bluuebunny/crossref_metadata_2025 huggingface dataset:
POLE‐AND‐ZERO DESIGN OF DIGITAL FILTERS
Authors: Mooney Harold M.
Published: October 2002

A recent paper in Geophysics by Shanks (1967) provides an excellent introduction to recursive or feedback digital filters. These filters do not appear to have been widely used in geophysics; for example, a recent review paper on the use of digital filters in geophysics (Kertz, 1965) does not mention them. The topic takes on added interest with the recent appearance of several papers following the original paper of Dean (1958), applying digital filter concepts to the analysis of gravity and magnetic data.
On: “A Method of Computing Residual Anomalies from Bouguer Gravity Map by Relaxation Technique” by Dr. M. K. Paul (GEOPHYSICS, August 1967, p. 708–719)
Authors: Naidu P. S.
Published: October 2002

Dr. M. K. Paul has presented a very interesting method of separating the residual and regional fields from a Bouguer gravity map. In essence, his method is the estimation of the residual field along a closed curve enclosing, but away from, the anomaly by the usual least‐squares technique. Next he extrapolates the residual field on the boundary curve into the anomaly region by a relaxation technique.
CONTINUOUS SOUNDING‐PROFILING WITH A DIPOLE‐DIPOLE RESISTIVITY ARRAY
Authors: Bodmer René , Ward Stanley H.
Published: October 2002

Among the different four‐electrode arrays used in resistivity sounding and profiling, the dipole‐dipole array can provide, in some instances, advantages over the more conventional Schlumberger and Wenner configurations. Interpretation of data from Wenner and Schlumberger methods has been described by Compagnie Générale de Géophysique (1955), Mooney and Wetzel (1956), Zohdy (1964), and many others. The primary reason for using a dipole‐dipole array has been to minimize inductive coupling between the transmitting and receiving dipoles when performing frequency‐domain, induced‐polarization surveys (e.g., Marshall and Madden, 1959). This inductive coupling, as effected by the presence of the earth, produces spurious frequency‐dependent voltages in the measuring circuit. Such spurious voltages are small and only of importance when one wishes to calculate the percentage change in resistivity between two frequencies; they are usually much less than the 5 to 10 percent accuracy sought in most resistivity surveys. For this reason, and because the dipole‐dipole array leads to small measured potentials, it is seldom used in single‐frequency resistivity sounding or profiling. However, we shall demonstrate in this paper the manner in which the dipole‐dipole array may be used effectively for simultaneous sounding and profiling.
STRUCTURAL IMPLICATIONS OF MAGNETIC ANOMALIES NORTH OF THE BAHAMA‐ANTILLES ISLANDS
Authors: Bracey Dewey R.
Published: October 2002

Seven individual shipboard and airborne magnetic surveys have been conducted north of the Bahama‐Antilles Islands by the Naval Oceanographic Office during a 5.3 year period since 1962. A computed main magnetic field model was used to reduce the observed data from these surveys to a common magnetic residual chart. Two anomaly trends, each composed of linear magnetic features, dominate the residual chart. One trend strikes northeast‐southwest, the other at an angle of 80 degrees to it. By relating the residual magnetic anomalies to structural features observed on an experimental model developed by Billings and to actual structural features in the nothern Antilles, it is shown that the anomalies could be engendered by structural features resulting from an east‐west horizontal couple. The line of action of this couple is consistent with prevalent theories on continental drift.
RESISTIVITY INVESTIGATIONS OF LIMESTONE AQUIFERS IN JAMAICA
Authors: Vincenz S. A.
Published: October 2002

Resistivity depth probes were made over water wells in an alluvial interior valley of Jamaica, known as the Queen of Spain’s Valley. Several type of electrode configuration were used and the observations analyzed by a technique designed to select those suitable for interpretation. The results, interpreted by standard curve matching methods, compare favorably with available well logs. They suggest that the yield is a linear function of the resistivity of the water‐bearing strata. A straight line fitted to the data by the method of least squares provides a nomogram which can be used for predicting the probable yield at points where no wells exist. It is concluded that resistivity surveys can be of material assistance in a water‐supply drilling program in the area. It is found that interpretation by multilayer curve‐matching procedures gives more realistic results than analysis by classical two‐layer master curves used in conjunction with Hummel’s rule. The results suggest also that the Wenner array gives a better resolution of small resistivity contrast than the operationally more versatile Cooper array. A conclusion of more general nature states that Helmholtz’ reciprocity theorem of electrical conduction holds also for rock media.
DETERMINATION OF THE ELECTRICAL CONDUCTIVITY OF THE SEA BED IN SHALLOW WATERS
Authors: Bannister Peter R.
Published: October 2002

