bluuebunny/crossref_metadata_2025 huggingface dataset:
MULTICHANNEL LINEAR FILTERS FOR OPTIMAL REJECTION OF MULTIPLE REFLECTIONS
Authors: Cassano E. , Rocca F.
Published: October 2002

In a CDP family after normal moveout correction, multiple reflections generally have a residual moveout hyperbolically increasing with the distance from the shotpoint. Simple stochastic models of primary signal, coherent noise and noncoherent noise can be generated, from which auto and mutual‐correlation functions can be derived. The set of linear operators corresponding to the multichannel filter for optimal rejection of coherent and noncoherent noise can be obtained by solving the proper Wiener‐Hopf equations. In the solutions already published some difficulties may arise, due either to the precision of the computations necessary to avoid numerical instabilities, or to the restrictive hypotheses on which the model is founded (multiples having precisely known moveout). In this case the difficulties are overcome with a proper model and by solving the Wiener‐Hopf equations in the frequency domain at different frequencies; the time operators are obtained by means of a discrete Fourier transformation. With n‐fold coverage, q linear systems of n equations in n unknowns have to be solved; the matrices of the coefficients are symmetric or Hermitian depending on the model chosen. With 48‐fold coverage, a set of 48 operators having a length of about 0.5 sec can be obtained in less than 40 sec of computing time on an IBM 360/44 with Array Processor. Several curves are shown comparing (for each residual moveout) the attenuation of the multiple obtained by multichannel filtering versus the one obtained by straight stacking. It can be seen that the increase of attenuation of the coherent noise may amount to 15–25 db in a broad range of moveouts. The filter can be designed to reject multiples having residual moveout in one or two disjoint intervals, or having fixed or varying amplitude.
AUTOMATED INTERPRETATION OF MAGNETIC ANOMALIES USING THE VERTICAL PRISM MODEL
Authors: Whitehill D. E.
Published: October 2002

The vertical prism model is frequently used for interpretation of magnetic anomalies. Such interpretations have been carried out previously by manual comparison of observed anomalies to atlases of prism model anomalies. An automated technique for computerized interpretation of magnetic anomalies using the three‐dimensional vertical prism model was applied to model data and two magnetic anomalies for intrusive bodies. The vertical‐sided, three‐dimensional prism model characteristically yields a range of models of near equally good fit to an anomaly; these prisms may have considerable variation in physical parameters. The results indicate the necessity for a computerized prism model interpretation technique which can adequately consider the range of possibilities for interpretation.
RESISTIVITY, SELF‐POTENTIAL, AND INDUCED‐POLARIZATION SURVEYS OF A VAPOR‐DOMINATED GEOTHERMAL SYSTEM
Authors: Zohdy A. A. R. , Anderson L. A. , Muffler L. J. P.
Published: October 2002

The Mud Volcano area in Yellowstone National Park provides an example of a vapor‐dominated geothermal system. A test well drilled to a depth of about 347 ft penetrated the vapor‐dominated reservoir at a depth of less than 300 ft. Subsequently, 16 vertical electrical soundings (VES) of the Schlumberger type were made along a 3.7‐mile traverse to evaluate the electrical resistivity distribution within this geothermal field. Interpretation of the VES curves by computer modeling indicates that the vapor‐dominated layer has a resistivity of about 75–130 ohm‐m and that its lateral extent is about 1 mile. It is characteristically overlain by a low‐resistivity layer of about 2–6.5 ohm‐m, and it is laterally confined by a layer of about 30 ohm‐m. This 30‐ohm‐m layer, which probably represents hot water circulating in low‐porosity rocks, also underlies most of the survey at an average depth of about 1000 ft. Horizontal resistivity profiles, measured with two electrode spacings of an AMN array, qualitatively corroborate the sounding interpretation. The profiling data delineate the southeast boundary of the geothermal field as a distinct transition from low to high apparent resistivities. The northwest boundary is less distinctly defined because of the presence of thick lake deposits of low resistivities. A broad positive self‐potential anomaly is observed over the geothermal field, and it is interpretable in terms of the circulation of the thermal waters. Induced‐polarization anomalies were obtained at the northwest boundary and near the southeast boundary of the vapor‐dominated field. These anomalies probably are caused by relatively high concentrations of pyrite.
On: “Statistical Models for Interpreting Aeromagnetic Data” by A. Spector and F.S. Grant (GEOPHYSICS, v. 35, p. 293–302).
Authors: Gudmundsson Gudmundur
Published: October 2002

I do not understand the reply of Spector and Grant (1971) to my Note on their paper “Statistical Models for Interpreting Aeromagnetic Data.” As I suspect that this may partly be the consequence of their misunderstanding the Note, I shall try and explain my points in more detail.
THE COMPLEX WIENER FILTER
Authors: Treitel Sven
Published: October 2002

The ordinary time or space domain Wiener filter is conventionally obtained for real‐valued inputs and real‐valued desired outputs. Situations exist for which these variables are complex valued. It is possible to develop a solution for the resulting complex Wiener filter in terms of complex autocorrelation and crosscorrelation functions. An algorithm for the efficient evaluation of the complex filter weights is also available.
COHERENCE FUNCTIONS FOR MAGNETOTELLURIC ANALYSIS
Authors: Reddy I. K. , Rankin D.
Published: October 2002

