDERATIONS ON INDUCED POLARIZATION
Authors: Henkel John H.
Published: October 2002

The phenomenon of induced polarization is treated as an induced electromotive force, leading to a polarization constant which is included in the effective resistivity of polarizable materials, such as earth materials. The transmission of sinusoidal electromagnetic waves through a polarizable earth is treated for frequencies above 30 cps. The polarization induced in a two‐layer earth is treated theoretically and calculations are presented.
SURFACE SHIP GRAVITY MEASUREMENTS ON THE TEXAS A. AND M. COLLEGE SHIP, THE “HIDALGO”
Authors: LaCoste Lucien
Published: October 2002

Gravity meter readings taken at sea are disturbed by the motion of the ship and thus involve accelerations thousands of times greater than the permissible error. These accelerations must be averaged out and/or correction made for them. A meter which allows this to be done is described, and data obtained on a surface ship are presented with such checks as were available.
DIRECT INTEGRATION OF CONTINUOUS VELOCITY LOGS
Authors: Manchee E. B.
Published: October 2002

The number of conventional check shots associated with a continuous velocity log (CVL) well survey may be reduced to one or perhaps to zero if the integration of CVL travel times can be made at least as accurate as check‐shot times. This should be possible if mud lag and instrumental lag can be taken into account. By the use of a centralizer on the CVL sonde and a caliper log, mud lag may be calculated. A digital computer may be used to simplify reduction of the data. Results to date are encouraging but further testing of the method will be necessary.
LATERAL VELOCITY VARIATIONS NEAR BOREHOLES
Authors: Hicks Warren G.
Published: October 2002

Difficulties occur in obtaining accurate two‐receiver velocity logs in formations sensitive either to damage by exposure to drilling mud or to mechanical stress relief. Some shales are so altered by the drilling operation that their elastic properties are modified. Vertical velocity measured immediately adjacent the boreface is lower than if it were measured at a greater radial distance from the bore. These damaged shales require relatively deep penetration by the acoustic signal; consequently, the transmitter‐to‐first‐receiver spacing in a two‐receiver velocity logging system should be long enough to refract the sound waves through virgin formation. Experiments in one predominantly shaly section show a difference of almost 10 percent between times measured using transmitter‐to‐first‐receiver spacing of 4.3 ft compared to 8.8 ft. A limited amount of field data suggest that sodium montmorillonite is the clay type most sensitive to hydration and swelling. Studies of areal prevalence of the shale damage problem are incomplete.
SEISMIC RECIPROCITY
Authors: Knopoff Leon , Gangi Anthony F.
Published: October 2002

The reciprocity relationship describing the relations among the fields resulting from the interchange of point sources and receivers may be extended to the seismic case. Seismic reciprocity can be described either in terms of the scalar product of the vectors representing the excitation of the source and the field at the receiver, or in terms of a Green’s tensor describing these two quantities. Theoretical reciprocity relations give no information concerning reciprocity in the cases for which the scalar product vanishes. A simple experiment in the vector case demonstrates that reciprocity is not obtained when the scalar product of the two vectors vanishes.
PHOTOMECHANICAL METHOD OF FREQUENCY ANALYSIS OF SEISMIC PULSES
Authors: Howell B. F. , Andrews A. B. , Huber R. E.
Published: October 2002

The harmonic analysis of a seismic pulse performed by a photomechanical wave analyzer is analytically related to the Fourier integral analysis of this aperiodic pulse. The calculated integral spectra of three simple analytical pulses are compared to the spectra obtained from this wave analyzer. The integral analyses of two seismic pulses are compared to the harmonics obtained through numerical analysis. The frequency range of the integral analysis which can be performed by this analyzer is from 5 to 300 cps. Integral analyses of recorded seismic pulses up to 6 inches long (0.10 sec) and with peak amplitudes up to 0.69 of an inch can be performed by this analyzer. From an evaluation of the equipment and operational errors involved in the measurements, the integral approximations obtained from the analyzer are estimated to be accurate on the average to within one db in the frequency range of 5 to 100 cps. For frequencies greater than 100 cps this accuracy will decrease. This is as great an accuracy as is usually obtained by numerical analysis.
ELIMINATION OF SEISMIC GHOST REFLECTIONS BY MEANS OF A LINEAR FILTER
Authors: Lindsey J. P.
Published: October 2002

