bluuebunny/crossref_metadata_2025 huggingface dataset:
Important design considerations for inboard airborne magnetic gradiometers
Authors: Hardwick C. D.
Published: October 2002

The advantages of magnetic gradiometry as an adjunct to total field mapping are generally recognized and a few aircraft have been equipped with gradiometers. These gradiometers are derived from high‐sensitivity total‐field magnetometer systems that are in themselves subject to certain errors that can usually be tolerated in conventional surveys. However, in a gradiometer, where very large total‐field values are differenced, these errors can, in many cases, greatly exceed the basic accuracy required of the system. There are two principal sources of error in inboard gradiometer systems. The first, and most significant, results from the inevitable magnetic interference of the aircraft or from the inability of currently available compensation systems to deal with the magnetic interference adequately. Passive methods of compensation are not sufficiently comprehensive for gradiometry and the active compensation systems currently in use, which were designed for military applications, cannot guarantee compensation at zero frequency (dc) or at the very low frequencies of interest to the geophysicist concerned with long‐wavelength anomalies. The second source of error is the frequency‐counting technique usually employed to convert a Larmor frequency to ambient total field. The counting process is somewhat analogous to digital sampling at a relatively low rate and as such affords little protection against aliasing from higher frequency interference sources, including components at aircraft maneuvering frequencies. This Paper, using examples, illustrates the two types of error. A list of design criteria is presented and several techniques are described for realizing these criteria. Finally, compensation and survey line results are shown for a three‐axis gradiometer system in the National Research Council of Canada’s Convair 580. This aircraft uses nonoriented cesium magnetometers, one in each wingtip and one at the tip of the tail fin. Compensations over the entire normal maneuver envelope of the aircraft on all headings give typical standard deviation errors of 3 mγ/m from dc to 1 Hz. Thus, the system is capable of measuring gradients down to nongeologic background levels.
Non‐oriented cesium sensors for airborne magnetometry and gradiometry
Authors: Hardwick C. D.
Published: October 2002

Optically pumped magnetometers are characterized by an optimal angle between their optical axes and the direction of the magnetic field they are sensing. Departure from the optimal angle causes a shift in the Larmor frequency with a corresponding error in the scalar value of the magnetic field being measured. To minimize this error, magnetometers are conventionally either mounted in multiple sensor clusters such that the errors tend to cancel, or they are mechanically oriented to maintain the optimal angle with respect to the magnetic field vector. Recent cesium vapor magnetometers using a split‐beam technique have a sufficiently flat error characteristic that they can be flown in a non‐oriented or “strap‐down” configuration. This configuration has advantages with respect to conventional methods in terms of reduced size and weight and of greatly reduced cost. This paper describes two fixed orientations for a particular split‐beam magnetometer and calculates the allowable maneuver envelope for all dip angles from 0 to 90 degrees. It is shown that the residual orientation errors can best be handled by the conventional type of magnetic interference compensation model that must, in any case, be implemented in digital form for high‐sensitivity magnetometry or for any type of gradiometry. The National Aeronautical Establishment (NAE) of the National Research Council of Canada has flown strap‐down magnetometers in a three‐axis gradiometer array in a Convair 580 for several years. Results for the entire normal maneuver envelope of the aircraft, including 30 degree bank turns, have equalled or surpassed those obtained with oriented magnetometers. Several typical maneuver compensation results are presented that gave root‐mean‐square (rms) (one‐sigma) residual errors as low as 0.03 gammas (γ) for total field and 3.5 mgammas/m (mγ/m) for lateral gradient.
Results from open‐hole and cased‐hole vertical seismic profiles
Authors: Lewkowicz J. F. , Reischman Richard , Walsh John J.
Published: October 2002

Data and preliminary results are presented from two vertical seismic profiles (VSPs) performed first in an open well and subsequently after the well was cased. Correlation between the VSPs and a seismic line recorded at a distance of 2 184 ft from the well are examined. Field results indicated that recorded waveforms were degraded in intervals of the well where cement bonding was poor, as might be expected. Analysis of the data revealed tube waves present in the cased‐hole VSP.
The horizontal loop electromagnetic response of a thin plate in a conductive earth: Part II—Computational results and examples
Authors: Hanneson J. E. , West G. F.
Published: October 2002

