bluuebunny/crossref_metadata_2025 huggingface dataset:
A STUDY OF CASED AND OPEN HOLES FOR DEEP‐HOLE SEISMIC DETECTION
Authors: Van Sandt D. R. , Levin F. K.
Published: October 2002

A study is made of the relative merits of recording seismic signals in cased and open boreholes. Simultaneous measurements of seismic signals in adjacent cased and open holes are compared. It is shown that, in general, the same natural earth noise is recorded in both holes, and the response to the high‐amplitude unidirectional signal is the same in both holes. The conclusion is that the casing in a borehole has no detectable effect upon a seismic signal if the casing is cemented to the borehole wall and wall‐coupled geophones are used.
THE INFLUENCE OF CONDUCTIVITY INHOMOGENEITIES UPON AUDIO‐FREQUENCY MAGNETIC FIELDS
Authors: Jewell T. R. , Ward S. H.
Published: October 2002

Audio‐frequency magnetic fields originate mainly from a world‐wide distribution of energy sources associated with thunderstorms. This energy is propagated over great circle routes within an earth‐ionosphere waveguide, and in passage is attenuated to varying degrees dependent upon the reflective properties of the waveguide. Various investigators have shown that the intensity and direction of the observed electromagnetic waves exhibit characteristic diurnal variations as a function of frequency, time of year, and geographical location. The results of this study demonstrate that the diurnal variations, particularly with regard to direction, are affected by the presence of a large, linear conductivity inhomogeneity in the vicinity of the recording site. Preliminary investigations were carried out near a mineralized fault zone roughly approximating a planar surface of unknown conductivity. No apparent effects upon the intensity or direction of 140‐ or 590‐cps fields were observed. Subsequent experiments adjacent to a large, elongated, pyritized rhyolite formation which contained several massive sulphide ore bodies of high relative conductivity demonstrated quite well the effect of the conductivity inhomogeneity upon the 140‐cps magnetic fields. The diurnal variations of magnetic field azimuth and intensity where relatively unaffected by the conducting body agree reasonably well with the results of other investigators. In addition, they show a good correlation with the estimated diurnal variations constructed from available information on the distribution of energy sources. The differences between the diurnal variations at different locations relative to the conductivity inhomogeneity are reconciled by adopting a theoretical model based upon the known geology. The diurnal behavior of the horizontal field direction at positions relative to the model which correspond to actual recording locations are then calculated and compared with the observed curves. Apparently, the character of the diurnal variation of the 140‐cps magnetic field azimuth is affected by the relative position of the recording site to the conductivity inhomogeneity; the geometry and propagation constant associated with the conductor or conductors; and the relative variation of the primary field direction about the mean axis of conductivity.
CALCULATING THE PARAMETERS USED IN TELLURIC CURRENT PROSPECTING
Authors: Y‐Shu Li
Published: October 2002

In telluric current prospecting, the amplitudes of variations of the telluric‐electric field are measured simultaneously at a fixed base station and a roving field station. In sedimentary basins, variations in electrical conductance through the sedimentary column may be studied by comparing the amplitude of electric field variations at the roving station with the amplitude of the same electric field variations at the base station. The parameter ordinarily used in interpreting telluric current data is μ, the ratio of electric field intensity at a roving station location to the electric field intensity at the base station. Under favorable conditions, the ratio μ is directly proportional to the total longitudinal conductance of the rock layers above insulating basement rock.
A NEW DATA‐PROCESSING TECHNIQUE FOR THE ELIMINATION OF GHOST ARRIVALS ON REFLECTION SEISMOGRAMS
Authors: Schneider William A. , Larner Kenneth L. , Burg J. P. , Backus Milo M.
Published: October 2002

A new data‐processing technique is presented for the separation of initially up‐traveling (ghost) energy from initially down‐traveling (primary) energy on reflection seismograms. The method combines records from two or more shot depths after prefiltering each record with a different filter. The filters are designed on a least‐mean‐square‐error criterion to extract primary reflections in the presence of ghost reflections and random noise. Filter design is dependent only on the difference in uphole time between shots, and is independent of the details of near‐surface layering. The method achieves wide‐band separation of primary and ghost energy, which results in 10–15 db greater attenuation of ghost reflections than can be achieved with conventional two‐ or three‐shot stacking (no prefiltering) for ghost elimination. The technique is illustrated in terms of both synthetic and field examples. The deghosted field data are used to study the near‐surface reflection response by computing the optimum linear filter to transform the deghosted trace back into the original ghosted trace. The impulse response of this filter embodies the effects of the near‐surface on the reflection seismogram, i.e. the cause of the ghosting. Analysis of these filters reveals that the ghosting mechanism in the field test area consists of both a surface‐ and base‐of‐weathering layer reflector.
SEA MAGNETOMETER RECONNAISSANCE OF HUDSON BAY
Authors: Hood Peter
Published: October 2002