A technique is described for elimination of ghost reflections on magnetically recorded seismograph records by means of a linear filter. The application of this filter does not alter the character of primary reflections although eliminating the ghost reflections. The principal assumption made in the development of the technique is that the effect of AGC in altering the amplitude ratio of primary and ghost reflections is uniform for all record time. A realization of the required filter is given and a measurement technique is outlined for detecting the existence of ghost reflections based on the autocorrelation function of the seismograph trace.
PRELIMINARY REPORT — A STRATIGRAPHIC SEISMOGRAM
Authors: Savit Carl H.
Published: October 2002

To the present time, the great bulk of seismographic exploration has, perforce, been devoted to obtaining subsurface structural information. Stratigraphic information has not normally been forthcoming except insofar as stratigraphy could be deduced from structure. What little direct stratigraphic information was available came from velocity data resulting from refraction surveys or delta‐tee analyses of reflection results. Both these types of velocity‐derived stratigraphic information suffer from lack of resolving power.
THE METHOD OF GENERALIZED REFLECTION AND TRANSMISSION COEFFICIENTS
Authors: Spencer T. W.
Published: October 2002

The objective of this work is to provide a method for predicting the surface response of a stratified half space to the radiation from a localized source when neither the assumptions of the plane wave theory nor the assumptions of the normal mode theory are valid. The earth model consists of a finite number of perfectly elastic, homogeneous, isotropic layers separated by interfaces which are plane and parallel to one another. The method leads to an infinite series for the Laplace transform of the response function (displacement, velocity, stress, etc.) in a multi‐interface system. Each term in the series describes all the energy which traverses a particular generalized ray path between the source and the receiver. The specification of the mode of propagation across each stratum (either as an irrotational wave or as an equivoluminal wave) and of the sequence in which the strata are traversed serve to define a generalized ray path. A prescription is given for constructing the integral representation for the disturbance which has traversed such a path directly from the integral representation for the source radiation. The method therefore obviates the necessity for solving a tedious boundary value problem. The time function associated with each term can be obtained by using Cagniard’s method.
SOUND SPEED AND ABSORPTION STUDIES OF MARINE SEDIMENTS BY A RESONANCE METHOD—PART II
Authors: Shumway George
Published: October 2002

The relationships between sound speed and the more important physical properties of unconsolidated sediments have become known in recent years largely as a result of the studies of Hamilton, 1956; Hamilton et al., 1956; Shumway, 1956, 1958; Nafe and Drake, 1957; Sutton, Berckhemer and Nafe, 1957; and Laughton, 1954, 1957. The suite of samples used in this study is larger and more varied than the suites used in any of the earlier studies, and the measurements probably were more carefully taken. A large number of shelf and harbor sediments were used in addition to sediments from deeper water in the Pacific and Arctic Oceans.
NOMOGRAMS TO SPEED UP SEISMIC REFRACTION COMPUTATIONS
Authors: Meidav Tsvi
Published: October 2002

Nomographic solutions of a number of equations commonly employed in seismic refraction work are presented. The equations solved are: 1. Depth to a second layer, by means of the critical distance formula. 2. The critical angle and offset distance formulas. 3. True velocity of bedrock from the apparent up‐ and down‐dip velocities. 4. Depth to a second layer by means of the intercept‐time formula. This last solution is also applied to (a) finding the throw of a fault and (b) depth to an irregular second layer at each seismometer beyond the critical distance. Geometry of each of the nomograms is explained. Procedure for using each of the nomograms is described.
TRANSMISSION AND REFLECTION OF RAYLEIGH WAVES BY WEDGES
Authors: Knopoff L. , Gangi A. F.
Published: October 2002

Experimental observations have been made of the transmission and reflection of Rayleigh waves by wedges. Results are reported for Rayleigh waves in aluminum wedges. It is observed that the wave shapes of the transmitted and reflected waves differ from that of the incident wave and depend on the angle of the wedge. The change of shape is attributed to an interference between part of the incident wave‐form and the radiation from a line source placed at the vertex. A procedure is given for the calculation of the partition between the two terms.
ON A SUCCESSIVE APPROXIMATION METHOD FOR INTERPRETING GRAVITY ANOMALIES
Authors: Danes Z. Frankenberger
Published: October 2002