EM response profiles and Argand diagrams of the peak profile response have been calculated for the horizontal loop (Slingram) system and a large vertical plate target conductor in a conductive host medium. Five host models—a homogeneous half‐space and four two‐layer earths—simulate a range of field conditions and survey parameters. As the host medium conductivity is increased, the initial effect on the Argand diagram of the Slingram anomaly is a slight counterclockwise phase rotation and anomaly enhancement due to channeling of current from the host medium into the conductor. Further increases in host conductivity lead to strong clockwise phase rotations and greater anomaly enhancement. Clockwise phase rotations continue, but eventually the anomaly becomes attenuated when propagation losses suffered by the fields in transit through the host become more significant. When background values from survey profiles are used to appraise the conductivity layering of the host, appropriate Argand diagrams can be chosen to obtain from anomaly profiles estimates of conductor depth and conductance which are independent of prospecting frequency. Three case histories support this claim, one of which gives good agreement between model and field data over four octaves of frequency.
Accuracy of finite‐difference and finite‐element modeling of the scalar and elastic wave equations
Authors: Marfurt Kurt J.
Published: October 2002

Numerical solutions of the scalar and elastic wave equations have greatly aided geophysicists in both forward modeling and migration of seismic wave fields in complicated geologic media, and they promise to be invaluable in solving the full inverse problem. This paper quantitatively compares finite‐difference and finite‐element solutions of the scalar and elastic hyperbolic wave equations for the most popular implicit and explicit time‐domain and frequency‐domain techniques. In addition to versatility and ease of implementation, it is imperative that one choose the most cost effective solution technique for a fixed degree of accuracy. To be of value, a solution technique must be able to minimize (1) numerical attenuation or amplification, (2) polarization errors, (3) numerical anisotropy, (4) errors in phase and group velocities, (5) extraneous numerical (parasitic) modes, (6) numerical diffraction and scattering, and (7) errors and transmission coefficients. This paper shows that in homogeneous media the explicit finite‐element and finite‐difference schemes are comparable when solving the scalar wave equation and when solving the elastic wave equations with Poisson’s ratio less than 0.3. Finite‐elements are superior to finite‐differences when modeling elastic media with Poisson’s ratio between 0.3 and 0.45. For both the scalar and elastic equations, the more costly implicit time integration schemes such as the Newmark scheme are inferior to the explicit central‐differences scheme, since time steps surpassing the Courant condition yield stable but highly inaccurate results. Frequency‐domain finite‐element solutions employing a weighted average of consistent and lumped masses yield the most accurate results, and they promise to be the most cost‐effective method for CDP, well log, and interactive modeling.
On: “Reflection of spherical seismic waves in elastic layered media” by P. M. Krail and H. Brysk (GEOPHYSICS, June 1983, p. 655–664).
Authors: Danbom Stephen H. , Domenico S. Norman
Published: October 2002

This paper develops the wavefront curvature correction for the plane‐wave Zoeppritz coefficients. The authors imply that seismic reflection amplitude versus angle of incidence does not enhance detection of pore‐fluid‐caused amplitude anomalies (“bright spots”) as proposed in the scientific literature by Backus et al (1982), Ostrander (1982), and Stolt (1981). We believe certain parts of this discussion are incorrect. Points of disagreement are as follows. (1) The model parameters associated with Figure 6 (especially the values for the brine‐filled sandstone) are unrealistic and are not taken from Domenico (1977) as the authors have indicated. (2) The conclusion “… it is clear that the CMP gather by itself fails as a gas indicator” based on Figure 6 is incorrect. (3) A reflecting spherical wave having a radius of curvature equal to approximately one‐half wavelength (kr = 3) requires a shallow reflector unreasonable for petroleum exploration.
Toward a three‐dimensional automatic interpretation of potential field data via generalized Hilbert transforms: Fundamental relations
Authors: Nabighian Misac N.
Published: October 2002

The paper extends to three dimensions (3-D) the two‐dimensional (2-D) Hilbert transform relations between potential field components. For the 3-D case, it is shown that the Hilbert transform is composed of two parts, with one part acting on the X component and one part on the Y component. As for the previously developed 2-D case, it is shown that in 3-D the vertical and horizontal derivatives are the Hilbert transforms of each other. The 2-D Cauchy‐Riemann relations between a potential function and its Hilbert transform are generalized for the 3-D case. Finally, the previously developed concept of analytic signal in 2-D can be extended to 3-D as a first step toward the development of an automatic interpretation technique for potential field data.
Kirchhoff integral extrapolation and migration of nonplanar data
Authors: Wiggins J. W.
Published: October 2002