A total of 6,500 nautical miles of sea magnetometer data were obtained during the 1961 cruise of the M. V. THETA in Hudson Bay. Near the edge of the Bay the resultant magnetic profiles are made up of short‐wavelength, high‐amplitude anomalies whereas the anomalies in the center of the Bay are of longer wavelength and of lower amplitude. This is a reflection of the depth to the crystalline basement. This observation is confirmed by a quantitative interpretation of the magnetic profiles, which shows that, in general, the depths to basement are least around the margins of the Bay, with the greatest calculated depths, in excess of 10,000 feet, occurring in the area to the northeast of Churchill. The depth‐determination values also reveal that an asymmetrical troughlike basement feature occurs in the area between Southampton, Nottingham, Mansel, Coats, and Digges Islands.
DEPTH FACTORS AND RESOLVING POWER OF ELECTRICAL MEASUREMENTS
Authors: Evjen H. M.
Published: October 2002

The use of depth factors in electrical prospecting is critically reviewed. The use of depth factors is placed on a rational foundation. This is accomplished by assigning new physical significance to the so‐called apparent resistivities and potential drop ratios measured by electrical methods. It is shown that these measurables may be considered as broadly averaged representations of the distribution of electric images with depth. As a result, universal depth factors exist which may be calculated once for all for any given electrode arrangement. The depth factors are calculated for a number of commonly used electrode arrangements and are found to be smaller than generally assumed. The potential drop ratio method is shown to be superior to the apparent resistivity method in regard to depth of reference as well as to resolving power. The resolution by either method, however, is found to be entirely inadequate to give anywhere near the detail obtained with well logs. The new approach adopted in this paper leads to quite simple and practical methods of analysis and direct solution of the interpretation problem which are illustrated. These methods are shown to have a useful field of application also in gravitational interpretation. The extent to which a detailed analysis justifiably may be carried is briefly examined. From the point of view of practicability it is reaffirmed that the best field of application of the electrical methods is the investigation of horizontal changes such as the mapping of faults and salt dome flanks.
SEISMIC EXPLORATION IN EASTERN VENEZUELA
Authors: Ransone W. R.
Published: October 2002

A map and cross‐section are presented to give a brief and generalized idea of the area. A limited discussion of the geology and physiography is given. Working conditions are discussed with their particular relations with physiography, climate and state of development of the country. Emphasis is placed on Anzoategui because of the amount of work done in that area.
LOCATING AND DETAILING FAULT FORMATIONS BY MEANS OF THE GEO‐SONOGRAPH
Authors: Johnson Curtis H.
Published: October 2002

Mapping of faults has previously presented great difficulties for the reflection method. These difficulties have often arisen from a confused reflection pattern, rather than from entire absence of reflections near the fault. The use of the Geo‐sonograph in making a directional analysis of such confused patterns is described, and illustrated with several examples of faults located and mapped by this method.
CONVERSION OF ELECTRIC LOGS FOR SEISMIC TIME SECTIONS
Authors: Ivanhoe L. F.
Published: October 2002

Geophysicists are constantly faced with the problem of showing geological data at the proper scale on seismic “time” sections, in order to help their geological colleagues put the seismic data into proper spatial relationship. This is a critical problem with the modem seismic “record” sections which have both vertical (reflection times) and horizontal (spread length) scales that differ radically from the geologist’s conventional distance (feet or meters) scales.
SURFACE DISPLACEMENT PRODUCED BY AN UNDERGROUND FRACTURE
Authors: Mitra M.
Published: October 2002

The surface displacement due to an underground fracture has been evaluated in exact terms. Numerical results show that the passage of the fracture produces a permanent lateral displacement which increases with distance along the direction of motion of the fracture.
THE GRAVITATIONAL ATTRACTION OF A RIGHT RECTANGULAR PRISM
Authors: Nagy Dezsö
Published: October 2002

The derivation of a closed expression is presented to calculate the vertical component of the gravitational attraction of a right rectangular prism, with sides parallel to the coordinate axis. As any configuration can be expressed as the sum of prisms of various sizes and densities, the computation of the total gravitational effect of bodies of arbitrary shapes at any point outside of or on the boundary of the bodies is straightforward. To calculate the gravitational effect of the “unit” building element a subroutine called Prism has been developed, tested, and incorporated, in one program to calculate terrain corrections, and in another program for three‐dimensional analysis of a gravity field.
THE SIMULATION OF MAGNETIC AND GRAVITY PROFILES BY DIGITAL COMPUTER
Authors: Isaacs Kalman N.
Published: October 2002

A method is discussed and demonstrated which enables the magnetic or gravity profile of a two‐dimensional geologic structure section to be determined by a relatively small digital computer. This method has no restrictions with regard to inclination of the field, remanent magnetization, or complexity of the section. Its chief use is as an objective means of verifying, theoretically, the structural validity of a geologic section, interpreted from a magnetic or gravity profile.
IN‐SITU DETERMINATION OF THE REMANENT MAGNETIC VECTOR OF TWO‐DIMENSIONAL TABULAR BODIES
Authors: Ross H. P. , Lavin P. M.
Published: October 2002

