bluuebunny/crossref_metadata_2025 huggingface dataset:
AUTOMATIC THREE‐DIMENSIONAL MODELING FOR THE INTERPRETATION OF GRAVITY OR MAGNETIC ANOMALIES
Authors: Gerard A. , Debeglia N.
Published: October 2002

Transformation of gravity or magnetic anomaly maps into isodepth maps of a surface separating two homogeneous media may be accomplished by (1) systematically estimating an average depth and density or magnetization contrast for the surface and (2) using an iterative method to adjust local depths compared to the average depth of the surface. Average depth, density or magnetization contrast, and iterative adjustment of local depths are determined using the Fourier transform of the field to be interpreted and that of the field generated by an equivalent surface. This leads us to propose a method of estimating the average depth of the sources and a distribution function for the depths and then a complete and very economical algorithm for the calculation of the corresponding equivalent surface.
On: “On the Phase Constraint of the Magnetotelluric Impedance,” by D. Loewenthal (GEOPHYSICS, April 1975, p. 325–330).
Authors: Eckhard Donald H.
Published: October 2002

In his paper, Loewenthal presents what he calls a “simple proof” that the phase of the one‐dimensional magnetotelluric modified impedance lies between −π/4 and π/4. A much simpler proof exists (Eckhardt, 1968). My proof is buried as a rather minor feature of an obscure (alas) paper, and some of its sign conventions differ from standard, so I believe it appropriate to restate the proof here.
ELASTIC‐WAVE VELOCITY MEASUREMENTS IN ROCKS AND OTHER MATERIALS BY PHASE‐DELAY METHODS
Authors: Kaarsberg E. A.
Published: October 2002

The phase delay of a continuous sinusoidal elastic wave after transmission through a medium may be used to determine the velocity of propagation of the wave in the medium. The change in path length for a given frequency, or the change in frequency for a given path length, required to change the phase delay by integral multiples of 360 degrees is measured in the laboratory by the use of source and receiver piezoelectric transducers whose signals are applied to the horizontal and vertical deflection circuits of an oscilloscope. The accuracy of the method depends upon the accuracy with which the frequency of the transmitted wave and its path length through the medium (or change in path length) can be determined, provided the effect of extraneous signals (e.g., boundary reflections, multiple reflections, alternate modes of propagation, etc.) is negligible. The phase‐delay methods are illustrated and compared with conventional pulse methods by using both to make compressional‐velocity measurements in water and compressional‐ and shear‐velocity measurements in a high velocity basalt and in a low velocity dried mud sample. The results of the two methods agree to within a few percent. It is suggested that these phase‐delay methods may be especially well‐suited for making elastic‐wave velocity measurements in media with high attenuation of the waves propagated in them.
MAPPING PORPHYRY COPPER DEPOSITS IN THE PHILIPPINES WITH IP
Authors: Pelton William H. , Smith Peter K.
Published: October 2002

The induced polarization (IP) method has played a very important role in the recent discovery of several new Philippine porphyry copper deposits. It was found, however, that the electrical response measured over different deposits can be extremely variable, even in a limited geographical area. Four main classes of response have been recognized; these appear to arise primarily from different structural control and possibly from differing levels of erosion in the Lowell and Guilbert (1970) and Hollister et al. (1974) porphyry copper models. Since the distribution of economic mineralization is highly dependent upon erosional level and structural control. It is important in IP interpretation to (1) recognize the type of deposit (2) determine the position of controlling structural features, and (3) map the existing alteration zones, before attempting to define the location of economic mineralization.
THE VERTICAL ARRAY IN REFLECTION SEISMOLOGY—SOME EXPERIMENTAL STUDIES
Authors: Wuenschel Paul C.
Published: October 2002

There are several advantages in using vertical arrays for recording reflected signal. Signal‐to‐noise ratio can be controlled to any desired level when the noise is due to scattering from layers shallower than the depth to the array. By the use of vertical arrays, the band width of useable seismic energy can be increased, events can be properly identified, the signal that eventually produces near surface induced multiples can be measured, as well as the direct pulse radiated from the source and its accompanying ghosts. A field test documents these predictions.
NUMERICAL INVERSE MAGNETOTELLURIC PROBLEMS
Authors: Ku Chao C.
Published: October 2002

Marquardt’s maximum neighborhood method for computing the values of nonlinear parameters is applied to model magnetotelluric data to compute conductivity contrast, overburden thickness, and the depth extent of a conductor. The problems treated are two‐dimensional and the network solution is used to formulate the relationship between the EM fields and the nonlinear paramaters. The method appears to work much better than trial‐and‐error or master‐curve matching techniques. Its application to any particular magnetotelluric problem appears to be limited only by one’s experience and imagination.
THEORY AND EVALUATION OF THE LACOSTE AND ROMBERG THREE‐AXIS INERTIAL PLATFORM FOR MARINE GRAVIMETRY
Authors: Valliant Herbert D. , LaCoste Lucien J.B.
Published: October 2002

