bluuebunny/crossref_metadata_2025 huggingface dataset:
A CALIBRATED MODEL SEISMIC SYSTEM
Authors: Dampney C. N. G. , Mohanty B. B. , West G. F.
Published: October 2002

Simple electronic circuitry and axially polarized ceramic transducers are employed to generate and detect elastic waves in a two‐dimensional analog model. The absence of reverberation and the basic simplicity. of construction underlie the advantages of this system. If the form of the fundamental wavelet in the model itself, as modified by the linear filtering effects of the remainder of the system, can be found, then calibration is achieved. This permits direct comparison of theoretical and experimental seismograms for a given model if its impulse response is known. A technique is developed for calibration and verified by comparing Lamb’s theoretical and experimental seismograms for elastic wave propagation over the edge of a half plate. This comparison also allows a critical examination of the basic assumptions inherent in a model seismic system.
THE USE OF PSEUDONOISE SEQUENCES TO CODE A PULSED NEUTRON LOGGING SOURCE
Authors: Foster M. R. , Sloan R. W.
Published: October 2002

Pseudonoise coded sources producing a seemingly erratic sequence of closely spaced, low‐energy impulses have been used extensively to probe deterministic systems. Data processing of the system output results in a record which would have been produced by a source having much larger energy impulses at a much greater time spacing. This paper is a study of the merit of this approach in pulsed neutron logging where the system must be regarded as stochastic. It is concluded that for processes with an exponential decay, a pseudonoise coded source gives some improvement over conventionally coded sources for times shorter than the time constant of the process. For longer times it is decidedly inferior.
DOWNWARD CONTINUATION OF MOVEOUT‐CORRECTED SEISMOGRAMS
Authors: Claerbout Jon F. , Doherty Stephen M.
Published: October 2002

Earlier work developed a method of migration of seismic data based on numerical solutions of partial differential equations. The method was designed for the geometry of a single source with a line of surface receivers. Here the method is extended to the geometry of stacked sections, or what is nearly the same thing, to the geometry where a source and receiver move together along the surface as in marine profiling. The basic idea simply stated is that the best receiver line for any reflector is just at (or above) the reflector. Data received at a surface line of receivers may be extrapolated by computer to data at a hypothetical receiver line at any depth. By considering migration before stacking over offset, it is found that certain ambiguities in velocity analysis may be avoided.
COMPUTER GRAPHICS IN GEOPHYSICS
Authors: Smith Robert B. , Warnock John E. , Stanley William D. , Cole E. Randolph
Published: October 2002

It has become increasingly apparent that the large quantities of geophysical data being collected by modern instrumentation must be efficiently stored, processed, and displayed before meaningful interpretation can be undertaken. To meet these needs a man‐machine interactive computing system has been developed to provide the geophysicist with a direct communication link to the digital computer and methods for providing two‐ and three‐dimensional representations of geophysical data on cathode‐ray tubes (CRT). Gravity, magnetic, resistivity, and induced‐polarization responses have been modeled on the interactive system and displayed on a CRT. The interactive modeling processes allow the direct interchange of geophysical variables and data followed by the display of the recomputed models and corresponding theoretical responses. Model parameters can be monitored visually and continuously adjusted for a better solution. In addition, color‐shaded perspective views of three‐dimensional surfaces representing aeromagnetic, gravity, topographic, and correlative data also have been produced. Seismic data processing has been demonstrated for filter synthesis and convolution with interactive communication for program commands and data modification. Processed seismic sections have been displayed on a CRT in two‐dimensional, halftone views.
8‐HZ TELLURIC AND MAGNETOTELLURIC PROSPECTING
Authors: Slankis J. A. , Telford W. M. , Becker A.
Published: October 2002

Telluric and magnetotelluric surveys utilizing natural 8‐hz electromagnetic fields have detected resistivity variations associated with some proven sulfide deposits. From an operational point of view the telluric method is preferred to magnetotellurics. Either method, however, can detect a conducting dike, providing that its width is greater than its depth of burial. For surveys where the telluric field is measured in a direction perpendicular to the strike of the conductor, theoretical profiles derived from two‐dimensional conductivity structures agree well with the field results. For this polarization, it is thus possible to use a general model as an aid to the interpretation of survey data.
APPLICATION OF HILBERT TRANSFORMS TO MAGNETIC PROFILES
Authors: Shuey Ralph T.
Published: October 2002

Three related operations commonly performed on total‐field magnetic data are 1) conversion to vertical‐field anomaly, 2) reduction to the pole, and 3) computation of pseudogravimetric anomalies. This note shows that for profile or flight line data for which the source can be assumed to be two‐dimensional, all these operations amount to linear combination of the profile with its Hilbert transform.
ELECTRICAL NONLINEAR PHENOMENA IN ROCKS
Authors: Katsube T. J. , Ahrens R. H. , Collett L. S.
Published: October 2002