A multiinput linear system approach is used to study the magnetotelluric phenomena in the presence of lateral conductivity inhomogeneities in the earth. The three types of coherence functions (ordinary, multiple, and partial) are defined, and their use in magnetotelluric data analysis is illustrated with a field example. Partial coherence functions are used to determine the principal axes in the case of two‐dimensional type inhomogeneities, and as measures of three‐dimensionality in the case of non‐two‐dimensional type structures. The results obtained using coherence functions are compared with those obtained with the conventional tensor impedance method.
THE IP PHASE MEASUREMENT AND INDUCTIVE COUPLING
Authors: Hallof Philip G.
Published: October 2002

Recent work by Van Voorhis et al (1973), Zonge et al (1972), Pelton (1973), and others has begun to make available some information concerning the phase shifts that accompany the IP effects used in mineral exploration. A portable system is now available to measure phase shifts in the field in the frequency range from 0.05 hz to 1.25 hz with an accuracy of a few milliradians. Field work with this system confirms that the phase shift associated with the IP effect from metallic mineralization is relatively constant at all frequencies within this frequency range. The inductive coupling effects (for the dipole‐dipole electrode configuration) give phase shifts that increase approximately linearly with frequency for a uniform or layered earth. Theoretical solutions and scale modeling suggest that this is also a valid approximation for two‐dimensional resistivity variations of considerable extent. Therefore, it should be possible to use accurate phase measurements, at several closely spaced frequencies, to separate IP effects from inductive coupling effects.
MATCHED FILTER DETECTION OF ELECTROMAGNETIC TRANSIENT REFLECTIONS
Authors: Clay C. S. , Greischar L. L. , Kan T. K.
Published: October 2002

An electromagnetic transient signal is distorted as it travels in the conducting earth. At vertical incidence, the signal is stretched by a time that is proportional to the magnetic permeability, conductivity, and depth squared. The stretch is a function of the travel distance, and a set of filters can be constructed to match the waveforms of signals which have traveled to different depths. Surface waves travel from the source and can overlap the reflected signals. The reflected signals were estimated by subtracting an estimate of the surface wave and then passing the estimated reflections through a set of matched filters. This method was tested by means of numerical models and laboratory models. The latter were coaxial coils on aluminum plates. In both cases, the matched filters determined the depth of the reflected signals.
FORMATION VELOCITY AND DENSITY—THE DIAGNOSTIC BASICS FOR STRATIGRAPHIC TRAPS
Authors: Gardner G. H. F. , Gardner L. W. , Gregory A. R.
Published: October 2002

A multiplicity of factors influence seismic reflection coefficients and the observed gravity of typical sedimentary rocks. Rock velocity and density depend upon the mineral composition and the granular nature of the rock matrix, cementation, porosity, fluid content, and environmental pressure. Depth of burial and geologic age also have an effect. Lithology and porosity can be related empirically to velocity by the time‐average equation. This equation is most reliable when the rock is under substantial pressure, is saturated with brine, and contains well‐cemented grains. For very low porosity rocks under large pressures, the mineral composition can be related to velocity by the theories of Voigt and Reuss. One effect of pressure variation on velocity results from the opening or closing of microcracks. For porous sedimentary rocks, only the difference between overburden and fluid pressure affects the microcrack system. Existing theory does not take into account the effect of microcrack closure on the elastic behavior of rocks under pressure or the chemical interaction between water and clay particles. The theory of Gassmann can be used to calculate the effect of different saturating fluids on the P-wave velocity of porous rocks. The effect may be large enough in shallow, recent sediments to permit gas sands to be distinguished from water sands on seismic records. At depths greater than about 6000 ft, however, the reflection coefficient becomes essentially independent of the nature of the fluid. Data show the systematic relationship between velocity and density in sedimentary rocks. As a result, reflection coefficients can often be estimated satisfactorily from velocity information alone.
ACCURACY OF FINITE‐DIFFERENCE MODELING OF THE ACOUSTIC WAVE EQUATION
Authors: Alford R. M. , Kelly K. R. , Boore D. M.
Published: October 2002

Recent interest in finite‐difference modeling of the wave equation has raised questions regarding the degree of match between finite‐difference solutions and solutions obtained by the more classical analytical approaches. This problem is studied by means of a comparison of seismograms computed for receivers located in the vicinity of a 90-degree wedge embedded in an infinite two‐dimensional acoustic medium. The calculations were carried out both by the finite‐difference method and by a more conventional eigenfunction expansion technique. The results indicate the solutions are in good agreement provided that the grid interval for the finite‐difference method is sufficiently small. If the grid is too coarse, the signals computed by the finite‐difference method become strongly dispersed, and agreement between the two methods rapidly deteriorates. This effect, known as “grid dispersion,” must be taken into account in order to avoid erroneous interpretation of seismograms obtained by finite‐difference techniques. Both second‐order accuracy and fourth‐order accuracy finite‐difference algorithms are considered. For the second‐order scheme, a good rule of thumb is that the ratio of the upper half‐power wavelength of the source to the grid interval should be of the order of ten or more. For the fourth‐order scheme, it is found that the grid can be twice as coarse (five or more grid points per upper half‐power wavelength) and good results are still obtained. Analytical predictions of the effect of grid dispersion are presented; these seem to be in agreement with the experimental results.
INTERPRETATION OF WELL‐SHOT DATA
Authors: Dix C. H.
Published: October 2002

