bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulating true amplitude reflections by stacking shot records
Authors: Hubral Peter
Published: October 2002

Field seismograms resulting from concentrated sources provide the starting point for all acquisition, processing, analysis, and interpretation techniques in seismic exploration. As pointed out recently by Bortfeld (1982), Hubral (1983), and Krey (1983), certain benefits with respect to improving (a) pulse deconvolution, (b) poststack migration, and in particular (c) the estimation of interface reflection coefficients, can however be gained by transmitting so‐called normal waves to (generally curved) reflectors in a laterally inhomogeneous earth.
On: “Airborne gravity is here” by S. I. Hammer (GEOPHYSICS, v. 48, p. 213–223).
Authors: Steenland Nelson C.
Published: October 2002

It is well and good to bring airborne gravity to the attention of practicing geophysicists, but at the very least the presentation must be correct. The logical test of accuracy only can be found in the comparison of ground and airborne positive, caprock anomalies. Other evidence is illogical, i.e., measurements of how well the airborne system is repeating itself. The caprock example in Figure 8 best marks the nonauthenticity. The ground anomaly at High Island appears to be 4 mgal, estimating the position of the salt minimum to which the positive must be referred. (The actual value of the positive could have been provided in the text.) The airborne positive at 1000 ft asl, approximately 800 ft above ground level and 1100 ft above caprock (estimated from Figure 8, again not given), appears to be 1 mgal. Well, instead of modeling the ground anomaly, then computing the model’s anomaly at 1000 ft (an excellent method of continuation which may use a synthetic model if no subsurface facts were available, which doesn’t need gridded data, only modeling the observed points), the author resorts to a demonstrably erroneous method of filtered continuations, further confused by the absence of vertical and horizontal scales in Figure 9.
Three‐dimensional terrain corrections in resistivity surveys
Authors: Holcombe H. Truman , Jiracek George R.
Published: October 2002

A three‐dimensional finite‐element computer algorithm which can accommodate arbitrarily complex topography and subsurface structure has been developed to model the resistivity response of the earth. The algorithm has undergone extensive evaluation and is believed to provide accurate results for realistic earth models. Testing included comparison to scale‐model measurements, analytically calculated solutions, and results calculated numerically by other independent means. Computer modeling experiments have demonstrated that it is possible to remove the effect of topography on resistivity data even under conditions where such effects are large with respect to the subsurface responses. This can be done without resorting to lengthy and costly trial‐and‐error computer modeling. After correction, the data can be interpreted as if the anomalies are due only to subsurface structure. The results of case studies on field data measured in high‐relief topography indicate the following. (1) Pre‐ or postsurvey 3-D computer analyses of topographic effects are worthwhile and can be done for a small fraction of the survey cost. (2) Three‐dimensional terrain corrections can significantly improve the quality of subsurface interpretation in terrain geometries which cannot be modeled with a 2-D algorithm. (3) Terrain effects on collinear electrode arrays can be minimized by alignment of the arrays parallel to the strike of the topography (that is, along streamlines and ridgelines). The computing cost of performing 3-D analyses is relatively small compared to the man‐hour costs required to implement them. The total cost of a 3-D terrain correction is typically small compared to the survey cost, especially if one uses the new generation of large main‐frame computers.
On: “Acoustic reciprocity—A paradox” by E. Eisner (GEOPHYSICS, v. 48, p. 1132–1134) or “Eisner’s paradox resolved”
Authors: Dahlen F. A. , Odom R. I.
Published: October 2002

Eisner has concocted a clever geometrical arrangement that appears to contradict the principle of acoustic reciprocity. In essence his example consists of a truncated opaque ellipsoidal reflector with the source and receiver situated at the foci as shown in Figure 1. In the limit of geometrical optics it is evident that only half the energy emitted by a source at 2 will contribute to the reflected arrival at 1 while a substantially larger fraction of the energy emitted by a source at 1 will appear at 2. Naively it seems as if the geometrical amplitude of the reflected arrival at 2 should exceed that at 1, in apparent violation of the reciprocity theorem.
A case study of stratigraphic interpretation using shear and compressional seismic data
Authors: McCormack M. D. , Dunbar J. A. , Sharp W. W.
Published: October 2002

This paper describes the use of surface recorded compressional and horizontal shear wave seismic data to detect lateral changes in the physical properties of a clastic unit. Shear and compressional wave transit times were measured across a selected interval from CDP stacked sections derived from data collected along coincident shear and compressional seismic lines. At each surface position the ratio of the shear to compressional transit time across the target horizon is calculated. It is shown that lateral variations in this ratio, coupled with the behavior of the individual transit time curves, can be used to infer changes in the physical properties of a formation. The horizon selected for this case study was the lower Pennsylvanian Morrow formation which produces gas from channel sand bodies at the Empire Abo field, New Mexico. A detailed geologic section of the producing horizon was mapped along a seismic line oriented so that it crossed productive and nonproductive regions of the field. Shear and compressional Vibroseis® surveys were conducted along this surface profile using data acquisition parameters designed to produce comparable signal‐to‐noise ratios and resolution in both sets of field data. After processing, the shear and compressional interval transit times through the Morrow formation decreased in going from nonproductive to productive thicknesses of sand. Furthermore there is a proportionately greater decrease in the shear wave transit time than in the compressional transit time resulting in an overall decrease in the ratio of shear to compressional transit times. While several possible physical changes in the lateral properties of the reservoir could explain these observations, it is concluded that the primary mechanism causing these ratio changes is variation in the sand‐shale ratio within the Morrow formation.
Offshore electrical exploration of sedimentary basins: The effects of anisotropy in horizontally isotropic, layered media
Authors: Edwards R. N. , Nobes D. C. , Gómez‐Treviño E.
Published: October 2002

