bluuebunny/crossref_metadata_2025 huggingface dataset:
ON THE FIELD DUE TO A VERTICAL LINE SOURCE OF CURRENT GROUNDED TO EARTH
Authors: Bilinsky Solomon
Published: October 2002

An expression for the current density at any point in the earth due to a current in an infinitely long vertical wire is found for two types of current: (1) Simply periodic, (2) Rectangular impulse. These are given respectively by equations (11) and (13) below, where [Formula: see text] is the horizontal radial component and [Formula: see text] the vertical component of the current density vector at the point (r, z), which is at a distance R from the grounding point. These formulas hold for frequencies not too high or times not too small to allow neglect of the displacement current.
GEOPHYSICAL OPERATIONS IN THE NORTH SEA
Authors: Cook Ernest E.
Published: October 2002

During the last three years the discovery of the world’s second largest natural gas field at Groningen in the Netherlands has touched off in the North Sea one of the greatest competitive offshore geophysical operations in history. Before 1962, only minor amounts of geophysical work had been done there. Severe weather conditions were expected, but seismic operations have not been as much affected by weather as was originally anticipated. However, strong currents, making cable location uncertain, hampered reflection stacking and refraction operations. Location by radio was also a serious problem due to lack of sufficient available frequencies for a lane identification system. Seismic work was carried out safely in areas mapped as mine fields. Few problems with the fishing industry have so far been encountered. As a preliminary, the geophysics of the Groningen area are discussed. A gravity compilation of the North Sea shows that there are three major basins—the Northwest German Zechstein Basin, the British North Sea Basin, and the Norwegian North Sea Basin. The British Basin which contains Tertiary, Cretaceous, Jurassic, Triassic, Permian, Carboniferous, and older sediments shows considerable salt movement with salt domes, walls, and pillows being in evidence to within 30 miles of the eastern coast of England. Some examples of seismic record sections show the quality of data and the kind of structures encountered. Water reverberations were satisfactorily reduced by means of anti‐ringing procedures. Multiple reflections were frequently observed and often interfered with or obscured simple reflections. Refraction studies indicate that two main refractors, the Upper Cretaceous Chalk and the Upper Magnesian Limestone of the Permian, are present over most of the British Basin. Mapping the key basal Permian reflector is made difficult by deterioration of the reflection under areas of salt growth. Stacking sometimes enhances this reflection. Also intrusions of Permian salt into the Mesozoic beds give rise to large and rapid changes in thickness of the overlying low‐velocity Tertiary and high‐velocity Cretaceous chalk sections. A correction system for these large lateral velocity changes is described.
SPECTRAL STUDIES OF MARINE GEOPHYSICAL PROFILES
Authors: Neidell Norman S.
Published: October 2002

Gravity, magnetic, and echo‐sounding data profiles from widely separated single‐track surveys in the Atlantic and Indian Oceans were subjected to spectral studies. A two‐dimensional sea floor with a recurring structural pattern seemed a model able to explain the computed results. In testing the model, the most important quantity to be determined from the spectra was the “ripple frequency” or interpeak interval. During the course of this work, a number of practical problems connected with the computation of spectra were considered. In addition, the spectral methods were also shown to be tools capable of investigating more complex geophysical theories. The experience and techniques of this investigation can be of value in treating analogous problems which arise in geophysical exploration.
AFMAG—APPLICATIONS AND LIMITATIONS
Authors: Ward S. H. , O’Donnell J. , Rivera R. , Ware G. H. , Fraser D. C.
Published: October 2002

The repeatability of AFMAG measurements has ranged from good to bad since the introduction of the method to industry in 1958. In some applications, e.g., mapping massive sulfide “dikes,” the repeatability is generally good, while in other applications, e.g., mapping broad fault zones, the repeatability is notoriously poor. In an endeavor to elucidate the factors controlling repeatability of AFMAG dip angle readings, we have conducted a continuing program of research in the State of California since 1961.
WAVE ATTENUATION AND INTERNAL FRICTION AS FUNCTIONS OF FREQUENCY IN ROCKS
Authors: Attewell P. B. , Ramana Y. V.
Published: October 2002

Published and unpublished data on attenuation and internal friction in rocks are calculated and plotted against frequency in the range [Formula: see text] to [Formula: see text] cps. It is shown that for [Formula: see text] [Formula: see text] [Formula: see text] [Formula: see text] The assumptions and limitations in the data analysis are outlined in the text.
RECURSION FILTERS FOR DIGITAL PROCESSING
Authors: Shanks John L.
Published: October 2002

Digital filters are most often applied to discrete time series by convolving the time series with the unit impulse response, or weighting function of the filter. Each output point computed is a weighted sum of a finite number of the input points. It is also possible to recursively filter a time series. That is, we can compute each output point as a weighted sum of input points plus a weighted sum of previously computed output points. The advantage of this technique is that certain filtering operations can be performed much faster using recursion as compared to convolution. A recursive filter can be expressed as a ratio of two polynomials of the z‐transform variable. Given the rational z‐transform for a particular filter, the recursive rule or algorithm for that filter can be derived. There are various ways to determine the rational z‐transform for specific types of filtering. Using our knowledge of the z‐transform, we can design filters by specifying the roots of the two rational polynomials. Also, we can use the relationship between the Laplace transform and the z‐transform to convert desirable analog filter equations into their digital equivalents. We can use least‐square techniques to approximate a desired impulse response with a recursion formula. Often the resulting recursion formula will be considerably more efficient in use than the original convolution operation. Wiener filters also can be expressed as rational z‐transforms. In many cases the resulting recursion formulas will be quite simple, providing efficient and rapid digital processing.
MAPPING GILSONITE VEINS WITH THE ELECTRICAL RESISTIVITY METHOD
Authors: Hays Walter W. , Nuttli Otto W. , Scharon Leroy
Published: October 2002