Evidence is presented in this paper on the electrical nonlinear phenomena occurring above a certain charge density at the electrode plates (critical charge density, [Formula: see text]) for a serpentinite sample with dry electrode contact and a galena sample using moist electrode contact. The critical charge density for the serpentinite sample is [Formula: see text] [Formula: see text]. In terms of critical current density, it is [Formula: see text] at [Formula: see text] hz, or [Formula: see text] at 10 hz. The galena sample shows two critical charge densities, where one is [Formula: see text] and the other is [Formula: see text]. In terms of critical current density, it is [Formula: see text] at [Formula: see text] hz, or [Formula: see text] at 10 hz for the first critical charge. The concept of “critical charge and nonlinearity” is similar to that of “decomposition voltage” of electrodes immersed in an electrolyte. The decomposition voltage is a characteristic of conductors immersed in liquids, and it varies according to the type of conductor. It is thus possible that the nonlinear study of minerals could form a basis of a new geophysical technique, especially in borehole logging, for detecting not only the existence, but also the type, of minerals in the ground.
INTRODUCTION
Authors: Ward S. H. , Nelson P. H. , Sill W. R.
Published: October 2002

A Symposium on the Electrical Parameters of Rocks, sponsored by the Utah Geophysical Society, (SEG Local Section) and the Department of Geological and Geophysical Sciences of the University of Utah, was held at the University of Utah, March 15–17, 1972. The purpose of the symposium can best be stated as an attempt to bring together a group of scientists, from different subdisciplines of geophysics, involved in measurement of the electrical parameters of rocks. It was expected that persons concerned with oil and gas, mining, geothermal, groundwater, crustal and upper‐mantle, or lunar exploration could benefit by a symposium which stressed diversity of applications of knowledge. This proved to be the case.
ON THE ANISOTROPIC ELECTRICAL CONDUCTIVITY OF ENSTATITE AS A FUNCTION OF PRESSURE AND TEMPERATURE
Authors: Dvořák Z. , Schloessin H. H.
Published: October 2002

The two principal conductivity coefficients σ 〈100〉 and σ 〈001〉 in the (010) plane of enstatite have been determined from measurements between 20 to 60 kb, which includes a region of polymorphic transition from orthorhombic to monoclinic structure. Values of σ 〈100〉 exhibit a relative minimum near 35–40 kb for temperatures >330K. They increase from [Formula: see text] at 293K and 24 kb to [Formula: see text] at 457K and 56 kb. Values of σ 〈001〉 increase from [Formula: see text] at 293K and 24 kb to [Formula: see text] at 470K and 56 kb. Activation energies of 0.42–0.65 eV can be considered typical of conductivities in the extrinsic phase of ionic conductors or polar semiconductors. Using results for the pressure and temperature variations of the anisotropic thermal conductivity coefficients, Q can be resolved into a dominant energy term V of the order of 0.6 eV, required for the increase in the number of charge carriers, and a minor energy term U of the order of 0.07 eV, involved in the migration of charges or charged crystal defects. The density of charge carriers varies between [Formula: see text] and [Formula: see text]. The mobilities are the order of [Formula: see text], indicating a small mean free path and large effective mass of the charge carriers involved in the conduction process between 300 and 500K.
VIBROSEIS REFRACTION WEATHERING TECHNIQUES
Authors: Geyer Robert L.
Published: October 2002

In general, Vibroseis® field techniques that are optimum for reflection mapping are the worst possible for recording the “first‐break” times needed for refraction weathering calculations. The purpose of this paper is to describe linearly swept frequency (LSF) vibratory field procedures for enhancing Vibroseis refraction data. The resulting optimum refraction techniques are contrasted with optimum reflection techniques and a few examples of the refraction results are shown.
NTH‐ROOT STACK NONLINEAR MULTICHANNEL FILTER
Authors: Kanasewich E. R. , Hemmings C. D. , Alpaslan T.
Published: October 2002

A nonlinear multichannel filter is developed which appears to be particularly useful for enhancement of seismic refraction and teleseismic array data. The basic filter involves the extraction of the Nth root of each element in the matrix forming the data set, where N is any positive integer, and the Nth power of the summation over the channels. The filter is effective in reducing random noise, whereas identical signals which are in‐phase on all channels are retained at the expense of some distortion. The output from this nonlinear filter has far greater resolution in specifying phase velocity than any multichannel linear filter we have employed. Examples of theoretical and actual field seismograms are presented after various forms of filtering to illustrate their effectiveness.
EQUIVALENT SOURCES USED AS AN ANALYTIC BASE FOR PROCESSING TOTAL MAGNETIC FIELD PROFILES
Authors: Emilia David A.
Published: October 2002

Equivalent sources are useful in processing total magnetic field profiles. A lines‐of‐dipoles distribution, obtained by solving the linear inverse problem, provides an analytic base for computing the following quantities from an observed field: first and second vertical derivative fields, upward‐ and downward‐continued fields, field reduced to the pole, amplitude spectrum of the field, and band‐passed field. A theoretical example demonstrates the validity of the approach, and a field example shows that reasonable results are readily obtained.
ANALYTIC MODELS FOR THE INTERPRETATION OF ELECTRICAL SURVEYS USING BURIED CURRENT ELECTRODES
Authors: Snyder Donald D. , Merkel Richard M.
Published: October 2002