The form of the Kirchhoff integral commonly used for migration of seismic data assumes that the observation surface is a flat, horizontal plane. This restriction is not necessary in theory or practice. A useful integral expression can be obtained for an arbitrary observation surface by use of the Kirchhoff approximation for relating the field on the observation surface to its normal derivative. Kirchhoff migration is usually carried out by an integral that represents summation over receivers for a given shot. Summation over shots is not explicitly handled. By invoking the principle of reciprocity and applying the Kirchhoff integral twice, one arrives at an integral expression that explicitly represents shot and receiver summation, contains the factors needed to compensate for nonplanar survey geometry, and can be used for wave extrapolation as well as migration. Adaptation of this integral to handling 2-D or line survey data is straightforward and, with the usual approximations concerning spherical wave spreading, it accomplishes 2-D wave extrapolation and migration. By tabulating both average and rms velocity functions, the Kirchhoff integral can be used to handle cases where the velocity changes vertically and the survey surface has vertical relief. The approximations involved are only those usually required in calculations of rms velocity traveltime. Migration is illustrated by treatment of two model data sets. One data set has shots and receivers extended horizontally with the receiver path undergoing strong elevation changes and the shot path flat. Migration of this data set illustrates the effect of the geometrical weighting factors and the effectiveness of the rms velocity approximation. The second data set is a vertical seismic profile (VSP) with shots extended horizontally and receivers extended vertically down a single well. Migration of this VSP data yields a useful image for a reasonable distance away from the well. Migration of nonplanar survey data is quite practical with geometrical artifacts that are manageable. The technique has use for surveys taken over nonplanar topography, for imaging from VSP data, and in a dual role for treatment of velocity anomalies, i.e., first as a wave extrapolation procedure to move below the anomaly and then as a migration procedure applied to the nonplanar extrapolated data.
Inversion of seismic reflection data in the acoustic approximation
Authors: Tarantola Albert
Published: October 2002

The nonlinear inverse problem for seismic reflection data is solved in the acoustic approximation. The method is based on the generalized least‐squares criterion, and it can handle errors in the data set and a priori information on the model. Multiply reflected energy is naturally taken into account, as well as refracted energy or surface waves. The inverse problem can be solved using an iterative algorithm which gives, at each iteration, updated values of bulk modulus, density, and time source function. Each step of the iterative algorithm essentially consists of a forward propagation of the actual sources in the current model and a forward propagation (backward in time) of the data residuals. The correlation at each point of the space of the two fields thus obtained yields the corrections of the bulk modulus and density models. This shows, in particular, that the general solution of the inverse problem can be attained by methods strongly related to the methods of migration of unstacked data, and commercially competitive with them.
Inversion of borehole normal resistivity logs
Authors: Yang Fang‐Wei , Ward Stanley H.
Published: October 2002

This paper reports on an investigation of the inversion of borehole normal resistivity data via ridge regression. Interpretation is afforded of individual thin beds and of complicated layered structures. A theoretical solution is given for a layered model containing an arbitrary number of layers in the forward problem. Two forward model results for resistive and conductive thin beds indicate that for high‐resistivity contrasts, the departure between true and apparent resistivity may be more important than the effects caused by the variations in borehole diameter and mud resistivity. Four normal resistivity logs were chosen to test the inversion scheme. Two of the logs were theoretical logs with and without random noise added, and the remaining two were field examples. Theoretical model results and field examples indicate that the inverse method can be used to obtain the resistivity for each layer when the boundary position is known, but it also can be used to obtain the thickness and resistivity for each layer simultaneously.
THE COMPUTATION AND MAPPING OF SEISMIC REFLECTION DATA
Authors: Widess M. B. , Haskell N. A.
Published: October 2002

The fundamental computation equations for dip shooting are reviewed. From a consideration of the curvature of the wave front of the reflected wave an expression is derived for the angle of arrival in terms of the reflection time difference over a finite spread. The effect of curvature of the reflecting bed is discussed. An outline is given of a rigorous computational procedure which takes full consideration of the three dimensional aspect of seismic interpretation and which is based on the use of time cross‐sections and maps. Since this procedure is presented on the basis of a velocity equation which is a function only of depth, it is indicated that a modification must often be made to account for the deviation of the direction of velocity gradient from the vertical.
THE HISTORY AND DEVELOPMENT OF SEISMIC PROSPECTING
Authors: Weatherby B. B.
Published: October 2002

An historical outline of the development of modern seismic prospecting methods is presented. Particular attention is given to a review of the very early work which laid the foundations upon which a rapid growth of the art took place when the facilities for adequate instrumentation became available.
A BRIEF HISTORY OF THE GRAVITY METHOD OF PROSPECTING FOR OIL
Authors: Eckhardt E. A.
Published: October 2002

An historical outline of the development of the gravity method of prospecting is presented. While the very early gravity work with pendulum equipment was for geodetic purposes alone, the increased instrumental sensitivity made available with the development of the torsion balance by Eoetvoes (1888), led in 1915, to the first gravimetric survey for oil prospecting in the Egbell field, Czechoslovakia. The application of the method to prospecting in the United States started in 1924 with the discovery of the Nash dome in Coastal Texas by a torsion balance survey. The development and use of improved pendulum equipment is discussed and finally the recent marked increase in gravity work resulting from the introduction of the gravimeter about 1930 to 1935.
FIFTEEN YEARS OF GEOPHYSICS: A CHAPTER IN THE EXPLORATION OF THE UNITED STATES AND CANADA, 1924–1939
Authors: Macelwane, James B.
Published: October 2002