The electric and magnetic field components produced by horizontal dipole antennas (both electric and magnetic) located within the upper layer of a two‐layer conducting earth are derived for the quasi‐near range. This range is defined as that in which the measurement distance is much greater than an earth skin depth but much less than a free‐space wavelength. Ionospheric effects are neglected. It is assumed that the transmitting and receiving anterenna depths are much less than their horizontal separation, and that the fields in the horizontal direction vary only slightly in a distance of one skin depth. It is well known that if the conductivity and thickness of the first layer (sea water) are known, the conductivity of the bottom layer (the sea bed) may be determined from magnetic field measurements alone. However, when extremely low‐frequency magnetic field measurements are performed at sea, the movement of the magnetic field sensors in the static magnetic field of the earth (which is many times stronger than the field to be measured) introduces a very strong noise component. It is argued that electric field measurements are preferable because the induced noise component is smaller. It is shown that the sea bed conductivity may be determined by measuring only the horizontal electric field components produced by a subsurface horizontal magnetic dipole antenna.
THE EQUIVALENT SOURCE TECHNIQUE
Authors: Dampney C. N. G.
Published: October 2002

The inherent ambiguity of potential field interpretation can be put to advantage. Bouguer anomaly measurements on an irregular grid and at a variety of elevations can be synthesized by an equivalent source of discrete point masses on a plane of arbitrary depth below the surface. By keeping the depth of the plane within certain limits relative to the station spacing, we can ensure that the synthesized field closely approximates the true gravity field in the region close to and above the terrain. Once the equivalent source is obtained, the projection of the Bouguer anomaly onto a regularly gridded horizontal plane is easily done. In addition, the equivalent source can then be economically used to carry out vertical continuation. The technique is illustrated by a hypothetical example and a case history of a local gravity survey in precipitous topography.
SCATTERING BY A RANDOM FIELD OF SURFACE SCATTERERS
Authors: Levin Franklyn K. , Robinson Donald J.
Published: October 2002

A study of elastic wave scattering by a random distribution of surface scatterers has been carried out with a three‐dimensional seismic model. The model consisted of 143 round brass rods cemented to the surface of a large concrete slab. By recording the signals from each detector position with and without scatterers present, we looked at the scattered energy in the absence of the direct signal. We found that (1) the separation for which signals from two detectors ceased to resemble each other (coherence distance) was about 0.3 to 0.4 wavelength of predominant coherent noise, (2) the coherence distance is independent of scatterer size for scatterers with diameters in the range 0.05 to 0.4 wavelength, (3) the amount of scattered energy increases with scatterer size, and (4) all components of motion appear in the scattered energy with about the same average amplitude.
On: “Quantitative Evaluation of a Stabilized Platform Shipboard Gravity Meter” by T. R. LaFehr and L. L. Nettleton (GEOPHYSICS, February, 1967, p. 110–118).
Authors: Grearson Lloyd S.
Published: October 2002

In their paper, LaFehr and Nettleton have reported a test conducted to evaluate the LaCoste and Romberg stable platform gravity meter.
MINING GEOPHYSICAL ACTIVITY IN 1968
Authors: Hood Peter J. , Kellog William C.
Published: October 2002

Data on worldwide geophysical activity in mining exploration programs is collected annually by the SEG Geophysical Activity Committee as part of a continuing service to the industry. The data are obtained by sending out questionnaires to all organizations which are known to be carrying out mining geophysical work; the organizations canvassed include mining companies, contractors, government departments, and universities. The questionnaire is divided into three sections, namely ground methods, airborne methods, and research. The data compiled from the questionnaires shows that world‐wide utilization of mining geophysical methods reached a new high in 1968 with total expenditures of U.S. $41,367,011. This was divided as follows: ground methods—$20,398,288; airborne methods—$14,937,733, and research—$6,039,990. For the ground methods, most money was spent on induced‐polarization techniques ($6,735,315); geochemical methods were second in reported expenditure ($3,437,712). The statistics for the past seven years show that induced‐polarization methods have gradually outstripped the electromagnetic methods as the principal ground prospecting tools used in the search for sulfides. Since 1966, utilization of ground electromagnetic methods has definitely declined and in 1968 more than four times as much money was spent on induced polarization as on electromagnetic methods. The expenditure for mining geophysics research reached a new high of $6,039,990 in 1968 which is an increase of 48 percent from 1967. The leading research activity was the magnetic method, and in second place was electromagnetic method research. The amount of money spent on mining geophysics research increased by 54 percent in the United States, and significant increases also occurred in Canada and Europe. In 1968, the total reported geophysical line‐mileage flown was 1,575,835 line‐miles, with aeromagnetic surveying again accounting for most line‐mileage (837,068). Typical line‐mileage costs in 1968 for the various airborne geophysical methods were: aeromagnetic U. S. $6.75; airborne electromagnetic U. S. $16; combined airborne electromagnetic/magnetic U. S. $24; airborne radiometric U. S. $7. Airborne radiometric line‐mileage showed a nine‐fold jump from 1967 to 1968, when the reported line‐mileage was 400,559
VELOCITY SPECTRA—DIGITAL COMPUTER DERIVATION APPLICATIONS OF VELOCITY FUNCTIONS
Authors: Taner M. Turhan , Koehler Fulton
Published: October 2002