A method of interpreting well‐shot data is presented having the advantage that the velocity distribution calculated can be made to fit the data with any desired closeness. In practise, the method is not difficult to apply. With discrete data, a unique interpretation evidently cannot be given, but in the case of ideal data, including also surface refraction data, the velocity distribution is uniquely determined.
ON SEISMIC PATHS AND VELOCITY‐TIME RELATIONS
Authors: Mott‐Smith M.
Published: October 2002

It is shown that only a few very simple velocity‐depth laws lead to path equations that can be integrated, and that none of them are suitable for application, either because there are no empirical velocities to which they conform sufficiently closely, or because the difficulties of fitting them to an empirical velocity are too great. It is shown that there are velocity‐time relations that do conform closely to the usual form of an empirical velocity, relations which can be easily fitted to the latter, which are not difficult to handle, and which have other advantages. Two ways of applying the seismic paths are discussed.
ON THE STRATEGY AND TACTICS OF EXPLORATION FOR PETROLEUM, III
Authors: Rosaire E. E.
Published: October 2002

In the preceding paper of this series, it was pointed out that three of the Principles of War, those relating to the Offensive, Surprise, and Security, were applicable to Exploration for Petroleum. The following discussion examines the similar applicability of two other Principles of War, the first, that relating to the Objective, and the fourth, that relating to Economy of Force. These are as follows.
AN AREAL PLAN OF MAPPING SUBSURFACE STRUCTURE BY REFRACTION SHOOTING
Authors: Gardner L. W.
Published: October 2002

Under suitable geologic conditions, arrival events of refraction wave trajectories following a relatively high speed “marker horizon” may be observed and identified and the associated travel times may be measured on refraction setups having detectors located at suitable distances from shot point locations. On each refraction setup, the sum of the two “delay times” at positions offset from the shot point and detector position may be determined. Depth of the marker horizon and the location of the “offset position” at which this depth applies are dependent on and determinable from “delay time” if the seismic wave speeds in the geologic section overlying the marker horizon are known. By arranging two or more refraction setups angularly disposed so that they have a common location for one offset position of each setup, differential delay times and corresponding differential depths at the free offset positions may be determined. By a geometrical arrangement of three setups in a triangular manner such that each offset position of each line is coincident with one offset position of another line, absolute delay times and depths at the common offset positions may be determined. By coordinating these two types of geometrical arrangements in shooting over an area, the depths and undulations of the marker horizon may be determined.
A PROPOSED GEOPHYSICAL METHOD FOR ORIENTING CORES
Authors: Vacquier Victor
Published: October 2002

A new method of orienting cores is suggested, in which a fluid suspension of magnetic particles is made to solidify and adhere firmly to the formation at the bottom of the well. While the substance is still liquid, the magnetic particles line up in the direction of the earth’s magnetic field so that upon solidifying a magnetized body is tightly cemented to the rock. The formation is then cored and the top‐most core carrying an inclusion of the polarized material is oriented by means of a magnetometer. When such a core is recovered there can be no doubt as to the fidelity of the result.
DISCUSSION AND COMMUNICATIONS
Authors: Rosaire E. E.
Published: October 2002

In Geophysics, Vol. IV, No. 3, July 1939, p. 225, in discussing Dr. Horvitz’ paper on Geochemical Prospecting, Mr. A. J. Teplitz considers that the “halo” pattern (of higher hydrocarbon concentrations in the soil over an oil field) is in direct contradiction with the findings of the Russian workers. Mr. Teplitz goes on to say that “Their (Sokolov and his associates, supposedly,) investigations have always indicated that the higher hydrocarbon concentrations overlie the petroliferous deposits rather than surround them.”
A BOUGUER GRAVITY ANOMALY MAP OF SOUTH AMERICA
Authors: Breville Gerald L.
Published: October 2002

A recently prepared Bouguer gravity anomaly map of South America is the first gravity anomaly representation of the entire South American continent. The technical paper which accompanies the map was presented by Breville et al. (1973). A copy of the map and technical paper may be obtained by writing for DMAAC Technical Paper No. 73-2 to the following address: Director, DMA Aerospace Center, ATTN: RDN, South Annex, St. Louis AFS, Mo 63125.
DIRECT INTERPRETATION OF GEOELECTRIC MEASUREMENTS BY THE USE OF LINEAR FILTER THEORY
Authors: Niwas Sri
Published: October 2002