In 1972, the Earth Physics Branch of the Canadian Department of Energy, Mines and Resources acquired a LaCoste and Romberg Inc. air/sea gravimeter complete with three‐axis inertial platform. Since then, in cooperation with the manufacturer, a thorough evaluation of this system for regional applications has been undertaken over two field seasons. In 1972, the instrument was operated over the Halifax Test Range where direct comparisons with the underwater measurements indicated that the system as constituted gave results which were accurate from ±5 to ±10 mgal (±1σ), with position updates at about two hour intervals. In general, the errors were greatest when the gyro precession rates were large and hardware modifications were made to reduce this effect. A second survey in Queen Charlotte Sound was made in 1973 to evaluate improvements designed to reduce the gyro precession rates. An analysis of crossover differences indicated that accuracies in measuring gravity of 3 mgal (±1σ) using the inertial platform under open sea conditions is achievable. The accuracy of the Eötvös correction determined from inertial data is intermediate between that obtained using Loran‐C in the rho‐rho mode and hi‐fix.
ELECTROMAGNETIC SOUNDING WITH SUSCEPTIBILITY AMONG THE MODEL PARAMETERS
Authors: Thomas Lindsay
Published: October 2002

The response of electromagnetic sounding systems is insensitive to the ordinary variations in magnetic susceptibility. However, in layered earth models susceptibilities of more than 0.005 may affect the system response appreciably, and susceptibility values of this magnitude might be determined by model fitting to parametric sounding observations. Electromagnetic soundings might, therefore, be used to reveal the existence of remanent magnetization in some cases. These conclusions are drawn from numerical studies with theoretical models.
TELLURIC ANOMALIES CAUSED BY SHALLOW STRUCTURES: ELLIPSOIDAL APPROXIMATIONS
Authors: Lee Tien‐Chang
Published: October 2002

An analytical solution for the electric field near an ellipsoid immersed in a semiinfinite medium is obtained to estimate the effect of shallow structures on telluric measurements. The determinant J of the Jacobean, which transforms telluric field from base to roving stations, may change across the boundary of different media by a factor equal to their resistivity ratio. At distances beyond one length of the intermediate axis from the edge of the ellipsoid, both the J-anomaly and the polarization of the normalized telluric field become negligible.
SPECTRAL SIGNATURES OF PARTICULATE MINERALS IN THE VISIBLE AND NEAR INFRARED
Authors: Hunt Graham R.
Published: October 2002

The utility of multispectral remote sensing techniques for discriminating among materials is based on the differences that exist among their spectral properties. As distinct from spectral variations that occur as a consequence of target condition and environmental factors, intrinsic spectral features that appear in the form of bands and slopes in the visible and near infrared (.325 to 2.5 μm) bidirectional reflection spectra of minerals (and, consequently, rocks) are caused by a variety of electronic and vibrational processes. These processes, such as crystal field effects, charge‐transfer, color centers, transitions to the conduction band, and overtone and combination tone vibrational transitions are discussed and illustrated with reference to specific minerals. Spectral data collected from a large selection of minerals are used to generate a “spectral signature” diagram that summarizes the optimum intrinsic information available from the spectra of particulate minerals. The diagram provides a ready reference for the interpretation of visible and near infrared features that typically appear in remotely sensed data. In the visible‐near infrared region, the most commonly observed features in naturally occurring materials are due to the presence of iron in some form or other, or to the presence of water or OH groups.
REMOTE SENSOR APPLICATIONS TO TECTONISM AND SEISMICITY IN THE NORTHERN PART OF THE MISSISSIPPI EMBAYMENT
Authors: O’Leary Dennis W. , Simpson Shirley L.
Published: October 2002

A comparative study of Landsat images, SLAR image strips, and Skylab photographs was made to help develop a regional tectonic model and to evaluate the seismic hazards of the northern part of the Mississippi embayment as far south as the Ouachita front. The geomorphic character and distribution of lineaments reflects the structure and tectonics of the area. The fairly consistent trends of northwest‐oriented lineaments suggest the influence of jointing, whereas the variable north‐ and northeast‐trending groups suggest a variety of influences including jointing, faulting, and surficial factors that control stream orientation. A number of northeast‐trending faults are relatively recent, or have significant surface displacements, as they disrupt slopes in equilibrium with drainage. Seismic and aeromagnetic data are available for part of the area. Well‐defined trends of epicenters near New Madrid conform to a pronounced lineament pattern along the Mississippi River. Over a small area near New Madrid, a trend analysis was made of the totals of length‐width ratios of magnetic anomalies grouped in 10-degree intervals. Anomaly trend distributions show some correlation with the azimuth frequency diagrams of lineaments in the embayment. These data suggest basement control along gradients of magnetic anomalies where they parallel mapped lineaments. The tectonic pattern of the area is generally revealed in the landscape, of which the lineaments seem to be a particularly important part. The Mississippi River flows along an axis of active subsidence, possibly actually controlled by tectonic events along the west edge of the embayment, including uplift of the Ozark dome. The tectonic evolution seems to be one of an episodically opening wedge, combining strike‐slip movement along the north and west side of the embayment and step‐faulting down to the east.
TIME‐TO‐DEPTH CONVERSION IN THE PRESENCE OF STRUCTURE
Authors: Gerritsma P. H. A
Published: October 2002