Multifold ground coverage by seismic techniques such as the common reflection point method provides a multiplicity of wave travel path information which allows direct determination of root‐mean‐square velocities associated with such paths. Hyperbolic searches for semblance among appropriately gathered arrays of traces form the basis upon which velocities are estimated. Measured semblances are presented as a velocity spectral display. Interpretation of this information can give velocities with meaningful accuracy for primary as well as multiple events. In addition, the velocity data can help correctly label events. This paper outlines the fundamental principles for calculating velocity spectra displays. Examples are included which demonstrate the depth and detail of geological information which may be obtained from the interpretation of such displays.
SPATIAL FILTERING IN THE WAVE‐VECTOR DOMAIN
Authors: Black D. I. , Scollar I.
Published: October 2002

Spatial data can be represented in the two‐ dimensional frequency (wave‐vector) domain. This much simplifies the achievement of any desired transfer response in a linear digital filter. Similar results have been obtained approximately by convolution (also termed correlation) in space with a weighted coefficient set. The equivalence of wave‐vector filtering and a special form of convolution filtering is demonstrated. Given the availability of a fast Fourier transform, computational advantage lies with the former. The regional trend is removed from data prior to wave‐vector filtering, and this combined procedure gives one solution to problems arising at the edge of the data. In the appendix, a computationally convenient form of the two‐dimensional fast Fourier transform is given for arrays with side lengths not restricted to powers of two. Considerable savings in computing time and storage allocation are shown when real data are used.
A NEW METHOD FOR DIFFERENTIAL RESISTIVITY SOUNDING
Authors: Zohdy Adel A. R.
Published: October 2002

A new method designated TDS (transverse differential sounding) is proposed for obtaining high resolution direct current resistivity soundings in the field. The measured apparent resistivity is designated TDR (transverse differential resistivity) and is defined by [Formula: see text], where [Formula: see text] and L are the Schlumberger apparent resistivity and electrode spacing, respectively. A collinear symmetric system (B′AMNBA′) consisting of two pairs of current electrodes and one pair of potential electrodes is used for measuring the value of [Formula: see text] directly in the field. For the interpretation of TDS curves obtained in the field, theoretical sets of master curves are computed either directly from the formal expression for [Formula: see text] in terms of Stefanesco’s kernel function and Bessel functions or by transformation of Schlumberger (VES) and/or dipole‐dipole sounding curves. Conversely, TDS and VES curves computed for the same horizontally stratified earth models and for corresponding electrode spacings are easily transformed, by means of simple relations, into radial, perpendicular, and parallel dipole‐dipole, or LDS (longitudinal differential sounding) curves. It is mathematically established that TDS curves, for horizontally stratified laterally homogeneous media, are identical to sounding curves obtained with a parallel dipole‐dipole array at an azimuth angle of 60 degrees, and that negative values of apparent resistivity [Formula: see text] are measured for certain geoelectric sections. Examples of theoretical TDS and LDS curves for two‐, three‐, and four‐layer earth models compared with corresponding VES curves illustrate the higher resolving power and greater probing depth of differential soundings. The field procedures and electric current requirements described for TDS and other types of soundings show that seven apparent resistivity values, corresponding to four electrode arrays, can be measured at each electrode setup.
DETERMINATION OF SIGNAL AND NOISE STATISTICS USING CORRELATION THEORY
Authors: Dash Bibhu P. , Obaidullah K. Ahmed
Published: October 2002

A seismic trace may be represented as the sum of a signal and noise series. Each of the series may further be represented by convolution of a finite wavelet and a random series. With this representation, and provided that the signal and noise are uncorrelated, it is possible, in theory, to extract signal and noise statistics from two adjacent traces of a reflection seismogram. Some experiments are shown on model seismic traces, and it is shown that within the time‐duration of the seismic wavelet, the estimates of signal and noise statistics are reasonable for low signal‐to‐noise ratio. There remains, however, the problem of determining the optimum time lengths of the estimates.
APPLICATION OF THE KALMAN FILTER TO CONTINUOUS SIGNAL RESTORATION
Authors: Bayless J. W. , Brigham E. O.
Published: October 2002

The Kalman filter is applied to the inverse filtering or deconvolution problem. The derivation given of the Kalman filter emphasizes the relationship between the Kalman and Wiener filter. This derivation is based on the representation of systems by state variables and the modeling of random processes as the output of linear systems excited by white noise. Illustrative results indicate the applicability of these techniques to a variety of geophysical data processing problems. The Kalman filter offers exploration geophysicists additional insight into processing‐problem modeling and solution.
FORMATION EVALUATION BY INSPECTION WITH THE BOREHOLE TELEVIEWER
Authors: Zemanek Joe , Glenn E. E. , Norton L. J. , Caldwell R. L.
Published: October 2002

The televiewer takes an oriented acoustic picture of the inside of the well bore in the form of a continuous well log. The resulting log is a representation of the well bore wall as if it were split vertically along magnetic north and laid out flat. Both induced and natural fractures are defined in remarkable detail. Well bores can be filled with any homogeneous liquid such as lease crude oil, water, or drilling mud. Resolution of the tool is sufficient to locate fractures 1/32 inch wide. Examples of natural fractures in extremely fractured formations, in shales, and also drilling‐induced fractures are discussed. Fracture orientation information useful for local and regional tectonic studies is presented.
THE GRADATIONAL DENSITY CONTRAST AS A GRAVITY INTERPRETATION MODEL
Authors: Gendzwill D. J.
Published: October 2002

