bluuebunny/crossref_metadata_2025 huggingface dataset:
SEISMIC STUDIES ON ANTARCTIC ICE SHELVES
Authors: Thiel Edward , Ostenso Ned A.
Published: October 2002

Three methods have been employed at Little America V to determine seismic velocity as a function of depth in the Ross ice shelf: (1) refraction‐shooting interpreted by means of the Herglotz‐Bateman‐Wiechert integral, (2) seismic logging of a deep borehole, (3) Robin’s formula relating P wave velocity to the density and temperature measurements made in the borehole. Results of the three methods are compared, discrepancies are discussed, and the preferred P and S velocity‐depth profiles are obtained. From the profiles and the measurements of density in the borehole, a complete determination of the elastic properties of the ice shelf is possible. Small velocity differences are obtained at depth by the methods of refraction‐shooting and borehole‐logging. The effects of crystal orientation on seismic velocity is discussed and found to be sufficient to account for the differences. A new determination of the variation of seismic velocity in ice as a function of temperature is made on the basis of recent experimental work.
AIRBORNE GEOPHYSICAL RECONNAISSANCE IN THE CANADIAN ARCTIC ARCHIPELAGO
Authors: Gregory A. F. , Morley L. W. , Bower Margaret E.
Published: October 2002

Profiles of total magnetic intensity and gamma radioactivity were obtained along a series of widely‐spaced flight lines across the main tectonic regions of the Canadian Arctic Archipelago. The interpretation of these data and the calculated depths‐to‐basement substantiate the recognized regional structures, confirm general geological continuity between the islands, and provide some additional structural detail. Results of particular interest are the maximum depths‐to‐basement in the sedimentary basins (10,000 ft or greater), the interpretation of the structure of the Precambrian arches, the extent of nonbasement igneous activity, the apparent absence of disturbed ferromagnetic rocks on the Polar Continental Shelf except near the edge where a few deep basement‐type anomalies were observed, and the occurrence of anomalous radioactivity in certain sedimentary rocks on Bathurst Island. Remarkable magnetic anomalies, which in profile show a central minimum with marginal maxima, are characteristic of known gypsum domes in the Sverdrup Basin. It is concluded that reconnaissance geophysical surveys of comparable geological regions can provide much information concerning the structure and lithology of the area. In particular, this survey reveals geophysical contrasts of significance in the planning of future investigations in the archipelago and points out a number of interesting anomalies which require further study.
AN APPROACH TO INVERSE FILTERING OF NEAR‐SURFACE LAYER EFFECTS FROM SEISMIC RECORDS
Authors: Goupillaud Pierre L.
Published: October 2002

This paper suggests a scheme for compensating the effects that the near‐surface stratification, variable from spread to spread, produces on both the character and the timing of the seismic traces. For this purpose, accurate near‐surface velocity information is mandatory. This scheme should greatly reduce the correlation difficulties so frequently encountered in many areas. It may also be used to enhance the resolving power of the seismic reflection technique. The approach presented here is based on the rather restrictive assumptions of normal incidence, parallel equispaced plant reflectors, and noiseless conditions.
THE EFFECT OF SHOTPOINT MEDIUM ON SEISMIC COUPLING
Authors: Adams W. M. , Swift L. M.
Published: October 2002

Two experiments on the coupling of seismic waves to the surrounding medium have been conducted. One was performed at two different overburden depths in a tunnel complex at the Nevada Test Site in the Oak Spring Tuff. The other was done at a depth of 800 ft in the Carey Salt Mine at the Winnfield Salt Dome near Winnfield, Louisiana. Free‐field measurements within 400 ft of the chemical charges indicate that three to four times more energy was propagated into the elastic zone in the tuff than in the salt. There is some indication that in tuff the amount of energy transmitted into the elastic zone was dependent upon the lithostatic overburden pressure. Increasing the overburden pressure by a factor of about five almost quadrupled the energy propagated into the elastic zone.
RESOLUTION AND CURVED PATH COMPUTATION OF STEEP DIP USING AN ELECTRONIC COMPUTER
Authors: Oksa Donald R.
Published: October 2002

A program for a basic IBM 650 electronic computer is discussed that resolves and computes four‐way seismic dip control. The velocity parameters are a separate entry to the program in order that prospect re‐evaluation, because of new velocity data, can be easily accomplished. Ample identification of specific reflection data is permitted. Although written specifically for curved path velocity assumptions, modification for different types of velocity functions would be a comparatively simple task.
LINEAR APPROXIMATION OF APPARENT RESISTIVITY FUNCTIONS
Authors: Unz M.
Published: October 2002

Linear expressions are sufficient for the approximation of resistivity functions, if the trend of the resistivity contrasts is taken into account. The maximum error in two‐layer systems has been determined. Application to more than two layers and interpretation methods are investigated. Three‐layer examples are discussed.
GEOPHYSICAL STUDY OF CENOZOIC GEOLOGIC STRUCTURES OF NORTHERN OWENS VALLEY, CALIFORNIA
Authors: Pakiser L. C. , Kane M. F.
Published: October 2002