Recent studies have shown that many rocks of the earth’s crust have a substantial component of remanent magnetization. Extensive sampling is required to determine adequately the remanent vector from small samples. A field technique has been developed (and tested on model data) for the in‐situ determination of the resultant (induced+remanent) magnetic vector of bulk volumes of rock, using a combined analysis of the gravity and magnetic fields of a disturbing body (Poisson’s Theorem). The potential fields are sampled adequately at a limited expenditure of time and effort in the field by utilizing the geometry of two‐dimensional bodies. The major limitation to the analysis is the removal of regional gradients and the estimation of the base levels of anomalies. Combined gravity and magnetic surveys were conducted over six diabase bodies in the Triassic Basin of Pennsylvania. The results of these surveys indicate a resultant direction of magnetization given approximately by: declination 2° W, inclination 41 degrees below the horizon. The corresponding direction of natural remanent magnetization has a declination of 1° W and an inclination of 28 degrees. The ratio of remanent to induced magnetization for the diabase is approximately two. These results have been used to provide a better interpretation of magnetic survey data over a magnetite deposit in the Triassic Basin.
THE DIGITAL COMPUTER’S ROLE IN THE ENHANCEMENT AND INTERPRETATION OF NORTH SEA SEISMIC DATA
Authors: Rockwell Donald W.
Published: October 2002

The North Sea, with its complex geology and rapidly varying seismic velocities, is a highly suitable proving ground for the new digital data enhancement and computational techniques. Record quality is good, although long and short period multiples often mask the desired deeper information (Permian or deeper). Considerable improvement in record quality has been attained by a combination of three or six‐fold stacking and inverse time‐domain or dereverberation filtering (deconvolution), as illustrated in the examples covering a representative selection of structural situations and record types (stratigraphic traps, faults, suspected reefs, and salt tectonics). The contribution of long‐operator time‐domain filtering is the most significant one to date of computer processing since good “brute” stacking can also be attained by analog means if care is taken. In the realm of velocity determination, the computer has been used to improve calculations of expanding spread data, taking into account ray‐refraction and thus providing more accurate interval velocities. Depth computations, using stratigraphy‐correlated multilayer velocity models, and velocity laws derived from these more accurate interval velocities, have been successfully carried out for over 100,000 North Sea “shotpoints” by computer and the results have been computer plotted. Computer programs are currently being perfected for such applications as automated velocity determination, multichannel filtered stacking, data migration, and plotting and contouring.
SEISMIC NOISE ESTIMATION USING HORIZONTAL COMPONENTS
Authors: Potter Thomas F. , Roden Robert B.
Published: October 2002

The use of seismometer arrays containing both horizontal‐ and vertical‐component instruments for attenuation of surface‐wave noise has been studied theoretically. If a process can be defined to estimate the vertical noise component by operating on the outputs of one or more horizontal‐component seismometers, the estimate may be subtracted from the vertical‐component record to improve signal‐to‐noise ratio. The exact waveforms of vertically‐incident signals must be preserved in an operation of this kind. Formulas are developed to describe the response of a system employing three components measured at a single point. This system is found to be useful only in cases where the noise is strongly directional. A physical separation between the vertical‐ and horizontal‐component instruments is necessary to resolve the difficulties caused by uncertainties in the sense of the propagation velocity vector and particle orbit vector. Formulas, derived for systems consisting of circular rings of radially‐oriented horizontals and a central vertical show, that useful noise rejection can be obtained even in the most unfavorable case of uniform azimuthal noise distribution. The performance of arrays of this kind is not affected very much by uncorrelated noise or Love‐wave noise. Comparisons with similar arrays containing only vertical‐component seismometers indicate that, for some of the noise models studied, the multicomponent array should provide useful noise rejection over a greater bandwidth and at longer wavelengths than an all‐vertical array with the same dimensions.
OPTIMUM MULTICHANNEL VELOCITY FILTERS
Authors: Sengbush R. L. , Foster M. R.
Published: October 2002

Optimum systems have been developed to correspond to the sub‐optimum moveout discrimination systems presented previously by several authors. The seismic data on the lth trace is assumed to be additive signal S with moveout [Formula: see text], coherent noise N with moveout [Formula: see text], and incoherent noise [Formula: see text], expressed [Formula: see text] where S, N, and [Formula: see text] are independent, second order stationary random processes and [Formula: see text] and [Formula: see text] are random variables with prescribed probability density functions. The signal estimate S⁁ is produced by filtering each trace with its corresponding filter [Formula: see text] and summing the outputs [Formula: see text] We choose the system of filters [Formula: see text] to make the signal estimate optimum in the Wiener sense (minimum mean‐square error of the signal ensemble). For the special cases discussed, the moveouts are linear functions of the trace number l determined by the moveout/trace τ for signal and [Formula: see text] for noise. Thus, the optimum system is determined by the probability densities of τ and [Formula: see text] together with the noise/signal power spectrum ratios [Formula: see text] and [Formula: see text]. In comparison, suboptimum systems are controlled completely by the cut‐off moveout/trace [Formula: see text]. Events whose moveout/trace falls within [Formula: see text] of the expected dip moveout/trace are accepted, and those falling outside this range are suppressed. Suboptimum systems can be derived from optimum systems by choosing probability densities for τ and [Formula: see text] that are uniform within the above ranges and letting [Formula: see text] be very large. Optimum systems have increased flexibility over suboptimum systems due to control over the probability density functions and the power spectrum ratios and allow increased noise suppression in selected regions of f‐k space.
SLR RECONNAISSANCE OF PANAMA
Authors: Viksne Andris , Liston Thomas C. , Sapp Cecil D.
Published: October 2002