Our electrical method for exploring beneath the sea consists of a long, vertical, bipolar ac source, extending downward from the sea surface to the bottom of the sea and a remote, encapsulated, microprocessor‐controlled magnetometer located on the sea floor. The amplitude and phase of the magnetic field are measured over a range of suitable frequencies and transmitter‐magnetometer separations. At the low‐frequency static limit, apparent resistivity curves, similar to standard Schlumberger resistivity sounding curves, are constructed as an aid in the direct interpretation of isotropic crustal resistivity. An intermediate relatively resistive or relatively conductive zone is detectable when the transmitter‐receiver separation exceeds the order of twice the depth to the zone. The physical property resolved by the method in an anisotropic crust, which has different horizontal and vertical resistivities, is the geometric mean of the two independent resistivities. The thickness of a layer is indeterminate. A layer with a coefficient of anisotropy f responds like an isotropic layer f times thicker. At higher frequencies, when induction in the sea water is significant, the apparent resistivity curves remain valid provided locally induced current flow does not dominate the galvanic flow in the crustal material beneath the sea. The presence of some locally induced current, at the electromagnetic resistive limit, is advantageous. It enables the coefficient of anisotropy f of an anisotropic zone to be determined jointly with the mean resistivity. An approximate direct scheme involves the calculation of the apparent anisotropy, a formula which, like the apparent resistivity formula, is a function only of field observations, in particular the phase difference between the measured magnetic field and the transmitted current. The depth of penetration and the resolution of mean resistivity and anisotropy are presented in terms of Fréchet kernels and resolving kernels. The kernels are analytic for the special case of a uniform crust. The shape of the Fréchet kernels for resistivity and anisotropy are different. At low frequency, this reflects the different behaviours of the galvanic in‐phase current flow and quadrature locally induced current flow. The sensitivity function for anisotropy vanishes identically at zero frequency. The inclusion of the complication of anisotropy for the interpretation of data collected over sedimentary basins is mainly for numerical convenience. The sediments themselves are unlikely to be anisotropic on a small scale. The anisotropic behavior is due to macro‐anisotropy, the grouping together of thin isotropic layers of different isotropic resistivities. Such a grouping is introduced into both forward and inverse computer algorithms when the resolving kernels about a given depth are wider than the thickness of a typical layer.
Electromagnetic response of a discretely grounded circuit
Authors: Wait James R.
Published: October 2002

In implementing electrical methods of geophysical exploration one is often faced with the spurious effects of extraneous targets. A good example is a metallic fence or similar conductive structure that is grounded to the earth at a discrete number of points. The quantitative estimate of response from such unwanted targets is very difficult. Furthermore, the precise nature of the configuration of the conductor is not known. Nevertheless a general procedure for dealing with such situations would be useful even if it is quite approximate. In an earlier investigation, Nelson (1977) constructed an ingenious circuit model to deal with the induced polarization (IP) response of a barbed wire fence that took account of the grounding impedances. He also referred to other studies that were never made available in the open literature. In Nelson’s formulation the fence was treated as a resistive network and all electromagnetic (EM) coupling effects were ignored. It seems worthwhile to extend and generalize his approach to allow for a wider application. This is the purpose of this analysis.
Models for thermoelastic attenuation of waves in heterogeneous solids
Authors: Armstrong Baxter H.
Published: October 2002

Some one‐dimensional models of a heterogeneous solid are presented consisting of a succession of slabs with different anharmonic properties. The equation for the temperature variation in these models due to passage of a longitudinal elastic wave can be solved exactly in the approximation of weak attenuation. The solutions are given in terms of the forced oscillation plus the temperature wave solutions to the homogeneous equation needed to match the boundary conditions of continuity of temperature and thermal current. Thermoelastic attenuation due to this temperature variation is compared to that of Zener’s classical approach. For periodic arrangement of slab properties or upon use of Zener’s boundary condition of vanishing thermal current, the temperature‐wave approach reproduces Zener‐type attenuation. However, a succession of slabs with a random, uncorrelated distribution of the Gruneisen constant leads to a new result with attenuation proportional to the three‐halves power of the wave frequency in the low‐frequency limit. The results are discussed in the context of seismoacoustic wave attenuation.
On: “Werner deconvolution for automated magnetic interpretation and its refinement using Marquardt’s inverse modeling” by C. C. Ku and J. A. Sharp (GEOPHYSICS, v 48, p. 754–774, June, 1983).
Authors: Rao V. Babu
Published: October 2002