A narrow gravity minimum anomaly of amplitude 30 mgals indicates that northern Owens Valley, California, a narrow fault‐bounded trough or graben filled with Cenozoic clastic deposits to a depth of as much as 8,000 ft. Seismic‐refraction measurements support this conclusion. Aeromagnetic and gravity measurements define a small, dense, and magnetic body buried by the valley fill of northern Owens Valley.
On: “GRAVITY AND AEROMAGNETIC EXPLORATION IN THE PARADOX BASIN,” BY NELSON C. STEENLAND (GEOPHYSICS, FEBRUARY, 1961, PP. 73–89)
Authors: Byerly P. Edward
Published: October 2002

Following are some comments concerning Dr. Steenland’s discussion of gravity data in Professional Papers 316-A and 316-C of the U.S. Geological Survey. Dr. Steenland’s cylinder calculations are entirely dependent upon the regional anomaly removed. He obtained an anomaly of −40 mgal over Paradox Valley, whereas we obtained a residual anomaly of −32 mgal. The bottom 4,000 ft of Dr. Steenland’s cylinder contribute −5 mgal to the anomaly. Dr. Steenland’s analysis supposes that the only effective density contrasts are those attributable to variations in salt thickness or isostatic compensation.
A COMPREHENSIVE SYSTEM OF TERRAIN CORRECTIONS USING A DIGITAL COMPUTER
Authors: Kane M. F.
Published: October 2002

The system of terrain corrections uses an electronic digital computer for much of the calculation. A new method using a desk calculator is suggested for terrain effects arising from close‐in topography. The present system covers a square area 40 km by 40 km, with the station at the center. A selected group of corrections that were compared with conventionally computed ones agree within 0.1 mgal. The speed of the operation using the Datatron 220 computer is considerably faster than the conventional method of computing.
AN ANALYSIS OF SOME SECOND DERIVATIVE METHODS
Authors: Daneš Z. F. , Oncley L. A.
Published: October 2002

Second vertical derivative methods developed by Henderson and Zietz, Elkins, Rosenbach, and Danes are applied to various mathematical harmonic fields and the response of the methods is investigated. All methods give good results as long as the wavelength of the anomaly is large compared with station spacing. As the wavelength decreases, Elkins’ formulae give progressively lower values. Rosenbach’s values decrease slightly, but values are still very close to correct. Henderson and Zietz’s values are about halfway between the above ones. Danes’ results can be made correct to the degree of accuracy of the input data.
BASIC THEORY OF INDUCTION LOGGING AND APPLICATION TO STUDY OF TWO‐COIL SONDES
Authors: Moran J. H. , Kunz K. S.
Published: October 2002

The basic idea of induction logging is reviewed and the use of the geometrical factor is explained. The rigorous theory based on Maxwell’s equations, is then formulated for the simple two‐coil sonde, which is the basic building block for multicoil sondes. For a homogeneous conducting medium this theory leads to simple solutions for the fields and current density. These simple solutions are exploited in detail to build up a firm understanding of the skin effect phenomenon. The theory is then applied to nonhomogeneous media and numerical results obtained through the use of the IBM 704 are presented. On the basis of this complete study some simple approximate results are presented which allow a rapid evaluation of skin effect in induction logging. The nonhomogeneous media considered in detail are two semi‐infinite beds, thin beds bounded by adjacent formations, and an infinite bed with a borehole or invasion. It is shown that the borehole gives a negligible contribution to the skin effect.
On: “RECENT TECHNIQUES FOR DETERMINATION OF IN SITU ELASTIC PROPERTIES AND MEASUREMENT OF MOTION AMPLIFICATION IN LAYERED MEDIA,” BY R.J. SWAIN (GEOPHYSICS, APRIL, 1962, PP. 237–241).
Authors: Nicholls Harry R.
Published: October 2002

Although I am in general agreement with Mr. Swain’s paper, there are several pitfalls inherent in the use of dynamic elastic constants which should not be ignored. The strength of materials and the elastic properties both undoubtedly depend on the rate of loading and/or the stress levels involved. It does not seem appropriate, therefore, to use dynamic in situ elastic properties for static design problems. The specific design problem at hand should determine the relative value placed on the use of static or dynamic elastic constants. The dynamic in situ values are generally more reliable than those obtained in the laboratory as indicated by Mr. Swain, although continued development of the laboratory pulse and critical‐angle method shows promise of improving the reliability of laboratory values.
On: “SEISMIC RECEPTORS,” BY H. R. PRESCOTT (GEOPHYSICS, JULY, 1941, pp. 221–244).
Authors: Dennison A. T.
Published: October 2002

The paper by Prescott is well known for its discussion of the steady‐state and transient response of seismic detectors. The purpose of this note is to point out that while the steady‐state curves shown in this paper are correct, the transient responses shown are erroneous.
A STUDY OF TWO‐DIMENSIONAL HEAD WAVES IN FLUID AND SOLID SYSTEMS
Authors: Dunkin John W.
Published: October 2002