Knowledge of geophysical methods of prospecting was not widely spread in America previous to 1924 and it was only late in that year that startling success in the discovery of oil‐bearing structures began to be achieved by them. Hence we may say without exaggeration that a new era of geophysics began with the year 1924.
TIDAL FORCE OBSERVATIONS
Authors: Wolf Alfred
Published: October 2002

Observations of the tidal variation of gravity at Tulsa, Oklahoma, are presented for the period of December 10 to December 16, 1939. A good correlation is found between observed and calculated values with the factor [Formula: see text].
Evaluation of anisotropy by shear‐wave splitting
Authors: Crampin Stuart
Published: October 2002

The polarizations of three‐component shear wavetrains carry unique information about the internal structure of the rock through which they pass: specifically, commonly observed shear‐wave splitting may contain information about the orientation of crack distributions. This information cannot usually be recovered from shear waves recorded at the free surface because of interference with the interaction of the shear wave with the surface, even for nearly vertical incidence. However, shear‐wave splitting in synthetic three‐component vertical seismic profiles, in some cases, may be interpreted directly in terms of the direction of strike of vertical cracks. Because the human eye is not skilled at identifying the phase relationships between three‐component seismograms played out conventionally as parallel time‐series, the polarizations are displayed in orthogonal sections of the particle displacements to facilitate recognition and evaluation of the shear‐wave splitting. Estimating the orientations of cracks, and hence of preferred directions of flow, by seismic investigations could be of crucial importance to production and reservoir engineering.
Inversion with a grain of salt
Authors: Lines Larry R. , Treitel Sven
Published: October 2002

Although least‐squares inversion is a useful tool in data analysis, nonuniqueness is an inevitable problem. This problem can be analyzed by considering the sensitivity of a model response to the parameter estimates. Such sensitivity methods produce extremal solutions which barely satisfy some resolution (or stability) criteria. Two closely related methods for producing such solutions are the “Edgehog” and “Most Squares” methods due to Jackson (1973, 1976). These techniques, which evaluate the “degree of nonuniqueness” in a least‐squares inversion, require only the information computed in a singular value decomposition (SVD) solution. While the “edgehog” and “most squares” techniques are mathematically similar, the “most squares” estimate is the simpler to compute. Both methods show that the credibility of an inversion depends on both the specified error criterion as well as on the properties of the Jacobian matrix associated with the least‐squares solution. The similar performance of these two closely related methods is demonstrated with the traveltime inversion of both synthetic and real vertical seismic profiles (VSPs). The sensitivity analysis of this inversion problem provides a quantitative measure of solution reliability.
Scaling relations in surface polarization modeling
Authors: Eskola Lauri
Published: October 2002

A large number of apparent resistivity and induced‐polarization (IP) model measurements have been performed in laboratories during the past decades using metal conductors immersed in an electrolyte solution. Guptasarma (1983) showed that the surface polarization that occurs on an electrolyte‐metal boundary has, at laboratory scale, a marked influence on apparent resistivity results, if the frequency of the transmitted current is lower than a specific limit and the frequency is dependent upon the dimensions and the physical and electro‐chemical parameters of the model.
Applications of gravity and magnetic surveys: The state‐of‐the‐art in 1985
Authors: Paterson Norman R. , Reeves Colin V.
Published: October 2002

There is a continuous large demand for gravity and magnetic surveys all over the world for a variety of exploration applications, all of which require the geophysicist to provide some new insight into the geology of an area at scales ranging from very large to very small. To achieve this objective, (a) surveys must be carried out accurately, and (b) their results must be interpreted in sympathy with what is already known of the geology. The methodology for acquiring and compiling data appears to be keeping pace with modern technology. Methods of quantitatively interpreting anomalies in terms of models of causative bodies are adapting rapidly to the burgeoning availability of computing power, from large, powerful machines to inexpensive and field‐portable microcomputers. Geologic interpretation, or the identification of physical property distributions in terms of realistic geologic models and processes, is still relatively neglected—in practice and, regretably, in the geophysical literature. Research into the relationships between physical rock properties—particularly magnetite distribution—and geology is gaining momentum, but research still lags behind the requirements of the conscientious geophysical interpreter.
The analysis of multichannel airborne gamma‐ray spectra
Authors: Grasty R. L. , Glynn J. E. , Grant J. A.
Published: October 2002

