bluuebunny/crossref_metadata_2025 huggingface dataset:
An examination of tube wave noise in vertical seismic profiling data
Authors: Hardage B. A.
Published: October 2002

Tube waves act as noise that camouflages upgoing and downgoing body wave events which are the fundamental seismic data measured in vertical seismic profiling (VSP). In two onshore vertical seismic profiles, the principal source of tube waves is shown to be surface ground roll that sweeps across the well head. Secondary tube wave sources revealed in these VSP data are the downhole geophone tool itself and the bottom of the borehole. Body wave signals are also shown to create tube waves when they arrive at significant impedance contrasts in the borehole such as changes in casing diameter. Computer simulated vertical geophone arrays are used to reduce these tube waves, but such arrays attenuate and filter body wave events unless static time shifts are made so that the body wave signal occurs at the same two‐way time at each geophone station. Consequently, actual downhole vertical geophone arrays are not an effective means by which tube waves can be eliminated. Power spectra comparisons of tube wave and compressional body wave events demonstrate that band‐pass filters designed to eliminate tube waves also suppress body wave signals. A simple but effective field technique for reducing tube waves is shown to be proper source offset. Using velocity filters to retrieve upgoing compressional events from VSP data heavily contaminated with tube wave noise yields in one example an agreement with surface measured reflections that is superior to that obtained from synthetic seismograms calculated from log data recorded in the same well.
COCORP: Southern Appalachian Area, Part II
Authors: Kaufman S.
Published: October 2002

The Consortium for Continental Reflection Profiling (COCORP) announces the availability of the data package and tapes for the Southern Appalachian area, part II, for the cost of reproduction. These data were obtained by COCORP whose operation is part of the U.S. Geodynamics Project sponsored by the National Academy of Sciences and funded by the National Science Foundation. The executive committee of the consortium consists of members from Cornell University, University of Houston, Princeton University, the University of Texas at Austin, and the U.S. Geological Survey. Cornell University is the operating institution.
Improvement of the conic prism model for terrain correction in rugged topography
Authors: Olivier Raymond J. , Simard Réjean G.
Published: October 2002

Terrain corrections for Bouguer gravity anomalies are generally obtained from topographic models represented by flat‐topped compartments of circular zones, utilizing the so‐called Hayford‐Bowie (1912), or Hammer’s (1939) method. Some authors have introduced improved relief models for taking uniform slope into consideration (Sandberg, 1958; Kane, 1962; Takin and Talwani, 1966; Campbell, 1980). We present a new model that increases the accuracy of the calculation of terrain correction close to the gravity station in rugged terrain, especially when conventional templates with few zones are used in field calculation.
On the resolution of statics, structure, and residual normal moveout
Authors: Marcoux Michael O.
Published: October 2002

The problem of separation of reflection times into the component parts of source static, receiver static, structure time, and residual normal moveout (RNMO) is presented. A new solution is derived which is valid for wavelengths ranging from a group interval at the short end to a distance equal to the separation between the full‐fold positions at the long end. In the absence of RNMO, this solution, though not unique, is, however, optimum with regard to stability against noise in general. In the presence of RNMO, the solution is most stable against white noise. Additionally, it is concluded that the underconstrained nature of the problem is of minor practical consequence. The new method is based on the two‐dimensional (2-D) spectrum of the reflection times considered as a function of the spatial variables of common depth point (CDP) and offset. The resultant equations yield a simple, explicit solution for each separate wavelength. The computation is rapid and directly controllable by selection of the desired wavelengths. Synthetic examples are used to demonstrate the properties of this method of solution.
Redox chemical remanent magnetization—A new dimension in exploration for sulfide deposits in volcanic covered areas
Authors: Bacon Lloyal O. , Elliot Charles L.
Published: October 2002

Redox chemical remanent magnetization (CRM) results from current flow associated with a redox potential cell. An active redox cell covered by later volcanics may continue in operation for a period of time, sufficiently long that the current flowing in the overlying volcanics will cause or assist in the remobilization of iron. The remobilization may be either in situ as an alteration of minerals or by actual migration as a ferrous hydroxide, with fixation at a higher Eh nearer the surface. Redox CRM will leave a characteristic pattern as a magnetic memory in the rocks. Measurement of the magnetic rock properties and interpretation of resulting patterns can be used to locate the ancient redox cell. Redox cells in nature are usually associated with oxidizing sulfide deposits. Under appropriate conditions, redox CRM can be utilized as an indirect method for sulfide exploration. Field results for two porphyry sulfide deposits and a massive sulfide deposit under approximately 400 m of post‐mineral volcanic cover demonstrate the feasibility of this technique. Field tests in nonsulfide areas indicate that pervasive occurrence of false redox CRM anomalies do not exist except in the presence of sulfides. Laboratory experiments in the simulation of the redox CRM concept give support to the theory.
Transient response of an induction logging tool in a borehole
Authors: Chew W. C. , Gianzero S. , Kaplin K. J.
Published: October 2002