The problem of transient wave propagation in a three‐layered, fluid or solid half‐plane is investigated with the point of view of determining the effect of refracting bed thickness on the character of the two‐dimensional head wave. The “ray‐theory” technique is used to obtain exact expressions for the vertical displacement at the surface caused by an impulsive line load. The impulsive solutions are convolved with a time function having the shape of one cycle of a sinusoid. The multiple reflections in the refracting bed are found to affect the head wave significantly. For thin refracting beds in the fluid half‐space the character of the head wave can be completely altered by the strong multiple reflections. In the solid half‐space the weaker multiple reflections affect both the rate of decay of the amplitude of the head wave with distance and the apparent velocity of the head wave by changing its shape. A comparison is made of the results for the solid half‐space with previously published results of model experiments.
AEROMAGNETIC PROSPECTING FOR IRON ORE ON EYRE PENINSULA, SOUTH AUSTRALIA
Authors: Mumme W. G.
Published: October 2002

The analysis of anomalies on the aeromagnetic survey maps of the Middleback Ranges and other areas of Eyre Peninsula in South Australia which were flown in the search for iron‐ore deposits has provided depth estimates which are in good agreement with known and later discovered geological facts. The average error involved in these estimates is of the order of 15 percent.
AN EVALUATION OF THE PEACE RIVER AEROMAGNETIC INTERPRETATION
Authors: Steenland Nelson C.
Published: October 2002

The magnetic basement map computed from the 1950 Peace River aeromagnetic survey of 27,000 sq mi in western Canada is compared with the subsurface Precambrian surface determined by 169 deep tests subsequently drilled. The computed map has a standard deviation of 6.5 percent and an average error of 7.4 percent. The errors of the computed depth estimates are biased toward the shallow side; that is to say, the depth estimates are generally not deep enough. Moreover, excessive errors, those over 20 percent, are conspicuously shallow. This asymmetrical distribution is attributed to the failure to utilize plate models in the interpretation, as both intrabasement and suprabasement anomalies may be identified in the observed field. Consequently, the comparison confirms the existence of the intrabasement magnetization units within the Precambrian by establishing the validity of the coefficients derived from them and used in computing the depth estimates.
MAGNETIC SUSCEPTIBILITY OF BEDDED IRON‐FORMATION
Authors: Jahren Charles E.
Published: October 2002

The susceptiblity of natural and artificial iron‐formation samples in the form of cylinders with length‐to‐diameter ratio less than 1.2 and susceptibility up to one cgs unit was measured in fields of the order of one gauss. Demagnetization factors calculated for the centers of the cylinders explain observed changes of apparent susceptibility with length and direction of magnetization. Layered samples made by stacking disks of magnetic and nonmagnetic material alternately show susceptibility as much as three times greater parallel to the layers than across them, with layer susceptibility as high as 0.5 cgs. This anisotropy increases with increasing layer susceptibility and is largely independent of the details of layering when less than half the volume of the sample is magnetic material. Natural layered samples show the same range of susceptibility anisotropy. The relationship between susceptibility, k, and volume percent magnetite, V, can be approximated by [Formula: see text] cgs, 10<V<40, corresponding to susceptibility 0.7 for pure magnetite. The along‐the‐layer susceptibility of a bedded sample containing 20 volume percent total magnetite is twice that of a homogeneous sample of the same overall magnetite content. For one percent total magnetite, it is three times as great.
A TECHNIQUE FOR SOLVING THE LOW‐VELOCITY LAYER PROBLEM
Authors: Thompson Jay F.
Published: October 2002

The advent of surface‐energy source methods and the increasing use of shallow‐hole shot patterns to obtain seismic data have complicated the determination of datum corrections. This is particularly true when spreads are offset from the station. At the expense of seismic party time, the data usually are acquired by shooting uphole surveys or special refraction profiles. This paper describes a refraction technique which uses a weight‐drop energy source and special recording instruments to obtain this vital information.
TIME‐DEPTH AND VELOCITY‐DEPTH RELATIONS IN WESTERN CANADA
Authors: Acheson C. H.
Published: October 2002

Every time‐depth curve studied so far in western Canada can be divided into at most a few segments, precisely represented by equations of the type [Formula: see text]. The particular values of a, b, and n that define each segment are of fundamental importance. Best values of these quantities have been determined by statistical methods at several places in the basin, and their areal variation is significant in terms of lithologic changes and basin position.
ADDENDUM TO “STUDIES OF ELASTIC WAVE ATTENUATION IN POROUS MEDIA” BY M. R. J. WYLLIE, G. H. F. GARDNER, AND A. R. GREGORY (GEOPHYSICS, OCTOBER, 1962, PP. 569–589)
Authors: Wyllie M. R. J. , Gardner G. H. F. , Gregory A. R.
Published: October 2002