The above paper on Werner deconvolution is very interesting. However, we notice the following discrepancies in the equations dealing with the seven‐point Werner operator with quadratic interference.
Limitations of large‐loop transient electromagnetic surveys in conductive terrains
Authors: Spies Brian R. , Parker Patricia D.
Published: October 2002

Conventional interpretation aids used in transient electromagnetic (TEM) exploration assume that the anomalous response is due to induction in a simple model such as a plate, sphere, or uniform layer. Estimates of conductivity, depth, and size are based on analyzing the profile shape and transient decay constant. In regions with conductive overburden or conductive bedrock, TEM responses obtained with the large‐loop configuration can often be very dependent upon the location of the transmitter loop, and the results can be easily misinterpreted. A series of scale‐model studies was carried out to investigate the influence of current channeling and gathering phenomena with the large fixed loop and moving single‐loop configurations of the TEM method. Models studied included resistive and step discontinuities in a horizontal conductive slab, and a vertical plate in electrical contact with conductive overburden. Large TEM anomalies can be observed at the edge of a conductive unit, when diffusing eddy currents migrating through surface conductors are channeled and become spatially localized. Current gathering occurs when the diffusing eddy currents are gathered into a locally more conductive zone. The most common geometric condition under which these effects are observed is when a conductor of long strike length is located outside a large transmitter loop. The TEM response is enhanced by channeling and gathering, and it is often stronger than simple inductive effects. Current channeling and current gathering phenomena are relatively unimportant when measurements are made within a large transmitter loop, or when a single‐ or in‐loop configuration is used. It is therefore recommended that multiple transmitter loop locations be used when surveying in conductive terrains with the largeloop configuration, or alternatively that the single‐ or in‐loop configuration be employed for additional geophysical control.
Simplified prediction of transient electromagnetic response
Authors: Becker A. , DeCarle R. , Lazenby P. G.
Published: October 2002

In general the target response to an electromagnetic system is a complex function of target geometry and of the geometrical and electrical parameters of the apparatus used to make the measurements. Thus, the prediction of system performance is usually done with respect to a particular target. Under some circumstances, however, one can predict the effect of changing one of the system parameters by assuming that the typical conductor in a given area is well represented by a shorted single‐turn loop. This simple, classical concept is shown to be valid for evaluating the effect of primary pulse width on target response to the Input airborne electromagnetic system. The justification of our basic technique for predicting this aspect of AEM system performance is provided by the analysis of Input surveys done in two areas of the Precambrian Shield in Canada. Repeat surveys were done in each area with a 1 ms and a 2 ms primary pulse width. The change in system performance that could be expected by doubling the pulse width was correctly predicted after an analysis of the 1 ms data.
The Newmont drill‐hole EMP system—Examples from eastern Australia
Authors: Boyd G. W. , Wiles C. J.
Published: October 2002

Surface and drill‐hole EMP (ElectroMagnetic Pulse) surveys are routinely carried out by Newmont in various countries around the world. Results from drill hole EMP surveys on three base metal deposits in New South Wales are presented and discussed. Interpretation of the EMP data is aided by computer modeling of the results to determine location, depth, strike, dip, and quality of conductors. Computer models for all three deposits give excellent agreement with geology and drill intersections.
The role of simple computer models in interpretations of wide‐band, drill‐hole electromagnetic surveys in mineral exploration
Authors: Dyck Alfred V. , West Gordon F.
Published: October 2002

Drill‐hole geophysical surveys are a means of extending the search for massive‐sulfide deposits to depths which are inaccessible to conventional surface techniques. The present investigation combines field and model studies of an electromagnetic (EM) prospecting method which utilizes a large, fixed transmitter loop with a downhole, axial‐component magnetic field sensor (solenoid). The system is shown to be well‐suited for detection of deeply buried massive‐sulfide conductors located in resistive host rock at appreciable distances from the drill hole. We propose that drill‐hole survey data collected with a wide‐band large‐loop EM system can be routinely used for estimating target parameters by forward modeling with two simple conductor shapes: a plate and a sphere in free space. Analysis of confined conductors is facilitated by “eigencurrent decomposition” of the induced current vortex into a set of noninteracting loops with simple RL‐circuit behavior. Solutions have been implemented in interactive computer programs which are fast, inexpensive, and sufficiently versatile to accommodate configurations and waveforms used in many practical EM systems. Both stationary and dynamic aspects of the induction process are exploited for diagnosis of three‐dimensional targets. Field studies at test sites in Sudbury and Noranda base‐metal mining areas with a commercial pulse EM system indicate that many important effects predicted by the model studies are, indeed, observable in survey data.
Radio‐frequency geotomography for remotely probing the interiors of operating mini‐ and commercial‐sized oil‐shale retorts
Authors: Somerstein Stephen F. , Berg Mike , Chang David , Chung Hwang , Johnson Hal , Richardson Bill , Pizzicara John , Salisbury Winfield W.
Published: October 2002