The long time transient response is developed for a point coil transmitter (vertical magnetic dipole) located in a wellbore surrounded by a homogeneous formation. A simple algorithm is derived for the apparent conductivity signal based on the difference between the voltage nulls in a pair of receiver coils which are displaced vertically from the transmitter along the axis of the wellbore. A justification of these results is provided by means of a direct numerical integration (double Fourier transform) and also two separate approximate approaches.
Ionospheric induced very low‐frequency electric field wavetilt changes
Authors: Thiel D. V. , Chant I. J.
Published: October 2002

Very low‐frequency (VLF) electric field wavetilt measurements were made continuously over a period of four months at a fixed location. Daytime and nighttime values of the wavetilt were found to be relatively constant. During sunrise and sunset the wavetilt decreased to 20 percent of the daytime value, making wavetilt measurements meaningless for two three‐hour intervals during a 24 hour period. A mechanism involving modal interference is postulated.This phenomenon may be a problem in other passive electromagnetic (EM) prospecting systems including audio‐frequency magnetic and audiomagnetotelluric.
Regression technique to remove temporal variation from geomagnetic survey data
Authors: Sander Eric L. , Mrazek Charmaine P.
Published: October 2002

A flexible and effective set of procedures has been developed to reconstruct the temporal variation occurring during a geomagnetic survey. These procedures use regression techniques and the field measurements made at survey track intersections. They have been applied to both ship and aircraft survey data collected in open ocean areas to remove from 60 to 80 percent of the temporal variation. This paper presents these procedures and the results for one ship and one aircraft survey.
Designing digital linear filters for computing resistivity and electromagnetic sounding curves
Authors: Das U. C.
Published: October 2002

Ghosh’s method of designing filters (1970, 1971a, b) for computing apparent resistivity and electromagnetic (EM) sounding curves over a layered earth requires a set of known input and output functions satisfying the corresponding convolution integral. The spectrum of the filter in amplitude |H(f)| and in phase φ(f) is determined as [Formula: see text], [Formula: see text], respectively. Inverse Fourier transform of the spectrum results in the filter function, which is sampled to derive the filter coefficients. For different electrode or coil configurations, different sets of input‐output functions are chosen. The criteria for choosing them were given in Koefoed et al (1972) and Anderson (1975). An alternative method of designing a filter was given by Johansen and Sørensen (1979) who obtained an explicit series expansion for the filter function and handled the tail of the infinite summation analytically. Following Ghosh’s method, Anderson (1979) designed the filters for Hankel transforms of orders 0 and 1 where both filters have identical abscissas. This avoids repetitious kernel evaluations because the kernels of many of the integral transforms of orders 0 and 1 encountered in electrical prospecting are related to one another by simple algebraic relationships. Table 1 presents a 14‐point filter (six intervals per log decade; abscissa of the filter coefficient [Formula: see text] is +0.1343155) designed by the author for Schlumberger curve computation.
Static corrections on the southeastern Piedmont of the United States
Authors: Bahorich Michael S. , Coruh Cahit , Robinson Edwin S. , Costain John K.
Published: October 2002

On the Piedmont of the southeastern United States, seismic reflection statics at different points in a typical common‐depth‐point (CDP) gather can differ from one another by more than 50 msec because of topographic relief in excess of 50 m, variations in the thickness of the weathered zone that can extend deeper than 50 m, and variation of velocity in the weathered zone of more than 100 m/sec. The ABCD method for computing statics is introduced to account for these velocity and thickness variations as well as the topographic effect. This method combines elevations and positions of source‐receiver points with times of first arriving refracted waves read from reflection correlograms or seismograms. It was tested in central Virginia where typical piedmont conditions are encountered. At four locations, ABCD statics are close to control values determined independently from refraction experiments. At 99 source‐detector points, ABCD statics differ by an average of 4 msec, (maximum of 19 milliseconds) from conventional elevation statics that do not account for local velocity thickness variations in the weathered zone. Where source‐receiver points were in line, modified ABCD statics were obtained from first arrival traveltimes without using elevation and position data. In this metamorphic terrane where clear reflections are difficult to record, ABCD statics appear to be more effective than conventional elevation statics for enhancing reflections on a seismic record section.
Gravity interpretation of the southern Wind River Mountains, Wyoming
Authors: Hurich Charles A. , Smithson Scott B.
Published: October 2002