The basic equations are presented for the gravity effect of a horizontal slab with density varying linearly between limits. A series of type curves and interpretation nomograms are presented showing the characteristics of the gradational model. A notable characteristic is the horizontal gravity gradient which is nearly constant over most of the width of the variable density zone. Maximum slope and curvature of the gravity effect vary directly with the density change, depth, and thickness of the slab and inversely with the width of the variable density zone. Practically identical gravity effects are sometimes produced by step models with different dimensions and density contrasts. However, if the variable density zone is sufficiently wide, the gravity effect cannot be approximated by a simple step model.
STATISTICAL STRUCTURE OF AEROMAGNETIC FIELD
Authors: Naidu Prabhakar S.
Published: October 2002

A study of the statistical structure of an aeromagnetic field over approximately four and a half thousand square miles of the Parry Sound and Huntsville area of southern Ontario has revealed the following: From the point of first‐order statistics, that is, mean, variance, skewness, and probability distribution function, the right half of the area which is occupied by hornblende or biotite migmatite is homogeneous and Gaussian, whereas the left half, which is mainly occupied by amphibolite gneiss, is generally inhomogeneous and non‐Gaussian. From the point of view of second‐order statistics, that is, the spectrum, the entire field appears to be inhomogeneous. This difference between statistical pictures is attributed to the fact that there are at least two preferred directions of strike, which influence the shape of the spectrum significantly.
SOME DATA PROCESSING RESULTS FOR A VERTICAL ARRAY OF TRIAXIAL SEISMOMETERS
Authors: Der Zoltan A.
Published: October 2002

A vertical array of three component (triaxial) seismometers was operated in an abandoned oil well near Grapevine, Texas. The experiment was designed to investigate the effectiveness of teleseismic P‐wave enhancement by utilization of all three components of motion at various depths within the well. Previous experiments with vertical arrays which only recorded the vertical component of motion showed that optimum processors did not significantly improve the signal‐to‐noise ratio (Roden, 1968). The reason for this poor performance was found to be a similarity in the changes of signal and noise properties with depth.
INVERSION OF SEISMOGRAMS
Authors: Eisner E.
Published: October 2002

Given that a seismic model consists of layers which are horizontally uniform but whose properties vary arbitrarily with depth, inversion consists of using recordings of the disturbance on the free surface and deducing from them the parameters which are depth dependent. An algorithm is presented which partially solves the problem of inverting seismograms through the use of appropriate averaging over range. Implications with regard to actual field techniques are discussed.
GRAVITY DIAGRAMS FOR THICKNESS DETERMINATION OF EXPOSED ROCK BODIES
Authors: Jacoby W. R.
Published: October 2002

Sets of diagrams of the normalized peak gravity effect of exposed rock bodies can be used to determine the depth to which the bodies extend. The advantages of the method are 1) straightforward and easy application, 2) clear display of the uncertainty of the result, and 3) applicability to any shape rock bodies.
SEISMIC REFLECTION AND MAGNETIC STUDY OFF BOMBAY, INDIA
Authors: Harbison R. N. , Bassinger B. G.
Published: October 2002

As part of the 1967 Global Expedition of the USC&GS Ship OCEANOGRAPHER, approximately 1700 km (920 nm) of reconnaissance seismic reflection and total magnetic field intensity traverses were made between 17° and 21° N latitude, over the shelf and slope off Bombay, India. This investigation was designed to determine why the shelf area off Bombay is as much as three times wider than the rest of the western Indian shelf. Near the landward edge of the continental shelf, the seismic reflection profiles show strata dipping west from Bombay. To the west, these profiles indicate that the configuration of the continental slope is influenced by anticlinal structures, suggesting that the western Indian shelf in the study area is a sediment‐filled structural basin. The greater shelf width off Bombay is due to the location of the anticlinal trends. Within the basin, a buried north‐south trending anticlinal high is present approximately 75 km (40 nm) west of Bombay in water depths of about 50 m (165 ft). The structural basin and subsurface anticlinal trend off Bombay may continue northward into the Cambay Basin, where similar features are present. This continuation may be locally interrupted by the westward extension of a fault zone from the Narbada Valley into the southern Cambay Basin. The magnetic field is relatively flat, except for a zone of high frequency anomalies, which may be caused by dikes extending from the Bombay coast westward for approximately 40 km (20 nm).
DIELECTRIC PROPERTIES OF DRY, GEOLOGIC MATERIALS
Authors: Saint‐Amant M. , Strangway David W.
Published: October 2002

A detailed investigation of the dielectric properties of powdered and solid dry rocks in the frequency range of 50 hz to 2 mhz has revealed the following general characteristics: 1) All dry rocks, powdered and solid, show an increase in both the dielectric constant and the loss tangent as frequency decreases and as temperature increases. This dispersion is believed to be due to polarization associated with charge buildup at grain boundaries or at grain imperfections. 2) Dry powdered rocks often show a thermally‐activated relaxation peak with a typical Debye‐relaxation character. This is due to the presence of pyroxene and biotite and may be associated with other minerals. The relaxation peak is not seen in solid rocks, where it is hidden by the low‐frequency dispersion. 3) At high frequencies, the loss tangent approaches a constant value which is frequency independent. This behavior is observed in many dielectrics and may be the result of a distribution of relaxation times.
PRINCIPLES OF DIGITAL MULTICHANNEL FILTERING
Authors: Treitel Sven
Published: October 2002