In a recent paper Lee and Green (1973) worked out a method for direct interpretation of electrical soundings made over a fault or dike (see Figure 1). They computed the kernel function using the method developed by Meinardus (1970). However, Koefoed (1968), while dealing with direct interpretation of electrical measurements made over a horizontally layered earth, showed that the relative variations in the apparent resistivity were not of the same order of magnitude in the corresponding kernel curve; thus, any method based on the determination of this function as the intermediate step would lead to a loss of information and hence to incorrect interpretation. Koefoed (1970) introduced a function T(λ) called the resistivity transform (a function related to the kernel function) as an intermediate step. Ghosh (1971) used linear filter theory and gave a simple and quick procedure to obtain the T(λ) function from the apparent resistivity field curve. He cited the properties of the T function as, (1) it is solely determined by the layer distribution; (2) it is an unambiguous representation of the [Formula: see text] function; and (3) for small and large values of 1/λ it approaches the [Formula: see text] curves.
REPRESENTATION THEOREMS FOR MULTIREGIONAL ELECTRODYNAMIC DIFFRACTION: PART I. THEORY
Authors: Won I. J. , Kuo J. T.
Published: October 2002

Generalized representation theorems are developed for electromagnetic diffraction in a domain which is arbitrarily divided into two or more different regions. Each region has different electromagnetic parameters and is assumed to be linear, homogeneous, isotropic, and of finite conductivity. The theorems apply to any harmonic input source without imposing any restrictions on the geometry of the diffracting surfaces. The total diffracted field can be expressed in terms of three newly introduced quantities: a geometrical factor matrix for each diffracting surface, a propagation matrix for each region, and an interaction matrix between two adjacent surfaces. The resultant solution is in an integral matrix form which can be evaluated either analytically or numerically depending upon the complexity of the geometry. The solution can be reduced to a simpler one for a smooth scatterer on whose surface the radii of curvature are longer than the source wavelength. Both the two‐ and three‐dimensional forms of the representation theorems are formulated and specifically developed in a cylindrical coordinate system for cylindrical diffracting surfaces. The present formulation can be applied to detailed studies on the effects of a half‐space, surface topography, a weathering layer, and a disseminated ore zone surrounding the main deposit; these geometries occur in almost all practical mineral environments.
DIP‐DEPENDENT REVERBERATION OPERATOR
Authors: Spencer T. W. , Bhambani D. J.
Published: October 2002

Using ray theory and assuming zero offset between source and detector, we have derived an expression for reverberation sequences in two‐dimensional models involving a water layer with a dipping bottom. The operators exhibit novel departures from those for the familiar case of a horizontal water bottom. The number of water‐layer multiples which contribute to the reverberation operator is limited by the geometry anti decreases rapidly with a small increase in dip. Moreover, for a given dip, the time scale of the reverberation operator is compressed linearly as source‐receiver position is moved in the up‐dip direction. The result is the familiar phenomenon of increasing “dip” of later multiples on seismic time sections. The introduction of dip causes a splitting of the arrival spikes in the zero‐dip operator. Splitting is a simple function of spike number but results in a complicated interfingering of spikes associated with multiples of different orders. The spikes derived from a particular multiple fan out over a time interval which increases with dip and multiple order. Whether the predicted effects cause significant changes in the structure of the zero‐dip reverberation operator depends principally on the dip and the impedance contrast. For small dip, significant differences are confined to the later multiples. These multiples have non‐negligible amplitudes only for high‐impedance contrast situations. As the dip increases, splitting and interfingering occur even in the earlier multiples. Consequently, departures from the zero‐dip operator are expected to be significant in areas characterized by small dip and quite persistent reverberation and in areas where the dip is large. Under these conditions, the reverberation operator is not minimum‐delay, and standard dereverberation techniques which assume this property are not optimal for resolution improvement. In high‐impedance contrast areas, the departure from the zero‐dip structure is particularly pronounced for reflections which propagate in the down‐dip direction at large angles with respect to the vertical. Narrow‐band filtered reverberation operators calculated for high impedance contrasts exhibit irregular beating patterns, late reflection times, phantom reflections, and time‐varying frequency content.
MAGNETIZATION‐INCLINATION ESTIMATION FROM PRISM‐LIKE ANOMALY PROFILES
Authors: Emilia David A. , Massey Robert L.
Published: October 2002

Magnetization inclination in a two‐dimensional source body that produces a prism‐like magnetic anomaly, i.e., an anomaly that appears to be due to a two‐dimensional, rectangular prism, can be estimated by solving the nonlinear inverse problem of a horizontal equivalent‐source layer with unknown magnetization‐amplitude distribution and magnetization inclination. The equivalent source layer is chosen to be symmetric about the anomaly’s degree‐of‐symmetry point, determined by the method of Lamontagne, and the computed magnetization inclination is that of the source of the prism‐like anomaly. Horizontal orientation of the magnetization vector is assumed known. A theoretical discussion and theoretical examples show the validity of the approach. A field example shows that the method has internal consistency and that it will have practical application.
MAGNETOTELLURIC RESPONSE OF LATERALLY INHOMOGENEOUS AND ANISOTROPIC MEDIA
Authors: Reddy I. K. , Rankin D.
Published: October 2002