SLR (Side‐Looking Radar) was used in lieu of optical photography for reconnaissance of the Darien Province of Panama and parts of Northwest Colombia, and for the construction of geoscience products thereof. An AN/APQ‐97 side‐looking radar was used to produce high‐resolution imagery of an area containing approximately 6600 sq mi in 4 hours flying time—an area, furthermore, that is almost perpetually cloud covered. The SLR imagery was used to prepare an uncontrolled mosaic and a series of geoscience overlays, including: 1. surface drainage 2. surface configuration 3. vegetation 4. engineering geology. This study provides a complete overview of Darien Province and demonstrates the capability of SLR to gather geoscience data in an area that is notorious for the difficulties that its persistent cloud cover poses for the acquisition of usable, optical, aerial photography.
MAGNETIC PROPERTIES OF ULTRAMAFIC ROCKS FROM RED MOUNTAIN, CALIFORNIA
Authors: Saad Afif Hani
Published: October 2002

A representative ultramafic intrusion at Red Mountain southeast of San Francisco, California was sampled for the purpose of studying the remanent magnetism and susceptibility of ultramafic rocks as functions of serpentinization. Due to the dependence of grain density upon the degree of serpentinization, the former was taken as a measure of the latter. Grain densities were accurately determined from bulk densities by taking into account the porosity and amount of adsorbed water contained in these rocks. In general, the porosity was found to increase with serpentinization. The magnetic data suggest that the remanent magnetization is mainly a chemical remanent magnetization acquired during the process of serpentinization whereby iron atoms, released from the silicate structure of the paramagnetic olivine and pyroxene, are oxidized to form ferrimagnetic magnetite. The intensity of both remanent and induced magnetization increases exponentially with serpentinization. However, the remanent magnetization becomes highly unstable as the rock becomes intensely serpentinized. This is due to the growth of magnetite grains to multi‐domain size and their oxidation, in some cases,to maghemite. The intensity of magnetization also depends on mode of occurrence and state of oxidation of the magnetite, and on original rock composition. Serpentinized dunites were found tobe less magnetic than equally serpentinized peridotites, because the olivine in the dunites has a lower iron content than the olivine and pyroxene in the peridotites. Therefore, it is expected that some serpentinized ultramafic bodies will have highly variable magnetic anomalies associated with them, while others of dunitic composition will have low magnetic anomalies.
COMPUTER RAY TRACING THROUGH COMPLEX GEOLOGICAL MODELS FOR GROUND MOTION STUDIES
Authors: Yacoub Nazieh K. , Scott James H. , McKeown F. A.
Published: October 2002

A computer program has been developed to study the effect of subsurface geology on the propagation of seismic body waves. The program, which is based on ray theory and on numerical solution of Zoeppritz’s equations for any angle of incidence at elastic boundaries, traces seismic rays and energy distribution through geological models of any complexity. The geological models are composed of rock units that are assumed to be isotropic, perfectly elastic, and homogeneous, and are two‐dimensional in distribution. The ray tracing program was applied to two geologic models of the crust, that are more than 100 miles long, starting at drill hole UE20F in the Nevada Test Site. The results, displayed as raypath plots and plots of the amplitude of vertical displacement at the earth’s surface, graphically illustrate the strong influence of the spatial distribution and elastic property variations of rock units in raypath distribution.
NEW METHODS FOR THE DIRECT INTERPRETATION OF MAGNETIC ANOMALIES CAUSED BY INCLINED DIKES OF INFINITE LENGTH
Authors: Koulomzine Th. , Lamontagne Y. , Nadeau A.
Published: October 2002

Most anomalies encountered in ground and airborne magnetic surveys are caused by rock and/or ore masses whose shapes are either dike‐like and sheet‐like structures of infinite length or limited parallelepipedoïdal prisms of varying dips, widths, lengths, and depths of burial of the top and bottom faces. The quantitative interpretation problem consists essentially in determining all of the above parameters through a complete analysis of the field profile. Our research group has discovered new methods with which to perform this interpretation. These techniques are both analytically correct and very fast. First, the field profile is decomposed into its symmetrical and antisymmetrical components by a new method that uses arbitrarily chosen conjugate points. Our procedure does not require any previous knowledge or assumption of the zero datum level or of the center of the body. Second, the symmetrical and antisymmetrical components of the field profile are analyzed separately. In the case of dike or bed‐like structures, the entire procedure takes but a few minutes and requires a single set of six master curves. The case of the interpretation of parallelepipedoïdal prisms is still under study and will be presented shortly in a second paper.
On: “The Gradational Density Contrast as a Gravity Interpretation Model” by D. J. Gendzwill (GEOPHYSICS, April 1970, p. 270–278)
Authors: Naidu P. S.
Published: October 2002

