bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of pipelines on spectral induced‐polarization surveys
Authors: Parra Jorge O.
Published: October 2002

The formal electromagnetic coupling solution for a dipole-dipole electrode array configuration has been modified to include cultural coupling in a uniform conducting half‐space. Solutions are obtained for survey lines oriented at an arbitrary position and angle with respect to a cylindrical structure. The convergence properties of the general mutual impedance solution are analyzed using a low‐frequency approximation which is useful in predicting cultural anomalies in the frequency range of spectral IP surveys as long as all significant dimensions are less than one skin depth. Both interfacial polarization and induced currents in the cylindrical conductor are considered in examining the behavior of the overall spectrum as seen by an external observer. Spectral responses for dipole-dipole arrays oriented perpendicular and parallel to the buried conductor show that the phase shift is the most diagnostic parameter for pipe depth and survey data distortion. The results also show that field survey procedures can be devised to minimize such interference effects when the pipe position is known.
On: “The signature of an air gun array: Computation from near field measurements including interactions” by A. Ziolkowski, G. Parkes, L. Hatton, and T. Haugland (GEOPHYSICS, v. 47, p. 1413–1421, October, 1982).
Authors: Safar M. H.
Published: October 2002

I would like to make two comments regarding the discussion on interaction between air bubbles given by Ziolkowski et al. The first concerns their statement that their approach for treating interaction is exactly the same as my approach (Safar, 1976), namely, that interaction is treated as a modulation of the hydrostatic pressure just outside the air bubbles. I would like to emphasize that, in fact, this was the approach used by Giles and Johnston (1973) and not the approach that I used in my paper. Since the problem of interaction between seismic sources forming an array is of considerable importance from the operational viewpoint, I give a summary of the analysis which I gave in my paper. Consider the case of two identical air guns placed at the same depth. When only one gun is fired, one air bubble is produced. From Newton’s third law, the effective pressure acting on the pulsating air bubble is not equal to the hydrostatic pressure as was stated by Ziolkowski et al., but equal to the hydrostatic pressure [Formula: see text] plus the dynamic pressure exerted by the surrounding water which is given by [Formula: see text], (1) where the dot denotes differentiation with respect to time t, v(t) and [Formula: see text] are the air bubble instantaneous volume and radiation impedance.
Tests of piezoelectric and pulsed‐radio methods for quartz vein and base‐metal sulfides prospecting at Giant Yellowknife Mine, N. W. T., and Sullivan Mine, Kimberley, Canada
Authors: Sobolev G. A. , Demin V. M. , Narod B. B. , Whaite P.
Published: October 2002

In the summer of 1983, G. A. Sobolev and V. M. Demin of the U.S.S.R. came to Canada to demonstrate techniques using piezoelectrical and other mechanoelectrical effects for in‐mine geophysical surveys. We performed field trials using Soviet and Canadian equipment in the Giant Yellowknife Mine, N. W. T., and the Sullivan Mine, Kimberley, B.C., Canada. The principle of the technique involves detonating an explosive charge and subsequently measuring the strength and delay time of electromagnetic fields generated by excited minerals. Ore grade gold zones at the Giant Yellowknife Mine occur within large quartz lenses in a shear zone in Archean volcanics. Our tests there produced strong signals in the 8 kHz range, which were very similar to piezoelectrical signals produced by quartz samples, in the laboratory at the University of British Columbia. The Sullivan orebody is a 160 million ton iron‐lead‐zinc sulfide deposit lying comformably in Proterozoic rock. Our tests there yielded some very strong electromagnetic signals with frequencies as high as 350 kHz. The short delay times of these signals preclude any possibility that the signals were simple seismic phenomena generated at the antenna sites. Sobolev et al., (1982a, b) suggest that such high‐frequency signals are produced by a highly nonlinear discharge phenomenon occurring at the grain boundaries of naturally occurring semiconductors such as galena, sphalerite, and pyrrhotite.
The signature of an air gun array: Computation from near‐field measurements including interactions—Practical considerations
Authors: Parkes G. E. , Ziolkowski A. , Hatton L. , Haugland T.
Published: October 2002

We have refined our system for calculating the signature of an interacting air gun array from near‐field measurements of its pressure field. We use an iterative technique to calculate a notional array of noninteracting sources from the near‐field hydrophone measurements. The notional signatures form the basis for calculating the array signature in any direction. The success of our iterative technique depends upon prudent positioning of the hydrophones, one close to each air gun. In normal operation the forward motion of the hydrophones and upward motion of the air gun bubbles are important effects which must be included in the equations. A linear model for this motion is adequate and improves the method significantly. The vertically traveling “far‐field” signature calculated by our extended method matches an equivalent “far‐field” measurement very closely. We present array signatures obtained in very bad weather conditions (force 8). In this extreme test the signatures are very stable from shot to shot. Therefore it is not necessary to calculate the array signature every shot; however, continuous recording of near‐fields should still be carried out as a check on signature stability.
The characteristics of reflection traveltimes for some layer models
Authors: Byun Bok S.
Published: October 2002