The lack of agreement between magnetotelluric field measurements and the calculations based on essentially two‐dimensional models with either anisotropy or lateral inhomogeneity necessitates a more complex model of the earth than has been previously considered. The Galerkin finite‐element method is applied to a two‐dimensional structure with a tensor conductivity. The importance of considering conductivity as a tensor is illustrated by a model consisting of an anisotropic, conducting dike embedded in an anisotropic half‐space. This model can be distinguished from an isotropic model by the nonvanishing diagonal elements of the impedance tensor, the ellipticity indices, and the skew.
COMPRESSIONAL VELOCITIES OF PARTIALLY SATURATED, UNCONSOLIDATED SANDS
Authors: Elliott S. E. , Wiley B. F.
Published: October 2002

The possibilities inherent in reflection amplitude anomalies or “bright spots” on seismic cross‐sections as indications of gaseous hydrocarbons have stimulated widespread interest. To help evaluate these possibilities, laboratory measurements were made on specimens of compacted Ottawa sand of approximately 30 percent porosity. The measurements were made at five different water saturations (0 percent and 100 percent, and approximately 9 percent, 45 percent, and 85 percent) and net confining pressures to 10,000 psi. The gas was air. Measurements were made with acoustic pulses with a peak frequency of about 700 khz.
A CLASS OF CONVOLUTIONAL TIME‐VARYING FILTERS
Authors: Pann Keh , Shin Y.
Published: October 2002

A linear time‐varying filtering process usually is realized by applying a number of time‐invariant filters to overlapping time regions of a trace and transitionally merging these different regions. We describe the design of a different type of linear time‐varying filter where the frequency spectrum of the filter is shifted along the frequency axis as a function of time without appreciable change in the spectrum shape. The design is based on a given time‐invariant filter with the desired spectrum shape. For long filter length, the design procedure is rather complicated. Furthermore, only a constant rate of frequency shift is possible. However, for many practical situations where the frequency shift over the filter length is much less than the bandwidth of the filter, the process of time‐varying filtering can be further simplified without appreciable frequency error. In fact, time‐varying filtering is achieved through modifying the complex signal representation of the original time invariant filter by a preapplication of a specified time‐varying frequency‐shifting operator and a post‐application of a corresponding frequency restoration operator. Under this short filter approximation, the more general time‐varying filter in which the rate of frequency shift is nonuniform can be realized. Behavior of the convolutional time‐varying filters is demonstrated by examples of filtering of sampled sinusoidal signals.
A WESTERN VIEW OF MINING GEOPHYSICS IN THE U.S.S.R.
Authors: Zietz Isidore , Eaton Gordon P. , Frischknecht Frank C. , Kane Martin F. , Moss Calvin K.
Published: October 2002

A five‐man delegation of American geophysicists visited the U.S.S.R. for a month during the early fall of 1973 as members of an official technical exchange program in mining geophysics and potential field methods. The state of the art of mining geophysics in the Soviet Union has progressed significantly and exhibit, among others, the following strong points: (1) The total national effort in the field is well coordinated, minimizing costly and time‐consuming duplication typical of the industrial approach of the Western world. (2) Planning of programs is well coordinated between geologists and geophysicists even at the highest level. (3) The approach to a mineral exploration program is well integrated, beginning with regional crustal studies and continuing on an increasingly more detailed scale to methods in defining and analyzing specific ore bodies. (4) The development and use of a number of innovative techniques that are essentially unknown in the West.
On: “The magnetotelluric method in the exploration of sedimentary basins” by Keeva Vozoff (GEOPHYSICS, February 1972, p. 98–114).
Authors: Jupp D. L. B. , Vozoff K.
Published: October 2002

A number of errors in the section dealing with the Tipper have been pointed out. Some alternative definitions and additional properties have also become apparent.
NUMERICAL SOLUTIONS OF THE RESPONSE OF A TWO‐DIMENSIONAL EARTH TO AN OSCILLATING MAGNETIC DIPOLE SOURCE
Authors: Stoyer C. H. , Greenfield Roy J.
Published: October 2002

A finite difference formulation is developed for computing the frequency domain electromagnetic fields due to a point source in the presence of two‐dimensional conductivity structures. Computing costs are minimized by reducing the full three‐dimensional problem to a series of two‐dimensional problems. This is accomplished by Fourier transforming the problem into the x-wavenumber [Formula: see text] domain; here the x-direction is parallel to the structural strike. In the [Formula: see text] domain, two coupled partial differential equations for [Formula: see text] and [Formula: see text] are obtained. These equations resemble those of two coupled transmission sheets. For a requisite number of [Formula: see text] values these equations are solved by the finite difference method on a rectangular grid on the y-z plane. Application of the inverse Fourier transform to the solutions thus obtained gives the electric and magnetic fields in the space domain. The formulation is general; complex two‐dimensional structures containing either magnetic or electric dipole sources can be modeled. A quantitative test of accuracy is presented which compares the finite difference results to analytic results for a magnetic dipole on a homogeneous half‐space. In addition, the computed results for a two‐dimensional model are qualitatively compared to published results for a three‐dimensional analog model. Synthetic field data for surveys over several different bodies of anomalous conductivity are presented. Two of these demonstrate the nonuniqueness of single frequency data interpretation. Results also show that the characteristic form of the response given by the anomalous body can be heavily dependent upon the structure of the host medium. This is especially true for horizontal magnetic dipole source surveys.
AN AUTOMATIC METHOD OF DIRECT INTERPRETATION OF MAGNETIC PROFILES
Authors: Jain Sudhir
Published: October 2002

