bluuebunny/crossref_metadata_2025 huggingface dataset:
RELATIVE RESOLVING POWER OF FOUR‐POINT RESISTIVITY CONFIGURATIONS
Authors: Unz M.
Published: October 2002

The ease of interpretation of four‐point resistivity measurements depends on the ratio of power electrode spread to probe spacing. Reduction of the probe spacing improves the resolubility of the resistivity graphs. It also reduces the error of the linear approximations. Field conditions, such as measuring sensitivity and maximum impressed ground voltage, limit the spacing ratio in practice. A two‐layer system has been investigated. The trend of the resistivity profiles at various spacing ratios and their evaluation capabilities are discussed.
COMPUTER ANALYSIS OF GAMMA‐RAY LOGS
Authors: Scott J. H.
Published: October 2002

A computer program recently developed determines [Formula: see text] grades for hypothetical half‐foot thick layers that are assumed to comprise a uranium ore zone represented by a gamma‐ray log anomaly. First approximations of the grades of the half‐foot layers are made proportional to the gamma‐ray log deflections corrected for instrument dead time. The grades of the hypothetical half‐foot layers are then adjusted by an iterative process until a synthetic anomaly calculated from them converges with the real gamma‐ray log anomaly. When a good fit is obtained, the [Formula: see text] grades assigned to the hypothetical layers at that time are taken as the correct values. Advantages of the computer method over conventional “hand” methods include greater speed, lower cost, and more useful results.
GEOPHYSICAL RESEARCH AND PROGRESS IN EXPLORATION
Authors: Van Melle F. A. , Faass D. L. , Kaufman S. , Postma G. W. , Seriff A. J.
Published: October 2002

This is the fifth of a series of review papers on the subject of research and progress in geophysical exploration. In the three years since the last presentation of this nature, the most salient trend in geophysical exploration has been one toward full utilization of the available data. Modern data‐processing equipment and techniques of analysis are being introduced while theoretical studies, many growing out of communication theory, attack seismic stumbling blocks such as multiple reflections, ghosts, diffractions, and other unwanted events. The vistas of innovation opened by digital recording and processing has spurred several operators to intensive study of the possibilities and the cost of digital methods. The use of seismic energy sources other than conventional dynamite explosions is increasing, as are special shooting techniques aimed at ghost elimination. Synthetic seismograms from continuous velocity logs are being brought to bear increasingly on stratigraphic problems; inverse convolution techniques seek to reconstruct the CVL from better quality seismograms, and research on synthetic seismograms continues at a high level. Applications of the rubidium vapor magnetometer have been reported both in exploration for minerals and in exploration for oil. The accuracy of the airborne gravimeter has been improved but is still insufficient for exploration purposes. The shipborne companion instrument is in use as an exploration tool. Existing well logging methods have undergone further development, and modern methods of electronic data processing are being applied increasingly. A new logging technique, nuclear magnetism logging, promises to give an indication of porosity above a certain pore size. A new interest has developed in natural electromagnetic fields and in the degree of coherence between electric and magnetic field components. Several of the projects sponsored by Vela Uniform, the U. S. Government Project created to achieve adequate detection of underground nuclear blasts, by the U. S. Atomic Energy Commission, and by other agencies have exploration potential (e.g., long seismic and electrical profiles, theoretical studies, certain instrument developments). Academic research in all branches of geophysics is quite active, with major benefits deriving from modern data handling techniques and from the new defense interest in the earth sciences.
THE PROBLEM OF VERTICAL FAULTS IN TELLURIC CURRENT PROSPECTING
Authors: Y‐Shu Li
Published: October 2002

At present in Africa, America, Canada, and Russia the telluric current method is widely used in oil field exploration. In this paper I shall discuss the problem of a two‐dimensional vertical fault having a definite throw (H‐h) (Figure 1(b)). The solution of this problem is obtained by means of the conformal transformation method.
MODERN GEOPHYSICAL METHODS FOR SUBSURFACE WATER EXPLORATION
Authors: Breusse J. J.
Published: October 2002

The use of exploration geophysics in the discovery of oil is well known, but applications of these methods in exploration for subsurface water are less publicized and less well known, despite the fact that geophysics has been employed in this field for more than thirty years and has been developing continually. Consequently, this paper is oriented towards exploration geo‐geophysical methods for ground‐water supplies. After a summary of the history of the methods, the paper examines the geophysical methods employed with their respective advantages and drawbacks: electrical prospecting using “electrical soundings” and resistivity mapping, shallow refraction, spontaneous polarization, and induced polarization. The principal problems that may be resolved by geophysics are reviewed, as well as practical considerations concerning the composition of a crew, its production, and costs. Some examples of hydrology surveys are presented to illustrate different types of problems that may be encountered. In conclusion, a large expansion in geophysical exploration and particularly electrical methods is expected as the need for sub‐surface water supplies increases in various regions of the world and the existing deficient supplies become known in industrialized nations.
AEROMAGNETIC INTERPRETATION ZELTEN FIELD AREA SIRTE BASIN, LIBYA
Authors: Steenland Nelson C. , Hinrichs F. W. , Navazio Fred
Published: October 2002