Time‐to‐depth conversion has long since been one of the objectives of seismic interpretation. Unfortunately, there is not a straightforward procedure that derives a depth section from a time section. This is due to the fact that reflection times depend upon the positions and shapes of the interfaces as well as upon the interval velocities, which are both unknown. A program will be described to handle this problem under fairly general conditions. An estimated depth section is updated in an iterative way, depending on the differences between reflection‐time data of the input and that derived with ray tracing from the estimated depth section. The program starts with the first layer and, after completion, continues with the second, etc. The main assumptions are that there is no crossdip and that the interval velocity within each layer is a linear function of depth and lateral position.
THE GEOSAT COMMITTEE, INC.
Authors: Brant Arthur A.
Published: October 2002

The Geosat Committee was formally incorporated as a nonprofit organization under the laws of California on December 2, 1976. The Geosat Committee is supported by contributing memberships restricted to companies. It seeks to further the incorporation of pertinent, geologically related objectives into the ongoing governmental remote sensing programs, as an aid to resource exploration and utilization, and to engineering applications.
SEISMIC RAYPATH MIGRATION WITH THE POCKET CALCULATOR
Authors: Michaels Paul
Published: October 2002

Migration of seismic diffractions and reflections on corrected record sections or contoured reflection time maps is possible with the most basic of programmable pocket calculators. Such an algorithm using a linear velocity function is given for the Hewlett Packard HP-25 calculator. Adaptation to other scientific calculators of at least 49-step program memories should be easy. Although the mathematics of migration are well understood, programming of pocket calculators requires a formulation involving the fewest possible number of steps. The formulation presented here requires only 6.8 sec to execute. The inputs are a time to the event and a dip in time for the wavefront segment to be migrated. Outputs are the horizontal and vertical coordinates of the migrated point as well as the dip of the reflector. Advantages of the calculator method are many. First, the calculator can be used to determine if and where migration is necessary for a set of data. Only those wavefronts that need migration are migrated. Existing sections can be used without the need for any special processing. One does not need to wait long for the results. Finally, the routine is valid for contour migration of time maps. This is perhaps the best application since one is not restricted to the plane of a single profile.
SEISMIC WAVE ATTENUATION IN NONRESOLVABLE CYCLIC STRATIFICATION
Authors: Spencer T. W. , Edwards C. M. , Sonnad J. R.
Published: October 2002

Situations arise in seismic exploration in which thick zones generate no well‐defined reflection events even though the sonic logs exhibit significant velocity stratification. In many cases, reflections are not generated because the stratification is so fine that it cannot be resolved by the seismic frequency band. Nonresolvable sections in which the impedance structure is cyclic in character, such as sand‐shale sequences, are shown to exhibit frequency dependent attenuation which differs from the linear dependence commonly attributed to absorption. This difference may be exploited to separate the absorptive and stratigraphic components of attenuation and to compare their relative magnitudes. Only in depth intervals in which the reflection coefficients are unusually large can stratigraphic attenuation account for observed attenuation of the order of a few tenths of a decibel per hundred feet. Relations are derived for determining the effect on the stratigraphic attenuation of scaling the parameters which characterize the section.
RECENT ADVANCES IN URANIUM EXPLORATION WITH ELECTRONIC ALPHA CUPS
Authors: Warren Roy K.
Published: October 2002

We compare data from a new radon measuring device, based on electronic alpha cups with data from the nonelectronic, plastic alpha cups commonly used in exploring for uranium. A burial time of three days appears to be adequate for the electronic alpha cups to collect exploration data equivalent to those from plastic sensing devices. The advantages of the electronic cups are short time of burial and instant read‐out in the field; the major disadvantage is the capital expenditure required for the instruments. It is possible the cups may be useful for copper exploration also.
SIMPLIFIED MAGNETIC INTERPRETATION OF THE GEOLOGIC CONTACT AND THIN DIKE
Authors: Stanley John M.
Published: October 2002

A simple procedure has been applied to the interpretation of magnetic profiles measured perpendicular to the strike of contact or dike‐like structures. The method is very easy and rapid to apply, and is applicable to airborne or ground level data. It does not necessarily require computing facilities but may be automated if desired. As the interpretation requires only a relatively short length of traverse (usually less than 3 depth units) in close proximity to the structure, the procedure results in a high immunity to neighboring disturbances. A worked example using theoretical data shows its effectiveness.
INDUCED‐POLARIZATION EFFECTS FROM GROUNDED STRUCTURES
Authors: Nelson P. H.
Published: October 2002