A new method for quantitative interpretation of gravity anomalies is presented. The disturbing body is represented by a finite number of vertical prisms arranged on a pre‐determined, regular grid. The horizontal dimensions of the individual prisms are small enough that they can be approximated by vertical‐line mass elements at the axis of the prisms. Formulas for gravity due to one prism are derived and, for the case of Gulf Coast salt densities, plotted on Figure 1. Gravity due to the whole body is an algebraic sum of the contributions of all prisms at the appropriate depths and distances. This method makes possible the direct interpretation by successive approximations by introducing proper geologic limitations. All the numerical work can be conveniently done on a high‐speed digital computer. The method is especially suitable for features with predominant vertical dimension such as salt domes and igneous plugs. It gives at least the same, and possibly higher, degree of accuracy as the graphical dot chart methods and, carried out on a digital computer, should be about two thousand times faster.
THEORY OF NEUTRON LOGGING I
Authors: Tittle C. W.
Published: October 2002

An analytical theory of epithermal neutron logging is presented. One‐group diffusion theory is applied to the slowing down of neutrons from a point fast neutron source in infinite continuous media, in a single cylinder, and in concentric cylinders representing a fluid‐filled borehole and the surrounding formation. Numerical results are given for the epithermal neutron flux in a water‐filled hole six inches in diameter, passing through limestone of 10 percent or 30 percent porosity. Preliminary semiquantitative agreement is obtained with the relative response of a commercial logging instrument in the range of 10 to 100 percent porosity.
AN EVALUATION OF THE GRAVITY CONTROL NETWORK IN NORTH AMERICA
Authors: Behrendt J. C. , Woollard G. P.
Published: October 2002

Observations with a LaCoste and Romberg geodetic gravimeter having a very low nearly linear drift rate, a high reading precision, and a world wide range were made at approximately three hundred sites in order to check and extend the gravity control network in North America. The sites occupied were mostly at former gravimeter bases located at airports, harbors, universities, and pendulum stations. The instrument was calibrated against the North American standardization range of pendulum measurements from Paso de Cortes, Mexico, to Fairbanks, Alaska, using the weighted mean values of the observations established with the U. S. Coast and Geodetic Survey, Cambridge University (England), and Gulf‐University of Wisconsin pendulum equipment. A statistical evaluation of the precision of the network based on the reoccupations at 40 major control stations gives an estimated standard deviation of 0.08 mgal. The airport network of bases previously reported by Woollard (1958) that was established with high range Worden gravity meters was found to require a systematic correction of 0.3 mgal per 1,000 mgal change because of the difference in calibration standard used. The adjusted values for the forty airport stations reoccupied agree on the average to 0.2 mgal with the results of this study. The reoccupations of the old pendulum observation sites of the U. S. Coast and Goedetic Survey suggest that much of this network is in error by over 3 mgals. Descriptions of sites occupied and the principal facts for position, elevation, observed gravity, and free‐air and Bouguer anomalies are presented.
UNDERGROUND AND SURFACE GRAVITY SURVEY, LEADWOOD, MISSOURI
Authors: Algermissen S. T.
Published: October 2002

A surface gravity survey consisting of 214 stations covering approximately four square miles was conducted over and adjacent to the North Leadwood Mines at Leadwood, Missouri. A corresponding survey of 278 stations was carried out in the mine workings. A method of reducing underground gravity observations is outlined. The principal factors limiting the accuracy of the underground observations are given. Methods for determining rock densities are described. A comparison of the surface and underground gravity maps shows that major Precambrian knobs were revealed by both surveys. Smaller structures not shown on the surface map were revealed by the underground survey. Anomalous density areas between the level of the two surveys were easily located.
THE DIURNAL PROBLEM IN AEROMAGNETIC SURVEYING IN CANADA
Authors: Whitham K. , Niblett E. R.
Published: October 2002