The Wind River range is the largest Laramide uplift in Wyoming and is cored by Precambrian rocks thrust over sedimentary rocks of the Green River Basin to the southwest. The origin of these uplifts is in dispute; it has been ascribed to horizontal compression along thrusts or vertical movement along high‐angle reverse faults. Therefore, the attitude of the bounding fault is critical to understanding the mechanics of deformation. A gravity study based on 1800 stations has been carried out to attack this question and to complement a COCORP deep crustal reflection study. Smoothed free‐air gravity anomalies are highly positive over the Wind River range and near zero over the adjacent margins. Bouguer gravity anomalies range from −252 and −225 mgal in the Green River basin and Wind River basin, respectively, to −150 mgal in the basement core of the Wind River range. The bulk density of sedimentary rocks in the adjacent basins is the critical parameter for gravity interpretation of the deep structure and ranges from 2.25 to [Formula: see text], based on density logs. Gravity modeling demonstrates that most of the +85 mgal gravity anomaly associated with the uplift is caused by the sediment‐basement density contrast, but about 18 mgal has a deeper source. This can be accounted for by offset of more dense middle crust along a thrust fault dipping at about 40 degrees. Three‐dimensional (3-D) modeling confirms this interpretation along the southern part of the range. No significant offset of the Mohorovicic discontinuity can be present. This suggests that the thrust either flattened out or the lower crust deformed by homogeneous ductile deformation. Gravity interpretation gives information on deep structure not found in the COCORP reflection data and further confirms the compressive nature of this Laramide uplift.
Theoretical and practical aspects of absorption in the application of in‐seam seismic coal exploration
Authors: Krey Th. , Arnetzl H. , Knecht M.
Published: October 2002

During the last two decades, the detection of coal seam discontinuities by seismic waves guided by the seam has become a special branch of exploration seismics in Europe. Waves consisting purely of SH motion (so‐called waves of Love type) are of special interest, and the rather high frequencies of the Airy phase, in thin seams, are most important because they present very high reflectivity at seam interruptions. Absorption increases with frequency in most layers, and therefore attenuates the high Airy‐phase frequencies more severely than the earlier low‐frequency part of the guided waves. Another fact additionally impairs the Airy‐phase signal: the quality factor Q is much lower in coal than in the schists and sandstones of the Carboniferous country rock. Unfortunately, most of the energy of the Airy phase is transferred by the coal, whereas the lower frequencies have their main energy conveyed by the country rock above and below the seam. In order to allow a better understanding of the influence of absorption on Love‐type seam waves, several simplified computations were carried out for the fundamental mode of a seam typical for the northwest‐German Ruhr area. The assumptions are as follows: The quality factors [Formula: see text] for coal and [Formula: see text] for the country rock do not depend upon frequency; higher powers of [Formula: see text] and [Formula: see text] can be neglected; and the distance from the source is large enough to allow the two‐dimensional plane‐wave case to be considered. The mathematics resulting from these assumptions and adequate data processing of transmission records provides the possibility to determine the quality factor [Formula: see text] of coal in‐situ, although the thickness of the seam may be much smaller than the wavelengths involved. [Formula: see text] may become of interest for practical mining problems.
Field development with three‐dimensional seismic methods in the Gulf of Thailand—A case history
Authors: Dahm C. G. , Graebner R. J.
Published: October 2002

A three‐dimensional (3-D) marine seismic survey was conducted in the Gulf of Thailand to aid in the development of a gas field indicated by three wildcat wells which had been located by seismic reconnaissance programs shot over a period of several years. The key to successful exploration in the area, basically a hinge line play, was a detailed understanding of the complex faulting controlling the hydrocarbon traps. Since the prospect lies 160–220 km offshore, some specialized surveying techniques were employed to achieve the required positioning accuracy. About 1280 km of seismic data were recorded at 100-m line spacing over a roughly rectangular block covering about [Formula: see text]. The 48‐fold data were processed using a 3-D wave equation migration algorithm yielding a set of seismic traces representing the data vertically below a grid of depth points spaced at [Formula: see text] by 100 m. The results of the 3-D program showed greater fault resolution and structural delineation. The interpretation developed from a series of horizontal slices provided by the 3-D processing further improved fault resolution. Five wells, drilled on the basis of the 3-D survey, are productive and closely tie the seismic data. Initial studies of amplitude patterns of key reflectors, combined with interval velocities from seismic derived logs, appear to offer the potential of direct detection of productive gas zones thicker than 25 to 30 ft. The 3-D seismic data are being utilized for planning additional development wells and potential platform locations.
Field appraisal with three‐dimensional seismic surveys offshore Trinidad
Authors: Galbraith Robert M. , Brown Alistair R.
Published: October 2002