Gish‐Rooney type electrical resistivity equipment was employed to map the location and structural attitude of gilsonite veins in the Uinta Basin, southeast of Vernal, Utah. Horizontal profiles at right angles to the strike of the dike‐like gilsonite veins were made using the Wenner electrode configuration. Very sharp anomalies with a peak apparent resistivity value of the order of 20,000–25,000 ohm‐cm were observed. The peak values were measured over the center of the vein, but smaller auxiliary highs were also observed, symmetrically spaced on each side of the vein. The position of the auxiliary high varied as a function of the electrode spacing and was found to occur when the current stake was located in the gilsonite. The observed field profiles were compared with various theoretical profiles computed from theoretical apparent resistivity expressions derived through the use of images. The best correlation was obtained when the theoretical model was assumed to have an infinite resistivity relative to that of the surrounding material. This correlation supports the conclusion that the gilsonite veins, because of their extremely large resistivity and their vertical attitude, act as a vertical insulating sheet to the flow of electrical current in the ground.
APPLICATIONS OF TWO‐DIMENSIONAL LINEAR WAVELENGTH FILTERING
Authors: Zurflueh Ernst G.
Published: October 2002

Wavelength filters permit the separation of anomalies of different horizontal extents on magnetic and gravity maps. They can be used equally well for structural and topographic maps or for any other type of data amenable to gridding. Practical filters were developed following two series of cutoff wavelengths. Of special interest is the small size of the filters as compared to the steepness and smoothness of their wavenumber characteristics. Filters for any desired wavelengths can easily be derived from the described coefficient sets. Examples are given for the application of filters which can be used as regional, residual, and band‐pass filters. The resultant maps are in the same units as the original maps. A given map can be divided into several wavelength components, and the sum of the components is equal to the original data. The method is shown to be versatile, and second‐derivative calculations can be approximated by a properly chosen residual filter.
THE NATURAL GAMMA‐RAY FLUX: IN‐SITU ANALYSIS
Authors: Doig Ronald
Published: October 2002

A fully portable transistorized gamma‐ray spectrometer has been constructed, and used to investigate the nature of the gamma ray activity at the surface of rock outcrops. Gamma‐ray photopeaks of [Formula: see text] and members of the U and Th series have been identified, along with strong fallout activity dominated by the 0.75 Mev activity of [Formula: see text]. A method has been devised for measuring, in situ, the K, U, and Th contents of rocks. Calibration accounts for the interference between the radioelements, and for background radiation. The following estimates of accuracy and sensitivity are for five‐minute counting intervals: 5 percent plus 0.1 percent K, 10 percent plus 0.2 ppm U, 10 percent plus 0.5 ppm Th. The main advantages of the method are its speed and versatility and the very large sample analyzed. A number of surveys have been performed to demonstrate some of the applications of the instrument. The major project of this series is detailed mapping of the K, U, and Th distribution in the Preissac granite of northwestern Quebec. In addition to its use as a petrologic tool, the technique is eminently suited to prospecting for U and Th, and the quantitative evaluations of occurrence of these elements.
VELOCITY OF COMPRESSIONAL WAVES IN POROUS MEDIA AT PERMAFROST TEMPERATURES
Authors: Timur A.
Published: October 2002

Measurements of velocity of compressional waves in consolidated porous media, conducted within a temperature range of 26 °C to −36 °C, indicate that: (1) compressional wave velocity in water‐saturated rocks increases with decreasing temperature whereas it is nearly independent of temperature in dry rocks; (2) the shapes of the velocity versus temperature curves are functions of lithology, pore structure, and the nature of the interstitial fluids. As a saturated rock sample is cooled below 0 °C, the liquid in pore spaces with smaller surface‐to‐volume ratios (larger pores) begins to freeze and the liquid salinity controls the freezing process. As the temperature is decreased further, a point is reached where the surface‐to‐volume ratio in the remaining pore spaces is large enough to affect the freezing process, which is completed at the cryohydric temperature of the salts‐water system. In the ice‐liquid‐rock matrix system, present during freezing, a three‐phase, time‐average equation may be used to estimate the compressional wave velocities. Below the cryohydric temperature, elastic wave propagation takes place in a solid‐solid system consisting of ice and rock matrix. In this frozen state, the compressional wave velocity remains constant, has its maximum value, and may be estimated through use of the two‐phase time average equation. Limited field data for compressional wave velocities in permafrost indicate that pore spaces in permafrost contain not only liquid and ice, but also gas. Therefore, before attempting to make velocity estimates through the time‐average equations, the natures and percentages of pore saturants should be investigated.
ITERATIVE THREE‐DIMENSIONAL SOLUTION OF GRAVITY ANOMALY DATA USING A DIGITAL COMPUTER
Authors: Cordell Lindrith , Henderson Roland G.
Published: October 2002