The IP response and the apparent resistivity resulting from a buried current pole in the presence of a stratigraphic target and a three‐dimensional target have been studied. The targets were modeled using a layered model to simulate the stratigraphic target and a buried sphere model to simulate the three‐dimensional target. The results show that there is a substantial increase in the response of the target measured at the surface for current electrode depths of greater than half the depth to the top of the target. A larger anomalous response is of particular importance when dealing with deeply buried targets from which little or no response is measured using conventional surface electrode methods. Furthermore, the results indicate that a survey around a drill hole containing a current electrode can be used to outline mineralization in the immediate vicinity of the drill hole. Some empirical observations resulting from our study are presented which relate the lateral offset of the target from the drill hole and its depth to characteristics of the anomaly pattern as measured on the surface.
CROSSCORRELATION METHOD FOR EVALUATING AND CORRECTING SHIPBOARD GRAVITY DATA
Authors: Lacoste Lucien J. B.
Published: October 2002

In shipboard gravity meter operation it is important to be able to evaluate the data and to make corrections if the gravity meter is not performing perfectly. These objectives can be achieved to some extent by (1) describing the ship motion with the terms of a power series in ship accelerations and velocities, and (2) using the crosscorrelations between these terms and observed gravity to evaluate performance and to obtain a corrected gravity that does not correlate with any of the power series terms. The method makes use of the simultaneity of the variations of observed gravity with those of the power series terms. The method is applied to some actual gravity data taken with a gravity meter that was not performing properly. The results show clearly the poor performance and indicate the cause. Computed corrections greatly reduce the noise in the results and give a systematic correction. Tests of the gravity meter after it was returned for repair confirm the results of the crosscorrelation analysis and show that the computed corrections are accurate.
TWO METHODS FOR COMPUTER INTERPRETATION OF MAGNETIC ANOMALIES OF DIKES
Authors: Rao B. S. R. , Murthy I. V. Radhakrishna , Rao C. Visweswara
Published: October 2002

Two computer‐oriented methods are presented in this paper for interpreting the magnetic anomalies of a dipping dike. In the first method, horizontal derivatives of the observed magnetic anomalies are calculated which define a single linear equation of the type [Formula: see text], where G is the horizontal gradient of the anomaly, and X is the distance of the gradient measured from any convenient point in the profile. Seven normal equations are derived and the coefficients [Formula: see text] to [Formula: see text] are solved from which the various parameters (Figure 1 of the body are obtained as [Formula: see text] [Formula: see text] [Formula: see text] and [Formula: see text]. In the second method, the method of iteration, the magnitudes and positions of the maximum and minimum anomaly points are located in the profile, and the approximate parameters of the dike are computed from these shape characteristics. The resulting anomalies are then calculated and compared with the observed data. The errors at each point of observation are solved for the changes to be given to the initial values. The process is repeated until the sum of the squares of the errors is less than a specified allowable error. Important features of the methods developed here are that all the observed data are utilized in the course of interpretation, and no input to the computer is required except the anomalies and their distances measured from a convenient point of observation in the profile.
USE OF WAVEFRONT CURVATURE TO RELATE SEISMIC DATA WITH SUBSURFACE PARAMETERS
Authors: Shah Pravin M.
Published: October 2002

A Dix‐type formula relating the normal moveout velocity (VNMO) to the parameters of a two‐dimensional, dipping bed, multilayered model of the earth is discussed. The formula results from (1) showing that the VNMO is related to the radius of curvature of the wavefront emerging at the surface after reflection from some interior horizon, and (2) expressing this radius in terms of the parameters of the earth model. An example with severely dipping layers is used to compute the VNMO from the formula, which is then compared to the value of moveout velocity obtained from a synthetic velocity gather generated at the same point in the model.
NONINTERCHANGEABILITY OF SOURCES AND RECEIVERS IN A HETEROGENEOUS MEDIUM
Authors: Balachandran K.
Published: October 2002

The starting point in the design of source patterns, seismometer groups, and multifolding parameters is a wave test. A wave test is usually performed with the source at a fixed location and the seismometers planted along a line at increasing distances from the source. If more coverage is desired than that provided by the length of the wave‐test cable, the cable is moved further from the source and source initiation is repeated. A wave test conducted with the source at a fixed location and the receivers planted at locations covering a range of distances from the source is here designated a normal wave test. Sometimes a wave‐test spread is kept fixed and the source is moved throughout a range of distances at intervals equal to the spread length to obtain the desired coverage. This may be termed a “semitransposed” wave test. It is sometimes desirable to perform a wave test in which a single receiver is kept at a fixed location and the source points are spaced at appropriately small intervals. This type of wave test is designated as a transposed wave test in this report. Principal interfering waves frequently can be identified by the use of a transposed wave test in certain areas where normal wave‐test sections indicate a confused mixture of waves having apparent velocities both toward and away from the source point. Comparison of transposed and normal wave‐test seismograms taken in such areas shows that source‐receiver interchangeability does not hold for the heterogeneous medium.
COHERENCE AND STRUCTURE
Authors: Mack H.
Published: October 2002

The wavefield generated at the surface of the earth from a surface seismic source can be represented as a function F(f, k) in the frequency wave‐number domain. This function depends upon the source and the propagation characteristics of the underlying structure. For any particular frequency the spatial coherence as observed at two points [Formula: see text] and [Formula: see text] on the surface is defined as: [Formula: see text]
REFRACTION DATA FROM SINGLE‐ENDED REFRACTION PROFILES
Authors: Cunningham A. B.
Published: October 2002