Aero Service Corporation, Fairchild Aerial Surveys, and Gravity Meter Exploration Company cooperated in the production of an interpreted aeromagnetic survey of the Sirte Embayment of Libya in December, 1960. During the course of the program, six east‐west traverses spaced at 3.0 km were placed across the area of the Zelten field, Libya’s first oil field. These were tied by north‐south lines every 40 km. The sampled area was 15×80 km in size, or 1,200 sq km. The survey was made with a Gulf‐type flux‐gate airborne magnetometer at an elevation of 600 m.
INTRABASEMENT REFLECTIONS IN SOUTHWESTERN ALBERTA
Authors: Robertson Glenn
Published: October 2002

Deep reflections at a time from about 3 to 5 sec were observed in the course of routine reflection seismic work in southwestern Alberta. These reflections are not multiples but come from within the Precambrian basement at a depth from about 7.5 to 14 km. The nature of the reflecting surface cannot be determined from available data but it is shallower than the depth for the Mohorović and Conrad discontinuities reported from a nearby long refraction profile. Additional similar observations by the geophysical industry could be made at little extra cost and could add to our understanding of the structure of the earth’s crust.
THE FARGO FIELD—A CASE HISTORY
Authors: Faust L. Y.
Published: October 2002

The seismic discovery of the Fargo Field, Wilbarger County, Texas, followed routine correlation procedures as practiced in 1936. Management contributed the policy of permitting record quality to determine the area explored. Gravity confirmed the seismic interpretation which was confirmed also in most details by subsequent wells. Seismic coverage averaged three townships per month at a cost of $28 per profile and $87 per square mile for the three‐mile square grids employed. The question is examined whether this comparatively low‐cost technique might be modernized into an effective reconnaissance tool. A line of the key data was transferred to magnetic tape (at some loss) and a few modern techniques employed. Compositing of these data was unpromising. Cross‐correlation of the composited data failed to yield sufficient improvement. A quasi‐continuous variable‐density presentation of 33 percent bed coverage shows the structure and provides some detail. The results suggest that further research might succeed in developing a useful method.
REPLY BY AUTHOR TO DISCUSSION BY T. C. RICHARDS
Authors: Hammer Sigmund
Published: October 2002

Dr. Richard’s comments are mainly concerned with regional gravity interpretation. This is only partly pertinent to the “stripping” method and apparently misses the point of caution near the end of my paper. Isostatic and regional studies, to which Dr. Richards refers, usually are not concerned with detailed gravitational influences of local geologic structure. Applications of “stripping” to explore for individual deep prospects requires detailed geological information of a different order of magnitude.
STUDY OF A THREE‐DIMENSIONAL SEISMIC DETECTION SYSTEM
Authors: Broding R. A. , Bentley‐Llewellyn N. J. , Hearn D. P.
Published: October 2002

As part of the VELA program for evaluating systems for detecting underground nuclear events, a three‐dimensional seismic array was instrumented and operated for over a year. Three seismometers spaced 1,320 ft apart were clamped to casing in each of five abandoned oil wells near Tryon, Oklahoma, and their outputs individually recorded on magnetic tape and conventional oscillograms. By time‐shifting and compositing to achieve optimum signal coherence, signal‐to‐noise improvement factors somewhat greater than [Formula: see text] times the ratio for a surface detector were obtained in the signal spectrum from 4 to 10 cps, with lower improvement factors in the 1‐ to 4‐cps range. Studies of the nature of the site noise and its attenuation characteristics with depth are presented together with autocorrelation, power‐spectrum, and coherence plots. To a depth of approximately 1,300 ft, the noise decay is exponential and characteristic of the fundamental Rayleigh mode. Below this depth the pattern is more complex and may be representative of higher mode Rayleigh and body waves. Expressions were derived for the theoretical comparison of the relative directional response of two‐ and three‐dimensional arrays. This study was sponsored by the Air Force Office of Scientific Research.
PRINCIPLES OF DIGITAL FILTERING
Authors: Robinson E. A. , Treitel S.
Published: October 2002

The digital computer is a versatile tool that may be used to filter seismic traces. Conventional filtering is performed by means of analog‐type electronic networks, whose behavior is ordinarily studied in the frequency domain. Digital filtering, on the other hand, is more fruitfully treated in the time domain. A digital filter is represented by a sequence of numbers called weighting coefficients. The output of a digital filter is obtained by convolving the digitized input trace with the filter’s weighting coefficients. The mechanics of digital filtering in the time domain are described with the aid of discrete z‐transform theory. These ideas are then related to the more familiar interpretation of filter behavior in the frequency domain. An important criterion for the classification of filters is the notion of “minimum phase‐lag.” This paper ends with a new and simple presentation of this concept.
BONDING OF LAYERS IN TWO‐DIMENSIONAL SEISMIC MODELING
Authors: Toksöz M. Nafi , Schwab Fred
Published: October 2002