Experimental potassium, uranium, and thorium gamma‐ray spectra covering a range of equivalent aircraft altitudes from 0 to 112 m were derived from measurements on large radioactive concrete calibration pads using plywood sheets to simulate the absorption effects of the air. A mathematical analysis of all three radioelement spectra showed they are composed of two basic spectral components which are added in different proportions dependent on the altitude of the aircraft above the ground. Above an energy of 0.662 MeV, the two‐component spectral model represented accurately the observed gamma‐ray spectrum from the ground. Below this particular energy, cesium-137 from atomic weapons fallout was found to contribute significantly to the airborne spectra. A theoretical analysis of the calculated spectrum from typical crustal material showed that by multispectral fitting above the cesium-137 energy, the thorium and uranium concentration errors could be reduced by approximately 25 percent compared to the standard three‐window method. Similar increases in accuracy were found experimentally by analyzing a series of 1 s airborne data recorded over a uniformly radioactive test strip. The two‐component model was also used to investigate the effect of changing the windows on the standard three‐window method. For typical crustal material, no significant increase in uranium accuracy was achieved by changing the windows from those recommended by the International Atomic Energy Agency.
Scattering characteristics of elastic waves by an elastic heterogeneity
Authors: Wu R. , Aki K.
Published: October 2002

Elastic wave scattering by a general elastic heterogeneity having slightly different density and elastic constants from the surrounding medium is formulated using the equivalent source method and Born approximation. In the low‐frequency range (Rayleigh scattering) the scattered field by an arbitrary heterogeneity having an arbitrary variation of density and elastic constants can be equated to a radiation field from a point source composed of a unidirectional force proportional to the density contrast between the heterogeneity and the medium, and a force moment tensor proportional to the contrasts of elastic constant. It is also shown that the scattered field can be decomposed into an “impedance‐type” field, which has a main lobe in the backscattering direction and no scattering in the exact forward direction, and a “velocity type” scattered field, which has a main lobe in the forward scattering direction and no scattering in the exact backward direction. For Mie scattering we show that the scattered far field is a product of two factors: (1) elastic Rayleigh scattering of a unit volume, and (2) a scalar wave scattering factor for the parameter variation function of the heterogeneity which we call “volume factor.” For the latter we derive the analytic expressions for a uniform sphere and for a Gaussian heterogeneity. We show the relations between volume factors and the 3-D Fourier transform (or 1-D Fourier transform in the case of spherical symmetry) of the parameter variations of the heterogeneity. The scattering spatial pattern varies depending upon various combinations of density and elastic‐constant perturbations. Some examples of scattering pattern are given to show the general characteristics of the elastic wave scattering.
A simplification of the Zoeppritz equations
Authors: Shuey R. T.
Published: October 2002

The compressional wave reflection coefficient R(θ) given by the Zoeppritz equations is simplified to the following: [Formula: see text] The first term gives the amplitude at normal incidence (θ = 0), the second term characterizes R(θ) at intermediate angles, and the third term describes the approach to critical angle. The coefficient of the second term is that combination of elastic properties which can be determined by analyzing the offset dependence of event amplitude in conventional multichannel reflection data. If the event amplitude is normalized to its value for normal incidence, then the quantity determined is [Formula: see text] [Formula: see text] specifies the normal, gradual decrease of amplitude with offset; its value is constrained well enough that the main information conveyed is [Formula: see text] where [Formula: see text] is the contrast in Poisson’s ratio at the reflecting interface and [Formula: see text] is the amplitude at normal incidence. This simplified formula for R(θ) accounts for all of the relations between R(θ) and elastic properties first described by Koefoed in 1955.
An algorithm for computation of resistivity soundings over inhomogeneous layers
Authors: Dasgupta S. P.
Published: October 2002

Calculation of dc resistivity sounding curves for a multilayer earth with transition layers has been treated by several authors since Mallick and Roy (1968). However, derivation of the kernel function for such problems has remained difficult for more than three‐layer models for want of a proper algorithm. The problem was first solved by Pekeris (1942) in the case of uniformly resistive layers. Other forms of recurrence relations for the kernel function of a half‐space containing such homogeneous layers were forwarded by Flathe (1955), Kunetz (1966), and Koefoed (1968). Patella (1977) considered the kernel function for a half‐space which contained a series of alternate layers, one having a linearly varying conductivity with depth while the other was homogeneously conductive. Koefoed (1979a) considered the case of a half‐space containing a transition layer situated anywhere among a series of homogeneous layers and possessing a resistivity that changed linearly with depth. In this article a very general form of algorithm is developed for generating the kernel function for a layered half‐space containing any number of transition layers having an arbitrary resistivity distribution [Formula: see text] in such ith layer. This new general form is very similar to the homogeneous form derived by Pekeris (1942).
An analysis of the spectrum of a gravity anomaly over an inclined dike
Authors: Agarwal B. N. P. , Ramaiah D. Sita
Published: October 2002