Cross‐borehole, radio‐frequency geotomographs were made across two different‐sized, operating, experimental, underground, in‐situ, oil‐shale retorts. The tomographs taken of the smaller retort were of a plane 16.7 m wide by 18.0 m high bisecting the retort. The measurements were taken at a frequency of 25 MHz and showed excellent correlation of high signal attenuation with the high‐temperature zones. Measured attenuation coefficients (inverse skin depth) at 25 MHz ranged from [Formula: see text] across the cool, unrubbled, sill‐pillar, and between [Formula: see text] for the high‐temperature 370–700° C zone. Image resolution was approximately 1 m. The signal attenuation across the lower retort region was also found to correlate well with the movement and concentration of condensed water. The conventional algebraic deconvolution method (ART) was modified for limited perspective and finite beam width, and gave results which were in good agreement with thermocouple data. The measurements made on the larger retort were taken over a period of 33 days of retort burn and mapped the attenuation coefficients in a vertical plane 90 m wide by 48 m high at a frequency of 1.5 MHz. At this frequency, attenuation coefficients in the cool, dry retort regions were between 0.12 and [Formula: see text], while regions containing a high moisture content had coefficients of from 0.15 to [Formula: see text]. In the regions encompassing the retorting and combustion zones, attenuation coefficients were between 0.15 and [Formula: see text]. Some additional effects on the attenuation measurements were observed due to nearby thermocouple piping. The movement of the contours of attenuation coefficient with time followed temperature changes, though the paucity of thermocouples in the tomographic plane only allowed a marginal correlation to be made. Overall results suggest that radio‐frequency geotomography can be a useful tool for mapping in‐situ moisture concentrations and temperature fronts in an operating in‐situ oil‐shale retort.
Cross‐hole magnetometric resistivity (MMR)
Authors: Nabighian M. N. , Oppliger G. L. , Edwards R. N. , Lo B. B. H. , Cheesman S. J.
Published: October 2002

The last decade has seen a growing acceptance of the magnetometric resistivity (MMR) method as a viable exploration technique in various geologic environments. Until recently, MMR exploration was carried out with both current electrodes and recording magnetometer located on the surface of the Earth. Significant improvements in anomaly amplitude can be achieved by lowering the recording magnetometer inside a drill hole. In contrast, the lowering of current electrodes beneath the surface does not always improve surface MMR responses. The advantages of locating the magnetic detector in a drill hole are illustrated numerically, anomaly calculations being carried out with a novel yet simple integral equation technique for a plate‐like body. The practicality of the cross‐hole MMR technique is demonstrated with a successful case history. Massive sulfide mineralization is mapped at a depth exceeding 500 m.
Application of inverse filtering to gamma‐ray logs: A case study
Authors: Bristow Q. , Conaway J. G. , Killeen P. G.
Published: October 2002

The application of digital inverse filter deconvolution techniques to seismic data has been routine for many years. More recently these techniques have been extended to natural gamma‐ray logging in order to improve the spatial resolution of the recorded logs (Czubek, 1971; Conaway and Killeen, 1978a,b). Early work in this field (Scott, 1963) involved an iterative procedure which required repeated processing of an entire log data set. Such a technique does not lend itself to continuous on‐line deconvolution of a log while the logging operation is in progress. The inverse digital filter approach, by contrast, is particularly well suited for implementation in a computer‐based borehole logging data acquisition system. Such a system had been developed at the Geological Survey of Canada (G.S.C.) by 1978 and was described by Bristow and Killeen (1978) and Bristow (1977, 1979).
On: “Ionospheric induced very low frequency electric field wavetilt changes” by D. V. Thiel and I. J. Chant (GEOPHYSICS, Jan., 1982, p. 60–62)
Authors: Wait James R.
Published: October 2002

With all due respect to Singh and Rankin (1983), I believe that their criticism of the paper by Thiel and Chant (cited above) needs clarification. Contrary to the assertion by Singh and Rankin, the quoted formula for the function [Formula: see text] (incorrectly called a wavetilt) can indeed depend upon the mode of propagation. The factor sin θ, appearing in the quoted expressions, is the sine of the complex angle of the incident wave whether it be a ground wave or a downgoing ionospherically reflected wave. This point is central to the long‐standing dispute between the “Cagniard school” of magnetotelluric sounding and those who believe that source field characteristics can play a role. Thiel and Chant correctly pointed out that the elliptical polarization of the downgoing wave can mess up the results. But possibly they overlooked that this effect, in a surface impedance measurement, is minimized compared with a true wavetilt measurement. The latter senses the vertical electric field with a vertical (i.e., whip) antenna, whereas in a true surface impedance measuring scheme both horizontal E and horizontal H fields are sensed. Actually Thiel and Chant measured surface impedance, so the elliptical polarization of the downgoing wave probably plays a minor role.
Wave‐equation velocity analysis
Authors: González‐Serrano Alfonso , Claerbout Jon F.
Published: October 2002