The author cites the following equation, from Novosolitskii (1965), giving the gravity field due to a slab where the density variation is along the x coordinate only, [Formula: see text] (1) and gives a solution for a special case of linear density variation over a limited zone. Even for such a simple variation, the expression is far too complicated.
SENSOR‐ARRAY PROCESSING WITH CHANNEL‐RECURSIVE BAYES TECHNIQUES
Authors: Farrell Edward J.
Published: October 2002

Arrays of seismometers, hydrophones, and electromagnetic receivers have several signal processing problems in common. This paper is concerned primarily with source location and secondarily with signal extraction. The basic problem can be described as follows: A transient signal from an event is detected in the outputs of the sensor array. We determine the location of the source from the temporal positions of the signal in the array outputs. Further, if the signal is unknown, we estimate it. The approach taken here differs from previous investigations in three ways: (i) a Bayes estimation approach is used, (ii) the estimates are evaluated recursively with respect to channels, and (iii) a time‐domain approach is used, as opposed to a frequency‐domain approach. The proposed estimation technique is optimum with respect to a large class of loss functions, since it is based on the expectation of the posterior distribution. Recursive evaluation of the posterior expectation has several advantages. At each step we have the optimum estimate of the unknown parameters and the corresponding covariance matrix. A channel selection rule and stopping rule are defined in terms of the covariance matrix. Further, having an optimum estimate at each step permits simplification of the processing; e.g., the search interval may be limited to the most probable region of the parameter space. Such techniques significantly decrease the processing time and increase the rate of convergence. Equations are developed for the known‐signal case with planar and spherical wavefronts, and results are presented from a computer simulation. Subsequently, equations for the unknown‐signal case are presented with simulation results.
EXPERIMENTAL STUDIES OF SOURCE‐GENERATED SEISMIC NOISE
Authors: Jolly R. N. , Mifsud J. F.
Published: October 2002

Source‐generated seismic noise has been studied in several areas of Oklahoma and Texas having distinctly different near‐surface layering and velocities. Surface‐wave velocities, particularly those of the propagating modes, are closely related to near‐surface layer thickness and velocity. Modal structure is strongly influenced by the shooting parameters, of which charge depth is the most important. The Rayleigh or first propagating mode and the third propagating mode are readily identifiable on the noise records and agree satisfactorily with theory, if frequency‐dependent attenuation and modal overlapping are taken into account. Leaky modes are identifiable on the basis of their high phase and group velocities but to date have defied quantitative comparison with theory.
A LOW‐NOISE SEISMIC METHOD FOR USE IN PERMAFROST REGIONS
Authors: Rackets H. M.
Published: October 2002

Dynamite charges of ten lb or greater detonated in permafrost regions at depths less than fifty ft below the ground surface tend to create frost breaks, a phenomenon of strong seismic energy from an outward flow of minute cracks in the permafrost caused by the detonation. The frost breaks obliterate reflected energy arriving at the same time. A seismic method was devised which minimizes frost breaks and allows a reduction of the massive equipment currently required either to drill shotholes or for the application of a nondynamite surface energy source. Advantages expected for the method are a cost reduction of 10 percent or more by reduction of drilling equipment and the ability to employ light‐weight portable seismic crews in remote areas without large mileage programs (300 to 500 miles) being necessary to justify the cost of introducing a crew. Also, light‐weight portable crews could be used to determine optimum operational parameters in virgin areas. Since dynamite is the most concentrated available energy source, a seismic experiment was conducted by detonating individual one‐lb dynamite charges at depths 5 ft below the permafrost surface. To maximize the number of small charges, they were detonated at each geophone location (156 one‐lb shotpoint locations per mile). On a total energy per mile basis, the energy from the small charges compares favorably with the energy expended by the conventional method of [Formula: see text] 25‐lb shotpoint locations per mile or [Formula: see text] lb. The reflected energy was recorded digitally and processed by applying vertical summing plus 12‐fold horizontal CDP stacking. The total profile distance was sufficient to allow for one‐half mile of fully stacked subsurface coverage. The experimental results compare favorably with those from the conventional method, and the small charges detonated at shallow depths were safe both from the point of view of human safety and ecological care.
DEPENDENCE OF IN‐SITU COMPRESSIONAL‐WAVE VELOCITY ON POROSITY IN UNSATURATED ROCKS
Authors: Watkins Joel S. , Walters Lawrence A. , Godson Richard H.
Published: October 2002

The relation of in‐situ compressional‐wave velocities to porosities, determined by seismic refraction for unsaturated near‐surface rocks from different areas in Arizona, New Mexico, and California, is grossly similar to relations determined by other investigators for water‐saturated rock and unconsolidated sediments. The principal difference is that in the porosity range 0.0–0.2, compressional waves travel somewhat more slowly in unsaturated rocks than in water‐saturated rocks, and much more slowly, in the porosity range 0.2–0.8. The function, ϕ=−0.175 ln (α)+1.56, where ϕ is the fractional porosity and α is the compressional‐wave velocity, was obtained as a least squares fit to the experimental data. Bulk densities are reported for all samples; moisture contents are reported in some instances.
EXTENDED TERRAIN CORRECTION TABLES FOR GRAVITY REDUCTIONS
Authors: Douglas Jesse K. , Prahl Sidney R.
Published: October 2002