The well‐known equations for the total magnetic field due to thin sheets and the edges of a thick body (Werner, 1953) can be programmed to compute automatically the depth to the top, susceptibility contrast, and the dip of these features from a given total magnetic field profile. The synthetic anomalies show that in ideal cases the depths can be determined to the accuracy of 10 percent or better, provided the source of the anomaly can be identified as a bounding edge or the thin sheet. It has also been found that the anomalies due to edges approximately one depth unit apart in horizontal direction can be resolved. Vertically, the interpretation of shallow bodies is not affected by the presence of deeper bodies. However, the deeper bodies can be located only when they cause anomalies much stronger than those associated with shallow bodies, or when the shallow bodies are displaced from the deep edges horizontally by a distance equal to or greater than the depth to the top of deep edges. The shallow high‐frequency anomalies tend to mask the interpretation of deeper anomalies rather than cause erroneous estimates. Susceptibility contrasts can be estimated reasonably accurately only when the dips are about 40 degrees or greater. The dip estimates are accurate to within 10 percent.
NEAR‐BOTTOM OBSERVATIONS OF 4 KHZ ACOUSTIC REFLECTIVITY AND ATTENUATION
Authors: Tyce Robert C.
Published: October 2002

A quantitative 4-kHz seismic profiling system has been developed as part of the Deep Tow Instrumentation System of the Marine Physical Laboratory at Scripps Institution of Oceanography. The system utilizes a sea‐going digital computer system aboard ship for real‐time processing and display of the quantitative near‐bottom reflectivity data. Preliminary results from the system are presented for four diverse areas of the Pacific Ocean floor: the Samoan Passage, the Carnegie Ridge, the San Diego Trough, and, in particular, an abyssal hill region several hundred miles west of San Diego where the system was first operated in real time during a survey in May 1974. These results exhibit a number of substantial variations in observed reflectivity on a small horizontal scale. To be more specific, they show (1) several db variations in layer reflectivity for horizontal distances less than 100 m for well‐defined reflectors; (2) abrupt changes of more than 7 db in subsurface reflectivity, which are generally coincident with topographic breaks in slope; (3) several db variations in reflectivity resulting from topographic focusing by buried reflectors, suggesting larger variations for surface observations; and (4) consistently diffuse returns from volcanic basement at relatively low energy levels which are substantially reduced by thin sediment cover, suggesting significant scattering at this frequency. In addition, calculations of acoustic attenuation in the sediment column have been made from the quantitative data by determining reflectivity differences for a well‐defined layer as a function of depth of burial. These calculations give a value of 0.36 db/m for clayey silt in the San Diego Trough, which is consistent with measured values. A value of 0.12 db/m is obtained for calcareous ooze on the Carnegie Ridge. This low value for attentuation suggests that significant lithification may have occurred in these relatively young, high‐porosity sediments. Sediment physical property analyses show relatively high velocities in these calcareous sediments, suggestive of greater rigidity.
GEOPHYSICAL ACTIVITY IN 1975
Authors: Espey H. R.
Published: October 2002

This report provides statistics on worldwide use of geophysical methods in 1975. Data were obtained primarily through a survey questionnaire which was mailed out to more than 1500 companies, government agencies, and universities that use geophysical techniques for petroleum exploration, oceanography, engineering, mining, geothermal exploration, and groundwater exploration. Response to the survey was excellent and provided detailed information on more than 2100 geophysical surveys. Data on unit costs, methods used, and line‐miles covered are believed to be more accurate this year as a result of better cooperation from industry in filling out the questionnaires. Computer processing was utilized in tabulating the statistics to provide increased accuracy and detail. Data not supplied on the questionnaire for costs or line mileage were estimated on the basis of worldwide averages to produce a more comprehensive report.
SIGNAL DESIGN IN THE “VIBROSEIS”® TECHNIQUE
Authors: Goupillaud Pierre L.
Published: October 2002

After more than twenty years of field practice, the most widely used signal in the Vibroseis technique remains the original so‐called linear sweep. To many, it seems that substantial improvement should be expected from taking advantage of more sophisticated signal design techniques. The purpose of this paper is to present a tutorial survey of the basic concepts involved in signal design, tying together the known features of the linear sweep with those of the more general type. Several particular types of nonlinear sweeps are analyzed and the results of a field test seem to confirm the conclusions drawn from the conceptual analysis. After a brief consideration of still another type of nonlinear signals, a particular variety of pseudorandom sweeps, the author concludes by giving his views of what might be the role of nonlinear signal design in the future development of the Vibroseis technique.
On: “Early Magnetic Interpretation,” by L. L. Nettleton (GEOPHYSICS, June 1976, p. 575–576)
Authors: Pirson S. J.
Published: October 2002

In the recent article by Nettleton, I believe there is an error in the geographic coordinates of the Oklahoma City field. I would appreciate if you would check this and make the correction if necessary.
MICROEARTHQUAKE INVESTIGATION OF THE MESA GEOTHERMAL ANOMALY, IMPERIAL VALLEY, CALIFORNIA
Authors: Combs Jim , Hadley David
Published: October 2002