Velocity sensitive (wide propagation angle) seismic data do not comply with the rms small propagation angle approximation. A hyperbolic velocity spectrum and Dix’s equation cannot use wide‐angle arrivals to estimate interval velocity accurately. The linear moveout method measures interval velocity exactly in stratified media. Snell midpoint coordinates are constructed to image the data before velocity estimation. Energy focuses at the arrival coordinates of a fixed reference Snell wavefront as required by the linear moveout method. The image of a common‐midpoint seismic gather in Snell midpoint coordinates, for a nonvertical reference Snell wave, defines the wave‐equation velocity spectrum. Two important properties of the velocity spectrum are locality, where energy is a local function of velocity; and linearity, that is invertible using linear transformations. Velocity sensitivity of wave equation extrapolation operators increases with angle of propagation. In Snell midpoint coordinates, angles are measured relative to an arbitrary slanted reference Snell wave. At this particular angle wave‐equation operators are exact, independent of velocity. Approximations of the wave equation in Snell midpoint coordinates satisfactorily image wide‐angle energy. To compute the velocity spectrum, we use the 15‐degree finite‐difference wave equation in the frequency domain. This equation is insensitive to the downward continuation velocity. This formulation resolves multivalued, wide velocity spectrum data using inhomogeneous, offset and depth dependent, downward continuation velocity. Stepout filtering concurrent with downward continuation eliminates wide‐angle propagation energy not modeled by the 15‐degree wave equation.
The role and application of entropy terms for acoustic wave modeling by the finite‐difference method
Authors: Dablain M. A.
Published: October 2002

The significance of entropy‐like terms is examined within the context of the finite‐difference modeling of acoustic wave propagation. The numerical implications of dissipative mechanisms are tested for performance within two very distinct differencing algorithms. The two schemes which are reviewed with and without dissipation are (1) the standard central‐difference scheme, and (2) the Lax‐Wendroff two‐step scheme. Numerical results are presented comparing the short‐wavelength response of these schemes. In order to achieve this response, the linearized version of an exploding one‐dimensional source is used.
On: “The normal vertical gradient of gravity” by J. H. Karl (GEOPHYSICS, 48, 1011–1013).
Authors: Swain C. J.
Published: October 2002

The implication of the author’s hypothesis, that the conventional free‐air correction factor is difficult to justify and can lead to large errors (e.g., 14 mGal from 300 m of topographic relief), would be very serious indeed for many interpretations of gravity data if it were true. He predicts a normal vertical gradient of 0.264 mGal/m near sea level, 14 percent lower than the conventional theoretical value. However, precise measurements of the free‐air gradient near sea level have been reported (Kuo et al., 1969) which differ by less than [Formula: see text] percent from the theoretical value; moreover these differences correlate with small local (isostatic) anomalies. My own observations at Leicester, England (elevation 100 m) and Nairobi (elevation 1 650 m) (made with students) also differ by less than [Formula: see text] percent from the theoretical values and again the differences correlate with small local anomalies. If these values represent the normal free‐air gradient, it would appear that the author’s analysis must be wrong. The formula he derives gives, correctly, the mean vertical gradient at some level over and within the Earth to a good approximation. This can be seen simply by considering the well‐known formulas for the gradient at a point within the Earth where the density is ρ [Formula: see text] and at a point outside the Earth [Formula: see text] and taking averages at this radius. However, the average value has no practical significance. It does not apply to any point on the Earth’s surface; it is merely a mean.
THE SEISMIC WAVE ENERGY REFLECTED FROM VARIOUS TYPES OF STRATIFIED HORIZONS
Authors: Muskat M. , Meres M. W.
Published: October 2002

Two applications are made of the reflection and transmission coefficients reported in the preceding paper. These concern the effect of the angle of incidence upon the fraction of incident energy returning to the surface, and the effect of velocity stratification upon the energy return.
CHANGE OF REFLECTION AMPLITUDE AND CHARACTER WITH GEOPHONE DEPTH
Authors: Houghton H. M.
Published: October 2002

The relation between character and amplitude of reflection and the geophone depth was investigated at several locations in Oklahoma. It is shown that the variation of reflection amplitude down to a depth of the order of 200 feet is, in general, what would be expected from changes of the acoustic impedance of the strata traversed. Changes in reflection character are illustrated by tracings from the records obtained.
PARTNERSHIP BETWEEN GEOLOGY AND GEOPHYSICS IN PROSPECTING FOR OIL
Authors: Eckhardt E. A.
Published: October 2002

It is my privilege to extend to the American Association of Petroleum Geologists, on this celebration of its silver anniversary, the felicitations of the Society of Exploration Geophysicists. The contemplation of twenty‐five years of sound and progressive history is no doubt a source of deep satisfaction to those who founded the Association and to many others who helped to make it grow. Simultaneously, the Society which I am here representing is celebrating its tin anniversary. When this coincidence first came to my attention, I hoped to find that the alloying of silver and tin, a noble and a base metal, produces a product with useful properties not possessed by either component alone. The rhetorical advantages which would have followed are obvious. The metallurgical literature, however, dashed this hope so that this allegorical approach to my subject had to be abandoned.
TYPICAL ELECTRICAL PROSPECTING METHODS
Authors: Rust W. M.
Published: October 2002