Subsurface layer thicknesses and velocities may be obtained from reverse refraction profiles or by special groupings of traces which effectively fabricate synthetic reverse profiles. Especially in offshore areas, and elsewhere, single‐ended profiles are normally used for data gathering. A method for determining refraction information directly from these single‐ended profiles has been developed, which eliminates the need for special field procedures or data handling when refraction information is desired. A comparison of the expected errors from this and the conventional method is given. Finally, an example of static corrections derived by applying this method to the first arrivals of ordinary reflection data is shown.
MAGNETIC RELAXATION AND THE ELECTROMAGNETIC RESPONSE PARAMETER
Authors: Olhoeft G. R. , Strangway D. W.
Published: October 2002

Relaxation processes are important in the study of electromagnetic prospecting techniques. These processes can be the result of magnetic fields due to eddy currents in electrical conductors or due to displacement currents from dielectric processes or magnetic fields associated with direct magnetic loss phenomena. In the past, most attention has been focused on eddy‐current losses since this is the dominant effect. Dielectric losses also may be of importance, particularly at high frequencies, where the conductivity is extremely low or where the dielectric properties become anomalously high. In general, however, little attention has been given to magnetic losses and the effect they might have on the electromagnetic response parameter. Magnetic minerals do have a measurable magnetic loss due to various mechanisms. This effect has been measured in a frequency range from 125 to 5000 hz for a variety of samples. In synthetic samples with varying amounts of magnetite dispersed in a nonmagnetic matrix the effect is linearly proportional to the magnetite content. The magnetic loss causes a distinct, but small, change in the electromagnetic response of a body. The effect can be detected, provided the peak frequency of magnetic loss is lower than the peak frequency of electrical loss. The most diagnostic feature is the reversed phase relationship from the electrical response (for constant geometries).
THE MAGNETIC INDUCED POLARIZATION (MIP) METHOD
Authors: Seigel Harold O.
Published: October 2002

The magnetic induced polarization (MIP) method derives information relating to the induced polarization characteristics of the earth through measurements of the magnetic fields associated with galvanic current flow. In the time domain, transient magnetic fields due to polarization current flow are measured. In the frequency domain, either the change in magnetic field with frequency or the phase shift of the magnetic field at a single frequency may be measured. The MIP method responds to regions of anomalous polarization, rather than providing physical property information. It tends to emphasize induced polarization effects in highly conducting bodies. It has special merit in certain problem areas, for example, where highly conducting overburden exists, or where the surface conditions render ground contact difficult. The results of field tests are presented where surveys using the MIP method have been carried out over “massive” and disseminated types of sulfide bodies or graphitic zones.
SEISMIC NOISE MEASUREMENTS IN YELLOWSTONE NATIONAL PARK
Authors: Iyer H. M. , Hitchcock Tim
Published: October 2002

In September and October, 1972 the U. S. Geological Survey made an investigation of seismic noise associated with the known geothermal phenomena in Yellowstone National Park. Eighty‐four stations, each recording for at least 48 hours, were operated. All major geyser basins were covered by the experiment. L-shaped three‐element arrays, three‐component stations, and single vertical component stations were operated. Four eight‐element mobile arrays were operated to study propagation characteristics of the noise. Preliminary analysis of data shows that high noise levels are associated with all the major thermal areas in the park. An elongated band of high noise envelops Lower and Upper Geyser Basins; noise levels are high around Norris Basin, Mammoth Hot Springs, Sulphur Mountain, and Mud Volcano; and a strong noise field exists around Lower and Upper Falls of the Yellowstone River. The seismic waves generated by the waterfalls have very different spectral characteristics from the waves associated with geothermal activity. The geothermal noise is predominantly in the spectral band of 2–8 hz, whereas the waterfall noise is predominantly around 2 hz. A mobile array operated near Norris Basin showed coherent wave trains radiating from seismic sources in the basin. Seismic noise measured around 50 m from Old Faithful Geyser showed amplitude fluctuations that followed the eruption cycles of the geyser. A few minutes after each eruption, the noise level starts rising slowly in ramplike fashion. Twenty to thirty minutes before the next eruption, sharp bursts of noise activity occur with increasing rapidity and continue for a few minutes after the eruption. The predominant energy of seismic noise generated by Old Faithful is at frequencies well above 8 hz. We postulate that only such high frequency noise is generated by the surface activity of geysers and hot springs and that the lower frequency noise found in and around the geyser basins is generated by a deeper convection system associated with the geothermal activity.
THE APPLICATION OF HOMOMORPHIC DECONVOLUTION TO SHALLOW‐WATER MARINE SEISMOLOGY—PART I: MODELS
Authors: Stoffa Paul L. , Buhl Peter , Bryan George M.
Published: October 2002