Bhimasankaram et al. (1977) used Fourier spectrum analysis for a direct approach to the interpretation of gravity anomaly over a finite inclined dike. They derived several equations from the real and imaginary components and from the amplitude and phase spectra to relate various parameters of the dike. Because the width 2b of the dike (Figure 1) appears only in sin (ωb) term—ω being the angular frequency—they determined its value from the minima/zeroes of the amplitude spectra. The theoretical Fourier spectrum uses gravity field data over an infinite distance (length), whereas field observations are available only for a limited distance. Thus, a set of observational data is viewed as a product of infinite‐distance data with an appropriate window function. Usually, a rectangular window of appropriate distance (width) and of unit magnitude is chosen for this purpose. The Fourier transform of the finite‐distance and discrete data is thus represented by convolution operations between Fourier transforms of the infinite‐distance data, the window function, and the comb function. The combined effect gives a smooth, weighted average spectrum. Thus, the Fourier transform of actual observed data may differ substantially from theoretic data. The differences are apparent for low‐ and high‐frequency ranges. As a result, the minima of the amplitude spectra may change considerably, thereby rendering the estimate of the width of the dike unreliable from the roots of the equation sin (ωb) = 0.
Structural interpretation using refraction velocities from marine seismic surveys
Authors: Mitchell James F. , Bolander Richard J.
Published: October 2002

Subsurface structure can be mapped using refraction information from marine multichannel seismic data. The method uses velocities and thicknesses of shallow sedimentary rock layers computed from refraction first arrivals recorded along the streamer. A two‐step exploration scheme is described which can be set up on a personal computer and used routinely in any office. It is straightforward and requires only a basic understanding of refraction principles. Two case histories from offshore Peru exploration demonstrate the scheme. The basic scheme is: step (1) shallow sedimentary rock velocities are computed and mapped over an area. Step (2) structure is interpreted from the contoured velocity patterns. Structural highs, for instance, exhibit relatively high velocities, “retained” by buried, compacted, sedimentary rocks that are uplifted to the near‐surface. This method requires that subsurface structure be relatively shallow because the refracted waves probe to depths of one hundred to over one thousand meters, depending upon the seismic energy source, streamer length, and the subsurface velocity distribution. With this one requirement met, we used the refraction method over a wide range of sedimentary rock velocities, water depths, and seismic survey types. The method is particularly valuable because it works well in areas with poor seismic reflection data.
Field comparison of shallow seismic sources
Authors: Miller R. D. , Pullan S. E. , Waldner J. S. , Haeni F. P.
Published: October 2002

Choosing a seismic source for a shallow reflection survey can be the most pivotal decision for the engineering geophysicist. The intent of this paper is to present data that will assist in selection of a shallow seismic source best meeting the goals within the constraints of specific projects, particularly in areas where the water table is near the surface. The data were collected (and displayed as seismograms and amplitude spectra) for 15 different shallow seismic sources in October, 1985, at a single site in New Jersey; they show the different characteristics of each source. Considering the almost three orders of magnitude difference in total source energy between the largest and smallest source, we chose a display format that presented the data as objectively as possible, while still allowing direct source‐to‐source comparisons. Two strong reflections at about 100 and 130 ms probably mark the top and bottom of a clay unit 80 m below the surface at this site. Our previous work and that of our colleagues suggests that, given a specific set of site characteristics, any source could dominate the comparison categories addressed here.
Effects of porosity and clay content on wave velocities in sandstones
Authors: Han De‐hua , Nur A. , Morgan Dale
Published: October 2002

The ultrasonic compressional [Formula: see text] and shear [Formula: see text] velocities and first‐arrival peak amplitude [Formula: see text] were measured as functions of differential pressure to 50 MPa and to a state of saturation on 75 different sandstone samples, with porosities ϕ ranging from 2 to 30 percent and volume clay content C ranging from 0 to 50 percent, respectively. Both [Formula: see text] and [Formula: see text] were found to correlate linearly with porosity and clay content in shaly sandstones. At confining pressure of 40 MPa and pore pressure of 1.0 MPa, the best least‐squares fits to the velocity data are [Formula: see text] and [Formula: see text]. Deviations from these equations are less than 3 percent and 5 percent for [Formula: see text] and [Formula: see text], respectively. The velocities of clean sandstones are significantly higher than those predicted by the above linear fits (about 7 percent for [Formula: see text] and 11 percent for [Formula: see text]), which indicates that a very small amount of clay (1 or a few percent of volume fraction) significantly reduces the elastic moduli of sandstones. For shaly sandstones we conclude that, to first order, more sensitive to the porosity and clay content than is [Formula: see text]. Consequently, velocity ratios [Formula: see text] and their differences between fully saturated (s) and dry (d) samples also show clear correlation with the clay content and porosity. For shaly sandstones we conclude that, to first order, clay content is the next most important parameter to porosity in reducing velocities, with an effect which is about 0.31 for [Formula: see text] to 0.38 for [Formula: see text] that of the effect of porosity.
Three‐dimensional transient electromagnetic responses for a grounded source
Authors: Gunderson Brian M. , Newman Gregory A. , Hohmann Gerald W.
Published: October 2002