A very general review and discussion of electrical prospecting methods, particularly as applied to petroleum prospecting, is given. It is pointed out that while the electrical methods have not received the wide spread acceptance in this field that other methods enjoy, continued interest may well be justified. The opinion is expressed that the most optimistic sign of a brighter future for electrical prospecting for petroleum is the evidence being accumulated to support the theory of secondary effects in relatively shallow strata overlying deposits of oil and gas.
Two representations of the magnetotelluric sounding survey
Authors: Murakami Y.
Published: October 2002

One method to determine the resistivity structure of the earth, the magnetotelluric (MT) sounding method (Cagniard, 1953; Vozoff, 1972), is widely used especially for deep sounding. The surface impedance of the earth is measured for natural electromagnetic (EM) waves, and the apparent resistivity is calculated as a function of frequency. The MT sounding curve is a plot of the apparent resistivity with respect to the frequency. This representation of the sounding curve at larger abscissa (higher frequency) shows the shallower structure. Sometimes the sounding curve is plotted with respect to the period (the inverse of the frequency), so that the larger abscissa (longer period) corresponds to the deeper structure, as is the case for the Schlumberger sounding curve.
On: “Maximum uncertainty of interval velocity estimates” by Z. Hajnal and I. T. Serada (GEOPHYSICS, 46, November 1981, 1543‐1547).
Authors: Késmárky I.
Published: October 2002

In these remarks I draw attention to some practical aspects of the paper by Hajnal and Serada. Although conclusions drawn from equation (27) are clear and concise, the statistical analysis of equation (26) would lead to a more practical result. Taking the absolute values of all the terms on the right‐hand side of equation (26) may lead to a pessimistic overestimate of [Formula: see text].
A guide to current uses of vertical seismic profiles
Authors: Oristaglio Michael L.
Published: October 2002

Vertical seismic profiles (VSPs) are small‐scale seismic surveys in which geophones are lowered into a well to record waves traveling both down into the earth (direct waves from the surface source and downgoing multiples) and back toward the surface (primary reflections and upgoing multiples). VSPs thus contain information about the reflection and transmission properties of the earth with a coverage that depends upon the geometry of the VSP experiment and the structure near the well. This article describes the uses of VSPs in seismic exploration that have been published in the last three years and is designed to complement the more detailed surveys by Hardage (1983) and Balch and Lee (1984). When the earth is horizontally layered, the well is vertical, and the source is close to the wellhead, upgoing and downgoing waves recorded by the VSP travel vertically, and the VSP can be used to calibrate surface seismic sections by providing the time‐to‐depth curve and allowing a detailed analysis of reflection and transmission properties of the earth at a given location. These applications rely heavily on signal processing to separate the upgoing and downgoing waves and to study their relationships to data recorded at the surface. When the earth varies laterally or when the source is offset from the well, the VSP can be used to complement surface surveys by providing high‐resolution images of structure near the well. Current work has concentrated on forming images from the reflected waves by the methods of common‐depth‐point (CDP) stacking and migration. Tomographic methods for inverting the traveltimes and amplitudes of transmitted waves are also being developed and will become important when downhole arrays and powerful downhole sources are available. The most significant advance in the next few years, however, will be the development of a reliable three‐axis tool with internal devices for determining both the orientation of the tool and the quality of its coupling to the borehole wall.
Geotechnical and groundwater geophysics
Authors: Dobecki T. L. , Romig P. R.
Published: October 2002

Because of a change in emphasis from power plant siting to projects principally involving process and facility monitoring and certification of existing structures (dams, pipelines, etc.), geotechnical and groundwater geophysics is in quite a healthy state after some lean years following the demise of nuclear energy construction projects. The orders‐of‐magnitude jump in the computational capability of geophysicists working in these fields has overshadowed advances in instrumentation (e.g., digital enhancement seismographs), field methods (e.g., cross‐borehole EM), and interpretive procedures. The advent of powerful, affordable microcomputers has enabled expansion into applications demanding finer resolution and quicker turnaround of results. As a result, shallow seismic reflection, seismic and electromagnetic geotomography, and the complementary use of surface and borehole electrical resistivity and seismic data will soon be common if not dominant methods in geotechnical and groundwater investigations. Future trends point to increased emphasis on environmental and economic issues (e.g., toxic wastes or the stability of underground petroleum storage facilities), cross‐fertilization with petroleum reservoir engineering (process monitoring and detailed reservoir description), and greater involvement of computers in the planning, data acquisition, and interpretive phases of our projects. As computers take over more of the data collection‐processing‐interpretation sequence, one of the greatest challenges facing us will be to define the proper role of humans and to use the new technology wisely.
Synthetic vertical seismic profiles in elastic media
Authors: Dietrich Michel , Bouchon Michel
Published: October 2002