A consortium comprising Texaco Trinidad Inc., Trinidad and Tobago Oil Company Ltd., and Trinidad‐Tesoro Petroleum Company Ltd. commenced exploration in the South East Coast Consortium block offshore Trinidad in 1973. After four years of intensive exploration, a gas/condensate discovery was announced in early 1977 on the Pelican prospect. Later that year, in anticipation of the possible future need to site drilling/production platforms, a three‐dimensional (3-D) seismic survey was recorded over the prospect. This survey resulted in improvements in seismic record quality, multiple attenuation, and fault resolution. A coordinated geologic‐geophysical interpretation based on the 3-D seismic survey, a reevaluation of log correlations, and the use of seismic logs differed significantly from earlier interpretations. Because of this, it is anticipated that development of the field will need to be initiated in a different fault block from that previously envisioned. A second 3-D survey contiguous to the Pelican survey was recorded in 1978 over the Ibis prospect. Results show significant data enhancement in the deeper part of the section and improved fault resolution relative to previous two‐dimensional (2-D) control. The 3-D interpretation has revealed a much more complex fault pattern than originally mapped. Separate fault blocks will have to be individually evaluated, thus greatly increasing exploration risk.
On: “The interpretation of direct current resistivity measurements” by D. W. Oldenburg (GEOPHYSICS, April 1978, p. 610–625)
Authors: Guptasarma D.
Published: October 2002

Oldenburg made an excellent example of the application of linearized inverse theory to invert dc resistivity sounding data to fit a continuous vertical variation of resistivity. In the Introduction he mentioned that the Frechet kernels for resistivity are the same as the depth investigation characteristic function (DIC) used by Roy and Apparao (1971). In the second part of the paper, he showed that it is so for a uniformly conducting half space. He mentioned that the electrostatic analog which was used (by Roy and Apparao) becomes quite complex when a layered medium is introduced, and that the extension to a continuous ρ(z) would be a difficult task (p. 623).
On: “Application of Gauss’s method to magnetic anomalies of dipping dikes” by I. J. Won (GEOPHYSICS, February 1981, p. 211–215).
Authors: Kunaratnam K.
Published: October 2002

In a recent paper, Won discussed the application of Gauss’s method for obtaining the parameters of a dipping dike from its magnetic anomaly. He assumed the magnetization to be entirely induced and did not consider the effect of the presence of any permanent magnetism. In view of the reported agreement of the calculated dip angles with drilled results, the assumption seems to be valid in this particular case. If permanent magnetization in an unknown direction is present, neither the dip angle of the dike nor the susceptibility can be determined, although the other parameters of the dike (i.e., depth to the top, horizontal location, and thickness) can be deduced from the magnetic anomaly. The dip angle of the dike and the angle made by the transverse component of the resultant intensity of magnetization combine to form a single angle which alone can be determined uniquely from the magnetic anomaly. If the transverse component of the resultant intensity of magnetization is J and it dips at an angle α below the horizontal, then using the other notations given by Won it can be shown that [Formula: see text] and [Formula: see text] (Bruckshaw and Kunaratnam, 1963). For this reason, the magnetic anomaly due to an inclined dike of infinite depth extent and horizontal top surface is the same as that due to a vertical dike having the same top surface but for a modified direction and intensity of magnetization. The inclined dike anomalies can, therefore, be analyzed using the vertical prism models as well. If the magnetization is entirely induced, the dip angle of the inclined dike can be deduced from the direction of magnetization of the equivalent vertical dike.
Computed waveforms in transversely isotropic media
Authors: White J. E.
Published: October 2002

Radiation of elastic waves from a point force or from a localized torque into a transversely isotropic medium has been formulated in terms of displacement potentials, and transient waveforms have been computed by numerical Fourier inversion. For isotropic sandstone, this procedure yields P‐ and S‐wave pulses whose arrival times and magnitudes agree with theory. For a range of anisotropic rocks, arrival times of quasi‐P‐waves and quasi‐S‐waves agree with asymptotic theory. For extreme anisotropy, some quasi‐S‐wave pulses arrive at times which are not predicted by asymptotic theory. Magnitudes have not been compared with results of asymptotic theory, but decrease with distance appears to be in agreement. This Fourier inversion method gives near‐source changes in waveform which are not obtainable from the asymptotic theory.
Velocity estimation in laterally varying media
Authors: Lynn Walter S. , Claerbout Jon F.
Published: October 2002