In the paper published last year we noted that the shear velocity through liquid‐saturated rocks often appeared to exceed the shear velocity through the same rocks when dry. Our results were based on measurements made by the critical‐angle method on rocks subjected to heavy confining pressure. We felt constrained to observe that, if shear velocities through liquid‐saturated rocks were higher than through their dry counterparts, the applicability of Biot’s theory to any but low‐frequency resonating systems was open to question. Because Biot’s theory will be of maximum use only if it can be applied to systems in which velocities are measured by high‐frequency pulse techniques, our warning diminished the practical value of our other results.
MURAC, A MULTIPLE REFLECTION ANALOG COMPUTER
Authors: Silverman D. , Lash C. C. , Hadley C. F.
Published: October 2002

Studies of the relative strength of multiple reflections as compared to primaries indicate that many reflection events on field records picked as primaries were probably multiple reflections. This has made the identification of multiples a matter of first importance. One method for identifying multiples and primaries is to predict them on the basis of log information. In practice, this means to compute synthetic records which have primary and multiple reflections in essentially proper time and amplitude relation. Synthetic records which include multiples have been computed by both digital and analog computers. Each has advantages and limitations which make them complementary rather than competitive processes. The digital method using the IBM 704 provides completeness and accuracy but is not well adapted to the study of many small changes or tests. The analog computer reported here utilizes a new concept which makes it possible to achieve a speed and convenience not previously available. The design of MURAC features two delay lines and a series of circuits which provide appropriate amplitude relations at the interfaces between formations. One delay line carries up‐traveling components, one the down‐traveling, while the transmission and reflection at the 20 interface units inserted in the lines is accomplished by means of potentiometers, amplifiers, and switches. As a consequence, the computer is readily set up and is adaptable to a wide variety of problems. Examples of the use of MURAC are given in the report. In addition to the computation of synthetic records, MURAC is equipped with controls which provide for its use as a time‐domain filter. Sixty sampling points, each with voltage divider, amplifier, and polarity control make it possible to provide a wide variety of responses. These controls can also be used to make primaries‐only synthetic records with a fine pattern of interfaces. A wavelet generator is available as input for the MURAC. The wave shape desired is obtained by setting of controls and the period or time scale is adjusted as desired. For example, Ricker wavelets having breadths which vary by small steps from 4 to 100 ms can be obtained. The controls provide for 90 sampling points at uniform time intervals. The wavelets generated can be inserted into MURAC to simulate concentrated or distributed charge shots.
PROPAGATION OF A PULSE IN A FLUID SPHERE
Authors: Alterman Z. , Kornfeld P.
Published: October 2002

The exact solution obtained in a previous paper for the motion of a uniform compressible fluid sphere due to a pressure pulse from a point source situated below the surface is applied to a source at a distance of one‐eighth of the radius below the surface. Taking the sphere as a simplified model of the earth, this corresponds to a source at a depth of about 800 km, which is not far from the depth of a deep‐focus earthquake. The time variation of pressure due to the source is represented by the difference between two step functions with rounded shoulders. The surface velocity due to sources of different durations has been evaluated for eight angular distances. The solution exhibits step function type and “logarithmic” reftected pulses, which one would anticipate from the “steepest descents” analysis of Jeffreys and Lapwood. In addition, the solution reveals single diffracted pulses and groups of diffracted pulses which have no counterpart in ray theory. When geometrical optics allows a ray to appear only after a minimum range [Formula: see text] from the epicenter, the complete wave‐theoretical solution shows that these pulses show up earlier in the forbidden zones. Similarly, in the case where the geometrical optics predicts that a certain ray should appear only for ranges [Formula: see text], and should not appear for [Formula: see text], the wave‐theoretic solution shows that such a ray does appear, by diffraction, for some range of [Formula: see text]. Arrival times of the diffracted pulses of the first group increase with increasing θ, while for the second group they decrease with θ.
APPROXIMATION OF RESIDUAL TOTAL‐MAGNETIC‐INTENSITY ANOMALIES
Authors: Kontis A. L. , Young G. A.
Published: October 2002

Utilizing magnitude and direction observations of the magnetic field, the anomalous total intensity (FA), the residual total intensity (FR), and the projection of the anomalous force in the direction of the earth’s normal field (T) are computed for two profiles over Plantagenet Bank. It is demonstrated that for the large anomaly over Plantagenet, the difference between T and FR is relatively small, and that in general T is a good approximation of the residual anomaly.
THE ELUSIVE FIRST ARRIVAL
Authors: Hagedoorn J. G.
Published: October 2002