Errors introduced into aeromagnetic survey results because of geomagnetic time variations have been analyzed in two ways. In the first, statistical properties of average magnetic disturbance have been calculated for five locations well distributed in latitude in Canada. The autocorrelation functions of vertical magnetic field fluctuations have been used to estimate errors in aeromagnetic surveying as a function of the length of the base loop and the length of the traverse lines or profiles, for five latitudes. In practical applications, the r.m.s. errors are proportional to the square root of elapsed time. These results can be explained in terms of the shape of the autocorrelation functions, and models of disturbance which might produce them are discussed. In the second part experimental results are presented of a comparison of total field intensity fluctuations measured with recording proton precession magnetometers at two pairs of stations, one pair 26 miles apart and the other 94 miles apart. The measurements were taken in Western Canada south of the auroral zone. The r.m.s. differences in the time variations have been determined quantitatively for each hour for both pairs of stations; statistically, it was found that these differences were proportional to the r.m.s. level of magnetic activity and, in this region, to the separation of the stations. The frequency distributions of the r.m.s. differences between the two pairs of stations for the month of September 1959 have been determined. A comparison of the indirect and direct methods of correction indicates that scientifically, in this region, there is little to choose between them in terms of accuracy. Operational considerations are mentioned.
WICHITA MOUNTAINS SEISMOLOGICAL OBSERVATORY
Authors: Gudzin M. G. , Hamilton J. H.
Published: October 2002

The Wichita Mountains Seismological Observatory was engineered and is now being operated under the technical supervision of the Air Force Technical Applications Center (AFTAC) by The Geotechnical Corporation of Garland, Texas. The work is being performed as a part of Project VELA Uniform, under the overall direction of the Advanced Research Projects Agency (ARPA). The seismological equipment used is identical to that recommended in 1958 by the Conference of Experts for detecting violations of a possible agreement on the suspension of nuclear tests. The equipment recommended is quoted in Appendix I attached to this report. This is the first station to be built employing the recommended instrumentation and the capabilities of a control system using such instrumentation have not been determined. The observatory’s performance should be of great interest to delegates of the United States, United Kingdom, and U.S.S.R. who are currently negotiating in Geneva for a ban on nuclear testing.
SOME TECHNICAL ASPECTS OF REFRACTION SEISMIC PROSPECTING IN THE SAHARA
Authors: Layat C. , Clement A. C. , Pommier G. , Buffet A.
Published: October 2002

The refraction method of seismic exploration was initiated in 1952 in the Northern Sahara after several difficulties were encountered with the reflection method. The first tests showed the existence of a deeper marker bed having a velocity of about 6,000 m/sec (20,000 ft/sec) which later proved to be the eroded surface of the basement. Now refraction can be employed, at least in certain regions, in detailed surveys although many difficulties in interpretation still exist. Both field practices and methods of interpretation are discussed.
GEOPHYSICS AND CONGRESS
Authors: Miller George P.
Published: October 2002

I approach the task of addressing this distinguished group of Geophysicists with some trepidation. I am a layman, and I do not want to impart in any way that what I say can be taken in a technical sense. I salute you and I honor you and respect you for the great contribution you have made in the field of science. Thinking people in the world are now coming to know that, if we are going to survive on this planet, we must study it and know it. We must go into the air above, the depths of the ocean, and probe beneath the Earth’s crust to find her locked‐in secrets.
EXPERIMENTAL AND FIELD DATA FOR THE DUAL‐FREQUENCY PHASE‐SHIFT METHOD OF AIRBORNE ELECTROMAGNETIC PROSPECTING
Authors: Paterson Norman R.
Published: October 2002

With the growing maturity in airborne electromagnetic instrumentation a transition has taken place from rule‐of‐thumb application and interpretation methods to a more sophisticated over‐all approach. A greatly improved success rate in the location of massive sulphide bodies is illustrated by recent results with the dual‐frequency phase‐shift airborne electromagnetic method. Part of the sophistication process has been the improvement of quantitative methods of interpretation, based on both theoretical and model studies. A recent research program has been most useful in this regard. These studies have been carried out within the framework of an active exploration department and have been integrated with field results at various stages.
MODERN DEVELOPMENTS IN SHALLOW SEISMIC REFRACTION TECHNIQUES
Authors: Stam J. C.
Published: October 2002