Fences, power lines, pipelines, and other extensive man‐made structures with earthed grounds produce spurious induced polarization anomalies by acting as secondary sources and sinks of current. This problem is especially difficult to detect because the redistributed current waveforms are similar to the desired induced‐polarization signal. If the structure grounds were purely resistive there would be no problem; but because most grounds are polarized, currents flowing through them undergo phase shifts that contaminate the desired measurements. The patterns and magnitudes of such spurious anomalies can be calculated using a mathematical model that incorporates the position of each ground in relation to the transmitter and receiver dipole locations. The model also requires an estimate of the grounding impedance, the resistance per unit length of the structure, and the earth resistivity. All three parameters influence the received signal, but the effect of high earth resistivity is particularly dramatic in increasing the undesired phase shifts. Comparison of the predictions with field tests helps confirm the validity of the model.
BOREHOLE‐RADAR EXPLORATION IN A COAL SEAM
Authors: Cook John C.
Published: October 2002

The experimental use of “geologic radar” to explore through coal from mined passages underground has already been reported elsewhere (Cook, 1973, 1974). Low‐resolution borehole radar work in salt, a much more favorable medium, has also been reported (Holzer et al., 1972). The purpose of this paper is to report the first known results of borehole radar tests in coal. The exploration of coal seams via boreholes from the surface is potentially a technique of great practical importance. The borehole experiment was performed in conjunction with a month‐long program of radar tests in Australian collieries and quarries performed under the sponsorship of the third ICOGEO and supported by several Australian mining and research organizations. A report on that program is expected to appear in the Bulletin of the Australian Society of Exploration Geophysicists some time during 1977.
THE APPARENT RESISTIVITY TENSOR
Authors: Bibby H. M.
Published: October 2002

The scalar apparent resistivity was originally defined for simple linear electrode resistivity arrays such as Wenner and Schlumberger arrays. With the extension of resistivity surveying into bipole‐dipole arrays, several different and not obviously related definitions of apparent resistivity have been used by different authors despite similar field procedure. In this note we show that when a pair of current sources (or quadripole source) is used, and the corresponding electric field vectors are measured at each field station, the most comprehensive expression of the reduced data is as an apparent resistivity tensor. Other definitions of apparent resistivity can be simply related to this tensor. For example, the quadripole‐quadripole apparent resistivity of Doicin (1976) is one of the tensor invariants; the maximum and minimum resistivities defined by the rotating dipole method of Furgerson and Keller (1975) can be simply derived, and their geometric mean is shown to be the quadripole‐quadripole value.
DIP‐DOMAIN MIGRATION OF TWO‐DIMENSIONAL SEISMIC PROFILES
Authors: Robinson John C. , Robbins Thomas R.
Published: October 2002

Two‐dimensional migration of seismic time sections by a digital computer has been performed by a variety of techniques, ranging from computer‐simulated hand migration to a computer simulation of elastic wave propagation from the surface receiver positions back into the subsurface. Past processes have utilized the normal moveout equation, whereas the subject process is an application of its spatial derivative. “Dip‐domain” migration pertains to special sections where all events have the same space‐time slope, and the migration at each slope is approximated by a trivial function of the subsurface rms velocity and the associated vertical two‐way traveltime. The actual process is to (1) dip decompose a seismic section into so‐called “dip blocks” through the applications of appropriate dip‐discriminating coherency functions, (2) migrate the individual dip blocks, and (3) recompose the migrated dip components (dip blocks). The dip‐discriminating coherency functions are computed from a combination of an “algebraic” and an “absolute” dipping spatial mix of the seismic time section. The input seismic data are not themselves spatially mixed on the dip blocks, but, rather, they are reduced in amplitude by the coherency functions for the dip blocks of which they are not a coherent component. Migration formulas are developed in the Appendix for both spatially invariant and variable subsurface velocity functions. The subject process has some computational advantages. In addition, improved signal‐to‐noise ratios have consistently been obtained due to the dip bandpass (fan) filtering inherent to the process. Synthetic and field seismic examples are included.
DIRECT CURRENT RESISTIVITY MODELING FOR AXIALLY SYMMETRIC BODIES USING THE FINITE ELEMENT METHOD
Authors: Bibby H. M.
Published: October 2002

The finite element method is used to determine numerically the apparent resistivity anomaly caused by the presence of any body with a vertical axis of symmetry embedded in a uniform half‐space. The potential for a point source of current, and hence the apparent resistivity, is determined in the form of a Fourier series. The use of the finite element method enables certain classes of resistivity anisotropy to be modeled. Several examples of bipole‐dipole apparent resistivity enable us to examine assumptions that are necessarily made when inhomogeneities are approximated by models for which explicit solutions exist for the potential. An application to the Broadlands geothermal field suggests that the horizontal cross‐sectional area of the geothermal reservoir increases with depth, consistent with a decrease in the permeability with depth.
MINERAL DISCRIMINATION AND REMOVAL OF INDUCTIVE COUPLING WITH MULTIFREQUENCY IP
Authors: Pelton W. H. , Ward S. H. , Hallof P. G. , Sill W. R. , Nelson P. H.
Published: October 2002