This note extends the gravity terrain corrections for elevation differences beyond the tables originally published by Hammer (1939). Experience in the Rocky Mountain area has demonstrated to us the need for such an extension. The frustration encountered by the authors led to a computer program to calculate the terrain correction tables presented in this article. The mountain topography in western Montana is typical of an area not sufficiently regular to allow use of the less tedious inclined‐plane model presented by Sandberg (1958). The inclined‐plane and the cylinder models are designed for calculating the effects of local terrain and do not include a correctional factor for earth curvature. Large regional surveys require the Hayford‐ Bowie terrain correction zones. However, local surveys can be easily incorporated into these larger studies by Hammer to Hayford‐Bowie transition tables (Sandberg, 1959),
SPECTRA OF WATER REVERBERATIONS FOR PRIMARY AND MULTIPLE REFLECTIONS
Authors: Pflueger John
Published: October 2002

A theoretical study shows that passage of a seismic event through the water‐layer filter imposes amplitude characteristics on the resultant reverberating event which are independent of whether the event is a primary reflection or a multiple reflection. The phase characteristics of each order of event are, however, different. It is also shown that the reverberating sequence from a multiple reflection can be “whitened” by deconvolution but will still exhibit ringing. This phenomenon explains why some marine data, containing dominantly multiple reflections, are not amenable to deringing using standard deconvolution approaches. In addition, a quick method of obtaining the approximate reflection coefficient of the sea floor is derived.
THE ELECTROMAGNETIC FIELDS OF A SUBTERRANEAN CYLINDRICAL INHOMOGENEITY EXCITED BY A LINE SOURCE
Authors: Howard Allen Q.
Published: October 2002

The anomalous fields from a buried cylindrical inhomogeneity in an otherwise uniform half‐space are analyzed. The problem is rendered two‐dimensional by assuming that the uniform line source of current is parallel to the subsurface cylinder. The multipole scattered field coefficients are obtained from the numerical solution to the associated singular Fredholm integral equation of the second kind. The horizontal magnetic field amplitude, the vertical magnetic field phase, and the amplitude and phase of the ratio of horizontal to vertical magnetic fields are shown to be diagnostic of the location of the inhomogeneity. The results have possible applications to electromagnetic location in mine rescue operations and to geophysical prospecting.
Kε AND TAN δ SPECTRA OF DRY LUNAR ANALOG MEASURED BY VARIOUS TECHNIQUES
Authors: Ryu Jisoo , Ward S. H. , Nash W. P. , Buzzell D.
Published: October 2002

For a two‐terminal measurement on a rock sample in a dry state whose dielectric loss tangent is less than unity, contact substitution without the insertion of any material between the electrode plates and the sample should be used to yield accurate measurements. The insertion may disturb measurements on the dielectric properties of the dry rock sample. A proposed optimum technique is useful when rock samples are not free of moisture and helps overcome electrode effects. Due to similarities in mineralogical composition and dielectric behavior, Duluth hornfels can serve as a terrestrial analog to some lunar basalts. The dielectric constant spectra of the Duluth hornfels measured during various stages in preparing a sample indicate that high dispersion at low frequencies is attributed mainly to the moisture content of the sample. Its intrinsic dielectric constant was found to be about 13, while its dielectric loss tangent linearly decreases from 0.07 at 100 hz to 0.019 at 100 khz in a log‐log plot. No Debye relaxation process was observed on the Duluth hornfels sample at room temperature over the frequency range used.
THE L/M PARAMETER OF TIME‐DOMAIN IP MEASUREMENTS—A COMPUTATIONAL ANALYSIS
Authors: Swift C. M.
Published: October 2002

The L/M parameter measured in some time‐domain IP systems has been analyzed theoretically by calculating the behavior of this parameter as a funcfion of intrinsic IP response and homogeneous earth EM coupling response. Calculated time‐domain voltage waveforms exhibit graphically the presence of (1) intrinsic IP response, (2) positive EM coupling response, and (3) negative EM coupling response. Quantitatively, L/M remains constant at unity for varying values of intrinsic IP response, and an anomalous L/M value indicates the presence of coupling.
On: “Computer Graphics in Geophysics” by Robert B. Smith, John E. Warnock, William D. Stanley, and E. Randolph Cole (GEOPHYSICS, October 1972, p. 826–838)
Authors: Steenland Nelson C.
Published: October 2002

Your colored pictures, Figures 6a, c, 7a, b, c, probably look good to an interior decorator but they certainly aren’t as articulate as Figure 6b but maybe most practitioners don’t use data quantitatively.
THE FORWARD MAGNETOTELLURIC PROBLEM FOR AN INHOMOGENEOUS AND ANISOTROPIC STRUCTURE
Authors: Abramovici Flavian
Published: October 2002