Given gridded gravity anomaly values and certain limiting restrictions on the mass distribution, a three‐dimensional structural model can be calculated automatically from gravity anomaly data by successive approximations. The causative body is optionally assumed to be either flat‐topped, flat‐bottomed, or symmetrical about a horizontal plane. Density contrast and the position of a horizontal plane delimiting either the top, base, or midsection of the causative body must be specified. A first approximation of structure is obtained by means of the Bouguer slab relationship. The gravity field of this first model is calculated and at each grid point the ratio of observed to calculated gravity is used to modify the first structural model, thus leading to a second approximation of structure. The process is iterated until a satisfactory agreement between observed and calculated gravity is obtained.
ON INDUCED ELECTRICAL POLARIZATION AND GROUNDWATER
Authors: Bodmer René , Ward S. H. , Morrison H. F.
Published: October 2002

Clay horizons and other clay‐bearing unconsolidated sediments are potential sources of induced‐polarization anomalies. If such anomalies may be detected above system noise, the induced‐polarization method may be of value for in‐situ classification of unconsolidated sediments encountered in hydrological projects. One such project exists in Santa Clara County where near‐surface unconsolidated sediments are frequently considered as potential recharge areas. Of four areas surveyed with induced‐polarization apparatus in Santa Clara County, only two yielded significant frequency‐effect anomalies, and in each of these two the frequency effects were of the order of 3 percent. These anomalous frequency effects may be related to clayey gravels. The dipole‐dipole array, with spreads of 10 ft and 20 ft, was typically used in the study.
BICUBIC SPLINE INTERPOLATION AS A METHOD FOR TREATMENT OF POTENTIAL FIELD DATA
Authors: Bhattacharyya B. K.
Published: October 2002

A method for the generation of bicubic spline functions is presented in this paper. From this method it becomes apparent that these functions derive their potential strength in accurate and reliable representation of two‐dimensional data by maintaining continuity of the variable and its slope and curvature throughout the area of observation. The results obtained by computing horizontal and vertical derivatives with model and field data illustrate the exceptional accuracy achieved with spline functions. The piecewise cubic polynomial functions expressing observed data analytically in space are used to estimate amplitude and phase spectra of magnetic anomalies. At relatively long wavelengths the amplitude spectrum thus calculated displays remarkable similarity with the true spectrum and is found to be superior to that obtained with two‐dimensional Fourier series expansion. A cubic spline method is also presented for computing values of an observed variable at equispaced points along two orthogonal directions with the help of irregularly distributed data. The interpolation technique applied to field data shows high resolution by maintaining the separation of neighboring anomalies and the small‐scale features. The shapes, peaks, and troughs of both large and small amplitude anomalies are faithfully reproduced. The gradients of the magnetic field do not undergo any appreciable distortion. It can thus be concluded that cubic splines are a reliable and accurate method of interpolation.
GEOPHYSICAL ACTIVITY IN 1968
Authors: Allen S. J.
Published: October 2002

Total expenditures reported for geophysical surveys for all purposes on a world‐wide basis during 1968 were $800.6 million, up 1 percent from the $790.9 million reported in 1967. This comprises $734.0 million for petroleum exploration, down 1 percent from the $744.9 million reported in 1967; $41.4 million for mining exploration, up 28 percent from the $32.3 million reported in 1967; $20.3 million for groundwater, engineering, and construction, up 62 percent from the $12.6 million reported in 1967; and $4.9 million for oceanographic surveys, up 325 percent from the $1.1 million reported in 1967. The expenditures for petroleum exploration were 91.7 percent of the total for all purposes, compared with 94.1 percent reported in 1967.
GRAVITY STUDIES OVER THREE EVAPORITE PIERCEMENT DOMES IN THE CANADIAN ARCTIC
Authors: Spector Allan , Hornal Robert W.
Published: October 2002

Reconnaissance gravity surveys over three evaporite piercement domes in the Canadian Arctic Islands have been interpreted. Each dome was considered as a right‐vertical cylinder divided into two homogeneous regions, a high density anhydrite zone [Formula: see text] overlying a low density gypsum and/or rock salt zone [Formula: see text]. The cylinder is surrounded by a sedimentary sequence which has a uniform density of [Formula: see text]. Interpretation involved a least‐sum‐of‐squares approach to estimate the thickness of the anhydrite and gypsum‐rock salt zones. The three sets of estimates gave a range of 200 to 550 m for the anhydrite thickness and a range of 700 to 5500 m for the vertical extent of domes. In each case the depths were less than expected on the basis of estimates from seismic and geological data. Possible explanations for this are: (a) the cross‐sectional area of each dome decreases with depth; (b) the existence of a transition zone where a gradation occurs between the high and low density zones; and (c) the effective density contrast of the low density zone is less than [Formula: see text].
STRUCTURE OF THE BEAUFORT SEA CONTINENTAL MARGIN
Authors: Wold R. J. , Woodzick T. L. , Ostenso N. A.
Published: October 2002