In constructing two‐dimensional layered seismic models, the most commonly used technique is to butt‐join the layers with some bonding agent. Whether or not this type of bonding satisfies the “welded‐contact” hypothesis which is assumed in theoretical treatments is investigated experimentally. Two pieces of a steel plate are bonded together with several agents (water, castor oil, Vaseline, epoxy resin, Devcon plastic steel, and two different solders), and the reflections of the normally incident compressional and Rayleigh waves from the interface are measured. From these, it is found that the bonds do not satisfy the “welded‐contact” condition. Furthermore, comparison with the theoretically computed values demonstrates that the observed reflection coefficients are, in general, too large to be solely due to the contrasts between the elastic coefficients of the plate and the bonding materials. Imperfect adherence of the bonding agent to the edges is offered as an explanation. Theoretical curves are presented to demonstrate the significance even the least possible effect of the bonding layer on the phase velocities of dispersive Rayleigh waves.
HOW TO EVALUATE EXPLORATION PROSPECTS
Authors: Rummerfield Ben F. , Morrisey Norman S.
Published: October 2002

Analytical evaluations raise petroleum exploration from the realm of educated guessing to a quantitative decision level that is compatible with modern business techniques. Management and explorationists can thus appraise the merits of an area and/or exploration program and expect to derive optimum results with a minimum risk. All petroleum exploration programs have one common goal: To find and exploit reserves of oil and gas at a profit. Today the economic factors are playing a dominant role in the highly competitive world petroleum situation. In order to justify his existence in the forecast 70‐billion‐dollar exploration effort during the next ten years, the scientist must translate his thoughts into terms the nontechnical business man or executive can readily grasp. The obvious common language is dollars and cents in terms of anticipated profits. These economic terms transcend the semantics barrier that normally exists between the executive and the oil finder. Various methods are discussed to show how geophysicists and geologist can convert exploration factors into anticipated profit‐to‐risk ratios. The authors include examples. Significant factors contributing to a successful exploration program are: (1) The exploration and economic analysis must be compatible with, and integrated into, modern business techniques. That is, the analysis must enhance the executive’s ability to make decisions. (2) The explorationist must recognize and avoid “marginal ventures,” because 60 percent of the wells completed in the United States are submarginal economically. (3) The laws of probability must be taken into consideration when establishing an exploration program. (4) To insure success, a company must hold risks to a minimum. This can be accomplished, in part, by participating in a large number of potentially profitable ventures, and/or by taking only a part of each drilling venture rather than the entire deal. (5) Anticipated profit‐to‐risk cost ratios can be estimated for many areas. Oil companies can use this information in evaluating and accepting wildcat prospects that have at least double the normal odds of developing into a profitable oil field. The scientist who applies quantitative analyses skillfully will quickly achieve both recognition within his company and the status of a key decision‐maker in his company’s exploration program.
MAGNETIC INVESTIGATIONS OF A RING DIKE, BUELL PARK, ARIZONA
Authors: Dubois R. L. , Carey W. W.
Published: October 2002

Remanent, induced, and total‐magnetic properties are measured for a suite of specimens from a north‐trending ring dike, Buell Park, Arizona. The direction of remanent magnetism is essentially across the dike in an easterly or westerly direction and has low‐angle positive or negative inclinations. The remanent magnetism has an average value of [Formula: see text]. The total magnetism is controlled in part by the remanent magnetism. A ground magnetometer survey, with anomalies exceeding 10,000 gammas, was made for the area and profiles are presented. The results of these measurements, which suggest a shallow magnetization, are correlated with rock‐magnetic measurements with good agreement. The importance of remanent and induced magnetism in the understanding of anomalies is emphasized. Variability of the remanent magnetism is related to a magnetism acquired after the thermal‐remanent magnetism of the rocks and most likely is related to a secondary magnetization caused by lightning.
MAGNETIC AND TECTONIC REGIONALIZATION ON TEXADA ISLAND, BRITISH COLUMBIA
Authors: Hall D. H.
Published: October 2002

A significant correlation exists between direction patterns in magnetic and tectonic trends on the northern two thirds of Texada Island, British Columbia, Canada. The patterns were analyzed in further detail by dividing the area into regions based on amplitudes of magnetic anomalies and on patterns in the magnetic trends. A technique involving smoothing and crosscorrelation was applied in the examination of the relationships between the patterns. The regions established on a magnetic basis also are distinct on the basis of tectonic pattern. However, correlation between the magnetic and tectonic patterns varies from region to region within the area. The directions N 50° W, N-S, and E-W, common throughout the coastal area of British Columbia, are the most widespread in the patterns studied on Texada Island. A distinctive zone, identified as a fault zone cutting across the island with a trend of N 20°W stands out in the patterns.
COLLECTION AND ANALYSIS OF PACIFIC OCEAN‐BOTTOM SEISMIC DATA
Authors: Schneider William A. , Farrell Patrick J. , Brannian Ross E.
Published: October 2002