The impedance tensor corresponding to the magnetotelluric field for a nonisotropic one‐dimensional structure is given in terms of the solutions of a sixth‐order differential system. The conductivity tensor is three‐dimensional. Its components depend upon depth only in an arbitrary manner such that the corresponding matrix is positive definite. The impedance tensor components are found by a numerical integration procedure based on a set of one‐step methods and a variable step‐size to insure a given accuracy in the final result. Calculations were made for three models having sharp boundaries and also transitional layers. The first of these models has a middle layer of high conductivity, sandwiched between two layers of linearly varying conductivity, while in the second model the middle layer has a very low conductivity. In the third model the conductivity tensor is three‐dimensional and is linearly varying in one of the layers.
ON THE CALCULATION OF FILTER COEFFICIENTS FOR MAXIMUM ENTROPY SPECTRAL ANALYSIS
Authors: Andersen N.
Published: October 2002

The maximum entropy method (MEM) for spectral analysis was suggested by Burg (1967). Its mathematical properties have been discussed in detail by Lacoss (1971), Burg (1972), and Ulrych (1972b) who found that the MEM in general is superior to the more conventional methods of spectral estimation. It has, for example, better resolution and gives more realistic power estimates, especially for short data records. However, the application of the method in the analysis of more complicated geophysical data series is reported in a surprisingly small number of papers. Ulrych (1972a) successfully used the MEM for the analysis of data on long period geomagnetic reversals.
SOME EFFECTS OF SPATIAL FILTERS ON SIGNAL
Authors: Hawes William S. , Gerdes Leo
Published: October 2002

Synthetic CDP records with source and receiver arrays are generated from three “earth models” to make a first order examination of (1) some factors which determine the degree of reflection waveform similarity as a function of source‐receiver distance, and (2) degrading effects upon signal stacking and analyses which result from changes in waveform. The factors include the mutual reflection interference due to irreversible normal moveout convergence and the phase distortion and amplitude attenuation resulting from action of the spatial filter on the signal. The arrays examined have lengths of 0–1500 ft (in 250 ft increments) at distances from 0–9200 ft and from 1 to 32 array elements. Raypath tracing employing Snell’s law was used to create the synthetics. The records so generated used to depict: (1) Reflection convergence and crossover; (2) signal distortion and attenuation as a function of array type, array length, and source‐detector separation distance; (3) signal distortion from dynamic correction; (4) degradation of velocity analysis as function of array length; and (5) degradation of deconvolution as a function of source‐detector separation and array length. The results show the general degree to which wide angle recording using spatial filters precludes maintenance of constant waveform and the way in which processing results become increasingly disturbed with increasing recording distances and array lengths.
To: “Two methods for computer interpretation of magnetic anomalies of dikes”, GEOPHYSICS, v. 38; p. 710–718.
Authors: Murthy I.V. Radhakrishna
Published: October 2002

I would like to bring to your attention some errors on the left column of p. 715 of our paper “Two methods for computer interpretation of magnetic anomalies of dikes”, Geophysics, v. 38; p. 710–718. Unfortunately, they were not discovered at the time of proof reading. I request you to incorporate the following corrections in a future issue of Geophysics.
HYDROPHONE STREAMER NOISE
Authors: Schoenberger M. , Mifsud J. F.
Published: October 2002

Experiments were performed to determine the noise characteristics of a hydrophone streamer that had incorporated a number of noise reduction features. In the original system, the channels to which the depth‐controller birds were attached were 3 to 4 times noisier than nonbird channels. Fortunately, the bird noise is near‐field and is eliminated simply by increasing bird/hydrophone separation to 9 ft. On this cable, no other discrete noise sources are evident. The boat, propulsion system, lead‐in cable, tail buoy, and ambient sea conditions (moderate seas) do not generate significant noise at towing speeds above 5 knots. The noise on individual hydrophones not near birds is mainly random with only a small coherent component traveling horizontally through the water from the direction of the boat. However, since the 145-ft hydrophone arrays of 20 detectors are much more effective in reducing random noise than coherent noise, the array output consists of approximately equal portions of each. A twofold decrease in the total noise‐to‐signal ratio would result from doubling the array length (to 290 ft) while maintaining the same hydrophone density. This would result in a four to fivefold decrease in the coherent noise‐to‐signal ratio and a 30 percent decrease in the random noise‐to‐signal ratio. Additional noise reduction would result from increasing the hydrophone density and decreasing the motion sensitivity of the hydrophones. (The streamer hydrophones are not the motion canceling type.) At a towing speed of 5.3 knots, the noise level recorded on an array (not near a bird) is equivalent to pressures of 1 μbar. In normal operations with an 8-gun sleeve exploder source, a stacked section signal‐to‐towing noise ratio of 3 was obtained at 3.0 sec. However, the towing noise increases as the cube of the boat speed, and the S/N ratio would decrease by a factor of 11 if the boat speed were doubled. Conversely, decreasing the boat speed by 18 percent would double the signal‐to‐towing noise ratio.
A NOTE ON THE SEISMIC‐ELECTRIC EFFECT
Authors: Thompson R. R.
Published: October 2002

An experiment is described which apparently eliminates electrode surface effects as a cause of the seismic‐electric effect. Evidently a volume variation of some electrical property or properties of the earth is necessary to explain the experimental results.
SURFACE‐WAVE DISPERSION APPLIED TO THE DETECTION OF SEDIMENTARY BASINS
Authors: Tatham Robert H.
Published: October 2002