In first‐arrival refraction work, the initial deflection of the first loop of the signal arriving from the shotpoint must be readily recognised against a background of seismic disturbances due to sources other than the explosion. The accuracy of timing a first arrival is determined by this signal‐to‐noise ratio. It depends primarily on the location of shot and receivers, the size of the charge, and the existing ground unrest at the time of registration. Experiments carried out with these variables kept constant, by recording at the same location from the same shot, show how much the signal‐to‐noise ratio also depends on the characteristics of the recording equipment used. The best signal‐to‐noise ratio is certainly not obtained when the transmission curve of the entire system, comprising geophone, amplifier, and galvanometer, peaks at the apparent dominant frequency of the refraction signal. Practical examples show that the signal‐to‐noise ratio can be improved considerably by using recording systems that transmit a band of frequencies extending many octaves below the observed dominant frequency. The inception of an oscillatory signal was found to be particularly sensitive to the characteristics of a recording system. A seismometer, for example, will transform a starting sine wave with a frequency equal to the natural frequency of the seismometer into a signal with a first loop that is about half as high and half as long as the succeeding loops, the latter moreover being advanced by one‐quarter period. This relative constriction of the initial part of a signal is called the “cramping” effect. Such an effect will weaken a refraction first arrival relative to simultaneously arriving later parts of noise signals. This explains why a cramping effect will impair the signal‐to‐noise ratio. A cramping effect can, of course, be avoided by using a recording system with a flat frequency response. The opposite effect, which can be expected to improve the signal‐to‐noise ratio, could obviously be achieved by using systems with relatively increased low‐frequency response. The practical limit to this improvement would be set by the low‐frequency noise that is enhanced by this procedure.
REGIONAL VELOCITY GRADIENT FROM NORMAL MOVEOUTS
Authors: Knox William A.
Published: October 2002

It is well known that an average‐velocity‐depth function can be computed directly from reflection times and normal moveouts, but, in current practice, results of such computations yield only a single function for an area as a whole, without disclosing the presence of a velocity gradient if it should exist. Normal moveout is a function of velocity; therefore, if moveout changes for a selected reflection are posted on a base map and contoured, contour values will be indicative of concomitant changes in velocity. To be of use quantitatively, raw normal moveouts must be adjusted for effects of known variables and a judgment must be made of the likelihood that changes in normal moveout are due to significant velocity changes and not to random timing errors. A field example is presented to illustrate application of the concept.
DATUM CORRECTIONS IN GLACIAL DRIFT
Authors: Patterson A. R.
Published: October 2002

It is well known that velocity inversions and surface‐to‐surface‐reflection‐time delays are common in glacial drift. The surface‐to‐surface‐reflection‐time delays are correlative with relatively low velocity material and are believed to be associated with the elastic properties of the material adjacent to the shot. Datum corrections, when these phenomena are encountered, are best made by curved‐path weathering methods of the Blondeau type.
THE RESOLUTION OF COMBINED EFFECTS, WITH APPLICATIONS TO GRAVITATIONAL AND MAGNETIC DATA
Authors: Elkins Thomas A. , Hammer Sigmund
Published: October 2002

A simple but rigorous and quite general mathematical method is given for finding the minimum separation of two nearby bodies at which their observed combined effect indicates the presence of two separate bodies. Geophysical applications of the method are illustrated by investigating the resolution of gravity and torsion balance data for the two limiting cases of spheres and infinite horizontal cylinders, the resolution of the vertical magnetic intensity for infinite rectangular plugs, and the direct interpretation of the infinite horizontal rectangular block. The possible usefulness of such analysis in the selection of a geophysical field method and in the choice of station spacing is discussed.
FREE‐AIR GRAVITY ANOMALY MAP OF THE GULF OF MEXICO AND ITS TECTONIC IMPLICATIONS, 1963 EDITION
Authors: Dehlinger Peter , Jones B. R.
Published: October 2002

As part of a continuing program, Texas A. and M. University has been making a surface‐ship gravity survey of the Gulf of Mexico. The 1963 free‐air anomaly map of the Gulf is the second in a series of maps resulting from these investigations; it includes not only a larger area than the first map, but also measurements having considerably higher accuracies. The present map indicates, as the first one suggested, that the Gulf of Mexico is essentially in isostatic equilibrium but contains local mass anomalies. Two cross sections of crustal layers were constructed which are consistent with the gravity observations and published seismic‐refraction results. One section extends from Galveston, Texas, to the Yucatan Peninsula, and the other from the Sigsbee Deep to Florida. Both sections consist of four to six layers, in which layer densities were assumed to be uniform laterally and the density below the Mohorovičić discontinuity constant. Densities of the layers were converted from seismic velocities using the Drake and Nafe curves. A free‐air anomaly profile across the Cayman Trench is included.
RADIATION RESISTANCE OF A HORIZONTAL CIRCULAR LOOP OVER A FINITELY CONDUCTING GROUND AND APPLICATION OF RESULTS TO GEOPHYSICAL EXPLORATION OF CONDUCTING SHEET‐TYPE DEPOSITS
Authors: Negi Janardan G.
Published: October 2002

A general analysis for radiation from a circular current‐carrying loop (with its axis vertical) in the presence of a homogeneous conducting earth is presented. The exact expressions for the electromagnetic fields and power radiated are derived for this case and then reduced to computable forms. The analysis is further extended to a case when the loop is situated over a thin highly conducting sheet. It is believed that the results can be useful for stratigraphic investigation of the earth’s crust and also for the geoelectric exploration of conducting sheet‐type deposits.
INDUCED‐POLARIZATION LOGGING IN THE SEARCH FOR NATIVE COPPER
Authors: Bacon Lloyal O.
Published: October 2002