Numerical simulations of vertical seismic profiles in flat‐layered elastic media using the discrete wavenumber method are presented. The effect of source‐borehole sep‐ aration on recorded wave types and amplitudes is studied. For nonzero source offsets, transverse and converted waves become very important and can be more energetic than the direct compressional arrivals. A systematic comparison of results from acoustic and elastic simulations shows that the acoustic approximation is quite valid for a zero source offset but becomes inadequate when the configuration of the source and vertical geo‐ phone array is two‐dimensional. Recording of both pressure and displacement allows a simple separation of transverse and compressional arrivals as long as the effect of the borehole on the incoming waves can be neglected.
Radar measurements of borehole geometry on the Greenland and Antarctic ice sheets
Authors: Jezek K. C.
Published: October 2002

A method for measuring the geometry of boreholes in glaciers has been developed and tested in Greenland and Antarctica. Coordinates of points along the borehole are determined by lowering a passive radar target into the borehole and then tracking the target from three surface stations. Comparison of geometry interpreted from radar data and from a conventional inclinometry experiment indicates that radar data can be used to estimate average borehole inclination and azimuth but cannot be used to measure details of the borehole geometry that are revealed by conventional inclinometry surveys. Random error introduced by variations in the physical properties of the glacier and electrical noise in the radar unit limit measurement accuracy, but the accuracy can be improved by establishing additional surface radar stations around the borehole. These experiments demonstrate the utility of the radar method and suggest the possibility of deploying permanently installed radar targets in ice sheets to measure intraglacial movements.
Direct hydrocarbon detection using comparative P-wave and S-wave seismic sections
Authors: Robertson James D. , Pritchett William C.
Published: October 2002

Two field experiments in the Sacramento basin and one in the Green River basin demonstrate that comparative P-wave and S-wave CDP seismic sections can be used to detect gas directly in sandstone reservoirs. The lines in the Sacramento basin were shot over producing gas fields known to correlate with amplitude anomalies on P-wave sections. Reflections of comparable strength are present on the P-wave and S-wave sections at lithologic boundaries, but gas‐saturated zones correlating with P-wave bright spots show no equivalent S-wave amplitude anomalies. The responses are consistent with laboratory observations that P-wave velocity is more sensitive to the introduction of gas into liquid‐saturated pore space than S-wave velocity. The line in the Green River basin was shot over a relatively deep gas field producing from an overpressured reservoir not associated with a conventional P-wave bright spot. The P-wave reflection strength of the reservoir is about 50 percent greater than the S-wave reflection strength, whereas the P and S strengths of other major reflectors are comparable. The three field tests show that an S-wave section validates a P-wave bright spot attributed to gas saturation when there is no anomalous amplitude at the equivalent S-wave event and that the technique is useful for verification of subtle as well as strong amplitude anomalies.
Curie point depths of the Island of Kyushu and surrounding areas, Japan
Authors: Okubo Y. , Graf R. J. , Hansen R. O. , Ogawa K. , Tsu H.
Published: October 2002

As part of a comprehensive, nationwide evaluation of geothermal resources for Japan, the first of the Curie point depth maps, covering the island of Kyushu, has been prepared. The map was created by inverting gridded, regional aeromagnetic data. Two satisfactory algorithms were developed to invert the gridded data based upon a distribution of point dipoles. The first algorithm estimates [Formula: see text],[Formula: see text], and [Formula: see text], the coordinates of the centroid of the distribution, by computing a least‐squares fit to the radial frequency of the Fourier transform; the second algorithm estimates centroid depth only by computing a least‐squares fit to the squared amplitude of the frequency estimates. The average depth to the top, [Formula: see text] of the collection of point dipoles, was estimated by a variation of the second algorithm. The depth to the bottom of the dipoles, inferred Curie point depth, is [Formula: see text]. The depth estimates are hand contoured to produce the final map. The Curie point depth map is then compared to regional geology and heat flow data, and to a limited set of gravity data. Good correlations are found between the Curie point depths and the heat flow and regional geology. A spatial correlation observed between gravity and Curie point depths is considered a secondary, structural effect. Locations of the currently operating geothermal power plants correspond to the shallowest Curie point depths. Based on these comparisons, we conclude that the methods provide geologically reasonable results which are usable in a nationwide geothermal assessment program.
On ratio and crossover points in the interpretation of refraction profile data
Authors: Rao T. K. S. Prakasa , Rao P. Koteswara , Subrahmanyam K.
Published: October 2002