Seismic surface‐wave velocities are greatly affected by crustal structure. Because there is a strong contrast in the physical properties of clastic sediments and underlying basement materials, surface‐wave dispersion provides a fast, convenient, and inexpensive means of detecting sedimentary basins and estimating their thickness. Model calculations and published reports of explosion studies indicate that sedimentary thicknesses as shallow as 500 m (∼1650 ft) should be detectable by analysis of routinely recorded earthquake seismograms. This study demonstrates the use of seismic surface‐wave dispersion to detect sedimentary basins and to estimate their thickness. The technique is used first for the Mississippi embayment region of the U.S. Gulf Coast, where the crustal structure is known and the results can be verified, and then applied to offshore Greenland, where the crustal structure is unmapped but a sedimentary basin is suspected. The data used are available seismograms of natural earthquakes and, for the Gulf Coast area, an underground nuclear explosion. Because this technique requires only existing, readily available data and may be applied to many regions of the world, it offers an attractive reconnaissance tool in petroleum exploration. In the present study, surface‐wave dispersion and the effects of shallow crustal structure are reviewed in light of this application, and the advantages and limitations of the technique are explored.
DETECTION OF THIN LAYERS BY RADIO INTERFEROMETRY
Authors: Rossiter J. R. , Strangway D. W. , Annan A. P. , Watts R. D. , Redman J. D.
Published: October 2002

Recent theoretical work on the radio interferometry technique for dielectric layers less than one free space wavelength thick has indicated that there is a thickness for which no interference is observed. This thickness is about 0.2 free space wavelengths for ice, and it lies between the thickness that allows one single mode to propagate in the layer (>0.2 wavelengths) and a thickness that is essentially transparent to the wavelength being used (⩽0.2 wavelengths). Field work was done on the Juneau Icefield, Alaska; frequencies from 1 to 32 Mhz were used. At 1 and 2 Mhz, an interference pattern typical of a half‐space of ice with a dielectric constant of 3.3 was observed, while at 4 Mhz essentially no interference was seen. At higher frequencies, the interference observed is typical of that of a layer overlying a half‐space. The upper layer can be interpreted to have a thickness of 15 or 20 m (0.2 wavelengths at 4 Mhz) with a dielectric constant of 2.4. These results are for a layer of snow over a half‐space of ice. The technique is therefore of potential interest in interpreting the nature of the snow layer overlying a glacier.
SYMMETRY PROPERTIES OF THE SURFACE IMPEDANCE TENSOR FOR STRUCTURES WITH A VERTICAL PLANE OF SYMMETRY
Authors: Fischer Gaston
Published: October 2002

For sites located in a vertical plane of reflection symmetry, the elements of the magnetotelluric impedance tensor Z satisfy a specific symmetry relationship in addition to the well‐known vanishing of the skew S: The diagonal tensor elements [Formula: see text] and [Formula: see text] both have the same argument α, invariant under rotation, as the sum [Formula: see text] of the off‐diagonal elements. Furthermore, it is shown that Z tensors exist which obviously do not satisfy the requirements of reflection symmetry, even though they yield S ≡ 0. The new condition adds a stringent requirement for consideration when one tries to assess from field data whether a structure does, or does not, possess a vertical plane of reflection symmetry; the condition should prove valuable especially because the solution of the inversion problem, which leads from Z(ω) to the conductivity structure σ(r), is generally not unique.
A DIRECT COMPUTATION OF GRAVITY AND MAGNETIC ANOMALIES CAUSED BY 2- AND 3-DIMENSIONAL BODIES OF ARBITRARY SHAPE AND ARBITRARY MAGNETIC POLARIZATION BY EQUIVALENT‐POINT METHOD AND A SIMPLIFIED CUBIC SPLINE
Authors: Ku Chao C.
Published: October 2002

A computational method, which combines the Gaussian quadrature formula for numerical integration and a cubic spline for interpolation in evaluating the limits of integration, is employed to compute directly the gravity and magnetic anomalies caused by 2-dimensional and 3-dimensional bodies of arbitrary shape and arbitrary magnetic polarization. The mathematics involved in this method is indeed old and well known. Furthermore, the physical concept of the Gaussian quadrature integration leads us back to the old concept of equivalent point masses or equivalent magnetic point dipoles: namely, the gravity or magnetic anomaly due to a body can be evaluated simply by a number of equivalent points which are distributed in the “Gaussian way” within the body. As an illustration, explicit formulas are given for dikes and prisms using 2 × 2 and 2 × 2 × 2 point Gaussian quadrature formulas. The basic limitation in the equivalent‐point method is that the distance between the point of observation and the equivalent points must be larger than the distance between the equivalent points within the body. By using a reasonable number of equivalent points or dividing the body into a number of smaller subbodies, the method might provide a useful alternative for computing in gravity and magnetic methods. The use of a simplified cubic spline enables us to compute the gravity and magnetic anomalies due to bodies of arbitrary shape and arbitrary magnetic polarization with ease and a certain degree of accuracy. This method also appears to be quite attractive for terrain corrections in gravity and possibly in magnetic surveys.