The complex cepstrum is investigated mathematically and through models for functions of interest in shallow‐water marine seismology. Association of the slowly varying components of the phase spectrum with the source replaces the usual minimum‐phase assumption. This is analogous to the usual treatment of the amplitude spectrum. Complex cepstrum expressions are developed for an arbitrary (but minimum‐phase) reflector series water‐column multiple generator, and simplified bubble‐pulse oscillation. While the complex cepstrum of all functions is of infinite extent, removing only the first n nonzero complex‐cepstrum contributions of a decaying, impulsive, periodic time function (such as the water‐column multiple generator) serves to eliminate the first n multiples entirely in the time domain and reduces the remaining multiples to at most 1/(n+1) of their original value. A new method of computing the continuous, ramp‐free phase spectrum required for complex‐cepstrum analysis is developed on the basis of the derivative of the phase curve.
CEPSTRUM ALIASING AND THE CALCULATION OF THE HILBERT TRANSFORM
Authors: Stoffa Paul L. , Buhl Peter , Bryan George M.
Published: October 2002

Schafer (1969) has pointed out that the Hilbert transform approach used in computing the minimum‐phase spectrum of a given amplitude spectrum corresponds to a special case of complex‐cepstrum analysis in which the phase information of the original function is ignored. The resulting complex cepstrum is an even function. Since a minimum‐phase function has no complex‐cepstrum contributions for T<0, its even part must exactly cancel the odd part for T<0. Thus, by setting all complex‐cepstrum contributions for T<0 equal to zero and doubling all contributions for T>0, we obtain the complex cepstrum of the minimum‐phase function corresponding to the original function. However, the DFT-calculated complex cepstrum is an aliased function (Stoffa et al., 1974). Thus some negative periods will appear at positive locations and vice versa. Appending the original function with zeros will reduce the aliasing. Shuey (1972), in computing the Hilbert transform for magnetic data, indicates that the computation breaks down near the end of the profile, or at long cepstrum periods. This is precisely the point in the even cepstrum where aliasing will have its greatest effect.
To: “Radius of Investigation in dc resistivity well logging” by A. Roy and R. L. Dhar, GEOPHYSICS, v. 36, no. 4, p. 754–760.
Authors: Roy Amalendu
Published: October 2002

Due to inadequate precision in computation with small values of radius r, the radial investigation characteristic (RIC) curves in the above paper are incorrect for values of r less than about 0.15. Fortunately, this has not affected the positions of the maxima which determined the radii of investigation in that paper.
On: “An Automatic Least‐Squares Multimodel Method for Magnetic Interpretation” by Peter H. McGrath and Peter J. Hood (GEOPHYSICS, April 1973, p. 349–358)
Authors: Al‐Chalabi M.
Published: October 2002

McGrath and Hood present a magnetic interpretation method whereby the search for a solution is carried out in the (hyper) space of n parameters defining the shape and position of an assumed model. The problem is an optimization problem and should be viewed within the general context of nonlinear optimization techniques. McGrath and Hood simply present one optimization method. The usefulness of individual methods is limited. One could similarly propose the use of the method of rotating coordinates (Rosenbrock, 1960), the “complex” method (Box, 1965), Davidon’s methods (Fletcher and Powell, 1963; Stewart, 1967; Davidon, 1969), etc. We currently have a wealth of these methods at our disposal. In fact, the use of these methods for magnetic interpretation has already been presented (Al‐Chalabi, 1970). As this and subsequent work indicated (Al‐Chalabi, 1972), these methods should be used as an integral group for interpreting magnetic and gravity anomalies. The exclusive use of individual methods is inefficient. Studies performed on objective functions used in magnetic and gravity interpretation have shown that the behavior of these functions in the parameter hyperspace is extremely complicated. Consequently, the search for a solution requires different strategies at different stages between the initial estimate and the ultimate solution (Al‐Chalabi, 1970, 1972).
On: “Bouguer Gravity and its Geologic Evaluation in the Western Part of the Bengal Basin and Adjoining Area, India” by S. K. Choudhury and A. N. Datta (GEOPHYSICS, August 1973, p. 691–700)
Authors: Gupta Amalendu
Published: October 2002

Choudhury and Datta have rightly commented that the distribution of the Rajmahal Trap which has fairly high density [Formula: see text] will have appreciable bearing on the quantitative interpretation of the gravity data in Bengal Basin. The authors have assumed in their model uniform thinning of the trap layer to the east. This has oversimplified the problems, and instead of relating the anomalies to the distribution of these igneous rocks, they have attributed them to the presence of Gondwana sediments below the trap, disregarding their own statement.
EXPLORATION GEOPHYSICS IN THE U.S.S.R.
Authors: Kleinkopf M. Dean , Balch Alfred H. , Frischknecht Frank C. , Hovey Richard , Savit Carl H.
Published: October 2002

Five American exploration geophysicists visited the USSR for a month during the late summer of 1971 as members of an official U.S.-USSR exchange program. The group was hosted by the USSR Ministry of Geology and saw 15 technical activities in six cities: Moscow, Leningrad, Tyumen, Kiev, Krasnodar, and Baku. During their stay, the Ministry of Geology assisted the American delegation in participating in the Proceedings of the 15th General Assembly of the International Union of Geodesy and Geophysics held at Moscow State University.
MAXIMUM‐ENTROPY SPATIAL PROCESSING OF ARRAY DATA
Authors: McDonough R. N.
Published: October 2002