The transition from single‐channel to multichannel data processing systems requires substantial modifications of the simpler single‐channel model. While the response function of a single‐channel digital filter can be specified in terms of scalar‐valued weighting coefficients, the corresponding response function of a multichannel filter is more conveniently described by matrix‐valued weighting coefficients. Correlation coefficients, which are scalars in the single‐channel case, now become matrices. Multichannel sampled data are manipulated with greater ease by recourse to multichannel z‐transform theory. Exact inverse filters are calculable by a matrix inversion technique which is the counterpart to the computation of exact single‐channel inverse operators by polynomial division. The delay properties of the original filter govern the stability of its inverse. This inverse is expressible in the form of a two‐stage cascaded system, whose first stage is a single‐channel recursive filter. Optimum multichannel filtering systems result from a generalization of the single‐channel least squares error criterion. The corresponding correlation matrices are now functions of coefficients which are themselves matrices. The system of normal matrix‐valued equations that is obtained in this manner can be solved by means of Robinson’s generalization of the Wiener‐Levinson algorithm. Inverse multichannel filters are designed by specifying the desired output to be an identity matrix rather than a unit spike; if this matrix occurs at zero lag, the least squares filter is minimum‐delay. Simple numerical examples serve to illustrate the design principles involved and to indicate the types of problems that can be attacked with multichannel least squares processors.
GLOSSARY OF TERMS USED IN WELL LOGGING
Authors: Sheriff Robert E.
Published: October 2002

The “Glossary of Terms Used in Geophysical Exploration” published in Geophysics in February 1968 met with some justified criticism because of the emphasis on seismic exploration for oil. The Addendum published in April 1969 attempted to restore balance as far as mineral exploration was concerned. But one major area of geophysics had still been slighted: that of well logging. This present addendum is intended to complete our coverage of geophysical exploration terms.
INTERPRETATION OF INDUCTION ANOMALIES ABOVE NONUNIFORM SURFACE LAYERS
Authors: Schmucker Ulrich
Published: October 2002

Using convolution integrals to account for the inductive coupling between surface layer and substratum, we derive the conductance of a nonuniform surface layer above a layered substratum by direct inversion from the system’s anomalous inductive response to natural geomagnetic variations. The conductivity in the substratum and the uniform conductance of the surface layer at some distance from the nonuniformity (two‐dimensional) must be known. The convolution method is also applied to the reverse problem: finding the anomalous inductive response for a given two‐dimensional nonuniformity. Our calculations are based on Price’s thin‐layer approximation.
AN EXPLANATION OF OBSERVED TIME DISCREPANCIES BETWEEN CONTINUOUS AND CONVENTIONAL WELL VELOCITY SURVEYS
Authors: Strick E.
Published: October 2002

Equations derived from the author’s mathematical (PL) model for the absorption of seismic energy by the earth are applied to explain statistical time discrepancies observed by Gretner between measurements using a continuous (CV) logging tool and conventional check shots. In essence, the predicted discrepancy is due to the pedestal effect that the author has fully described in an earlier paper. The magnitude of the predicted discrepancy not only agrees extremely well with the statistical observations but the corresponding theoretical Q’s for rock that are necessary to cause just such deviations are quite consistent with Q values expected in the shales and sandstones in which the observations were made. Our interpretation is also consistent with Gretner’s observation that the discrepancies are independent of the measured velocities. It is also shown that a mathematical model (TLF) based upon a truncation of a linear frequency attenuation function predicts time discrepancies just about as well as does the PL model but that the predicted values of Q at 40 hz are 60 percent larger. However, ambiguities still remain and the use of the TLF model is not recommended.
AUTOMATIC TREND ANALYSIS AND INTERPRETATION OF POTENTIAL FIELD DATA
Authors: Agarwal R. G. , Kanasewich E. R.
Published: October 2002

In this investigation, the fast Fourier transform is applied to the analysis of two‐dimensional data. In particular, a crosscorrelation function has been used to obtain trend directions of magnetic data. The technique is applicable to other geological or geophysical data. An empirical method has been devised to trace the trends by fitting a least squares third degree polynomial to the maximum coefficient in the correlation matrix. An example from Stony Rapids, Northern Saskatchewan illustrates the method.
ELECTRICAL AND SEISMIC PROPERTIES OF THE EARTH’S CRUST IN THE SOUTHWESTERN GREAT PLAINS OF THE U.S.A.
Authors: Mitchell B. J. , Landisman M.
Published: October 2002