An airlifted gravity survey was conducted in 1968 in the Beaufort Sea between Barter Island and Banks Island, south into the Mackenzie River Delta area and northward to about 74° latitude. The 1968 gravity data were combined with data from previous airlifted surveys and ice island T‐3. The major feature of the free‐air anomaly gravity map of this area is a more or less continuous 100 mgal high paralleling the coast from Barrow, Alaska, to the edge of the survey area north of Banks Island. The gravity high is explained by a thinning of the crust and a ridge in the basement rocks at about the 200 m isobath. This linear anomaly is broken by saddles off the Colville, Mackenzie, and Bernard Rivers, which are interpreted to reflect sedimentary fans built by the discharge of these rivers. Two‐dimensional crustal models constructed from gravity profiles indicate a narrow transition zone from ocean to continental crustal thickness, 55 km to 100 km shoreward of the 2000 m isobath. In a review of continental margin structure, Worzel (1968) found the transition zone to be centered under the 2000 m isobath. The departure from “normal” in the Beaufort Sea area may be explained by a greater accumulation of sediments seaward of the “structural” continental margins. This accumulation implies a faster rate of sedimentation and/or a greater age for the Beaufort Sea continental margins than for those analyzed by Worzel.
PREDICTION OF IN‐SITU ACOUSTIC AND ELASTIC PROPERTIES OF MARINE SEDIMENTS
Authors: Hamilton Edwin L.
Published: October 2002

This is one of several interrelated reports concerning sound velocity, elasticity, and related properties of marine sediments from several major environments in the North Pacific and adjacent areas. Previous reports were concerned with methods, measured properties (e.g., compressional‐wave velocity, density, porosity), environmental control, empirical relationships, elastic models for saturated, porous marine sediments, and the computation of elastic moduli. These measured and computed properties were referred to 1 atmosphere pressure and 23°C; all samples were from the sediment surface (0 to 30 cm). This report is concerned with the correction of laboratory values to in‐situ values, and the prediction of in‐situ values in the absence of any sediment data.
COLOR SONAGRAMS: A NEW DIMENSION IN SEISMIC DATA INTERPRETATION
Authors: Balch A. H.
Published: October 2002

We have developed a computer‐graphic‐photographic system which uses color mimicry to display the frequency spectra of seismic events simultaneously with their time‐varying waveforms. Mimicking the visible light spectrum, we have used red for the low frequencies and violet for the highs. The output of our system is a variable‐area‐wiggle‐trace seismic cross‐section. The waveforms are the same as those on a conventional section; however, the variable‐area part of the section appears in color. The color represents the frequency spectrum of the wavelets. Lateral changes in rock attenuation show up as color shifts on this type of display. Faults often stand out as interrupted color bands. Fault diffractions sometimes have a characteristic color signature. The cancellation of high frequencies due to misalignment of events on constant‐velocity stacks can show up in color. Loss of high frequencies due to slight lateral changes in moveout velocity, and consequent trace misalignment, is often indicated by a shift toward red on a color seismic section.
A NONLINEAR LEAST‐SQUARES METHOD FOR SEISMIC REFRACTION MAPPING—PART I: ALGORITHM AND PROCEDURE
Authors: Ocola Leonidas C.
Published: October 2002

An iterative inversion method (Reframap) based on the kinematic properties of critically refracted waves is developed. The method is based on ray tracing and assumes homogeneous and isotropic media and ray paths confined to a vertical plane through each source‐detector pair. Unlike the earlier Profile or Time‐Term Methods, no restrictions are imposed on interface topography except that it be continuous almost everywhere (in the mathematical sense). As in the preexisting methods, more observations than unknowns are assumed. The algorithm and procedure, on which the Reframap Method is based, generate apparent dips for each source detector pair at the noncritical interfaces from the slope of a least‐squares line approximation to the interface functional in the neighborhood of each refraction point. In turn, the dip and path along the critical refractor is, at every iteration, pairwise approximated by a line through the critical refracting points. The incidence angles are computed recursively by Snell’s law. The solution of the overdetermined, nonlinear multiple refractor time‐distance system of simultaneous equations is sought by Marquardt’s algorithm for least‐squares estimation of critical refractor velocity and vertical thickness under each element.
IMPACT OF OSHA ON GEOPHYSICAL OPERATIONS
Authors: Searcy Frank
Published: October 2002

The Williams‐Steiger Occupational Safety and Health Act of 1970 has placed new responsibilities on everyone involved in geophysical operations in the United States. This law applies in all 50 states, the District of Columbia, and territories under the jurisdiction of the United States. The declared congressional purpose of the act is “to assure so far as possible every working man and woman in the nation safe and healthful working conditions and to preserve our human resources.”
GEOSEISMIC HISTORY AND DEVELOPMENT OF RINCON FIELD, SOUTH TEXAS
Authors: Ashford Ted
Published: October 2002