The procedure of maximum‐entropy spectral analysis (MESA), used in the processing of time series data, also applies to wavenumber (bearing) analysis of signals received from a spatially distributed linear array of sensors. The method is precisely the use of autoregressive spectral analysis in the space dimension rather than in time. There are also close links to the predictive deconvolution method used in geophysical work, and to the process of constructing noise‐whitening filters in communication theory, as well as to least‐squares model building. In this note, we review the maximum‐entropy procedure pointing out all these links. The specific algorithm appropriate to a uniformly spaced line array of sensors is given, as well as one possible algorithm for use in the case of nonuniform sensor spacing.
Architecture of the Earth
Authors: Wendler A. P.
Published: October 2002

Daly’s earth model has a solid crystalline crust varying in thickness but not exceeding fifty miles. This crust encloses a continuous layer of vitreous rock, relatively thin, which rests on a denser vitreous material. The heavier substratum surrounds a so‐called “iron core” possibly crystallized by high pressures of the interior. This vitreous subcrust has some properties of a solid; hence, it behaves more or less like a rigid mass.
THE RELATIONSHIP OF GEOPHYSICS TO GEOLOGY
Authors: Kannenstine F. M.
Published: October 2002

In this paper, the term “geology” should be interpreted to mean petroleum geology; and the term “geophysics,” to mean the use of physical methods and observations to assist the geologist in the location of oil and gas deposits.
REPRESENTATION THEOREMS FOR MULTIREGIONAL ELECTRODYNAMIC DIFFRACTION: PART II. APPLICATIONS
Authors: Won I. J. , Kuo J. T.
Published: October 2002

With the aid of the representation theorems presented in Part I of this paper, the two‐dimensional diffraction problem of a circular cylinder of finite conductivity embedded in a medium of finite‐conductivity is treated. The solution is expressed in a closed form for an arbitrary electromagnetic source. Comparison of the present solution for the case of an electric line source with the previously known solution shows that these two solutions are compatible. However, the difference between the two solutions becomes considerable as the source approaches the scatterer.
DISTRIBUTION AND REGIONAL VARIATION OF DENSITY IN THE WESTERN CANADA BASIN
Authors: Maxant J.
Published: October 2002

An extensive density study based on gamma‐gamma (or density) logs from 435 wells in western Canada was undertaken in an area extending from the Rocky Mountains in the west to longitude 100°W in the east and from the international boundary in the south to latitude 56°N in the north. The logs, with, total footage of about 814,000 ft, were subdivided into nearly 34,000 intervals for computer input. In this paper density is studied without separation of measurements according to lithology. Uncompensated density logs yield an average density for the whole basin which is lower by [Formula: see text] than that from compensated density logs. The weighted average densities in the Western Canada Basin established from compensated density logs are as follows: [Formula: see text] for the section above sea level, [Formula: see text] below sea level, and [Formula: see text] without regard to depth. Large density variations both horizontally and vertically are demonstrated on several contour density maps. The prevailing northwesterly contour trends indicated on maps of density and by some statistics coincide with the strike of the strata. Generally, density above sea level increases toward the Rocky Mountains, whereas below sea level no general rule is apparent. The linear correlation coefficient between density and depth based on all compensated density logs is 0.46, and the regression line of density D (in [Formula: see text]) on depth below sea level d (in feet) has the equation D = 2.372 + 0.000052d ± 0.21. The mean value of the correlation coefficient between density and depth calculated independently in single wells is 0.45, and a similarly established mean for density increments with depth is [Formula: see text] per 10,000 ft. However, both local (in single wells) and regional density sample distributions are most often skewed to the left which suggests that, in general, the vertical density gradient is higher at smaller depths than at larger depths.
FIELD MEASUREMENT OF THE ELECTROSEISMIC RESPONSE
Authors: Long Leland Timothy , Rivers Wayne K.
Published: October 2002

The electroseismic response is a change in apparent resistivity induced by seismic excitation. It can be measured under field conditions with a conventional Wenner array and an explosive seismic source. In coastal plain sedimentary rocks near Gordon, Georgia, a Wenner array with 9-m electrode spacing measured a change in voltage of 100 to 300 μv per mm/sec of 15 hz vertical particle velocity at the surface. The response correlates best with the Rayleigh surface waves and compressional body waves. By assuming a layered medium which is excited uniformly by a seismic disturbance, we can use a Taylor series expansion of the voltage expression for a Wenner array in terms of layer resistivities to obtain estimates of the perturbations of the resistivities in the layers.
ON THE PHASE CONSTRAINT OF THE MAGNETOTELLURIC IMPEDANCE
Authors: Loewenthal D.
Published: October 2002

A simple proof that the phase of the one‐ dimensional magnetotelluric modified impedance lies between −π/4 and π/4 is given. The proof is based upon a rational polynomial representation of the impedance. The formalism from which the rational representation is obtained is outlined. An adjoint model is defined and it is shown to be pertinent for the proof. A numerical example is given.
GAMMA‐RAY SPECTROMETRY IN CENTRAL MOROCCO
Authors: Demnati A. , Naudy H.
Published: October 2002