A brief review is given of the development of seismic refraction equipment applicable to engineering problems. Specific aspects of the one‐channel and the multi‐channel equipments are compared and their relative usefulness discussed. Examples of some seismic surveys are presented.
OPTIMUM INVERSE FILTERS WHICH SHORTEN THE SPACING OF VELOCITY LOGS
Authors: Foster M. R. , Hicks W. G. , Nipper J. T.
Published: October 2002

A long‐spacing velocity log contains almost the same information as an ideal short‐spacing log, but in a distorted form with added noise. The distortion can be thought of as a moving average or smoothing filter. Its inverse, called a “sharpening” filter by astronomers, amplifies noise. If the inverse is to be useful, it must be designed with a balance between errors due to noise amplification and those due to incomplete sharpening. The Wiener optimum filter theory gives a prescription for achieving this balance. The result is called an optimum inverse filter. We have calculated finite‐memory optimum inverse filters using the IBM 704. We have applied them to actual data, digitized in the field, to produce synthetic short‐spacing velocity logs. These we have compared with their field counterparts. The synthetic logs have less calibration error and are free from noise spikes. The general agreement is good.
A HISTORY OF WELL LOGGING
Authors: Johnson Hamilton M.
Published: October 2002

The science of well logging, begun by Conrad Schlumberger in 1927 as an application of his work on resistivity measurements of the earth in surface exploration, has advanced further during the intervening thirty‐three years than even its creator could have imagined. From the early measurement of electrical properties by means of normal and lateral resistivity curves, modern logging has progressed through such changes as the introduction of wall‐contacting micro‐spacing devices, focused devices, dipmeters, and conductive and inductive techniques. Parameters other than electrical have been utilized, such as the radioactive properties of rocks both inherent and as a result of bombardment by both low‐ and high‐energy neutrons; the measurement of acoustic properties; the physical recovery of rock and fluid samples by wire‐line; etc. A chronological listing of these developmental steps of formation evaluation by downhole measurement is presented and the earlier ones are discussed in considerable detail. Each relates to a certain phase of development and is the answer to a certain need for evaluation of a specific zone or to overcome limitations of a prior method. It is hoped that this presentation of these tools, measurements, and methods, together with the interpretation techniques associated with them, will honor those who did the work and will inspire future logging engineers and analysts to even greater accomplishments than have preceded them.
BOUGUER GRAVITY CORRECTIONS USING A VARIABLE DENSITY
Authors: Grant F. S. , Elsaharty A. F.
Published: October 2002

The principle of density profiling as a means of determining Bouguer densities is studied with a view to extending it to include all of the data in a survey. It is regarded as an endeavor to minimize the correlation between local gravity anomalies and topography, and as such it can be handled mathematically by the method of least squares. In the general case this leads to a variable Bouguer density which can be mapped and contoured. In a worked example, the correspondence between this function and the known geology appears to be good, and indicates that Bouguer density variations due to changing surface conditions can be used routinely in the reduction of gravity data.
MAGNETIC ANOMALIES AND MAGNETIZATIONS OF THE BIWABIK IRON‐FORMATION, MESABI AREA, MINNESOTA
Authors: Bath Gordon D.
Published: October 2002

In a study of the relationship of magnetic anomalies to the magnetite content and structure of layered Precambrian iron‐formations, the effective induced and remanent magnetizations of the Biwabik iron‐formation were determined by analyses of aeromagnetic anomalies recorded 1,000 ft above the Biwabik in areas of known geology. The induced magnetization along the layers is relatively insignificant because the formation is almost perpendicular to the earth’s magnetic field. For the relatively unmetamorphosed iron‐formation of the Main Mesabi district, the dominant magnetization was found to be across the layers, about 0.012 gauss, and most likely induced. For the strongly metamorphosed iron‐formation of the East Mesabi district, the dominant magnetization was found to be along the layers, about 0.100 gauss, and remanent. These values are used as bases for explaining aeromagnetic anomalies from correlatives of the Biwabik iron‐formation in the Gunflint, Cuyuna, and Gogebic districts.
AIRBORNE ELECTROMAGNETICS IN REVIEW
Authors: Pemberton Roger H.
Published: October 2002