The Rincon field of South Texas is one of many important producing fields associated with the Frio‐Vicksburg flexure which is parallel and downthrown to the Sam Fordyce‐Vanderbilt fault zone. Exploration started at Rincon in 1935 and resulted in the discovery of Frio oil in 1938 and Vicksburg oil in 1950. Development of the shallow Frio and Vicksburg reserves is continuing today. The use of modern tools in the late 1960’s indicates there is potential for future development and extension of reserves at Rincon. Improved seismic techniques including Vibroseis and digital processing provided deep structural information that allowed the interpretation of the gross Rincon structure for the first time. This information set the stage for another round of drilling to seek the ultimate reserves in this old established field.
On: “Direct Interpretation of Two‐Dimensional Structural Faults From Gravity Data,” by Mrinal K. Paul, Sukumar Datta and Buddhadeb Banerjee (GEOPHYSICS, October 1966, p. 940–948)
Authors: Choudhury S. K. , Amaravadi R.
Published: October 2002

In their paper, Paul et al. have outlined a useful method by which the parameters of a two‐dimensional fault can be directly determined from the gravity data. The upward continuation of the gravity profile over the fault plays a very important role in this method. The necessary coefficients for the upward continuation and the theory involved in their determination form the appendix of the paper. The equation (9) in the appendix is incorrect since for any positive integral values of m and n, the coefficients Q(m, n) become quite large and negative, which will never satisfy the relation (10). The correct equation (9) should be [Formula: see text]
CONCENTRATIONS OF HYDROCARBONS IN THE EARTH
Authors: McDermott Eugene
Published: October 2002

An hypothesis of the origin and accumulation of oil and gas fields, arrived at from the data of soil analysis, is presented. This hypothesis also offers an explanation of the quite different concentrations of subsurface waters in different geologic provinces. Furthermore, it seeks to explain the variation of type of accumulation with respect to depth below the surface and deformation of the sediments in which the accumulation occurs. Lastly, the value of the method of soil analysis as an exploration device is indicated.
SHALLOW RESISTIVITY SURVEY AT SOUTH ELTON, LOUISIANA
Authors: Blondeau E. E.
Published: October 2002

A direct current resistivity survey covering about one and one‐half townships adjacent to the South Elton oil field is described. The contour map based on data taken 100′ from a current electrode shows many near surface anomalies while that based on data taken 1000′ from a current electrode is comparatively uniform. Conclusions reached were: First, that Eltran and direct current data show good qualitative agreement; second, that the large observed variations in near surface resistivity can hardly be attributed to mineralization emanating upward over structure; and third, that the number of anomalies found is so great that their direct association with deep structure is highly improbable.
DEEP CRUSTAL REFLECTIONS IN EUROPE
Authors: Dohr Gerhard P. , Meissner Rolf
Published: October 2002

Deep crustal reflected arrivals in the near‐vertical and the wide‐angle range have been recorded in various parts of Europe. A high resolution of deep crustal structures can be obtained by special seismic investigations. Reconnaissance surveys may be performed during routine prospecting work by using recording lengths up to 15 sec. Several profiles with multiple coverage and digital processing of the data demonstrate the possibility of obtaining seismic cross‐sections down to the Moho. In addition to near‐vertical reflection work, studies of wide‐angle arrivals at distances up to 150 km and more are most favorable for a calculation of seismic velocities in the deeper crust. Ray‐tracing programs have revealed low‐velocity zones at depths below 10 km in many continental crusts. From comparison of reflected amplitudes in the near‐angle and the wide‐angle ranges and from other observations, a lamellar structure in the deeper crust is often detected. Regional differences of various traveltime branches show characteristic crustal structures. It is shown that deep seismic sounding is useful for estimating the shape and development of sedimentary troughs and their connection to oil‐bearing structures, heat energy, and possible ore deposits.
On: “Gravity Interpretation Using the Fourier Integral”, by M. E. Odegard and J. W. Berg, Jr. (GEOPHYSICS, June 1965, p. 127–138)
Authors: Saha Jay Gopal
Published: October 2002

In their paper, Odegard and Berg claim that from the gravity anomaly due to a two‐dimensional vertical fault the density, the throw, and the depth can be determined uniquely by a Fourier transform method. It is true that the solution of the reverse problem for a two‐dimensional vertical step is theoretically unique. The derivation of the Fourier transform by the authors, however, is erroneous.
A RECURSIVE TIME‐VARYING BAND‐PASS FILTER
Authors: Nikolic Z. J.
Published: October 2002

Time‐varying, digital band‐pass filters are extensively used in seismic data processing, since the dominant frequency of reflected signals usually becomes lower and their bandwidth narrower with the passage of time. In routine seismic data processing, time‐varying, digital band‐pass filtering is stepwise.
GROUND AND AIRBORNE RESISTIVITY SURVEYS OF PERMAFROST NEAR FAIRBANKS, ALASKA
Authors: Hoekstra P. , Sellmann P. V. , Delaney A.
Published: October 2002