Correlations between radiometric, aeromagnetic, and geologic maps for the central part of Morocco are shown. Some geologic boundaries are closely followed by the total count. The relatively low sensitivity of the system (400 cu in detector, flown at 150 m) is adequate for producing radioactivity and radioelement distribution maps in this area with a high percentage of outcrop. Maps of thorium count, potassium count, and their ratio emphasize various geologic features. Uranium distribution maps are not presented because the area does not contain really characteristic anomalies in this channel.
On: “The Complex Wiener Filter” by Sven Treitel (GEOPHYSICS, April 1974, p. 169–173)
Authors: Berkhout A. J.
Published: October 2002

In this publication Treitel determines the complex normal equations and proposes a solution method (Robinson’s block‐Toeplitz algorithm). With respect to these results, I would like to draw attention to the paper Berkhout (1973). In Appendix B of that paper the complex normal equations were also derived: [Formula: see text]for n = 0, 1, ⋯ , N. In (1a), [Formula: see text] is the autocorrelation function of a complex input signal x(t), [Formula: see text] is the complex least‐squares filter of duration (N + 1)Δt, [Formula: see text] is the complex desired output, and Δt represents the sampling interval. Note that in matrix notation expression (1a) equals expression (3) of Treitel’s paper. In the case of least‐squares inverse filtering, (1a) takes on a different form.
DEEP CRUSTAL REFLECTION RECORDINGS USING “VIBROSEIS” METHODS—A FEASIBILITY STUDY
Authors: Fowler J. C. , Waters K. H.
Published: October 2002

This work on deep crustal reflections started in 1972. Possibilities for improvement in signal quality were noted and, in 1973, were incorporated in a new experiment. All work was done with 15.0–2.0 hz or 20.0–5.0 hz swept frequency signals 30 sec long. On most occasions, 240 sweeps were used—all recordings of each trace were added together. In order to further improve the signal/noise, six large vibrators, spaced closely together along a line at right angles to the source‐receiver line, were used simultaneously. The basic detector was an 8‐arm star, with radius 300 ft, each arm containing thirty 4.0 hz geophones. Two very good refraction events were recorded; they came from velocity discontinuities at depths of only a few thousand feet. Other long delayed energy, which might be deep crustal reflections, was observed. Due to the lack of complete subsurface coverage and possibly due to complexity of the structure, these events cannot be certainly identified as reflections.
DECONVOLUTION AND SPECTRAL ESTIMATION USING FINAL PREDICTION ERROR
Authors: Fryer Gerard J. , Odegard Mark E. , Sutton George H.
Published: October 2002

Least‐squares, zero‐lag inverse filters may be used for predictive deconvolution of stationary time series and for obtaining autoregressive or maximum entropy spectral estimates. The greatest problem in finding such an inverse filter is determining the optimum operator length for a given finite length of data. The identical problem of determining the correct order of an autoregressive model for the data has been solved by Akaike, whose final prediction error (FPE) statistic is a minimum for the optimum length model. This minimum FPE criterion may be applied to both single and multiple time series. The FPE procedure has been used successfully on simultaneous three‐component seismometer and hydrophone data for the detection of refracted arrivals from explosions up to 1350 km away and for estimation of spectra of microseismic noise observed at the time of each shot. The data were recorded with an ocean bottom seismometer.
THE USE OF GEOMAGNETIC FIELD MODELS IN MAGNETIC SURVEYS
Authors: Regan Robert D. , Cain Joseph C.
Published: October 2002

Global geomagnetic field models, usually computed from spherical harmonic series, are becoming more important in the reduction of magnetic surveys. When used correctly, a numerical model of sufficient complexity, including adequate secular variation correction, provides a suitable representation of the regional field. The best known and most widely used of the available field models is the International Geomagnetic Reference Field (IGRF). However, the IGRF may not be suitable for the reduction of all magnetic survey data because of its imperfect fit to the main field, particularly since 1968.
ELEMENTARY APPROXIMATIONS IN AEROMAGNETIC INTERPRETATION FOR APPLICATION IN LOW LATITUDES
Authors: Barker R. D.
Published: October 2002

Approximate methods for determining from the total field anomaly the depth to a point dipole and a line of dipoles were developed by Smellie (1967) and have frequently been applied in the interpretation of aeromagnetic data. The technique, involving the use of various characteristic curves, is very useful for interpretation in high magnetic latitudes but loses its applicability at latitudes lower than about 30 degrees. For this reason, a modification to the method presented by Smellie is proposed for application in surveys conducted near the magnetic equator.
AMPLITUDES OF SEISMIC WAVES—A QUICK LOOK
Authors: Hilterman Fred J.
Published: October 2002

A form of Kirchhoff’s wave equation is presented which is useful to the geophysicist doing an amplitude interpretation of seismic reflection data. A simple rearrangement of Kirchhoff’s retarded potential equation allows the reflection process to be evaluated as a convolution of the derivative of the source wavelet with a term called the “wavefront sweep velocity”. The wavefront sweep velocity is a measure of the rate at which the incident wavefront covers the reflecting boundary. By comparing wavefront sweep velocities for geologic models with different curvature, one obtains an intuitive feeling for the relation of diffraction and reflection amplitudes to boundary curvature. Also, from this convolutional form of the wave equation, the geometrical optics solution for the reflection amplitude is easily obtained. But more important, from the wavefront sweep velocity approach, a graphical method evolves which allows the geophysicist to use compass and ruler to estimate the effects of curvature and diffraction on seismic amplitude.
EM COUPLING, ITS INTRINSIC VALUE, ITS REMOVAL AND THE CULTURAL COUPLING PROBLEM
Authors: Wynn Jeffrey C. , Zonge Kenneth L.
Published: October 2002