The art of prospecting by electromagnetic (EM) methods has progressed so rapidly in the last decade that it would seem timely for the publication of a review on this field of activity. Airborne EM surveys have been conducted on a world‐wide basis for quite a number of years, and they are now accepted as a standard and proven exploration tool. Unlike, however, the field of airborne magnetics which has received a great deal of attention in publications in many technical journals, the airborne EM method has scarecely been touched upon, publication‐wise. A few papers have been presented in the past on specific types of EM prospecting instruments and methods, but to the writer’s knowledge, no attempt as yet has been made to incorporate all the various techniques and EM instruments in one article. This review is therefore primarily an attempt to set down for the reader a short history of airborne EM development, along with some details on each of the different types of EM systems or instruments which have flown in the past or are presently flying today.
THE IMPORTANCE OF REFLECTED REFRACTIONS IN SEISMIC INTERPRETING
Authors: Allenby R. J.
Published: October 2002

Even under excellent recording conditions, the interpretation of seismic data is often complicated by the presence of energy not attributable to simple subsurface reflection sources. One type of extraneous energy, more prevalent than commonly realized, is reflected refractions which often complicate seismic results in many areas of the San Joaquin and Sacramento Valleys of California. In localities where the subsurface layering is nearly flat, reflected refractions, because of their steeply dipping alignments, can be readily recognized, although not all of these steeply dipping alignments will be reflected refractions. The geophysicist’s real identification problem comes in areas of steeply dipping geological beds, especially when record quality is poor. His failure to recognize reflected refractions can result in a seismic interpretation having little or no relationship to the true subsurface geological picture.
CHEBYSHEV OPTIMIZED GEOPHONE ARRAYS
Authors: Holzman Mark
Published: October 2002

A theoretical discussion of Chebyshev arrays is presented which leads to useful and practical extensions of the previously published empirical expositions. It is shown that the wavelength (or, equivalently, the moveout) discriminating property of these arrays may be either quickly estimated or fully determined without the knowledge of the weighting parameters. Thus, the geophysicist may perform a great many “pencil and paper experiments” rapidly without the need to compute array weights, except when actually specifying a cable design. In addition, complex interrelationships between several tapered arrays are clarified, and, finally, a simplified, closed form equation for the array coefficients is derived, which has substantial computational advantages over the present high‐order polynomial expressions. To exemplify the use of the criteria and the equations introduced below, a sample array design is shown in some detail.
NUMERICAL ANALYSIS OF RELATIVE RESISTIVITY FOR A HORIZONTALLY LAYERED EARTH
Authors: Onodera Seibe
Published: October 2002

The method of calculating the relative resistivity, which is the ratio of the apparent resistivity to the resistivity of the upper layer, for a multiple‐layered earth is given by means of the expansion of the kernel function according to a complete system of normalized orthogonal polynomials. The method, which includes estimation of the accuracy to be expected, is illustrated by application to a three‐layer earth.
THE KERNEL FUNCTION IN THE SURFACE POTENTIAL FOR A HORIZONTALLY STRATIFIED EARTH
Authors: Roman Irwin
Published: October 2002

After a review of the derivation of the system of equations determining the surface potential over a horizontally stratified earth, the kernel determinants are evaluated by the use of a schematic representation for each of five earlier papers that use Bessel functions. A symbolic notation is introduced to express the principal functions, and a recursion formula is deduced for each of these functions. The kernel is expressed as the sum of contributions from each successive boundary. The results are correlated with those obtained by the method of Kelvin images and expressed as integrals with finite limits of integration.
On: “AN EVALUATION OF BASEMENT DEPTH DETERMINATIONS FROM AIRBORNE MAGNETOMETER DATA,” BY PETER JACOBSEN, JR. (GEOPHYSICS, JUNE, 1961, PP. 309–319).
Authors: Steenland Nelson C.
Published: October 2002

At the outset, it must be mentioned that the writer was Editor of Geophysics at the time of publication of the Jacobsen paper on aeromagnetics. Inasmuch as Gravity Meter Exploration Company made Interpretation A, Figure 3, p. 313, the writer kept “hands off” the entire affair and published the material completely without consideration of his opinions.