Data from 20 magnetotelluric stations were used to determine a resistivity‐depth distribution for the crust in western Texas. We computed one‐dimensional models, which adequately explained the apparent resistivities obtained from the elements of a rotated impedance tensor. Descriptions of the method of data processing and model computation are given. The derived model includes an anisotropic, low resistivity layer at depths slightly greater than 20 km. The resistivity model is compared with a seismic velocity interpretation for eastern New Mexico, and the low resistivity layer is inferred to coincide with a zone of low seismic velocity. The base of the low velocity‐low resistivity region occurs at the transition from intermediate to more basic rocks in the crust. The low resistivity values are interpreted as resulting from interstitial fluid. This fluid may cause low seismic velocities and may form a weak region in which continental earthquakes occur. Interstitial water at depth in the crust may be produced by the intersection of the crustal temperature‐depth curve and equilibrium boundaries of hydrothermal reactions.
WAVE PROPAGATION IN A LAYERED MODEL DUE TO POINT SOURCE LOADING IN A HIGH‐IMPEDANCE MEDIUM
Authors: Daniel I. M. , Marino R. L.
Published: October 2002

Dynamic photoelastic techniques were used to study the incident, reflected, and refracted waves in a layered model. The model, consisting of two birefringent layers with an impedance ratio of 2.2:1, was subjected to point source explosive loading in the high‐impedance medium. The isochromatic fringe patterns were photographed with a Cranz‐Schardin multiple spark camera operating at a rate of 200,000 frames per second. Propagation and attenuation characteristics of waves in both layers were studied.
DOWNWARD CONTINUATION USING THE MEASURED VERTICAL GRADIENT
Authors: Ackerman John
Published: October 2002

The availability of two independent measured quantities, the total magnetic field and the vertical gradient of the total field, allows a solution to the downward continuation problem not heretofore practical. The method proposed in this short note uses the Laplace equation and a Taylor series expansion but does not require the numerical solution of an integral equation derived from potential theory. Evjen (1937) discussed the use of this technique for the gravitational field; however, he did not advocate the method because of the practical difficulties involved in making the gradient measurement. It is the purpose of my note to point out the utility of this direct method and to discuss the details of its application to airborne gradiometer survey data now being collected.
THE ELECTROMAGNETIC RESPONSE OF A CONDUCTING DISK FOR USE IN AFMAG INTERPRETATION
Authors: Greenfield Roy J.
Published: October 2002

The buried conducting disk has been suggested by Ward et al (1968) as a suitable model for mineralized zones in the interpretation of AFMAG data. Using an integral equation method, we obtained the secondary magnetic field of a thin disk of arbitrary electrical conductivity. From the magnetic secondary field, we then computed AFMAG anomaly tilt‐angle profiles. These profiles in turn were used to construct interpretation diagrams, of the type introduced by Ward et al, for determining the depth of the disk and its thickness‐conductivity product. The interpretation diagrams obtained, which we show to be accurate, differ from those of Ward et al for intermediate values of the disk conductivity. An alternate method of interpretation, based on measurements of the phase difference between the horizontal and vertical magnetic fields, is also presented.
REDUCTION OF UNEVENLY SPACED POTENTIAL FIELD DATA TO A HORIZONTAL PLANE BY MEANS OF FINITE HARMONIC SERIES
Authors: Henderson Roland G. , Cordell Lindrith
Published: October 2002

Conventional reductions of gravity and magnetic data do not lead to values that are effectively on the same horizontal plane, although it is common practice to regard them so. In regions of high topographic relief, failure to take into account local differences in vertical gradients can result in appreciable error. In this study a method is developed for reducing to a common level gravity or magnetic anomaly data observed at unevenly spaced stations at various elevations above a reference plane. The reduction is effected by means of finite harmonic series approximations in which the coefficients are determined by matrix methods and least squares. Traditional Fourier methods are not applicable because uneven station spacing and relative vertical displacement of stations preclude the use of the orthogonality properties of the trigonometric functions. The number of terms required to represent the data adequately is discussed in terms of “cutoff” wavenumbers empirically determined from residual variance estimates. The method is illustrated by application to theoretical and field data.
VERTICAL ATTENUATION OF ANOMALIES IN AIRBORNE GRAVIMETRY
Authors: Hammer Sigmund
Published: October 2002

Analysis of components of the gravity anomaly field from basic one, two, and three‐dimensional masses demonstrates that all components of the same derivative order attenuate with flight elevation at the same rate for a given type of anomaly. Different types of anomalies attenuate with significantly different rates. The signal strength of an anomaly component is conveniently defined as the difference between values at adjacent maximum and minimum points along a profile. For a given type of anomaly, the signal strengths of all comparable components are roughly equal, within about half an order of magnitude. Relative attenuations of anticipated signal and terrain noise permit evaluation of favorable and unfavorable prospect areas for airborne gravity exploration. A corollary result is that the tolerance in flight elevation control is orders of magnitude less severe for gradiometer surveying than for airborne gravity surveying. The conclusions are relevant in the choice of the most advantageous component for possible development of airborne gravimetry.
THE ESTIMATION OF MAGNETOTELLURIC IMPEDANCE TENSOR ELEMENTS FROM MEASURED DATA
Authors: Sims W. E. , Bostick F. X. , Smith H. W.
Published: October 2002