The induced polarization method of geophysical prospecting has been in use for more than 25 years with varying degrees of success. Until recently, its two principle drawbacks were (1) the inability to distinguish between anomalous rock responses and, (2) inability to distinguish between these rock responses and inductive coupling. The first problem was solved by K. L. Zonge in 1972. Solutions to the coupling problem go back to 1932, and have been expanded and elaborated upon by successive authors since then. In most of these papers, electromagnetic coupling was separated into two functions, here designated as P, a purely inductive term, and Q, a grounding or purely resistive term. This paper extends this work into a study of the reflective coupling contribution and the effects of anisotropy. Two immediate results are: (a) the development of an ultra‐low‐frequency deep sounding technique for highly conductive overburden environments, and (b) a successful iterative technique for the removal of coupling from complex resistivity field data. A study was made of the effect of electrically conductive pipelines on induced polarization and complex resistivity data. It appears that the so‐called “pipeline effect” is a composite of several effects, including current focusing nonlinearities, electromagnetic induction, and complex electrode polarization. The pipeline effect is generally predictable, while the effect of a fence or an irregular conductive inhomogeneity is not as simple.
On: “Maximum‐Entropy Spatial Processing of Array Data,” by R. N. McDonough (GEOPHYSICS, December 1974, p. 843–851)
Authors: Adams James W.
Published: October 2002

McDonough has made a fundamental error when he concludes on page 849, “Thus the spectrum (62) can be computed from sufficiently many equally spaced samples of [Formula: see text], sampled in r, by using the above procedure … applicable to time series.”
TRAVELTIME CURVES FOR REFLECTIONS IN DIPPING LAYERS
Authors: Gangi Anthony F. , Yang Sung Jin
Published: October 2002

The traveltimes for reflected waves from plane, dipping layer interfaces for split‐spread arrays and CGP gathers are determined by a computationally efficient method. The computational efficiency is obtained by (1) interpolating the traveltimes for particular source‐receiver distances from least‐squares curves fitted to traveltime data rather than by using iterative ray‐tracing techniques, and (2) using the traveltime curves for a fixed source to determine those for other source locations and for CGP gathers over the same dipping layer interfaces. The additional computations necessary in this latter case are minimal when this approach is used. The traveltime curves for other source locations and CGP gathers are obtained by taking advantage of the fact that the traveltimes and travel distances for parallel rays are simply related for plane, dipping layer interfaces. For the fixed source array, standard deviations between 0.95 msec for the fourth interface (in a four layer model) and 9.5 msec for the first interface were obtained when the traveltimes t(x) were fitted to a second order curve: [Formula: see text]. These standard deviations decreased to 0.81 and 3.5 msec, respectively, when a third order curve [Formula: see text] was used. The standard deviations became 2.97 and 0 msec, respectively, for a second order curve in [Formula: see text]; and 0.95 and 0 msec, respectively, for the third order curve: [Formula: see text]. For a CGP gather over the same layers, the standard deviations were 0.84 and 0 msec for the fourth and first interfaces, respectively, when the traveltime data were fitted to a first order curve in [Formula: see text]. These standard deviations became 0.10 and 0 msec, respectively, when the curve fitted was: [Formula: see text]. Errors in the traveltimes for a fitted curve, [Formula: see text], in a CGP gather for the third interface at a depth of about 5000 ft were less than 0.5 msec for source‐receiver separations up to 5000 ft. For the same interface, errors were much less than 0.1 msec over the same separations for the fitted curve: [Formula: see text].
A SUBSURFACE ELECTROMAGNETIC PULSE RADAR
Authors: Moffatt David L. , Puskar R. J.
Published: October 2002

An electromagnetic pulse radar has been developed for investigation of subsurface geology and man‐made targets. The radar uses separate broadband dipole‐type antennas for transmission and reception. The antennas are well matched to the soil or rock surface. An orthogonal orientation of the antennas on the medium surface effectively decouples them and also prevents reflections from the air‐medium interface or any horizontal stratification from being seen. Two versions of the radar are used: small 6-ft dipoles with a shock‐type (250 picosec) pulse for shallow soundings and 24-ft dipoles with a 45 nanosec pulse for deeper soundings. Signatures of faults, joints, cavities, and lithologic contrasts in soft rock have been obtained with the radar, and these results are presented. A sampling oscilloscope acts as a receiver for the radar, and the target signatures are isolated portions of the time record whose time delays agree with the physical geometry and measured pulse velocities for the medium. For a large void at a depth of 20 ft in limestone, a frequency domain signature is also given to illustrate the potential of using both temporal and spectral signatures. Signatures of an exposed fault in a dolomite quarry are used via mapping measurements to delineate the direction of a minor fault. Signatures of two lithologic contrasts at depths of 40 ft in the dolomite are given. The signatures of a drift coal mine tunnel as measured from a hill 11 to 26 ft above the tunnel are shown. Unique features of the radar are enumerated and present capabilities are summarized.