In this note, mathematical expressions are developed for the coordinates of certain characteristic points which are easy to locate on the refraction segment of the time‐distance graph. The characteristic points are (1) intercept time, (2) critical point at which the reflection time equals the refraction time, and (3) crossover point at which the direct arrival time equals the refraction time. Any two of these are sufficient to determine the depth to a horizontal refractor as discussed by Nettleton (1940), Dobrin (1976), and Telford et al. (1976). Slotnick (1959) discussed the significance of the critical point. Considering the time‐distance relations for the case of two media with speeds [Formula: see text] and [Formula: see text] separated by a horizontal discontinuity at depth H, the slope of the time‐distance reflection hyperbola at the critical point is [Formula: see text]. At this point, the emergence angle is equal to the critical angle for velocities [Formula: see text] and [Formula: see text]. In other words, as one moves away from the shotpoint, the slope of the reflection hyperbola increases from zero to [Formula: see text] and passes through the value [Formula: see text] at the critical distance. The important result of Slotnick (1959) about the critical point has been further investigated, and useful relationships have been derived on the basis of it. When the refractor is inclined, the critical point can still be explained but it is difficult to locate. However, a ratio point may be used which lies on the segment obtained by joining intercept time and crossover point. The role of critical and ratio points is discussed in the interpretation of refraction profile data supported by exact mathematical expressions.
Synthetic finite‐offset vertical seismic profiles for laterally varying media
Authors: McMechan George A.
Published: October 2002

The analysis of vertical seismic profile (VSP) data is generally directed toward determination of rock properties (such as velocity, impedance, attenuation, and anisotropy) as functions of depth (that is, in a one‐dimensional model). If VSPs are extended to include observations from sources at multiple, finite offsets, then lateral variation in structure near the drill hole can be studied. Synthetic offset VSPs are computed by an acoustic finite‐difference algorithm for two‐dimensional models that include the main types of structural traps. These show that diagnostic lateral variations can be detected and interpreted in VSPs. In a VSP, lateral structure variations may produce changes in the type and number of arrivals, in amplitudes, in time and phase shifts, in interference patterns, in curvature of arrival branches, and in the focusing and defocusing of energy. All of these effects are functions of the positions of the source(s) and receiver(s); numerical modeling is a potentially useful tool for interpretation of VSP data from laterally varying structure.
Seismic Deconvolution Workshop Sponsored by the SEG Research Committee, July 18–19, 1984, Vail, Colorado
Authors: Schoenberger Michael
Published: October 2002

During July 18 and 19, 1984, the SEG Research Committee sponsored a research workshop on Seismic Deconvolution. This topic was also the subject of an earlier research workshop conducted during the half‐day following the 1981 SEG Annual Meeting in Los Angeles. Since this earlier workshop was felt to have only scratched the surface of an important subject, it was continued and expanded into the more recent workshop.
An autoregressive filter model for constant Q attenuation
Authors: McCarley Lon A.
Published: October 2002

The Earth’s filter response to seismic wave propagation can be approximated by constant Q attenuation and is dispersive or minimum phase. A finite‐length autoregressive (AR) filter model is a good representation for constant Q attenuation with minimum phase. Coefficients of the AR filter are the Wiener‐Levinson inverse filter coefficients for the sampled constant Q auto‐correlation. Conventional spike deconvolution approximates fairly well the inverse filter on this ground if the minimum‐phase attenuation law holds true. Wavelet response to attenuation was analyzed using the AR filter model. The amplitude of the filtered impulse response decreases at nearly 1/t, where t is traveltime, and is sensitive to the loss of low frequencies. The wavelets’ peak amplitude is fractionally time delayed with .92/Q. Velocity dispersion of seismic waves should not contribute to the mis‐tie observed between the conventional check shot and the borehole sonic log when the first arrivals are picked the same way.
A high‐precision gravity survey in the Delaware Basin of southeastern New Mexico
Authors: Barrows Larry , Fett John D.
Published: October 2002

Since 1974, the Department of Energy has been studying bedded salt deposits in southeastern New Mexico as a possible location for disposing of defense‐generated transuranic and low‐level radioactive wastes. The program, known as the Waste Isolation Pilot Plant, includes intensive geologic characterization of about [Formula: see text] and construction of an underground test facility. The gravity survey reported here is part of the geologic site characterization. The gravity survey was conducted to delineate structural features near and at the proposed site. However, during the survey the gravity field was found to be dominated by effects of lateral density variations within relatively flat‐lying strata. Particularly distinctive is a pattern of elongate negative anomalies about one‐half mGal in amplitude. Boreholes in the anomalies encountered normal stratigraphy and no unusual geologic structures. However, borehole densilogs showed lower densities and uphole velocity surveys showed lower acoustic velocities than are measured outside of the anomalies. The low densities adequately account for the observed gravity anomalies. The regional stratigraphy contains water‐soluble minerals (halite, polyhalite, anhydrite‐gypsum, carbonates). Much of this material has dissolved and the region has been identified as a karstland. At the site, dissolution is slowly affecting the Rustler formation overlying the main salt‐bearing units. The low rock densities, associated with the negative gravity anomalies, are interpreted as due to alteration in the vicinity of solution conduits within the Rustler formation. This interpretation is supported by (1) partial coincidence between the negative gravity anomalies and closed topographic depressions (alluvial dolines); (2) greater anhydrite‐to‐gypsum conversion detected in boreholes within the anomalies; and (3) solution conduits encountered in one of the boreholes.