In areas of large lateral variations in velocity, stacking velocities computed on the basis of hyperbolic moveout can differ substantially from the actual root mean square (rms) velocities. This paper addresses the problem of obtaining rms or migration velocities from stacking velocities in such areas. The first‐order difference between the stacking and the vertical rms velocities due to lateral variations in velocity are shown to be related to the second lateral derivative of the rms slowness [Formula: see text]. Approximations leading to this relation are straight raypaths and that the vertical rms slowness to a given interface can be expressed as a second‐order Taylor series expansion in the midpoint direction. Under these approximations, the effect of the first lateral derivative of the slowness on the traveltime is negligible. The linearization of the equation relating the stacking and true velocities results in a set of equations whose inversion is unstable. Stability is achieved, however, by adding a nonphysical fourth derivative term which affects only the higher spatial wavenumbers, those beyond the lateral resolution of the lateral derivative method (LDM). Thus, given the stacking velocities and the zero‐offset traveltime to a given event as a function of midpoint, the LDM provides an estimate of the true vertical rms velocity to that event with a lateral resolution of about two mute zones or cable lengths. The LDM is applicable when lateral variations of velocity greater than 2 percent occur over the mute zone. At variations of 30 percent or greater, the internal assumptions of the LDM begin to break down. Synthetic models designed to test the LDM when the different assumptions are violated show that, in all cases, the results are not seriously affected. A test of the LDM on field data having a lateral velocity variation caused by sea floor topography gives a result which is supported by depth migration.
Vertical gravity gradient surveys: Field results and interpretations in British Columbia, Canada
Authors: Ager Charles A. , Liard Jacques O.
Published: October 2002

Two vertical gravity gradient (VGG) surveys were completed by the authors during 1977 in British Columbia, Canada. The VGG method utilizes a LaCoste and Romberg model D gravity meter in conjunction with a small gradient tripod. The VGG method is practical under most field conditions using a 2 person crew and yields results precise to ±20 E or better. The VGG work indicates that the “free‐air” effect ranges between 2600–2800 E for southwestern British Columbia, which is somewhat lower than the theoretical value of 3086 E. The usefulness of the method in mining exploration is doubtful, especially in hilly or mountainous terrain where VGG values are shown to be very terrain sensitive. However, the importance of knowing the regional VGG variations is emphasized by the work over the Hat Creek coal deposit, British Columbia.
Two representations of the fan filter
Authors: Hildebrand Steve T.
Published: October 2002

The fan filter is a two‐dimensional (2-D) velocity filter that removes low apparent velocity events from seismic data. The convolutional representation was derived by Embree et al (1963) and by Fail and Grau (1963). Treitel et al (1967) extended the representation to include a recursive realization. Its basic use includes migration dip suppression, multiple suppression, slant stacking, and noise suppression in stacking.
Wave‐equation migration in the presence of lateral velocity variations
Authors: Castle Richard Jay
Published: October 2002

An algorithm for three‐dimensional wave equation migration in the presence of lateral velocity gradients is developed. The algorithm is based geophysically on the exploding reflector model and mathematically on integral solutions to the scalar wave equation. In addition, the velocity is assumed to vary slowly over a seismic wavelength. The primary application of the algorithm is to stacked sections. However, if the velocity is a function of depth only, the algorithm may be used to migrate and/or image common‐depth‐point gathers.
Formal gravity interpretation over the 800-m Darai escarpment in New Guinea
Authors: Anfiloff Vadim , Flavelle Alan J.
Published: October 2002

A gravity traverse over the 800-m escarpment at Darai in New Guinea is analyzed using the formal interpretation approach in which processing and interpretation are combined in a fully computerized procedure. The topographic anomaly is computed by modeling the air‐rock interface, and the density of the escarpment is deduced from Nettleton’s density profiles after making allowance for upward continuation. To aid in the analysis, synthetic density profiles are produced for escarpment models and for the real escarpment. The analysis shows that a fault is located near the base of the escarpment and the bulk density there is [Formula: see text]. There is no vertical continuation bump directly over the fault. The upthrown basement is at a depth of roughly 2500 m. The fault’s position near the base of the escarpment suggests a history of repetitive crustal movements.
Quantifying resolving power of seismic systems
Authors: Widess M. B.
Published: October 2002

A quantitative formulation of vertical resolving power of seismic exploration systems is presented and is offered as a proposed characteristic, or standard, resolving power identified with individual systems. The formulation broadens the classical concept of resolution by taking into account the reflection waveform and the noise, in addition to the classical time variable. The principal feature in the formulation is the stipulation that the intratrace distribution of reflections and of noise be treated as random (Gaussian) distribution, which is regarded as the most general representation for seismic sections as a whole. Through this quantification of vertical resolving power and therefore of intratrace reflection quality, a number of elemental reflection properties that have been described only qualitatively in the past are expressed by simple formulas. The quantification is consistent with the concept that the resolving power of a noise‐free zero‐phase system with a flat spectral band response is proportional to the bandwidth. The derived basic formula for the proposed characteristic resolving power is [Formula: see text], where [Formula: see text] is the maximum (absolute) amplitude of the signal wavelet of a seismogram interval, E is the energy of the signal wavelet, and noise is neglected. The quantification of the reflection properties, including taking the noise into account, stems from this formula. The classical concept of resolution, which considers only the time variable, such as the dominant period of signal wavelets, is applicable essentially only in cases of two noise‐free equal‐strength reflections. In contrast, the proposed formulation of resolving power accommodates a wide scope of applications and might be considered basic to seismic systems. I present theoretical material for evaluating the merits of the proposal. Suitable comparisons by seismic modeling would be useful in the overall evaluation.
Gravity models of the Albuquerque basin, Rio Grande rift, New Mexico
Authors: Birch Francis S.
Published: October 2002