In‐situ complex resistivity measurements over the frequency range [Formula: see text] to [Formula: see text] have been made on 26 North American massive sulfide, graphite, magnetite, pyrrhotite, and porphyry copper deposits. The results reveal significant differences between the spectral responses of massive sulfides and graphite and present encouragement for their differentiation in the field. There are also differences between the spectra of magnetite and nickeliferrous pyrrhotite mineralization, which may prove useful in attempting to distinguish between these two common IP sources in nickel sulfide exploration. Lastly, there are differences in the spectra typically arising from the economic mineralization and the barren pyrite halo in porphyry copper systems. It appears that all these differences arise mainly from mineral texture, since laboratory studies of different specific mineral‐electrolyte interfaces show relatively small variations. All of the in‐situ spectra may be described by one or two simple Cole‐Cole relaxation models. Since the frequency dependence of these models is typically only about 0.25, and the frequency dependence of inductive electromagnetic coupling is near 1.0, it is possible to recognize and to remove automatically the effects of inductive coupling from IP spectra. The spectral response of small deposits or of deeply buried deposits varies from that of the homogeneous earth response, but these variations may be readily determined from the same “dilution factor” [Formula: see text] currently used to calculate apparent IP effects.
VELOCITY ESTIMATION AND DOWNWARD CONTINUATION BY WAVEFRONT SYNTHESIS
Authors: Schultz Philip S. , Claerbout Jon F.
Published: October 2002

A “wave stack” is any stack over a common shot or geophone gather in which the moveout is independent of time. It synthesizes a particular wavefront by superposition of the many spherical wavefronts of raw data. Unlike the common midpoint stack, wave stacks retain the important property of being the sampling of a wave field and, as such, permit wave‐equation treatment of formerly difficult or impossible problems. Seismic sections of field data generated by wave stacks that synthesized slanted downgoing plane waves showed a similarity in appearance to the common midpoint stacks. In signal‐to‐noise ratio they lay between the single offset section and the midpoint stack. The angle selectivity of the slanted plane‐wave stacks permitted detection of a reflector that was not visible on either the midpoint stack or the raw gathers. Simple velocity estimation in slant frame coordinates differs only in detail from standard frame coordinates. Because of the wave field character of data that have been slant plane‐wave stacked, wave‐equation techniques can be used to generalize migration and velocity estimation to regions in which exist a strong lateral velocity inhomogeneity within the distance of a cable spread.
THE DEBUBBLING OF MARINE SOURCE SIGNATURES
Authors: Wood L. C. , Heiser R. C. , Treitel S. , Riley P. L.
Published: October 2002

We propose two novel approaches for the removal of bubble oscillations from marine seismograms. Both methods involve filters containing matched as well as least‐squares inverse components. The techniques are deterministic rather than statistical because we require explicit knowledge of the source pulse shape such as a far‐field signature. The first approach leads to the design of a matched filter in cascade with a two sided Wiener‐shaping operator. The second begins with the computation of the zero‐delay Wiener filter that is inverse to the source signature and continues with the autocorrelation of the resulting output. This performance is illustrated with seismic field data. Our techniques are general ones, applicable in principle to any type of source; however, the examples displayed use Maxipulse and Aquapulse seismic records. Acoustic oscillations caused by successive expansions and contractions of gas bubbles initiated by the seismic source can be attentuated effectively with these filters, and the process has been called debubbling by other investigators. Our procedures improve the definition of seismic reflections (resolution), and the debubbling procedure is a first step in data processing prior to applying other deconvolution and pulse compression techniques.
WAVEFRONT CURVATURE IN A LAYERED MEDIUM
Authors: Ursin Bjørn
Published: October 2002

For a layered elastic medium consisting of horizontal layers with constant velocity as shown in Figure 1, the wavefront radius of curvature is derived as a function of the source‐receiver separation. Newman (1973) derived the radius of curvature as a function of the ray parameter. We shall express the radius of curvature and the source‐receiver separation as functions of the ray parameter which will subsequently be eliminated to give the desired result. A similar technique has been used by Taner and Koehler (1969) to derive an approximate expression for the traveltime of the reflected waves in the same geometric model. The derived expression for the wavefront radius of curvature may be used to compensate for divergence effects in order to derive amplitude information from seismic data, as discussed by Newman (1973).
On: “Seismic Raypath Migration with the Pocket Calculator”, by Paul Michaels (GEOPHYSICS, August 1977, p. 1056–1063)
Authors: Hubral Peter
Published: October 2002

Many structural interpreters will welcome the calculator program for performing a time‐to‐depth migration of a seismic reflection or diffraction (as observed on a zero‐offset or stacked section) in a vertically inhomogeneous velocity medium.
Effect of hydrophone arrays on offshore Texas seismic signals
Authors: Arnold M. E.
Published: October 2002