A total of 500 hours of usable ocean‐bottom seismic data recorded on pressure and three components of velocity has been collected in three geographically separate areas of the Pacific Ocean at depths to 20,000 ft. These data are presently being analyzed to determine the extent to which monitoring seismic motion on the ocean floor can assist Project VELA UNIFORM goals of detection and identification of underground and underwater nuclear blasts. Analysis of three earthquakes and ambient noise recorded simultaneously on the ocean bottom and land reveals: 1. Ocean‐bottom signal‐to‐noise ratios are equal to or less than those seen at a comparative land station; 2. Ocean‐bottom signal and noise levels are higher than those obtained at the land station; and 3. Ocean‐bottom ambient noise power spectra increase in level towards the microseismic 6‐ to 8‐sec peak as do the land data. No strong directional ocean‐bottom noise components have been observed. Simultaneous recording of pressure and particle velocity affords the ocean‐bottom station a distinct advantage over its land counterpart, through exploitation of the relationships between pressure and vertical velocity which exist for various types of arrivals and modes.
A REVIEW OF THE RELATION BETWEEN PHYSICS AND GEOLOGY IN PETROLEUM EXPLORATION
Authors: Karcher J. C.
Published: October 2002

In any field of research, as soon as the fullness of knowledge peculiar to that field approaches the capacity of the individual for such knowledge, the individual in pursuing further researches in that field begins to specialize. The specific fields of physics and geology have increased in such magnitude that the mastery of branches of them requires the lifetime of the individual. Many scientists have been obliged to devote so much time to one single branch of science that they have sacrificed a knowledge of the science as a whole. We see the student of paleontology specializing in the field of geology, and the student of atomic structure specializing in the field of physics.
HYSTERESIS IN THE VELOCITY‐PRESSURE CHARACTERISTICS OF ROCKS
Authors: Gardner G. H. F. , Wyllie M. R. J. , Droschak D. M.
Published: October 2002

It is shown that in dry sedimentary rock the velocity of elastic waves depends not only on the present values of the internal and external pressures but also on the past pressure history of the sample. The internal and external pressures are shown to be equally effective in changing velocities, provided the pressure differential follows a pressure cycle previously impressed upon the sample.
ULTRASONIC SHEAR‐WAVE VELOCITIES IN ROCKS SUBJECTED TO SIMULATED OVERBURDEN PRESSURE AND INTERNAL PORE PRESSURE
Authors: Banthia B. S. , King M. S. , Fatt I.
Published: October 2002

Change in shear‐wave velocity for four dry sedimentary rocks has been studied as a function of the variation of both external hydrostatic pressure and internal pore pressure in the range 0 to 2,500 psi. The experimental method employs a beam of ultrasonic energy passing through a liquid in which a copper‐jacketed parallel‐sided slab of rock is rotated. The shear‐wave velocity is calculated from the laws of refraction and reflection of waves at a liquid‐solid boundary applied to the angle at which minimum energy is transmitted. The variation of shear‐wave velocity with pressure has been found to be a function of net overburden pressure, [Formula: see text], where [Formula: see text] hydrostatic pressure on the jacketed sample, [Formula: see text] pore pressure and n = a pressure‐dependent factor less than unity. The values of n at several differential pressures were chosen to yield a smooth curve passing through the displaced data points when the shear‐wave velocities were plotted as a function of net overburden pressure. Using the n values so obtained, the matrix compressibility [Formula: see text] for two of the sandstones has been calculated from the relation [Formula: see text]. The bulk compressibility [Formula: see text] for these two rocks had previously been obtained experimentally as a function of differential pressure. The values obtained for the matrix compressibility are in the range expected from a knowledge of the grain and cementing materials for these sandstones.
REFLECTION OF PLANE WAVES FROM A LINEAR TRANSITION LAYER IN LIQUID MEDIA
Authors: Gupta Ravindra N.
Published: October 2002

It is shown that care should be taken in using the term “velocity” in connection with wave propagation in inhomogeneous media. An expression is derived for phase velocity which depends on frequency and depth. Exact solutions are found for normal and oblique incidence, for plane‐wave propagation in a liquid medium in which density, ρ, and bulk modulus, λ, vary as follows: [Formula: see text] and [Formula: see text] where [Formula: see text], [Formula: see text], b, and p are arbitrary constants. It is shown that the geometrical optics approximation solution, valid for high frequencies, is the first term in an asymptotic expansion of the exact solution. The reflection coefficients are obtained for a linear transition layer between two homogeneous half‐spaces. Both first‐order and second‐order discontinuities in density and bulk modulus are considered at the boundaries of the transition layer.
DECOMPOSITION AND SUPPRESSION OF MULTIPLE REFLECTIONS
Authors: Watson R. J.
Published: October 2002