As part of a hydrologic study, gravity anomalies over the Albuquerque basin were modeled to determine the thickness of the low‐density Neogene basin deposits, the major aquifer of the basin. The models are constrained by substantial density log and well data. Fifteen two‐dimensional (2-D) models include four major density units. The upper unit (density [Formula: see text]) corresponds to the Neogene syn‐rifting deposits. Thickness of this unit averages about 1.5 km and exceeds 2.0 km in a strip along the east side of the basin. The thickness is about 1.0 km between the Albuquerque and the Santo Domingo basins. At the south end of the Albuquerque basin, the thickness is less than 0.5 km. The configuration of the remaining density units, less constrained by well data, raises significant questions about the history of the basin that cannot be answered with existing data.
Magnetic anomalies of two‐dimensional bodies in a magnetic half‐space
Authors: Sampaio Edson E. S.
Published: October 2002

The presence of magnetization in country rock modifies the anomaly caused by magnetic bodies. Such a modification is distinct from the self‐demagnetization effect of the body, and the concept of susceptibility contrast is not adequate to explain it. We can achieve an exact understanding of the problem by solving the potential function in three media: air, magnetic country rock, and magnetized body. This paper sets up the solution of this problem when the magnetized body is a circular cylinder with an infinitely long horizontal axis, for both a horizontal and a vertical inducing ambient field. It expresses the solution of Laplace’s equation in bipolar coordinates for the potentials in the form of Fourier series. Analysis of the vertical, horizontal, and total magnetic anomalies shows that neglect of country rock magnetization reduces the apparent causative body dimensions.
Analytic minimum entropy deconvolution
Authors: Ulrych Tad J. , Walker Colin
Published: October 2002

The standard model of the reflection seismogram assumes an impulsive, scalar, reflectivity series convolved with a seismic wavelet. Under certain, not unlikely, conditions a reflection coefficient can become complex and the resulting seismogram may be shown to contain phase‐shifted wavelets. When this is the case, scalar deconvolution approaches fail. To overcome this problem, Levy and Oldenburg (1982, this issue) have proposed a technique based on the analytic signal formulation. We couple the Levy and Oldenburg approach with a complex version of the minimum entropy algorithm and apply this new technique to synthetic and real examples.
Frequency resolution of Fourier and maximum entropy spectral estimates
Authors: Marple S. L.
Published: October 2002

An analytic determination of the frequency resolution for maximum entropy and conventional Blackman‐Tukey spectral estimates is made for the case of known autocorrelation. As the signal‐to‐noise ratio decreases, the maximum entropy resolution is no better than that achievable by the Blackman‐Tukey spectral estimate. The mean resolution of an ensemble of spectra constructed from sampled data sequences agrees with the analytic result.
The effect of near‐surface superparamagnetic material on electromagnetic measurements
Authors: Buselli G.
Published: October 2002

The development of an instrument that enables transient electromagnetic (TEM) measurements to be made to voltage levels of 1 μV/A and less has enabled the detection of an anomalous transient response in some areas with lateritic soil cover. This anomalous transient causes apparent resistivity values derived from the measured transient decay to decrease at late delay times in areas where the known geology indicates the values should increase with delay time toward the resistivity value of the basement. The main cause of the anomalous transient has been identified as the response of superparamagnetic material in the lateritic soil cover. Both field and laboratory measurements of the voltage M induced by this transient, show a [Formula: see text] time dependence. This is the same behavior reported previously for magnetic viscosity over a longer time scale. Measurements of magnetic susceptibility of material separated magnetically from soil samples taken at areas where a residual response is measured, show that over a wide temperature range (from −196°C to 590°C) the susceptibility increases with temperature, confirming the presence of superparamagnetic particles. The anomalous transient response is localized to within 3 m of the transmitter loop; it is consequently detected only by loop configurations where the receiver loop is in proximity to the transmitter loop. The effects caused by the presence of a superparamagnetic response within 3 m of the transmitter loop apply to all electromagnetic methods, whether the measurements are made in the time or frequency domain.
Spontaneous potential log response expressed as convolution
Authors: Nosal E. A.
Published: October 2002

A special case of spontaneous potential (SP) logging, which has a closed‐form solution, will be expressed as a convolutional operation. Such a formal demonstration serves two purposes. First, it separates the individual contribution of the tool from that of the earth. Second, it places this logging device within the mathematical context of signal analysis. The special case for which a closed‐form solution is known is that where all resistivities are equal. Fourier analysis applied to this solution leads to a product of two functions, of which one is identified as the contribution of the earth and the other of the tool.
Thin‐layer response and spectral bandwidth
Authors: de Voogd N. , den Rooijen H.
Published: October 2002