Six different estimates of the magnetotelluric impedance tensor elements may be computed from measured data by use of auto‐power and cross‐power density spectra. We show that each of the estimates satisfies a mean‐square error criterion. Two of the six estimates are relatively unstable in the one‐dimensional case when the incident fields are unpolarized. For the remaining four estimates, it is shown that two are unaffected by random noise on the H signal, but are biased upward by random noise on the E signal. The remaining two estimates are unaffected by random noise on the E signal, but are biased downward by random noise on the H signal. Computation of all of the estimates provides a measure of the total amount of noise present, as indicated by a stability coefficient for the estimates. In the absence of additional information as to the relative signal‐to‐noise ratios of the E and H signals, we suggest that a mean estimate be used. A numerical example is included.
On: “Geophysical Activity in 1969,” by S. J. Allen (GEOPHYSICS, February 1971, p. 189–196)
Authors: Hood Peter J.
Published: October 2002

On page 195 of the above article, there is an unfortunate erroneous statement made which has the effect of casting considerable doubt on the reliability of the statistics for mining geophysics expenditures published by the SEG prior to 1969. The erroneous statement to which I refer is “. . .in past years, mining costs were obtained by multiplying mining crew‐months by an assumed average cost per crew‐month.” W. C. Kellogg and myself were responsible for the compilation and reporting of the 1967 and 1968 SEG mining geophysics statistics, which were published in Geophysics in December 1968 and December 1969, respectively. On the questionnaire forms that were sent out for those years (and also for previous years), there were, in fact, blank columns appropriately labeled, in which expenditures were to be reported directly for ground and airborne surveys and research. Only in a few percent of the cases where the responding organization had for some reason omitted the costs were these calculated from professional man‐months or line‐miles of survey using averaged (not “assumed”) costs reported by other respondents. For example, the average costs for 1968 ground geophysical surveys appears on page 854 in the 1968 article (Geophysics, December 1969) as Table 3.
SEISMIC SIGNATURES OF LARGE AIR GUNS
Authors: Mayne W. Harry , Quay Roy G.
Published: October 2002

Large chamber air guns are a reliable and effective source of energy for marine seismic surveys. Air guns with chamber volumes of 300 and 1000 cubic inches demonstrate desirable low‐frequency responses and high absolute energy levels. Overall performance has been compromised, however, by the bubble effect. Previous attempts at minimizing the bubble response have resulted in loss of reliability, reduced power, or incomplete bubble suppression, or a combination thereof. In this paper, we present the results of experimental tests on air guns with 300 and 1000 cubic inch chambers and describe a divided‐chamber gun which greatly attenuates the bubble effect. Significant improvements in the width and flatness of the frequency spectra are demonstrated by analysis of the actual signatures obtained in deep water and with record sections comparing the results obtained with the standard and improved guns along an identical traverse. The bubble‐attenuating air gun simultaneously provides improved resolution, high absolute‐energy levels, and excellent reliability.
ACOUSTICAL HOLOGRAPHY USING DIGITAL PROCESSING
Authors: Hoover G. M.
Published: October 2002

A method for the reconstruction of hologram images by digital computer has been developed and its technical feasibility, demonstrated. The method for the detection of geological formations is applied to acoustic data from a series of two‐dimensional models of the earth. The models are constructed from 4 ft × 8 ft acrylic sheets and a radially pulsed (wide‐band) energy source is used. Values of elastic displacement scattered to the surface by the buried formations constitute the basic data from which the images are reconstructed. The images are displayed as computer plots of amplitude versus position. Narrow frequency bands ranging from 5 khz to 70 khz are selected from the wide‐band signals to form images at several different frequencies. Range‐gating in the time domain is demonstrated to be a useful technique for the removal of source‐generated noise. At all frequencies, the images demonstrate lateral and depth resolution near the diffraction‐limited values predicted by equations from classical optics. The method permits detection of objects with discontinuities that are sufficiently sharp to produce significant diffraction of the incident energy, such as a geological fault. Images of weakly diffracting objects, such as a triangular anticline, are more difficult to interpret because of strong ghost images of the source. The results show that the interference of diffracted and reflected energy can lead to erroneous interpretations of acoustic hologram images.
AN ALGORITHM FOR COMPUTATION OF SOURCE‐TO‐DETECTOR RANGES FROM THE TRAVELTIME OF MULTIPLY REFLECTED SOUND WAVES
Authors: Ocola Leonidas C.
Published: October 2002

We describe an algorithm for the computation of source‐to‐detector ranges from the traveltime of multiply reflected sound waves. Our technique is based on ray tracing the corresponding reflection multiple in an unfolded velocity‐depth function. It requires information about water depth in the vicinity of each reflection point, and the velocity‐depth function characteristic for the surveyed area. The point‐wise velocity information is transformed into a continuous function by assuming a linear gradient between sampling points. For each layer (i), horizontal offsets and traveltimes are computed from: [Formula: see text] and [Formula: see text] where [Formula: see text] is the ray parameter [Formula: see text] is the incidence angle), [Formula: see text] is the velocity gradient in the ith layer, and [Formula: see text] is the sound velocity at the ith point in the velocity‐depth function. The solution is found by iterating for that ray parameter (P*) which minimizes [Formula: see text] Here, [Formula: see text] is the observed traveltime and N is the number of layers in the unfolded velocity‐depth function. The horizontal distance (range) between the source and detector is given by [Formula: see text] The method has been tested with data gathered off Hawaii during the anisotropy Show experiment. Depths at reflection points were estimated from a surface fit to the bathymetry data, and ranges computed from one and two bottom reflections agree within 0.1 percent.
RECOMMENDED STANDARDS FOR DIGITAL TAPE FORMATS
Authors: Meiners E. P. , Lenz L. L. , Dalby A. E. , Hornsby J. M.
Published: October 2002