Multiple reflections constitute an important source of “noise” on seismograms. Because multiples are related to the primary reflectivity function r(t) in a very complicated way, the suppression of all multiple reflections on a single seismic trace is unlikely without a detailed knowledge of the reflectivity function itself. This paper describes a series of approximations in which multiple reflections are decomposed into component subsets. For certain types of velocity functions, one or two of these subsets form a sufficiently good approximation to the complete multiple process to at least predict the strongest multiples present on a field seismogram. The subset approximations to the complete multiple function include the first‐order surface multiples, i.e., three‐bounce multiples having a second reflection at the surface. The expression for this set of multiples is [Formula: see text], where the * stands for convolution, and [Formula: see text] is the surface reflection coefficient. Computations of this set are compared with the complete multiple function for logs from Alberta, Canada, and southern Mississippi. A further approximation, called the [Formula: see text], is valid in areas where a few reflection coefficients, r⁁(t), are responsible for the bulk of the multiple noise. [Formula: see text] consists of all first‐order surface multiples which have a contribution from the r⁁(t) zone. The above‐mentioned velocity logs are used to illustrate the [Formula: see text] approximation. The [Formula: see text] function is compared with the complete first‐order surface multiple function for the two logs. A method for suppressing the multiples described by the approximations is proposed. The technique is illustrated for the [Formula: see text] approximation but can be extended to higher‐order approximations. It consists of a positive feedback circuit in which a reflectivity function is simulated along with appropriate time‐variant gain adjustments. In order to realize the computation, it is necessary to find the time and amplitude of the reflection coefficients responsible for the large‐amplitude multiple reflections. Several methods for providing this information are discussed and a correlation search technique is illustrated with examples. Finally, the suppression technique is illustrated with live seismic data on two record sections from two different areas showing data before and after multiple suppression. In both cases the [Formula: see text] approximation was adequate.
PROJECT SHOAL VOLUNTEER TEAM PROGRAM
Authors: Weisbrich R. A.
Published: October 2002

SHOAL, the second underground nuclear test announced sufficiently well in advance to permit Volunteer Team participation, was detonated in Western Nevada on October 26, 1963. Twenty‐four Volunteer Teams from exploration seismograph crews, universities, research organizations, and independent observers joined in monitoring this event. Sixteen teams recorded energy from the explosion. Results obtained by these teams are summarized and reproductions of some of the seismograph records are included.
EARTH’S FREE MODES AND A NEW GRAVIMETER
Authors: Slichter Louis B.
Published: October 2002

Gravitational observations of free vibrations of the earth due to the Alaskan earthquake of March, 1964 were compared with corresponding results for the Chilean earthquake. In the graver spheroidal modes of periods greater than 7 minutes which were compared, the periods generally agreed within a few parts per thousand. The freemode spectrum has been used by Anderson, Landisman, and others to improve models of the earth. To fit both the free mode periods and the seismic data, Anderson’s modification of classical earth models introduces a more rapid increase of density in the upper mantle, and less rapid increase near the core. In addition to the precise set of numbers characterizing the free periods, the free mode observations supply another independent set of criteria in the form of the “Q” values for the different modes. The large Q value of more than 25,000 for the purely radial mode excited by the Alaskan quake is noteworthy. In gravity observations of the earth’s oscillations, of period one hour or more, and especially in observations of the semidiurnal and diurnal tides, the vertical component is observable with greater reliability than is the horizontal component. Improved instrument sites, probably drill‐hole sites, seem to be required. A laboratory model for a simple type of drill‐hole gravimeter incorporating a new efficient transducer due to R. Havens was shown. The instrument has a 20‐sec period, and in preliminary tests shows a sensitivity of a few microgals.
THE EFFECTS OF FLUID INVASION IN PULSED NEUTRON LOGGING
Authors: Allen L. S. , Mills W. R. , Caldwell R. L.
Published: October 2002

The weak dependence of the results of pulsed neutron logging on the presence of the oil well borehole suggests that thermal neutron die‐away measurements in an open hole may be capable of “seeing” beyond a limited amount of borehole fluid invasion. In order to obtain an estimate for the depth of investigation which can be expected from pulsed neutron logging when some invasion has occurred, multigroup neutron diffusion theory was used to calculate thermal neutron lifetimes for the open‐hole logging geometry. The transient, three‐group equations were solved with the aid of an IBM 7090 computer. Two cases of open‐hole fluid invasion were investigated: Fresh‐water filtrate invading a salt‐water‐saturated formation, and salt‐water filtrate invading a formation saturated with fresh water (or oil). In both cases the formation is assumed to be a 20 percent porosity sandstone and the fluid‐filled borehole is taken to be 20 cm in diameter. It is shown that for any given depth of borehole fluid invasion, up to the point of total masking, it is much easier to detect the presence of oil behind salt‐water invasion than to detect salt water behind a zone invaded by fresh water.
GEOTHERMAL SURVEYS IN SEDIMENTARY ROCKS NEAR GRANTS AND LAGUNA, NEW MEXICO
Authors: Byerly P. Edward
Published: October 2002

Data obtained from temperature surveys in drill holes over two uranium ore bodies are presented as maps of isotherms at depths of 100 ft or more. Temperature surveys in permeable ore bodies above the water table may reveal zones of downward migration of oxygenated water, or zones with a comparatively large retention of pore water. These zones may be associated with an increased amount of oxidation in an ore body which is not completely oxidized.
THE ANOMALOUS FREE‐AIR VERTICAL GRADIENT IN BOREHOLE EXPLORATION
Authors: Thyssen‐Bornemisza Stephen
Published: October 2002