A derivation is presented of the response of an embedded thin layer to a vertically incident seismic pulse. The reflected pulse has the shape of the time derivative of the incident‐wavelet, and its amplitude is proportional to the two‐way traveltime in the thin layer and to a factor depending upon the ratio of acoustic impedances. The influence of spectral bandwidth on interface resolution and thin‐layer response is investigated by means of zero‐phase signals, and a filtering philosophy is proposed which enables thickness estimation either from amplitude or from peak‐to‐trough time.
Seismic source decomposition
Authors: Stoffa Paul L. , Ziolkowski Anton
Published: October 2002

We exploit the differences that exist between the radiation fields of a point source and an array to design a time‐separated marine seismic source array with desired power spectral and directional characteristics, whose far‐field time signature is known precisely from measurements. The desired power spectral characteristics are created by firing a predetermined series of point source units sequentially, such that their time signatures do not overlap. The effective power spectrum of the whole series of time‐distributed signatures can be made to approximate the sum of the power spectra of the individual signatures and can, therefore, be designed to suit the desired application by the appropriate choice of source units. The desired directional characteristics of the array can be created by arranging the source unit separations such that each source unit reaches the desired spatial position at the prescribed firing instant. The key to the subsequent processing of the recorded data is to measure the pressure wave generated by each point source unit with a hydrophone placed close by, but in the linear radiation field. The position of this hydrophone relative to the source unit must be known accurately in all three dimensions. The depths of the source units and their relative spatial positions at the instants of firing must also all be known. From these measurements the far‐field signature of the sequence in any azimuth can be deduced, and the impulse response of the earth can be recovered by dividing the Fourier frequency spectrum of the recorded reflection data by that of the measured source unit sequence. This process is completely deterministic in nature and depends primarily upon our ability to monitor accurately the far‐field signature of each source unit. Field results from an initial evaluation of this method indicate that this measurement can be readily accomplished. The success of this technique is then ultimately dependent on the signal to noise ratio. [This method is the subject of a patent application.] We stress that, since the signature is known, we are not obliged to make any assumptions about the phase. In particular, we do not need to make the minimum‐phase assumption. We are therefore free to choose our parameters to optimize our desired power spectral and directional characteristics with complete disregard for the conventional requirement that the signature of an air gun source have a high primary‐to‐bubble ratio.
Electrical conductivity of dry lower crustal rocks
Authors: Kariya K. A. , Shankland T. J.
Published: October 2002

This study provides values of electrical conductivity of possible lower crustal materials to assist interpretation of lower crustal magnetotelluric soundings. We present mean values of conductivity measurements collected from the literature for dry mafic and silicic rocks in the temperature range of 500°C to 1000°C. We observe statistically significant differences between rock types: mafic rocks are better conductors than granites by about half an order of magnitude and within the mafic group, aphanitic (fine‐grained) rocks have higher conductivity than phaneritic (coarse‐grained) ones. “Best‐fitting” curves of log conductivity versus temperature are presented for each rock type to show mean log conductivity values together with standard deviations so that most probable temperature ranges can be inferred from conductivity. Because the laboratory rocks are dry, their conductivities are lower at a given temperature than they would be if fluids or volatiles were present; hence any temperatures inferred from magnetotelluric (MT) contivities are upper bounds.
Fast seismic inversion
Authors: McClary W. Keith
Published: October 2002

The equal traveltime layer model of a horizontally layered medium with waves at normal incidence is used to solve the inverse problem (determination of the reflection coefficients and fundamental polynomials from the surface response) by an efficient algorithm using [Formula: see text] computations for N layers. The idea is to reduce the N‐layer problem to two similar problems of N/2 layers each plus additional computations of type B and C. Type B computes the data for the second problem from the result of the first and type C combines the results. For [Formula: see text] the reduction is carried out k times leading to N trivial one‐layer problems plus more computations of type B and C, which are polynomial multiplications or discrete convolutions. The improvement over the [Formula: see text] computations required by recursive methods comes from using the fast Fourier transform to perform the convolutions.
Further discussion on attenuation and dispersion of electromagnetic wave propagation in fluid‐saturated porous rocks and applications to dielectric‐constant well logging
Authors: Pascal H.
Published: October 2002

This paper presents a more detailed analysis of some basic problems of electromagnetic wave propagation through a porous medium saturated with fluid, associated directly with quantitative interpretation of dielectric constant logging. The advantages and disadvantages of a new approach, in which fluid‐saturated porous rock is considered as a two‐phase system, are discussed and compared with those obtained from the single‐phase model. It is shown that the two‐phase model may provide a better interpretation of dielectric constant logging.
On the relationship between apparent resistivity functions in the case of complicated underground structures
Authors: Patella D.
Published: October 2002