In permafrost regions investigations for such geotechnical endeavors as route selection for roads and pipelines and site investigations for buildings and dam construction often require that a careful assessment be made of the presence or absence of frozen ground, of the ice content of frozen ground, and of the depth of frozen ground. In the vicinity of Fairbanks, Alaska, where the permafrost is discontinuous, ground and airborne methods of mapping electrical resistivity using radiowaves were tested as means of delineating permafrost. When the resistivity maps are compared with surficial geological data, the following conclusions are reached: (1) In areas of fine‐grained sediments, where the near surface sediments are relatively uniform, VLF resistivity delineates permafrost. (2) In areas where surface sediments vary widely (flood plains), VLF resistivity shows little information on permafrost conditions but can provide other important geotechnical information, such as, depth to bedrock, surface soil type, and layering. Comparison of the apparent resistivity derived from a surface impedance measurement at VLF on the ground with the apparent resistivity derived from an airborne measurement of wavetilt shows that the regional trends in the data agree, but the surface impedance measurements show much more local detail in ground conditions. When the surface layers are also frozen, the surface impedance method of measuring ground resistivity was found to have distinct advantages over conventional galvanic methods in terms of production and problems associated with probe contact resistance.
AUDIO‐FREQUENCY MAGNETOTELLURICS WITH A GROUNDED ELECTRIC DIPOLE SOURCE
Authors: Goldstein M. A. , Strangway D. W.
Published: October 2002

The conventional audio‐frequency magnetotelluric method has been useful in problems of shallow exploration to depths of a few kilometers. Because the natural sources are unpredictable in strength and direction, we have examined the possibility of doing magnetotelluric sounding utilizing the ratio of horizontal electric field to horizontal magnetic field from a controlled source. The source chosen for this study was a grounded electric dipole. The equations for the electric and magnetic fields around this antenna as a function of range and azimuth have been calculated for a half‐space and for a one‐layered earth. These calculations were checked for the half‐space case with an analog model and in the field at the Bonneville Salt Flats. In addition, layered cases were calculated, and a field example near Timmins, Ont., studied. Provided the distance of the observation point from the source is three skin depths (relative to the greatest resistivity in the section) or more, the conventional magnetotelluric interpretations can be applied. If the observations are closer than this, it is possible to use sets of type curves for the analysis. Such curves have been calculated. Lateral variations were studied in a model tank and by studying a massive sulfide deposit near Gooderham in Ontario.
CALCULATION OF THE MAGNETOTELLURIC TENSOR IMPEDANCE: ANALYSIS OF BAND‐LIMITED MT SIGNAL PAIRS
Authors: Grillot Larry R.
Published: October 2002

A method of analyzing the tensor nature of magnetotelluric fields from short independent sections of filtered MT data is presented in which: (1) horizontal field components are filtered at chosen periods; (2) short sections which show a well defined signal over 3–4 cycles are selected from the records; (3) the polarizations of these short record sections are plotted; and (4) Fourier coefficients calculated for section pairs showing differing magnetic field polarizations are then used to calculate the elements of the impedance tensor. Results of this analysis performed on MT field data show that the use of MT signal pairs yields uniform principal resistivities for calculations using several different section pairs of the same period. These results also illustrate internal consistency of the resistivity estimates over the complete period range of the data (30–7500 sec). A brief comparison of the MT pairs calculations and results derived from a least‐squares approach at a selected period shows good agreement between the two methods. The overall results suggest that the use of independent signal pairs in the determination of the tensor impedance could complement more traditional methods in some cases.
GEOPHYSICAL ACTIVITY IN 1974
Authors: Espey H. R.
Published: October 2002

The principal function of the SEG Geophysical Activity Committee is to gather and compile statistics on worldwide geophysical activity. Statistics are primarily obtained through the survey questionnaire which is mailed out at the beginning of the year to over 1500 companies, government agencies, and universities engaged in geophysical work. This year the returned questionnaires included information on more than 1400 geophysical surveys. All activity is not reported through the questionnaire survey; other methods are used to estimate missing data. Each member of the committee is responsible for a geographical area or a specific application. Committee members are thoroughly familiar with the area on which they report. Each member provides appropriate corrections to make the quantitative picture one that his knowledge and experience tell him it should be. The statistics reported on the questionnaire are thus augmented with company scouting reports, reports of scouting services, and by comparison with other published surveys.
DIGITAL CONVOLUTION FOR COMPUTING GRAVITY AND MAGNETIC ANOMALIES DUE TO ARBITRARY BODIES
Authors: Bhattacharyya B. K. , Navolio M. E.
Published: October 2002

The magnetic and gravitational potentials and fields due to arbitrarily shaped bodies with homogeneous magnetization and uniform density distribution are expressed as a convolution of the source geometry and the Green’s function. The Green’s function depends on the location of the observation point and on either the magnetization vector (in the case of the magnetic field) or the density (in the case of the gravitational attraction). A fast digital convolution algorithm is used for efficiently and accurately calculating anomalies caused by irregular bodies. The shapes of the calculated anomalies faithfully reproduce the exact shapes when the sampling interval selected for digitizing the source geometry and the Green’s function is less than one‐tenth of the depth of the source. In the digital convolution method for computing anomalies, it is unnecessary, for any given structure, to perform analytical integration of the dipolar magnetic field or the gravitational field of a point mass. One of the examples given in the paper deals with the computation of the magnetic anomaly due to the irregularly shaped Round Butte Laccolith, Montana. The results are found to be in satisfactory agreement with the observed aeromagnetic data. A new method is also described for calculating the magnetization vector associated with the laccolith and the datum level of the magnetic observations.
QUADRIPOLE‐QUADRIPOLE ARRAYS FOR DIRECT CURRENT RESISTIVITY MEASUREMENTS—MODEL STUDIES
Authors: Doicin Dariu
Published: October 2002