When the current in a grounded wire is terminated abruptly, currents immediately flow in the Earth to preserve the magnetic field. Initially the current is concentrated near the wire, with a broad zone of return currents below. The electric field maximum broadens and moves downward with time. Currents are channeled into a conductive three‐dimensional body, resulting in anomalous magnetic fields. At early times, when the return currents are channeled into the body, the vertical magnetic field is less than the half‐space field on the far side of the body but is greater than the half‐space field between the source and the body. Later the current in the body reverses; the vertical field is enhanced on the far side of the body and decreased between the source and the body. The horizontal magnetic field has a well‐defined maximum directly over the body at late times, and is a better indicator of the position of the body. The vertical magnetic field and its time derivative change sign with time at receiver locations near the source if a three‐dimensional body is present. These sign reversals present serious problems for one‐dimensional inversion, because decay curves for a layered earth do not change sign. At positions away from the source, the decay curves exhibit no sign reversals—only decreases and enhancements relative to one‐dimensional decay curves. In such cases one‐dimensional inversions may provide useful information, but they are likely to result in fictitious layers and erroneous interpretations.
Acoustic reflections from undulating surfaces
Authors: Hill N. R. , Lerche I.
Published: October 2002

We recently investigated the behavior of acoustic wave reflection in a medium with a single rough interface with acoustic speed [Formula: see text] and density[Formula: see text] above (below) the interface (Lerche and Hill, 1985). We used mean‐field techniques (Howe, 1971; Karal and Keller, 1964; Frisch, 1968) to find a closed‐form expression for the mean reflected field, and in particular we examined the case where the roughness is on a much smaller scale than the wavelength of the incident energy. We derived a closed‐form expression which is accurate to second order in roughness height.
Application of cross‐hole seismic measurements in site investigation surveys
Authors: McCann D. M. , Baria R. , Jackson P. D. , Green A. S. P.
Published: October 2002

Application of the cross‐hole seismic method to site investigation surveys for civil engineering developments is illustrated by case histories from the United Kingdom using the borehole sparker probe developed in McCann et al. (1975) as the seismic source. At Newborough, Anglesey, it is shown that this source generates both a compressional‐wave and a vertically polarized shear‐wave in the borehole; the measured velocity values are used to determine the dynamic elastic moduli of the saturated sand deposit at this site. At Cocking, Sussex and East Fleet, Dorset, the presence of an air‐filled void between two boreholes is shown to have a significant effect on the velocity of propagation and amplitude of the transmitted seismic signals. This work is extended at Maidstone, Kent, where the technique is used to confirm the presence of a cavity below the foundation of a house. In the Hot Dry Rock Geothermal Project at Rosemanowes, Cornwall, increased fracturing of the granitic rock mass following explosive stimulation in one of the exploration boreholes is shown to result in a significant reduction in the compressional‐wave velocity.
Prompt neutron‐gamma logging for coal ash in water‐filled boreholes
Authors: Charbucinski J. , Youl S. F. , Eisler P. L. , Borsaru M.
Published: October 2002

Use of the prompt‐neutron gamma technique to determine the ash content, depth, and thickness of coal strata was successfully demonstrated in water‐filled boreholes at six deposits in eastern Australia. The diameter of the boreholes varied from 96 mm (nominal diameter) to around 175 mm. The root‐mean‐square (rms) deviation between ash determinations based on regression analysis of logging data and laboratory assays for ash content varied between 1.4 and 2.8 weight percent. This variation is partially due to errors in sampling and chemical analysis (the errors are generally unknown). The optimum configuration for the logging probe consists of a 0.7 μg [Formula: see text] neutron source, a 51 × 51 mm BGO detector, and a source‐to‐detector spacing of 15 cm. This configuration gives the best vertical resolution as well as the smallest rms deviation for ash determination. Probe diameter was 70 mm and wall thickness was 3.2 mm. Regression analysis of the logging data was performed for each coal deposit to establish a calibration equation for the deposit. The calibration equations obtained in one of the deposits (Hunter Valley Deposit B) were applied to new boreholes in the same deposit. The rms deviation between the ash content predicted by the technique and the chemical analysis was ≃2 percent, which was consistent with the results for the calibration holes.
Maximum‐likelihood deconvolution: An optimization theory perspective
Authors: Goutsias John , Mendel Jerry M.
Published: October 2002