Microearthquakes associated with the Mesa geothermal anomaly were recorded for five weeks during the summer of 1973 using an array of six portable, high‐gain seismographs equipped with vertical‐component 1-sec natural period seismometers. Background seismicity of the area is thus determined prior to development for geothermal power and water. The local seismicity changed considerably over the recording period. Most daily activity was characterized by only one or two potentially locatable events, while two microearthquake swarms of two‐ and three‐day duration included as many as 100 or more distinct local events per day. Hundreds of small events (nanoearthquakes), some clustered in swarms, were recorded by each seismograph; however, most were not detected on four or more seismograms so that hypocentral locations usually could not be determined. Locations were determined for 36 microearthquakes having epicenters situated in the [Formula: see text] areal extent of the geothermal anomaly. Focal depths ranged from near‐surface to about 8 km. More than half of the located events have hypocenters greater than the 4.0 km which is approximately the depth to crystalline basement. Stress associated with the Mesa geothermal anomaly is relieved by a combination of continuous microseismic activity and intermittent microearthquake swarms. Based on the results of the present study, a new right‐lateral strike‐slip fault, the Mesa fault, was defined. First motion studies indicate strike‐slip faulting although there is no surface expression of the fault. The northwest‐southeast trending Mesa fault is an active fault functioning as a conduit for rising geothermal fluids of the Mesa geothermal anomaly. This investigation is another demonstration that geothermal areas are characterized by enhanced microearthquake activity.
MICROWAVE BRIGHTNESS SPECTRA OF LAYERED MEDIA
Authors: England A. W. , Johnson G. R.
Published: October 2002

Many natural media are layered at the scale of microwaves. Examples include frozen soil over moist soil, ice over water, and snow over soil. The microwave brightness spectra of such media may exhibit interference patterns. Such patterns have been observed for emission from fresh‐water ice but not for emission from snow or from seasonally frozen soil. Three factors which determine whether interference is detectable are the spectral resolution of the radiometer, the uniformity of layer thickness, and the distinctness of interfaces between layers. Analyses of these factors show that: (1) The radiobrightnesses of layered media vary sufficiently slowly with wave‐length that radiometers designed for the radio‐astronomy bands provide adequate spectral resolution; (2) a variability of layer thickness greater than 15 percent of the free‐space wavelength in the area viewed by the radiometer will effectively eliminate an interference pattern; and (3) a diffuse interface, whose thickness is 15 percent of the free‐space wave‐length, is transparent to microwaves so that effects of the interface will not appear in the radiobrightness spectrum.
INTERPRETATION OF APPARENT RESISTIVITY DATA FROM APODI VALLEY, RIO GRANDE DO NORTE, BRAZIL
Authors: Rijo Luiz , Pelton W. H. , Feitosa E. C. , Ward S. H.
Published: October 2002

One hundred and twelve Schlumberger vertical electrical soundings were made as part of a hydrogeological study in the Apodi Valley, Brazil. Most of the data have been interpreted using an automatic ridge regression inversion algorithm in conjunction with a fast digital filter forward algorithm. As a result, the inversion costs are very low. The increase in speed and accuracy in the evaluation of the forward problem has also allowed calculation of the Schlumberger apparent resistivity from potential differences, instead of the electric field. Consequently, there is no difficulty in the interpretation of data where very large receiver electrode (MN) spacings have been used or where discontinuities have been introduced by changing the MN spacing on a layered earth containing large resistivity contrasts. The soundings were conducted primarily to map the thicknesses of a known alluvial aquifer and a potential sandstone aquifer. These thicknesses have been determined to within an error of 20 percent as estimated from analysis of the parameter standard deviations and comparison with available drill hole information.
DECONVOLUTION OF TEMPERATURE GRADIENT LOGS
Authors: Conaway John G.
Published: October 2002

A complete system for continuous logging of borehole temperature gradients has been developed and subjected to field tests with prototype equipment. To meet the requirements of this system, a time‐domain operator was derived consisting of a smoothing term, a deconvolution term (to compensate for lag due to the thermistor time constant and probe velocity), and a gradient term. When convolved with raw field data, this combined operator will yield directly a high‐precision, high‐resolution temperature gradient profile. The system was field tested in two partially cased, water‐filled boreholes. There was good correlation between gradient logs and thermal resistivity profiles from laboratory measurements on core material, indicating that even in cased wells the gradient log is a good approximation of the thermal resistivity profile. For a lowering rate of 18 m/minute, the gradient profile exhibits a repeatability better than ±0.5°C/km. Comparison of the gradient profile with a fine‐scale geologic log indicates a stratigraphic resolution threshold on the order of 2 m for a 10–20 percent thermal resisitivity contrast. For isolated resistivity contrasts of 50–100 percent, the resolution is better than 0.5 m.
VERTICAL STACKING AS A REFLECTION FILTER
Authors: Levin Franklyn K.
Published: October 2002