The effect of hydrophone arrays in the recording of seismic signals during offshore Texas seismic marine experiments is judged by comparing traces of spatially tapered hydrophone array signals with traces that are combinations of simultaneously recorded wavetest hydrophone signals. Each spatially tapered hydrophone group array consists of 26 hydrophones nonuniformly spaced over 212 ft. The wavetest streamer section consists of 36 groups of two hydrophones, each pair connected in parallel and with hydrophones back‐to‐back for acceleration cancellation, with 5-ft spacing between groups. Reflection from deep subsurface interfaces are negligibly affected by hydrophone arrays except for very long arrays and/or long‐range distances. Consequently, the report is primarily concerned with the effects of simulated and real hydrophone arrays on first‐arrival signal and early subbottom reflections. Comparison of theoretical and actual seismic traces from an Aquapulse source for near range distances (835 ft) used in normal operations indicates that (1) near‐simultaneous arrival of the direct wave and surface reflection result in their virtual cancellation, (2) the early event with largest amplitude is associated with constructive interference between source and receiver ghost reflections, and (3) the “pseudo‐bubble” period effectively fixed the predominant frequency of all seismic events at values near 28 Hz. At medium range distances (4755 ft), such comparisons indicate that (1) first arrivals are refracted waves traveling in subbottom layers; (2) the water‐bottom reflection is beyond critical angle and is, therefore, complex; (3) the early events with largest amplitude are multiple reflections; and (4) at least two orders of water‐bottom multiples are identified. The attenuation of the high‐amplitude, first‐arrival signal that includes the water‐bottom reflection permits greater dynamic range in field recording and higher levels of “true” amplitude for later reflections without overload distortion of early events on playback. However, if improved resolution of reflection from moderate depths (∼4000 ft) is important, then arrays of length studied in this report (∼200 ft) should not be used to record signals at range distances greater than about 2000 ft because frequencies above 50 Hz are attenuated severely. Spectral analysis of wavetest records in the absence of signals shows that the wavenumber distribution of the noise is located along a slope line equivalent to 5000 ft/sec between wavenumbers that imply a spectral distribution of 30 to 100 Hz. Theoretical array response studies show that both the 36‐element Chebyshev array and the 26‐element spatially tapered array are superior to a 36‐element uniformly weighted array in rejection of seismic noise in the spectral range of 30 to 100 Hz.
Diurnal drift removal from aeromagnetic data using least squares
Authors: Yarger Harold L. , Robertson Robert R. , Wentland Robert L.
Published: October 2002

A relatively simple method of diurnal drift removal from aeromagnetic data which does not require the use of a base station is presented. The underlying assumption is that diurnal drift during flight is a smoothly varying low order polynomial in time. The polynomial coefficients are determined by minimizing residuals at flight‐line/tie‐line intersections using least squares. This procedure is applied to a conventional sensitivity aeromagnetic survey in northeastern Kansas. The results of the drift determinations compare favorably with independent knowledge of actual drifts such as the magnetic K indices and other measurements.
Magnetotelluric data analysis: removal of bias
Authors: Goubau Wolfgang M. , Gamble Thomas D. , Clarke John
Published: October 2002

Two new techniques for analyzing 4‐channel magnetotelluric (MT) data are described. These techniques produce estimates of the elements [Formula: see text] of the impedance tensor that are unbiased by noise in the autopowers of the electric and magnetic fields. Effectively, each technique uses one field channel as a reference signal that can be correlated with the other three channels. Method 1 obtains estimates for the [Formula: see text] in terms of crosspowers of the Fourier components of the electric and magnetic fields [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]. Method 2 is a generalization of method 1, and obtains estimates for [Formula: see text] in terms of weighted crosspowers. Both methods fail when the geology is one‐dimensional, or two‐dimensional with one electrode oriented along the strike direction. To obtain results that are stable for any geology and that are unbiased by autopower noise, at least five channels of data are required. To also minimize bias by correlated noises, one needs six channels of data, two channels of which are for fields measured at a site that is remote from the base MT station. The analysis of MT data using a remote magnetometer as a reference is discussed.
Resistivity‐porosity‐particle shape relationships for marine sands
Authors: Jackson P. D. , Smith D. Taylor , Stanford P. N.
Published: October 2002

A laboratory investigation has been made of formation factor‐porosity relationships (formation factor being the ratio of the resistivity of a porous medium to the resistivity of the pore‐fluid), using natural and artificial sand samples whose grains varied widely in both size and shape. All samples obeyed Archie’s law, [Formula: see text] (where FF is the formation factor and n is the porosity) including mixtures of two differently shaped particle types. The exponent m was dependent on the shape of the particles, increasing as they became less spherical, while variations in size and spread of sizes appeared to have little effect. The results have been combined to produce an FF/n relationship, with an error “envelope”, which may be applicable to marine sediments in general, being in agreement with published data for marine clays. It is also suggested that the exponent m may be a better measure of the “tortuosity” of porous media than the formulas quoted in the literature.
Wave equation migration with the phase‐shift method
Authors: Gazdag Jenö
Published: October 2002