For a quadripole‐quadripole array, in which current is sequentially injected into the ground by two perpendicular dipoles, an apparent resistivity can be defined in terms of the vectorial cross product of the two electric fields measured at the receiver site. Transform equations are derived (for horizontally layered media) which relate this apparent resistivity to the apparent resistivities obtained with conventional dipole‐dipole and Schlumberger arrangements. To evaluate the method, two mathematical models are used. The first model is a half‐space with an “alpha conductivity center,” and the second model is a half‐space with a vertical contact. For an idealized quadripole‐quadripole array, simple expressions are found for the apparent resistivity, which is shown to be independent of the orientation of the current quadripole. Theoretical anomalies calculated for the quadripole‐quadripole array are compared with those obtained for a dipole‐quadripole array. It is shown that whereas the apparent resistivity map for the dipole‐quadripole array varies greatly with the azimuth of the source dipole, the results obtained with the quadripole‐quadripole array consistently display a remarkable resemblance to the assumed distribution of true resistivity. This is especially true when the current quadripole is placed at a large distance from the surveyed area.
GRAVITY AND MAGNETIC FIELDS OF POLYGONAL PRISMS AND APPLICATION TO MAGNETIC TERRAIN CORRECTIONS
Authors: Plouff Donald
Published: October 2002

Computer programs based on the exact calculations of the gravity and magnetic anomalies of polygonal prisms are faster in operation and more accurate than previous programs based on the numerical integration of polygonal laminas. The prism programs also are of more general application than existing computer programs that are based on the exact gravity and magnetic effects of rectangular prisms. There are no restrictions on the use of the exact formula for the gravitational attraction of a polygonal prism, but the formulas for the magnetic effect are restricted in that demagnetization is not considered, and a finite answer is not obtained in the unrealistic circumstance where an observation point coincides with an edge of the prism. Least‐squares methods permit calculation of the gravity or magnetic effect of models without knowledge of the density or magnetization contrasts, respectively, by comparison of the observed anomalies with theoretical dimensionless values to determine contrasts as regression coefficients. The coefficient of correlation provides a goodness of fit estimate that helps model evaluation. After calculating a magnetic terrain correction for an outcrop of Quaternary dacite and andestite near Clear Lake, Calif., an improvement of the coefficient of correlation from 88 to the 92 percent level indicates that this volcanic unit probably extends at least 150 m beneath the surface. Application of a magnetic terrain correction to disconnected outcrops of Tertiary andesite, eliminates most of a prominent v-shaped magnetic anomaly south of the San Juan Mountains, Colo.
THE DERIVATION OF ABSOLUTE UNITS IN ELECTROMAGNETIC SCALE MODELING
Authors: Spies Brian R.
Published: October 2002

Scaled model studies often are used to determine the structure of secondary magnetic fields when the numerical approach is unreasonably difficult. For most purposes, it is sufficient to model the geometric configurations or relative components of the magnetic field and, neglecting displacement currents, the general modeling relation [Formula: see text] is valid. However, in some applications such as transient electromagnetic (TEM) modeling, it is necessary to determine the power level of the full‐scale system. An expression relating the voltage levels of the model and the full‐scale situation is derived as [Formula: see text]where the subscript m represents the model system. This expression must be used in order to compensate for changes in the modeling parameters.
MODEL FOR ESTIMATING ELECTRIC MACROANISOTROPY COEFFICIENT OF AQUIFERS WITH HORIZONTAL AND VERTICAL FRACTURES
Authors: Campbell David L.
Published: October 2002

Electrically resistant beds often contain water‐bearing fractures having a much lower resistivity than the beds themselves. This paper presents a model for estimating the macroanisotropy coefficient of such fractured beds.
GEOPHYSICAL METHODS FOR THE INVESTIGATION OF LANDSLIDES
Authors: Bogoslovsky V. A. , Ogilvy A. A.
Published: October 2002

Landslides occur extensively in all countries of the world. A landslide is a complex geologic body composed of a combination of layers having contrasting and gradational physical properties. In assessing the danger of landslides, it is of prime importance to investigate the structure of the landslide slope and its water saturation as well as the properties and status of the soils comprising the slope. The investigation and full evaluation of all these problems by traditional methods of engineering geology are sometimes impossible. Electrical and seismic methods are used to obtain the information needed to determine slope stability. Experience has been gained from longterm investigations carried out in various regions of the Soviet Union. Applications include evaluating geologic and hydrologic conditions related to the occurrence of landslides. Primary attention is devoted to the study of landslide slopes proper. The geologic structure of a landslide is considered in modeling it and determining the thickness of both the landslide body and the slip zone. The methods of self‐potential, resistivity, and temperature measurement are analyzed for characterization of the seepage flow through the landslide body. Self‐potential, resistivity, and temperature anomalies are associated with sites of increased landslide activity. Useful engineering properties of soils may be obtained from field and laboratory geophysical measurements. Measurement of changes of geophysical parameters with time are significant in assessing changes in the states of landslide soils. Observation of the direction and velocity of landslide movements is possible with magnetic and electrical methods. Examples of geophysical investigations of landslides in the Crimea, on the Black Sea coast of the Caucasus, and in the Volga River Valley are presented.
On: “Time Dependent Electromagnetic Fields of an Infinite, Conducting Cylinder Excited by a Long Current‐Carrying Cable” by S. K. Verma (GEOPHYSICS, April 1973, p. 369–379)
Authors: Singh Shri Krishna
Published: October 2002