In order to improve the signal‐to‐noise ratio of noisy seismic reflection data before velocity determination by computer, it’s common practice to sum several traces having the same source‐to‐geophone (SGD) separation but different CDP points. We say we form a vertical stack. When reflectors are horizontal planes, vertical stacking simply reduces the noise without distorting the reflection waveforms. However, when a reflector is a dipping plane, the individual reflections from that plane may be identical; but they arrive at slightly different times. As a result, the sum reflection is distorted: vertical stacking acts as a filter.
GRAVITATIONAL ATTRACTION OF A RECTANGULAR PARALLELEPIPED
Authors: Banerjee Buddhadeb , Das Gupta S. P.
Published: October 2002

In the inverse solution of a gravity problem when the direct method of interpretation is not effective, the interpretation is done by assuming a geometric model of the subsurface mass distribution based on geologic or some other considerations, calculating the gravity effect of the model, and finally matching with the observed anomaly curve. The model is repeatedly changed until there is an agreement of the theoretically calculated and observed anomaly curve. Again for estimation of terrain effect, it is necessary to compute the gravity from the known mass distribution. So the importance of direct potential problems in gravity should not be overlooked.
SHEAR ZONES IN NORTH EGYPT INTERPRETED FROM GRAVITY DATA
Authors: Riad S.
Published: October 2002

The Bouguer anomaly map for the northern part of Egypt was used for determining fault systems which are probably present in the area. These systems show the presence of a number of almost parallel shear zones, striking in a northwest‐southeast direction. Extrapolation of some of these zones is suggested in the Gulf of Suez area. The movement of the eastern side of each zone is thought to be right‐lateral to the southeast. The shear zones are probably related to the interaction between the European and African plates. They probably started developing in the Oligocene and are presently still active. The opening of the Gulf of Suez is thought to be mainly due to the action of these transcurrent faults.
SINGLE‐CHANNEL WHITE‐NOISE ESTIMATORS FOR DECONVOLUTION
Authors: Mendel Jerry M. , Kormylo John
Published: October 2002

The Wiener filtering approach to deconvolution is limited by certain modeling assumptions, which may not always be valid. We develop a Kalman filtering approach to deconvolution which permits more flexible modeling assumptions than the Wiener filtering approach. Our approach is applicable to time‐varying or time‐invariant wavelets as well as to nonstationary or stationary noise processes. We develop equations herein for minimum‐variance estimates of the reflection coefficient sequence, as well as error variances associated with these estimates. Our estimators are compared with an ad hoc “prediction error filter,” which has recently been reported on in the geophysics literature. We show that our estimators perform better than the prediction error filter. Simulation results are included, for both time‐invariant and time‐varying situations, which support our theoretical developments.
INTEGRAL FORMULATION FOR MIGRATION IN TWO AND THREE DIMENSIONS
Authors: Schneider William A.
Published: October 2002

Computer migration of seismic data emerged in the late 1960s as a natural outgrowth of manual migration techniques based on wavefront charts and diffraction curves. Summation (integration) along a diffraction hyperbola was recognized as a way to automate the familiar point‐to‐point coordinate transformation performed by interpreters in mapping reflections from the x, t (traveltime) domain into the x, z (depth domain). We will discuss the mathematical formulation of migration as a solution to the scalar wave equation in which surface seismic observations are the known boundary values. Solution of this boundary value problem follows standard techniques, and the migrated image is expressed as a surface integral over the known seismic observations when areal or 3-D overage exists. If only 2-D seismic coverage is available, wave equation migration is still possible by assuming the subsurface and hence surface recorded data do not vary perpendicular to the seismic profile. With this assumption, the surface integral reduces to a line integral over the seismic section, suitably modified to account for the implicit broadside integral. Neither the 2-D or 3-D integral migration algorithms require any approximation to the scalar wave equation. The only limitations imposed are those of space and time sampling, and accurate knowledge of the velocity field. Migration can also be viewed as a downward continuation operation which transforms surface recorded data to a deeper hypothetical recording surface. This transformation is convolutional in nature and the transfer functions in both two and three dimensions are developed and discussed in terms of their characteristic properties. Simple analytic and computer model data are migrated to illustrate the basic properties of migration and the fidelity of the integral method. Finally, applications of these algorithms to field data in both two and three dimensions are presented and discussed in terms of their impact on the seismic image.
ELECTROMAGNETIC METHODS FOR MAPPING SHALLOW PERMAFROST
Authors: Hoekstra Pieter
Published: October 2002

Two electromagnetic techniques, radiohm and magnetic induction, were evaluated for their ability to delineate and measure the thickness of shallow permafrost. Radiohm instruments in the VLF (15–30 kHz) and LF (200–400 kHz) band, and a magnetic induction instrument using horizontal coplanar coils, separated by a distance of 3.66 m and operating at a frequency of 39.2 kHz, were tested. Along test sites in the Mackenzie Valley, N.W.T. and in northern Alberta, boreholes were placed to verify the interpretation of the geophysical data. The results showed that both LF and the magnetic induction instrument accurately delineated permafrost. The effective depth of exploration of VLF radiation was, in general, too large for delineating boundaries of shallow permafrost. The resistivity layering over permafrost in the winter could be represented by a two‐layer model for computing the thickness of frozen ground.