Induced‐polarization logging of underground boreholes, in the footwall zones of the Osceola Mine, has provided an increased sample volume to detect the presence of copper mineralization as well as to provide information on geological contacts which aid in geologic interpretation and in the planning of mining operations.
A NEW DATA‐PROCESSING TECHNIQUE FOR MULTIPLE ATTENUATION EXPLOITING DIFFERENTIAL NORMAL MOVEOUT
Authors: Schneider William A. , Prince E. R. , Giles Ben F.
Published: October 2002

A new data‐processing technique is presented which utilizes optimum multichannel digital filtering in conjunction with common subsurface horizontal stacking for the efficient rejection of multiple reflections. The method exploits the differential normal moveout between primary and multiple reflections that results from an increase in average velocity with depth. Triple subsurface coverage is obtained in the field; the common subsurface traces are individually prefiltered with different filters and stacked. The digital filters are designed on the least‐mean‐square‐error criteria to preserve primaries (signal) in the presence of multiples (noise) of predictable normal moveout, and random noise. The method achieves wide‐band separation of primary and multiple energy with only a three‐point stack; it can work effectively with small normal moveout differences eliminating the need for long offsets and the attendant signal degradation due to wide‐angle reflections; it does not require equal multiple moveout on the triplet of traces stacked; and finally the method is not sensitive to small errors in statics or predicted normal moveout. The technique is illustrated in terms of synthetic examples selected to encompass realistic field situations, and the parameter specification necessary for the multichannel filter design.
A METHOD FOR COMPUTING THE TOTAL MAGNETIZATION VECTOR AND THE DIMENSIONS OF A RECTANGULAR BLOCK‐SHAPED BODY FROM MAGNETIC ANOMALIES
Authors: Bhattacharyya B. K.
Published: October 2002

In this paper is presented a new method for determining the following parameters of a uniformly magnetized body of rectangular prismatic shape: (i) horizontal dimensions, (ii) depths to the top and bottom of the body, and (iii) intensity and direction of magnetization. Accuracy in the computation of these parameters is highly dependent on the correct location of the center and on the determination of the major and minor axes of the body. An iterative method of calculations is used. This method is considerably aided not only by the second vertical derivative map of the observed total field but also by the total field reduced to the pole and its second vertical derivative map. The horizontal dimensions are determined by noting the location of the maximum of the odd component of the second vertical derivative about the center of the body. These dimensions are estimated with high accuracy when they are greater than the depth to the top of the body. The remaining unknown parameters of the body are calculated with the help of the first horizontal and vertical derivatives and the total field at the origin in the plane of observation which is directly above the center of the body. The present method also requires the total‐field value at the point one‐half unit of length above the origin. The factors affecting the accuracy in the calculation of the parameters are discussed in detail. With the help of high‐speed digital computers, this method can be used with great advantage for computation of the above parameters of magnetized bodies giving rise to a number of anomalies over a particular area.
STATIC FRICTION AS A SOURCE OF SEISMIC ATTENUATION
Authors: White J. E.
Published: October 2002

This paper discusses a particular mechanism for energy loss in granular media, involving sticking and sliding between grains at areas of contact. Earlier publications have developed expressions for attenuation in regular packings of spheres, attributing all the loss to sliding friction. The present work extends the model by incorporating static friction and thereby derives an expression for attenuation which is in better agreement with measured values. The attenuation coefficient for both shear and compressional waves is found to be: 1. Independent of vibration amplitude at low amplitudes; 2. Directly proportional to frequency; 3. Dependent on the difference between static and sliding coefficients of friction; and 4. Dependent on the packing of the granules.
STRUCTURE OF THE LOWER CONTINENTAL RISE HILLS OF THE WESTERN NORTH ATLANTIC
Authors: Ballard James A.
Published: October 2002

Data acquired during a recent seismic reflection survey of portions of the western North Atlantic have provided new information on the movement and deposition of sediments on the ocean floor. The area surveyed included portions of the lower continental rise hills which form the transition between the continental rise and the abyssal floor east of Cape Hatteras. The lower rise hills are ridges having a general east‐west orientation and an average length of 20 miles. Individual hills have 25 to 50 fathoms relief and are two to three miles wide. Each of the hills has a relatively short, steep out‐crop slope facing seaward and a shallow dip slope. Shallow subbottom reflectors dip opposite to the regional slope; deep reflectors are structurally independent of the upper reflecting layers. In profile, the ridges appear to be imbricate blocks with a maximum thickness of 100 fathoms. Ridges become progressively smaller seaward and are eventually onlapped by the horizontal beds of the abyssal floor. No fault structure is visible. The imbricate structure of individual hills, dip reversals between the continental rise and lower rise hills, dip reversals between the lower rise hills and the abyssal floor, and the apparent recent denudation of sediment cover from the continental rise indicate that the lower rise hills are gravitational glide blocks. Published literature on the engineering properties of marine sediments illustrates progressive gravitational gliding on slopes as low as 1:100.
DISPERSION OF BODY WAVES IN LAYERED MEDIA
Authors: GUPTA I. N.
Published: October 2002