Accurate methods for the solution of the migration of zero‐offset seismic records have been developed. The numerical operations are defined in the frequency domain. The source and recorder positions are lowered by means of a phase shift, or a rotation of the phase angle of the Fourier coefficients. For applications with laterally invariant velocities, the equations governing the migration process are solved very accurately by the phase‐shift method. The partial differential equations considered include the 15 degree equation, as well as higher order approximations to the exact migration process. The most accurate migration is accomplished by using the asymptotic equation, whose dispersion relation is the same as that of the full wave equation for downward propagating waves. These equations, however, do not account for the reflection and transmission effects, multiples, or evanescent waves. For comparable accuracy, the present approach to migration is expected to be computationally more efficient than finite‐difference methods in general.
Resolving capabilities of the inductive methods of electroprospecting
Authors: Kaufman A. A.
Published: October 2002

The relative sensitivities of the active and reactive components of a harmonic field are considered as well as that of transient fields to changes in the resistivity and dimensions of a conducting body. It is shown that the active and reactive components of a harmonic field have different resolving capabilities and, as such, different signal/noise ratios. The resolving capabilities of time and frequency domain techniques are compared in both high and low frequency ends of the spectrum, as well as early and late stages of the transient. The useful signal and the “geologic noise” are approximated by two conducting bodies immersed in an otherwise nonconducting medium.
Radar probing of Victorio Peak, New Mexico
Authors: Dolphin L. T. , Beatty W. B. , Tanzi J. D.
Published: October 2002

Ground‐penetrating radar and resistivity measurements were made on 20–31 March 1977 in support of limited exploration and excavation of Victorio Peak, White Sands Missile Range, New Mexico, by Expeditions Unlimited. The survey by SRI International (formerly Stanford Research Institute) confirms the existence of large caverns beneath the mountain as well as tunnels, fissures, and other features of geophysical interest, pertinent to the legends that a treasure cache was discovered within the peak in 1937. The uniqueness of the radar work reported is a result of lower‐than‐average radio‐frequency attenuation in the peak permitting sounding to depths as great as 400 ft.
Introduction
Authors: Keller George V.
Published: October 2002

The first exploration for geothermal energy in the U.S. apparently took place during the early 1920s in California and at Yellowstone Park in Wyoming. Despite this early start, by the 1950s, commercially developable geothermal steam had been identified only at one location, The Geysers in California. In the past 10 years, interest in developing geothermal energy has advanced rapidly. Many tens of wildcat wells have been drilled in the search for new energy sources. Many more tens of geothermal prospects are being evaluated. This evaluation of new prospects is beginning to have an impact on the science of geophysical exploration. Some geothermal reservoirs can be located merely by drilling close to thermal manifestations such as geysers and fumiroles. In many other cases, leakage of water in hot springs is far removed from the reservoir in which the hot fluids are present, and it is conceivable that many geothermal systems may exist without any surface manifestation. Development of these more or less hidden geothermal systems will require the services of geologists, geochemists, and geophysicists.
Alternative derivation of the Claerbout equations
Authors: Foster M. R. , Laird C. E.
Published: October 2002

This paper is basically an extension to higher dimensions of the one‐dimensional theory developed by Foster (1975). We begin by briefly reviewing these ideas. In one dimension, the equation of motion for the particle displacement u is given by [Formula: see text]where [Formula: see text] is the vertical traveltime, and [Formula: see text] is the reflectivity function. ρ and c are, respectively, the density and compressional velocity.
Technique for correcting for overburden effects in ground level radiometric surveys of uranium ore bodies
Authors: Dickson B. L. , Clark G. J. , McGregor B. J.
Published: October 2002

An experimental study of the effect of barren overburden on the gamma‐ray spectrum of uranium ore has been made. This has shown that counts in either of the two energy windows 0.56 to 0.64 MeV or 0.66 to 1.01 MeV may be used to correct for the decrease in total count rate as a result of overburden. The correction is limited to an overburden thickness‐density product of approximately [Formula: see text], which is equivalent to a correction factor of 16. Theoretical spectra have been modeled for a geometry similar to that used in the experiment and agree in the changes in spectral shape with overburden thickness. The calculations have also been used to evaluate the effects of ore thickness and to determine the useful limits of the correction factors. The method is mainly applicable to a mining environment where sufficiently high count rates exist.
Seismic imaging of deep crust
Authors: Phinney Robert A. , Jurdy Donna M.
Published: October 2002

We introduce here an integral two‐dimensional (2-D) scheme for the processing of deep crustal reflection profiles. This approach, in which migration occurs before stacking, is tailored to the unique character of the data in which nonvertically propagating energy is as important as vertically propagating energy. Since reflector depths range beyond 30 km, the horizontal displacement of reflections which occurs in migration can be as large as reflector depths; under these circumstances, the common‐midpoint (CMP) stack is inadequate. In our scheme, each common‐source trace gather is transformed into a set of traces (beams) corresponding to set of different incidence angles. A correction for wavefront curvature similar to the normal moveout (NMO) correction yields traces (focused beams) which are focused at image points along the direction of arrival. While the method is equivalent to the Kirchhoff integral migration method, and therefore to any complete continuation method, it gives rise to an intermediate data set which is characterized by the direction of arrival of the upward propagating energy. By a geometrical transformation of the beams and summation, we may synthesize images composed of a specified range of Fourier spatial components. Geologic examples suggest that complex structures in the basement may be most easily characterized by their local direction of layering, a quantity we may determine by this approach. Noise‐free synthetic data examples illustrate the limits of horizontal and vertical resolving power at mid‐crustal depths for any imaging method. Velocity determination is difficult at these depths due to the small NMO and may be possible only by evaluating the effects of velocity models on the imaged data. Examples of the imaged section from the COCORP test profile in Hardeman County, Texas, show a combination of horizontally continuous reflectors and an irregular pattern of scatterers with locally horizontal layering.
Analytical expressions for gravity anomalies due to homogeneous polyhedral bodies and translations into magnetic anomalies
Authors: Okabe M.
Published: October 2002