I have already discussed the error contained in this article by Verma (Singh, 1974). It was pointed out in that discussion that the electromagnetic field of an infinite conducting cylinder excited by an infinite current‐carrying line source has a significant contribution from the axially symmetric induced current in the cylinder giving rise to a secondary transverse magnetic field outside the n = 0 term in the scattered wave field (Singh, 1972a; 1972b; 1973). It was further pointed out that for a finite cylinder also, the n = 0 term is important as shown by Hill and Wait (1972) and Wait and Hill (1973). In his article, Verma had neglected the n = 0 term (like many earlier authors). In his reply (Verma, 1974) to my discussion, Verma accepts this error but, quoting a physical argument by Wait (1973), concludes that for a finite cylinder, except under certain extreme conditions, the contribution from n = 0 would be negligible (in a real problem, the cylinder as well as the line source must of course be finite). That this physical argument is not valid is borne out by Wait’s own later works (Hill and Wait, 1972; Wait and Hill, 1973) which were cited in my discussion but, apparently, were not read by Verma before replying. (Also, see Wait, 1974.)
GRAVITY EXPLORATION FOR GROUNDWATER AND BEDROCK TOPOGRAPHY IN GLACIATED AREAS
Authors: Carmichael Robert S. , Henry George
Published: October 2002

Gravity geophysics has useful application in mapping buried bedrock topography and in groundwater exploration in some specific geologic situations. One of these is the occurrence of groundwater in the sands and gravels of buried stream valleys on the bedrock surface, where there is an overburden such as glacial drift. We have evaluated the gravity method as an aid in glaciated terrain, to (a) locate local buried valleys for groundwater extraction, and (b) do reconnaissance surveying of the bedrock surface for a groundwater resource inventory. This has been done by modeling analysis and field research studies. For a gravity meter with sensitivity of .01 mgal, and with the standard corrections in field surveys, buried‐valley anomalies greater than about 0.14 mgal can be resolved. This requires that elevations be accurate to ±2 inches. For a drift/bedrock density contrast of [Formula: see text], valley depths greater than 45 ft can be identified. Data‐analysis techniques can be used to enhance the precision and applicability of the method, namely, (a) the use of theoretical diurnal earth‐tidal curves for gravity correction, in lieu of the less accurate and more time‐consuming empirical curves: (b) various least‐squares surface fittings to improve interpretation and compensate for errors in surface elevations; and (c) conversion of gravity anomaly data to bedrock elevations. Field studies here illustrate the use of gravity in locating a localized buried stream valley and in mapping bedrock under glacial drift in a reconnaissance survey of over 50 sq mi. In the latter, although elevation control was not available to precision, a useful residual anomaly map resulted from subtracting a 3rd degree (regional) surface from a 15th degree (smoothed‐elevation‐error) surface. The resulting gravity residual was accurate to ±0.15 mgal; the calculated relief on the bedrock surface is up to 250 ft, below about 200 to 300 ft of glacial drift.
THE FRONTIERS OF MINING GEOPHYSICS
Authors: Ward S. H. , Campbell R. E. , Corbett J. D. , Hohmann G. W. , Moss C. K. , Wright P. M.
Published: October 2002

The RANN Division (Research Applied to National Needs) of the National Science Foundation has embarked upon a program of encouragement and sponsorship of research on exploration for and exploitation of nonenergy nonrenewable resources. To define critical problems requiring research, RANN is sponsoring a series of workshops wherein prominent members of industry, academia, and government assemble to debate the issues and produce lists of subjects requiring research. The first workshop in this series, entitled “Workshop on Research Frontiers in Exploration for Nonrenewable Resources”, was held at Pennsylvania State University, University Park, PA, October 11 to 13, 1976. The second workshop, “Geophysics Applied to Detection and Delineation of Nonenergy Nonrenewable Resources”, reported here, was held in Salt Lake City, Utah, December 6 to 8, 1976.
OPTIMUM MIXED DELAY SPIKING FILTERS
Authors: Ford Wayne T.
Published: October 2002

Recent increased knowledge of mixed delay seismic source input wavelets requires extension of industry understanding of high‐resolution digital spiking filters. A close analysis of widely existing minimum delay deconvolution processes can be used to gain insights and estimates in mixed delay cases. This paper presents a time‐domain method wherein a double application of standard (zero delay spiking) deconvolution processes yields understanding of the mixed delay nature of a wavelet given for spiking. It is shown that such deconvolution of both a given mixed delay wavelet and the reversal of this given wavelet can be combined to estimate the complete variable delay suite of spiking filter performances for the given mixed delay wavelet. The intuitive basis of the method lies in the observation that the reversal of a wavelet is equivalent to inversion of the zeros of the wavelet Z‐transform in the complex plane. Thus, (zero delay spiking) deconvolution of the wavelet emphasizes the minimum‐phase portion of the wavelet while such deconvolution of the reversed wavelet emphasizes the maximum‐phase portion of the original wavelet.