The propagation of P and S waves is dispersive whenever the elastic medium considered departs from an infinite, homogeneous, isotropic, and ideally elastic medium. The dispersion effects are generally small but not always negligible. It is shown that, for propagation normal to a layered structure, shorter‐period body waves travel faster than the longer‐period body waves. This result is used as a possible qualitative explanation for observations left unaccounted for in several explosion and earthquake investigations.
SOME NUMERICAL TECHNIQUES FOR APPLICATION IN MINING EXPLORATION
Authors: Fraser D. C. , Fuller B. D. , Ward S. H.
Published: October 2002

Certain techniques of time series analysis may be applied to a space series, such as a geophysical profile, by utilizing a space frequency (or wavenumber) expressed in cycles by linear dimension. Among these techniques are the approximately rectangular band‐pass filter operators, the use of which may permit anomaly enhancement by removal of “noise” which lies outside the frequency band of interest. A type of spectral analysis, utilizing a band‐pass filter operator, is described whereby filter characteristics may be chosen without a priori assumptions regarding the spectral characteristics of anomaly and noise. The multiple crosscorrelation of m lines (m=4, 6,⋯) may yield a more useful output in the search for linear trends than the conventional paired crosscorrelation technique.
EXPERIMENTAL DEEP RESISTIVITY PROBES IN THE CENTRAL AND EASTERN UNITED STATES
Authors: Anderson L. A. , Keller G. V.
Published: October 2002

Nine deep resistivity probes were made in southeastern Nebraska, southwestern Iowa, central Pennsylvania, southwestern and northeastern New York, Vermont, New Hampshire, and Maine. The surveys were designed to determine the requirements for crustal‐scale resistivity surveys in areas where there is no thick overburden of conductive sedimentary rocks. The Schlumberger array was used for spacings up to one km, the dipole method for wider spacings. A source dipole moment in the range [Formula: see text] to [Formula: see text] amp‐m was used with the dipole method. Electric field measurements were made at distances up to 60 km from the source dipole. These separations were adequate to determine the presence of resistant basement rock, but not adequate to penetrate the crust. Penetration of the crust might have been obtained if the source dipole had been stronger by an order of magnitude, but experience gained in surveys made in northeastern New York and in central Vermont suggests that lateral changes in surface resistivities may present a difficult problem at large dipole spacings. In order to penetrate the highly resistive screening portion of the crust, galvanic resistivity surveys should be supplemented by inductive surveys which would not require such large separations of source and detection coils for the measurements.
DEEP RESISTIVITY PROBES IN THE SOUTHWESTERN UNITED STATES
Authors: Jackson D. B.
Published: October 2002

Eight deep resistivity probes have been made along a traverse extending from southwest Colorado to southern California to determine the electrical properties of the deep crust. The dipole method for measuring resistivity was used, and dipole spacings as great as 50 to 100 km were obtained at each survey location. In the Colorado Plateaus province, the electrical basement usually does not coincide with the top of the Precambrian crystalline basement, but rather, corresponds with the top of the carbonate rocks of Paleozoic age. Because of the large electrical conductance in the sedimentary column, resistant basement rock was detected at only three of the five sounding locations in the Colorado Plateaus province. At these three locations, the minimum possible basement resistivity was 2000 to 6000 ohm‐m. Electrical basement was detected at the survey locations in the Basin and Range province. The minimum possible basement resistivity was 600 to 1000 ohm‐m. A survey near Barstow, California, indicates that the resistant portion of the crust may be 9 to 10 km thick and may have a resistivity of 600 to 700 ohm‐m. A conductive zone underlies the resistant portion of the crust.
MODEL INVESTIGATION OF EXPLOSIONS IN PRESTRESSED MEDIA
Authors: Kim W. H. , Kisslinger C.
Published: October 2002

Seismic effects of explosions and rupture propagation in prestressed two‐dimensional models (Plexiglas, aluminum) as well as anisotropy produced by the stress field are investigated. An explosion in a prestressed medium releases a portion of the stored strain energy by one or more of the following mechanisms: (1) formation of directional cracking, especially in brittle materials, (2) release of strain energy in the elastic zone outside the cavity, and (3) rupture propagation. Phenomena associated with all of these mechanisms were observed in the present investigation. Explosions in prestressed Plexiglas produce cracks in preferred directions, the intensity of which increases with the applied stress. Explosions in prestressed aluminum sheets do not cause fracturing but rather plastic deformation about the explosion. Straight and branching modes of moving cracks initiated from explosions in prestressed Plexiglas can be explained on the basis of stress distribution ahead of the crack tips. Observed radiation patterns resulting from explosions in prestressed media indicate asymmetrical radiation fields which are a direct consequence of strain energy release for the case of aluminum and by the combined effects of directional cracking and energy release in the elastic zone for the case of Plexiglas. Explosions in prestressed media generate shear waves. The observed S‐wave magnitude increases sharply with the level of the existing stress field for a given amount of strain energy release. It is concluded that this phenomenon is attributable to the effective conversion of energy release to seismic radiation at high ambient stress fields. In other words, the effectiveness of S‐wave generation is governed by the rapidity with which the existing strain energy is released. A definite anisotropy effect was observed in prestressed models, but this effect is not large enough to affect wave propagation in the range of the tensile loads applied.
OPTICAL PROCESSING AND INTERPRETATION
Authors: Fitton John C. , Dobrin Milton B.
Published: October 2002