A large number of deconvolution procedures have appeared in the literature during the last three decades, including a number of maximum‐likelihood deconvolution (MLD) procedures. The major advantages of the MLD procedures are (1) no assumption is required about the phase of the wavelet (most of the classical deconvolution techniques assume a minimum‐phase wavelet, an assumption that may not be appropriate for many data sets); (2) MLD procedures can resolve closely spaced events (i.e., they are high‐resolution techniques); and (3) they can efficiently handle modeling and measurement errors, as well as backscatter effects (i.e., reflections from small features). A comparative study of six different MLD procedures for estimating the input of a linear, time‐invariant system from measurements, which have been corrupted by additive noise, was made by using a common framework developed from fundamental optimization theory arguments. To date, only the Kormylo and the Chi‐t algorithms can be recommended.
Determination of crustal interface topography from potential fields
Authors: Pilkington Mark , Crossley D. J.
Published: October 2002

We present a method for determination of the topography of an interface separating two uniformly but differently magnetized media. The technique is developed within the framework of discrete linear inverse theory and exploits the quasi‐linearity of the problem to derive a simple, efficient algorithm suitable for use on large gridded data sets. Variation of auxiliary parameters allows a suite of acceptable models to be produced rapidly; these parameters can be appraised in light of available geologic and geophysical evidence. Because very large matrices are involved when large amounts of data are interpreted, approximate resolution measures requiring no matrix inversion provide useful information concerning averages of the constructed topography.
Geoelectric structural dimensions from magnetotelluric data: Methods of estimation, old and new
Authors: Beamish David
Published: October 2002

A magnetotelluric (MT” sounding curve obtained at a given location may contain contributions from 1-D, 2-D, or 3-D geoelectric components, either singly or in combination. The modeling and interpretation of such data depend upon the ability to assess the degree of influence exerted by the three possible structural components. Six existing MT sounding curves provide case histories of the performance of both conventional and new methods for estimation of structural dimensions. For data from a single location, all the methods must be based on the horizontal rotation properties of the impedance tensor. Of the three conventional dimensional indicators considered, only one (skew) provides a degree of satisfactory performance for practical data. Three recently introduced, normalized dimensional weights appear to offer better performance. Solutions to the 1-D MT problem are central to the issue of providing dimensional constraints. The inverse theories established by Weidelt (1972) and Parker (1980) provide two tests that can be applied to establish the existence of 1-D solutions. The formalism in these theories provides the basis for a systematic method for investigating the dimensional properties of practical MT data.
Computation of apparent resistivities using an exponential approximation of kernel functions
Authors: Niwas Sri , Israil M.
Published: October 2002

We present a fresh approach to the mathematical computation of apparent resistivities in electrical prospecting. The method is based on an exponential approximation of the kernel function which reduces the integral equation for the potential over a layered earth to a simple algebraic equation. The coefficients in the approximation are obtained using a least‐squares inversion technique. A single, unified matrix equation allows computation of apparent resistivity values for arbitrary four‐electrode arrays over a layered earth. The unified G function automatically reduces to that of a symmetrical four‐electrode array and dipole array function with the proper interelectrode separation. Computations for some two‐, three‐, and four‐layer earth models (Schlumberger configuration), along with a few Wenner and radial dipole apparent resistivity values, demonstrate the versatility of this unified equation.
Magnetic susceptibility anomalies of lithosphere beneath eastern Europe and the Middle East
Authors: Arkani‐Hamed J. , Strangway D. W.
Published: October 2002

Standard inversion methods for inverting aeromagnetic anomalies into magnetic susceptibility contrast in the crust are based on the assumption that the core field is constant over the region considered. This assumption, however, does not hold over regions of a few thousand kilometer extent. An inversion technique is developed which takes into account the variations in the core field, both in direction and in intensity. It can also take into account the surface topography of a magnetic layer within the lithosphere. The technique is applied to a scalar magnetic anomaly map of eastern Europe and the Middle East, derived from the magnetometer satellite (MAGSAT) data. The magnetic susceptibility contrast map thus obtained delineates the major tectonic features of the region. Small‐scale Precambrian shields are characterized by high magnetic susceptibility. The Alpine‐Himalayan belt has a distinct magnetic signature. The collision zones are low‐susceptibility regions, whereas the continental plates within the belt have high magnetic susceptibility. The well‐known iron formation of Kursk is delineated by a strong magnetic susceptibility high. The magnetometer satellite has measured the Earth’s magnetic field at two distinct local times, dawn and dusk, and provided two independent magnetic anomaly maps. The scalar magnetic anomaly map is based on the common features of the two maps. A covariance technique is developed which selects these common features on the basis of the correlation spectra of the two maps.