Hammer (1950) and Smith (1950), in discussions about gravity measurements in vertical shafts and boreholes, have pointed out that gravitational effects originating in zones or layers above or below the gravity‐meter observation interval may easily produce anomalous free‐air vertical gradients. These anomalous gradients cannot well be corrected without the proper density information and, therefore, would represent a possible source of error in the interpretation of results.
STANDARD CURVES FOR MAGNETIC ANOMALIES OVER LONG HORIZONTAL CYLINDERS
Authors: Gay S. Parker
Published: October 2002

The magnetic anomalies in Z, H, and [Formula: see text] for the long horizontal circular cylinder are shown to belong to a single mathematical family of curves for all values of strike and all values of inclination of the magnetizing field, a characteristic that was previously shown to hold for long tabular bodies, or dikes (Gay, 1963). The complete family of standard curves has been constructed and is incorporated into an interpretational scheme based on superposition with observed magnetic profiles. Comparison of cylinder anomalies with dike anomalies shows only slight differences in the two types of curves, which would be very difficult, if not impossible, to detect using interpretational methods based on a few isolated points of a profile curve, such as half‐width, distance between maximum and minimum, etc. Curve‐matching, or superposition, appears to be mandatory for reliable quantitative interpretations.
GEOPHYSICAL INVESTIGATIONS IN THE AREA OF THE PERTH BASIN, WESTERN AUSTRALIA
Authors: Hawkins L. V. , Hennion J. F. , Nafe J. E. , Thyer R. F.
Published: October 2002

Marine geophysical investigations in the area of the Perth Basin lead to proposed changes in the structural control of the basin and of the structure within the basin. The main north‐south graben structure appears to be crossed by a series of major faults which trend roughly north‐northwest. A broadening of the basin to a width of just over 100 km (65 miles) to the west in the area between Perth and Harvey, which was earlier indicated by aeromagnetic results, appears to be produced by two such faults: The southern fault does not cross the graben but merges with it in the form of the Dunsborough Fault; the proposed northern fault crosses the graben to produce a break in continuity of the Darling Fault which marks the eastern margin of the basin. The northern fault appears to have caused a division of the main sedimentation axis of the Perth Basin into two near parallel axes within the widened section of the basin. The Dandaragan Trough which forms the eastern axis now appears to terminate at the Darling Fault between Pinjarra and Harvey. The western axis continues to the southeast into the Bunbury Trough. This suggests the presence of a basement and anticlinal ridge which may produce favourable traps for oil or gas within the basin. The western margin of the basin appears to be formed by an eastward‐tilted basement with associated faulting which, in places, assumes major proportions. A tentative estimate of sediment thickness of about 5.7 km (18,600 ft) is obtained from a seismic profile near the axis of sedimentation on the extension of the Bunbury Trough. This figure is our preferred interpretation, but it would have to be reduced to about 3.5 km should a 5‐km/sec layer turn out to be basement. The southward extension of the Darling Fault onto the narrow continental shelf appears to be observed with a throw of two km to the south of Pt. D’Entrecasteau.
FAULT CRITERIA
Authors: Campbell Francis F.
Published: October 2002

Examples of six criteria for fault interpretation of seismic data in East Texas and the Gulf Coast are presented on a series of maps and cross sections. The six criteria are: correlation of reflection horizons which bracket the producing objective, projection of a fault from a shallow bed with correlatable reflections to a deep producing objective, variation between geological correlations and geophysical dips, misclosure around a grid unit of seismic control, dip pattern along several lines of control, and diffraction. The diffraction example includes results of fault model experiments in the laboratory. Although each example was selected as a particularly clear illustration of one criterion, fault interpretation usually depends on a combination of several criteria. Additional criteria for fault interpretation may be used. The criteria presented are not infallible and they do not detect very small faults.
SEISMIC WAVE PROPAGATION IN LAYERED MEDIA IN TERMS OF COMMUNICATION THEORY
Authors: Treitel S. , Robinson E. A.
Published: October 2002

The problem of a normally incident plane P wave propagating in a system of horizontally layered homogeneous perfectly elastic plates is reformulated in terms of concepts drawn from communication theory. We show how both the reflected and transmitted responses of such a system can be expressed as a z transform which is the ratio of two polynomials in z. Since this response must be stable, the denominators of the z transforms describing the reflected and transmitted motion are minimum delay (i.e., minimum‐phase lag). If the layered medium is bounded at depth by a perfect reflector, then the reflected impulse response recorded at the surface is in the form of a dispersive all‐pass z transform. A dispersive all‐pass system is one whose z transform is the ratio of the z transform of a maximum‐delay wavelet to that of its corresponding minimum‐delay wavelet; hence, the amplitude spectrum of a dispersive all‐pass system is unity for all frequencies. This means that the amplitude spectrum of the reflected response is identical to the amplitude spectrum of the input wavelet used to excite the system. More specifically, all the energy put in is returned with the same frequency content, but is differentially delayed. The phase‐lag spectrum of the reflected response lies everywhere above the phase‐lag spectrum of the input wavelet. Thus, the all‐pass situation implies that the layered earth model considered here, while not able to alter the amplitude of the frequency components of the input wavelet, will introduce differential time delays with certain properties into each such component. Finally, since the reflected impulse response is an all‐pass wavelet, its autocorrelation is a spike of unit magnitude at τ=0, and zero for all other lags.
TELLURIC SOUNDING—A MAGNETOTELLURIC METHOD WITHOUT MAGNETIC MEASUREMENTS
Authors: Yungul S. H.
Published: October 2002