Complete analytical expressions for the first and second derivatives of the gravitational potential in arbitrary directions due to a homogeneous polyhedral body composed of polygonal facets are developed, by applying the divergence theorem definitively. Not only finite but also infinite rectangular prisms then are treated. The gravity anomalies due to a uniform polygon are similarly described in two dimensions. The magnetic potential due to a uniformly magnetized body is directly derived from the first derivative of the gravitational potential in a given direction. The rule for translating the second derivative of the gravitational potential into the magnetic field component is also described. The necessary procedures for practical computer programming are discussed in detail, in order to avoid singularities and to save computing time.
Integration of aeromagnetic data acquired at different times with varying elevations and line spacings
Authors: Bhattacharyya B. K. , Sweeney R. E. , Godson R. H.
Published: October 2002

The methods required for integrating magnetic data sets from surveys flown with contrasting specifications at different times are described. The first requirement in the process of integration is to bring the data sets obtained at different elevations to the same reference level. Various methods are available to continue magnetic fields from one horizontal plane to another. The procedure for continuation of the magnetic field from a draped surface to a constant barometric level requires an equivalent source representation of the field at all points of observation. Data sets continued to the same elevation over adjacent areas are found to exhibit mismatch along the area boundaries. The mismatch between data values along profiles running across a boundary can be explained by changes in level, gradient, and curvature of the fields in the two data sets in the neighborhood of the boundary. The method discussed for making the two fields compatible depends, basically, upon the adjustment of quadratic surfaces representing the fields on both sides of a boundary. Examples are also provided to demonstrate the usefulness of these methods in the preparation of an integrated data set at a reference level using data sets flown at different elevations above the ground surface in the state of Nevada.
Theory of microwave dielectric constant logging using the electromagnetic wave propagation method
Authors: Freedman R. (Bob) , Vogiatzis John P.
Published: October 2002

The composite dielectric constants of earth formations at microwave frequencies are strongly dependent on formation water saturations and relatively independent of water salinities. Therefore, microwave frequency dielectric constant logging offers an attractive new electromagnetic (EM) method of formation evaluation. The EM wave propagation method of dielectric constant logging attempts to deduce the dielectric properties of earth formations from phase shift and attenuation measurements of EM field, which have been propagated in the formation. A device which utilizes this method of well logging has been proposed by Calvert (1974) and Rau (1976) in two recent U.S. patents. We discuss the basic physics underlying the operation of a device of this type and describe the plane wave procedure discussed by these authors for relating the phase shift and attenuation measurements made by such a device to the formation dielectric properties. This procedure is suspect, since it is based on an unrealistic plane wave model which fails to treat the radiation field correctly and ignores the presence of a layer of mud cake which separates the antenna pad from the formation. To determine the errors likely to be inherent in using this procedure in practice, we consider several simple theoretical models of an EM wave propagation tool. Computer experiments performed on these theoretical models indicate that the apparent formation traveltimes obtained by using this procedure are semiquantitatively accurate with relative errors less than five percent in most cases. For our theoretical models, correction plots or departure curves are demonstrated which enable one to deduce the true formation traveltimes, given the apparent values and a knowledge of the dielectric properties and thickness of the mud cake. The problems which remain if this new method of logging is to attain its full potential (e.g., the accurate determination of formation fluid saturations) are discussed.
A new computerized method for finding effective velocity from reversed reflection traveltime data
Authors: Kaila K. L. , Krishna V. G.
Published: October 2002

A new computerized method is developed using a pair of direct and reversed reflection traveltime curves from two reciprocal shotpoints for finding effective velocity [Formula: see text] appropriate to the depth of the common‐reflection‐point (CRP) for a dipping reflector. For the two sets of traveltime data, least‐square fits are made to [Formula: see text] versus [Formula: see text], where T is the traveltime for recording distance X, θ is the dip of the reflector, and d is the perpendicular distance from the shotpoint to the reflector. The observed convergence of d for any assumed reflector dip θ makes it possible to scan through a wide range of θ values by an iterative process until the velocities computed by the direct and reversed traveltime data agree within the required tolerance limit, which then yields the appropriate effective velocity, and the corresponding θ is the true dip of the reflector. This method simultaneously yields an optimum migrated depth cross‐section. It is also possible to study, by this method, velocity distribution in both vertical and lateral directions in the subsurface area under investigation.