This paper is the result of work done by a subcommittee of the SEG Committee on Digital Recording Standards. It recommends an additional tape format for 9‐track field recording of seismic data and an optional use of 1600 bpi, as well as 800 bpi, for all field formats.
THE BRYAN FIELD—A SEDIMENTARY ANTICLINE
Authors: Oxley Marvin L. , Herlihy Daniel E.
Published: October 2002

Although the Bryan field has existed as a producing structure since 1958, this paper presents the first published data detailing the geologic history and origin of the structure. Drilling of the discovery well for the Jurassic (Cotton Valley) sediments was preceeded by a detailed seismic program, which outlined an anticlinal feature corresponding to the shallow (Cretaceous) production. On the assumption that the structure was a salt‐cored anticline, the discovery well was permitted for 17,000 ft but was drilled to 21,105 ft before reaching salt. A reevaluation of the data reveals the Bryan field as a classic example of the sediment‐cored anticline or turtleback structure. An unusual thickness of Haynesville sediments and a continued outward flow of salt into nearby salt‐cored anticlines combined to produce the sedimentary structure which is the subject of this study.
COMPUTER‐DESIGNED WIENER FILTERS FOR SEISMIC DATA
Authors: Robinson John C.
Published: October 2002

This paper is concerned with differences in the frequency content of signal and noise on seismic traces. In order to develop a filter which has applicability over some considerable spatial range, special consideration is given to basic differences in the shape and the frequency content of individual signal and noise wavelets (events) on these traces. Therefore, a so‐called “wavelet” Wiener filter is introduced which suppresses “noise” wavelets and enhances “signal” wavelets; this filter can be contrasted with an ordinary Wiener filter which discriminates between signal and noise on the additional basis of the statistics of the repetition of wavelets along the seismic trace. A technique for the automatic derivation of (wavelet) Wiener filters for seismic data by a digital computer is developed. The filters are time dependent to the extent that independent filters are derived at a sequence of data windows which are specified by the operator; criteria for selecting the window positions are given. The overall technique for the computer derivation of Wiener filters is demonstrated with synthetic and actual seismic data. A discussion of the wavelet Wiener filter and its relation to the ordinary Wiener filter is appended to this paper with a discussion of the effects of finite data windows on the eduction of the wavelet Wiener filter.
THE EFFECT OF A DIPPING CONTACT ON THE BEHAVIOR OF THE ELECTROMAGNETIC FIELD
Authors: Geyer Richard G.
Published: October 2002

Theoretical solutions for the electromagnetic response of a dipping interface in the field of normally incident plane waves are given in the form of inverse Lebedev‐Kontorovich transforms. When the lateral resistivity contrast becomes very large, the resulting integral solutions simplify considerably and allow ready numerical evaluation. The amplitude response of the vertical magnetic field seems most diagnostic of the structural attitude of sloping interfaces, even though the vertical magnetic field phase appears relatively insensitive to dip changes compared to horizontal electric field phase. The disturbance in the homogeneity of the field caused by the presence of an inclined contact is postulated to be due to cylindrically diffused waves generated by the dipping interface and propagating along the earth’s surface. It would then seem that formulation of plane‐wave impedances from orthogonal components of the surface electric and magnetic fields would only be applicable at distances from the interface which are large relative to a skin depth in either layer. The results presented here should prove to be useful in detecting and defining sloping interfaces or in avoiding their effects.
DECONVOLUTION OF REFRACTION SEISMOGRAMS FROM LARGE UNDERWATER EXPLOSIONS
Authors: White R. E. , Mereu R. F.
Published: October 2002

The P-wave spectra of seismograms from large underwater explosions are frequently dominated by reverberations. When this is so, a simple reverberation model similar to that of Backus (1959) gives a good approximation to the source spectrum. The basic wavelet determined by this method is not necessarily minimum‐delay. A few promising deconvolutions have been carried out, revealing a sequence of arrivals of comparable amplitudes separated by short time intervals. Synthetic seismograms which have been constructed from these spike sequences differ very little from the field records. However, the technique often yields output seismograms which are not easily interpreted. A study using synthetic seismograms suggests five reasons for this: 1) low signal‐to‐noise ratios, which result in too narrow a frequency band in which signal is predominant; 2) pulses arriving at the recorder having undergone phase changes during transmission, especially in combination with 3) very close spacing of the sequence of arrivals, causing overlap of the deconvolved output pulses; 4) the presence of arrivals which are not propagated along simple ray‐theory paths, and 5) poor estimation of the source parameters. Nonlinear effects and complex geology at the source are other possible causes of complications in the deconvolved records.