The basic theory and objectives of telluric sounding (TS) are about the same as those of the well‐known method of magnetotelluric sounding (MTS) (Cagniard, 1953). Both methods make use of the natural electromagnetic phenomena known as geomagnetic micropulsations to obtain crude “resistivity logs” from the surface down to great depths, without drilling, if the subsurface has mild structures, low dips, and lateral continuity in the electrical resistivity. Let the x-y plane of the Cartesian coordinates represent the surface of the earth. With MTS, the field operation consists of simultaneously recording the time variations of an arbitrary x component of the electric field, [Formula: see text], called a tellurogram, and that of the y component of the magnetic field, [Formula: see text], called a magnetogram, both at the same point where the downward information is desired. The main difficulty is in the measurement of the magnetic field variations with sufficient accuracy. The measurement of the electric field variations is very simple and expeditious. TS bypasses this difficulty, because it does not require the measurement of the magnetic field. With TS, the field operation requires two electric field recording units. One of these units remains at a “base station” where the subsurface is known from a well log, while the second unit is placed at a “field station” where one wishes to explore the subsurface. Thus, for each sounding, one obtains two simultaneous tellurograms. These are Fourier analyzed. The ratios of the electric field amplitudes as a function of frequency, combined with the resistivity log at the base station, furnish the MTS‐type data at the field station that are interpreted in the usual manner to yield a crude resistivity log at the field station. The primary objective of TS is the exploration of sedimentary basins. It may be preferable to MTS in certain cases and vice versa; it is not meant to replace MTS. The theoretical basis and the procedures of TS are discussed in this paper.
THE SEARCH FOR ORE DEPOSITS USING THERMAL RADIATION
Authors: Strangway D. W. , Holmer R. C.
Published: October 2002

Kennecott Copper Corporation has conducted several months of airborne infrared work in an attempt to detect patterns related to geology or the presence of oxidizing sulphides. The results encountered have shown that certain microclimatological effects are very important. Among these are the hill effect shown by small topographic features bathed in warm air under thermal inversion conditions. Major faults are evidently reflected in some cases as are areas of subterranean hot water. These observations have been confirmed by ground temperature measurements in holes two to three ft deep.
ATTENUATION OF CONTROLLED WAVE SEISMOGRAPH SIGNALS OBSERVED IN CASED BOREHOLES
Authors: Mack Harry
Published: October 2002

The possibility of a frequency‐dependent coupling, in the range 5–20 cps, between a deep‐hole casing and the surrounding medium, was studied experimentally. The source used was a Continental Oil Company VibroSeis vibrator and the receiver was a Geotechnical Corporation deep‐hole seismometer, which was developed for use in the VELA UNIFORM deep‐hole experiments. The seismometer was located at selected depths in the hole so that the coupling of the surrounding medium to the case was as varied as possible. The field seismogram was crosscorrelated with the original signal, and the nature of the attenuation of the received waveform is reflected in the amplitude and width of the correlation pulse. Smooth amplitude variation with depth and constant pulse width indicated that the coupling does not vary in this frequency range and that all measured attenuation can be explained by normal propagation phenomena. The pulse amplitude attenuation was obtained numerically using different constants of attenuation (assumed to be of a “constant Q” nature). The results were compared with the attenuation measured in the field, and hence the constant which best explains the field data was found. By this method, a value for 1/Q of 0.012±0.004 was derived over 1,800 m of fairly uniform sandy shales.
THE USE OF LINEAR PROGRAMMING TO FILTER DIGITIZED MAP DATA
Authors: Dougherty E. L. , Smith S. T.
Published: October 2002

The procedure used to discover subsurface formations where mineral resources may exist normally requires the accumulation and processing of large amounts of data concerning the earth’s fields. Data errors may strongly affect the conclusions drawn from the analysis. Thus, a method of checking for errors is essential. Since the field should be relatively smooth locally, a typical approach is to fit the data to a surface described by a low‐order polynomial. Deviations of data points from this surface can then be used to detect errors. Frequently a least‐squares approximation is used to determine the surface, but results could be misleading. Linear programming can be applied to give more satisfactory results. In this approach, the sum of the absolute values of the deviations is minimized rather than the squares of the deviations as in least squares. This paper describes in detail the formulation of the linear programming problem and cites an example of its application to error detection. Through this formulation, once errors are removed, the results are meaningful physically and, hence, can be used for detecting subsurface phenomena directly.
GRAVITY ANOMALIES OF TWO‐DIMENSIONAL FAULTS
Authors: Geldart L. P. , Gill Denis E. , Sharma Bijon
Published: October 2002