The pole‐pole, pole‐dipole, and dipole‐dipole are electrode arrays frequently utilized in geoelectrical soundings. In view of some classical applications, such as the transformation of sounding curves (Patella, 1974) and the computation of standard curves by means of digital linear filters (Ghosh, 1971; Kumar and Das, 1978), I derive in the present note the relationships existing between the apparent resistivity functions of the pole‐dipole and dipole‐dipole classes of arrays and the apparent resistivity function of the pole‐pole system in the case of complicated underground structures. Before doing this, I first review the main features of the above arrays.
Higher order auto‐spectra by maximum entropy method
Authors: Plaisted Robert Owen , Peña Hugo Gustavo
Published: October 2002

Higher order auto‐spectra, in particular bispectra and perhaps trispectra, are being used increasingly for analyzing various nonlinear interactions in the ocean, e.g., Herring (1980) and McComas and Briscoe (1980). The resolution of these spectra, as with conventional energy spectra, is frequently limited because short data records must be used. The purpose of this note is to present a maximum entropy (MEM) representation for higher order auto‐spectra which has the advantage of the superior resolving power of the MEM technique under these circumstances. The derivation is a generalizaton of the power spectra derived for a linear process (Box and Jenkins, 1970). We derive an MEM representation for bispectra and show that this result can be generalized to auto‐spectra of any order.
Velocity estimates derived from three‐dimensional seismic data
Authors: Owusu Kwame , Gardner G. H. F. , Massell Wulf F.
Published: October 2002

A new computer algorithm is described by which velocity estimates can be derived from three‐dimensional (3-D) multifold seismic data. The velocity estimate, referred to as “imaging velocity,” is that which best describes the diffraction hyperboloid due to a scatterer. The scattering center is best imaged when this velocity is used in the reconstruction process. The method is based on the 3-D Kirchhoff summation migration before stack. The implementation consists of two basic phases: (1) differentiating the input field traces and resampling them to a logarithmic time scale, and (2) shifting, weighting, and summing each resampled trace to a range of depth levels also chosen on a logarithmic scale. Peak amplitudes in the resulting image matrix give a time T and depth Z from which velocity is obtained using the relation [Formula: see text] The locus of constant velocity is a slanted straight line in the coordinate system of the matrix. In the usual application of migration for velocity analysis, each input trace of N samples is migrated for each of M constant velocity functions requiring [Formula: see text] moveout shift calculations. In the new method presented here, a constant shift is calculated for a given resampled trace, for each depth into which it is summed. This reduces the number of calculations per trace to about N, resulting in a significant improvement in computing efficiency. The operation of the algorithm is illustrated using synthetic and physical model data.
On: “Seismic array noise studies at Roosevelt Hot Springs, Utah geothermal area”, by E. J. Douze and S. J. Laster (GEOPHYSICS, v. 44, p. 1570–1583).
Authors: Asten Michael W.
Published: October 2002

Douze and Laster (1979) considered the problem of identifying the mode of propagating isotropic seismic noise by the characteristics of the cross‐spectrum between two geophones. For the vertical component of isotropic single‐mode Rayleigh wave noise, they derived the theoretical relation [Formula: see text] where ψ(ω) is the power spectrum, [Formula: see text] is the cross‐spectrum between geophones 1 and 2, Δx is the geophone separation, V is the phase velocity of the waves at angular frequency ω, and [Formula: see text] is the zero‐order Bessel function. They noted that the cross‐spectrum is purely real and “no apparent phase velocities can be obtained from the data.”
On: “Ionospheric induced very low frequency electric field wavetilt changes” by D. V. Thiel and I. J. Chant (GEOPHYSICS, January, 1982, p. 60–62)
Authors: Singh Ramesh P. , Rankin D.
Published: October 2002

The paper by Thiel and Chant reports the well‐known sunrise and sunset effect which in micropulsation studies is referred to as the dawn and dusk “chorus.” However, we disagree with the interpretation of these authors. Their choice of wavetilt as the diagnostic function is slightly misleading since they are essentially measuring at the surface of the earth despite the unfortunate location of their magnetic sensor at a height of 4 m and presumably in a building. In this case the wavetilt [Formula: see text] according to their equation (1) is equivalent to [Formula: see text] where [Formula: see text] is the impedance of the earth and [Formula: see text] is the impedance of free space. This result is independent of the mode of propagation but certainly at their distance of 4000 km is predominantly the sky wave except during the dawn and dusk periods. The turbulence due to the formation or breakup of the D‐layer in the ionosphere virtually destroys the ionospheric reflected component which is the dominant contributor to the field incident on the earth. We suggest that during most of the day the authors are measuring the impedance of the earth scaled by a numerical constant.