Although the use of optical techniques for enhancing seismic data has become well established, the applicability of these techniques to seismic interpretation is not so widely recognized. Optical processing is ideally suited for use as a direct aid to interpretation because of the precision with which filtering can be controlled and because of the flexibility made possible by the instantaneous visual presentation of the filtered data. Frequency relationships in seismic data have great value in interpretation, and optical techniques are particularly suitable for bringing out such relationships. The one‐dimensional optical transform displays a channel‐by‐channel spectrum of a seismic section from which useful geological information can be inferred. On such transforms significant effects can often be brought out which are not discernible on the corresponding record sections. Reefs, for example, often cause a thinning of overlying formations which gives rise to a high‐frequency anomaly on the transform, even at levels so shallow in the section that no evidence for reef effects is apparent to the eye on the original records. Characteristic frequency anomalies can also be observed over faults. One‐dimensional transforms from sections made over features of both kinds show diagnostic patterns that can be used as a basis for detection. The sharp cutoffs and flexibility available in optical filters make it possible to discriminate between conflicting events on record sections by frequency filtering alone. With proper monitoring, one can select those cutoff frequencies which bring out events that appear geologically most plausible. Multiple reflections, for example, can often be eliminated by frequency discrimination once the geophysicist identifies the primary reflections on the monitor. Often seismic records are discarded as useless, when in reality they are simply too complex to interpret because a large number of events, all potentially significant, overlap. Such events can be sorted out for possible use by optical filtering and concurrent monitoring. No other processing technique allows the geophysicist to do this so easily.
WHAT IS RESIDUAL GRAVITY?
Authors: Skeels D. C.
Published: October 2002

The term “residual gravity” has come to have two different meanings. On the one hand it is used in the original sense, to mean what remains of Bouguer gravity after subtracting a smooth regional effect. On the other hand, since about 1949 the term has been widely used to mean the values that result from the convolution of the Bouguer values with some weighting function which is, in effect, a two‐dimensional filter. The resulting anomalies are not equivalent to residual anomalies in the older sense of the term, and it is recommended that some other name be used; “convolution gravity” is suggested.
SHEAR‐WAVE RECORDING USING CONTINUOUS SIGNAL METHODS PART II—LATER EXPERIMENTATION
Authors: Erickson E. L. , Miller D. E. , Waters K. H.
Published: October 2002

The emphasis in this shear‐wave research was placed on determining the general quality of data which could be obtained in different areas and whether such quality was consistent with the main objective of getting information from the shear‐wave data which could not be obtained from the corresponding P‐wave data. Borehole data are presented to show that the SH source of vibrations generates a downward‐propagating, horizontally polarized shear wave. Shear velocities were determined for depth intervals of two to three hundred feet, but no absolute correlation between [Formula: see text] and lithology could be established. In the deeper sedimentary section, [Formula: see text] averages about one‐half; but in the low velocity, or weathering layer, the ratio may be as small as one sixth. All the reflection record problems which arise from wave propagation phenomena in the LVL are generally much worse for SH waves than for P waves because of the very small SH velocities in the LVL. Nevertheless, by using large source and receiver patterns and various processing techniques, interpretable SH reflection records were obtained in almost every test area. It has been possible to obtain a depth of penetration about equal to that of the corresponding P‐wave records, with some reservations. The relative quality to be obtained has proved unpredictable. Several examples of SH reflection sections are presented with the corresponding P reflection sections. Some of these field examples show definite differences between the corresponding P and SH reflection sections. Such differences represent new information which potentially can be useful to the exploration geophysicist.
DETERMINATION OF SEISMIC SYSTEM DISTORTION AND ITS COMPENSATION USING DIGITAL FILTERS
Authors: Gray Russell L. , Leitinger J. Hans , Hollister John C.
Published: October 2002

Distortion is inherent in recording seismic data. Although some distortion serves a useful purpose, distortion of desirable seismic events decreases resolution thereby reducing the effectiveness of the seismograph as an exploration tool. This paper describes an experimental‐computational technique to determine the distortion introduced by a seismic recording system. The technique utilizes a piezoelectric shaketable to obtain suitable input‐output pairs from which the velocity impulse response of the system is computed. Distortion introduced by the system is compensated by digital filters that are designed in the frequency domain. Nearly complete phase compensation is achieved by designing filters with phase characteristics that closely approximate the negative phase characteristics of the seismic system. Complete amplitude compensation is intentionally averted because of practical considerations. The degree of amplitude compensation deemed feasible is controlled by the relative frequency content of signal and noise. Synthetic examples which simulate field data indicate that approximate compensation filters are effective in removing much of the signal distortion introduced by the seismic recording system without decreasing the signal‐to‐noise ratio.