A simplified formula is given for the gravity effect of a horizontal semi‐infinite block truncated by a dipping plane. This formula is used to obtain curves illustrating the gravity anomalies for blocks having different thicknesses and depths truncated by planes dipping at various angles. By combining two blocks, results are obtained for faulted horizontal beds for a wide range of bed thicknesses and depths, fault displacements and dips. These should be useful as guides in interpreting fault anomalies, and in planning gravity programs intended to map faults. The most striking feature of the curves is the marked effect of the dip of the fault plane on the curves for faulted beds. The asymmetry of the fault curves is related mainly to the dip and can be used to determine dips between 30 and 90 degrees. If the dip of the fault, density contrast, and bed thickness are known, the depths to the bed on the two sides of the fault are given by the sizes and positions of the gravity maximum and minimum.
WAVEGUIDES, ARRAYS, AND FILTERS
Authors: Clay C. S.
Published: October 2002

Conventional plane wave array theory does not apply to arrays in an inhomogeneous medium. In a stratified waveguide such as the ocean there are many modes of propagation, and each of them is dispersive. As expressed in normal mode formalism, the transmission between vertical arrays can be considered as a filter problem. In this paper we consider the response of the array filters to an ambient noise field, and maximize the signal‐to‐noise ratio for transmission in a noisy waveguide. The resulting optimum, or matched array, filter is given by the conjugate of the product of the source function and the waveguide transmission function. The response of the matched array filter is weighted with the reciprocal of the noise power in each mode and the attenuation.
ATTENUATION MEASUREMENTS IN THE FIELD
Authors: De Bremaecker J. Cl. , Godson Richard H. , Watkins Joel S.
Published: October 2002

The amplitudes of the P head wave were measured on an aa lava flow, on unconsolidated cinders, and on compact limestone. The data are satisfied by the equation [Formula: see text], where A is the amplitude, d the distance, [Formula: see text] the attenuation coefficient for P waves, ν the frequency, and α the P wave velocity. By assuming a complex shear modulus μ* but a real λ one finds [Formula: see text], where β is the shear wave velocity. This formula is in reasonable agreement with published data.
APPLICATION OF FILTER THEORY AND INFORMATION THEORY TO THE INTERPRETATION OF GRAVITY MEASUREMENTS
Authors: Sax Robert L.
Published: October 2002

A wavenumber filter can be used in conjunction with the Hartley‐Tuller‐Shannon formula to bring about some reduction in the number of ambiguous contours on a gravity map. The filter focuses the interpreter’s attention on the maximum response to density changes within a given layer. At the same time, characteristics of the filter design, such as bandwidth and power of the desired output relative to other interfering outputs, can be used in the H‐T‐S formula to determine a minimum contouring threshold, such that the relative number of false anomalies and missed true anomalies is minimized. In addition, it is also possible to estimate a lower limit on the probability that an anomaly observed on a map originates in a target layer.
ATTENUATION AND DISPERSION OF SHEAR WAVES IN PLEXIGLAS
Authors: Jordan Neal F.
Published: October 2002

Wuenschel (1965) measured the attenuation of plate waves in a Plexiglas sheet, finding a value of [Formula: see text] for the attenuation coefficient at frequency f(cps). Using only the first term in the attenuation, he then employed the results of Futterman (1962) to predict the intrinsic dispersion which should accompany this attenuation. Good agreement was obtained between the predicted and observed dispersion. Changes in pulse shape with distance traveled proved to be a sensitive indicator of the presence and nature of the dispersion; when pulses at a location remote from the source were predicted on the basis of a pulse observed near the source plus the transmission characteristics of the material, inclusion of the Futterman dispersion substantially improved the match between predicted and observed waveforms.
EFFECT OF A FLUID CORE ON PROPAGATION OF AN SH‐TORQUE PULSE FROM A POINT‐SOURCE IN A SPHERE
Authors: Alterman Z. , Kornfeld P.
Published: October 2002

An exact solution is obtained for the displacement of the surface of a sphere consisting of a fluid core and an elastic solid mantle, due to an impulsive SH‐torque source. Theoretical seismograms are computed for several distances from a surface source and from a deep source, and are compared with results for a homogeneous sphere. The seismograms show reflected pulses of H‐type and of Lg‐type, and pulses after diffraction at the core and at the surface. In agreement with observation and as expected by steepest‐descent analysis, it is found that the Lg‐type pulses start before the ray arrival time. Traveltime and amplitude curves for reflected and diffracted pulses are given.
THE SEPARATION OF REMANENT FROM INDUCED MAGNETISM IN SITU
Authors: Goldstein N. E. , Ward S. H.
Published: October 2002

Remanent and induced magnetism both contribute to static field magnetic anomalies whereas only induced magnetism contributes to dynamic field magnetic anomalies. The theory whereby this phenomenon may be used to advantage for in‐situ separation of remanent from induced magnetism is presented as a prelude to observational evidence confirming the phenomenon. Four field experiments on Western States magnetic anomalies prove that it is possible to predict whether or not a given static field magnetic anomaly is primarily due to remanent or to induced magnetism. The limitations of the method include variability of micropulsation field direction, ellipticity, and intensity.