The characteristics of reflection time‐distance curves were examined mainly in terms of stacking velocity for various single‐layer models. There were basically different types: 1-D stratified layer, 2-D stratified plane dipping layer, and 2-D homogeneous curved layer. It is shown through numerical experiments that a highly stratified medium can lead to a large error in the estimates of the layer thickness if the stratification effect is ignored. This error can be reduced in part by approximating the medium with an elliptically anisotropic one. In the homogeneous but curved layer case, the conventional stacking velocity analysis yields an increased bias in velocity with increasing curvature of the reflecting interface and with increasing local dip of the interface at the normal incidence point. Unless this bias is corrected, the common Dix‐type velocity estimation technique can lead to a significant error in the depth estimates.
Response of a normal resistivity tool in a borehole crossing a bed boundary
Authors: Zhang G. J. , Shen L. C.
Published: October 2002

The fundamental boundary‐value problem involving a current electrode in an inhomogeneous medium is solved. Two integral equations are obtained by imposing the condition of conservation of electric currents. We emphasize the solution of one of the integral equations involving potentials in a step‐wise inhomogeneous medium. Two examples are shown. The first example is concerned with a normal tool in a well with mud conductivity [Formula: see text] in a thick bed of conductivity [Formula: see text]. In the second example the response of a 16 inch normal tool is calculated when it is in a well with mud conductivity [Formula: see text] crossing a boundary separating two beds with conductivities [Formula: see text] and [Formula: see text], respectively. Results agree with other theoretical data for the first case. For the second case they agree well with experimental data obtained from Schlumberger’s resistor network model.
On‐line movies of reflection seismic data with description of a movie machine
Authors: Ottolini Richard , Sword Charles , Claerbout Jon F.
Published: October 2002

On‐line movies are an exciting way to view reflection seismic data. We make a movie by slicing through a three‐dimensional (3-D) seismic data cube in a selected direction (Figure 1). By sweeping through the data fast enough, it is possible to get another perspective of what is going on in the data than by examining still frames. By twiddling various knobs and buttons, we can zoom onto a zone of interest, magnify it, rotate it, and change the color to emphasize a feature of interest.
The migration of common midpoint slant stacks
Authors: Ottolini Richard , Claerbout Jon F.
Published: October 2002

Reflection seismic data can be imaged by migrating common midpoint slant stacks. The basic method is to assemble slant stack sections from the slant stack of each common midpoint gather at the same ray parameter. Earlier investigators have described migration methods for slant stacked shot profiles or common receiver gathers instead of common midpoint gathers. However, common midpoint slant stacks enjoy the practical advantages of midpoint coordinates. In addition, the migration equation makes no approximation for steep dips, wide offsets, or vertical velocity variations. A theoretical disadvantage is that there is no exact treatment of lateral velocity variations. Slant stack migration is a method of “migration before stack.” It solves the dip selectivity problem of conventional stacking, particularly when horizontal reflectors intersect steep dipping reflectors. The correct handling of all dips also improves lateral resolution in the image. Slant stack migration provides a straightforward method of measuring interval velocity after migration has improved the seismic data. The kinematics (traveltime treatment) of slant stack migration is also accurate for postcritical reflections and refractions. These events transform into a p-τ surface with the additional dimension of midpoint. The slant stack migration equation converts the p-τ surface into a depth‐midpoint velocity surface. As with migration in general, the effects of dip are automatically accounted for during velocity inversion.
Complex seismic trace analysis of thin beds
Authors: Robertson James D. , Nogami Henry H.
Published: October 2002

Displays of complex trace attributes can help to define thin beds in seismic sections. If the wavelet in a section is zero phase, low impedance strata whose thicknesses are of the order of half the peak‐to‐peak period of the dominant seismic energy show up as anomalously high‐amplitude zones on instantaneous amplitude sections. These anomalies result from the well‐known amplitude tuning effect which occurs when reflection coefficients of opposite polarity a half period apart are convolved with a seismic wavelet. As the layers thin to a quarter period of the dominant seismic energy, thinning is revealed by an anomalous increase in instantaneous frequency. This behavior results from the less well‐known but equally important phenomenon of frequency tuning by beds which thin laterally. Instantaneous frequency reaches an anomalously high value when bed thickness is about a quarter period and remains high as the bed continues to thin. In this paper, complex trace analysis is applied to a synthetic model of a wedge and to a set of broadband field data acquired to delineate thin lenses of porous sandstone. The two case studies illustrate that sets of attribute displays can be used to verify the presence and dimensions of thin beds when definition of the beds is not obvious on conventional seismic sections.
Algorithm for error adjustment of potential field data along a survey network
Authors: Mittal P. K.
Published: October 2002

Gravity and magnetic surveys on sea, land, or air invariably exhibit discrepancies at intersections of survey lines. These “misties” have to be removed before any meaningful contour map can be prepared. The misties are caused by many errors. Errors in location fixing, instrumental drift, diurnal magnetic variations, inaccuracies in Eötvös corrections because of errors in heading, and speed of ship or aircraft play a major role. Foster (1970) proposed a method for statistical estimation of the misties at line intersections with special reference to aeromagnetic data. The method can be extended to all gravity and magnetic surveys on land, sea, or air. However, it requires an approximately rectangular grid of survey lines. In case the network is irregular and there are lines in various directions crossing each other, Foster’s method would not be applicable.
Geophone ground coupling
Authors: Krohn Christine E.
Published: October 2002

The geophone ground coupling for vertical and horizontal geophones has been measured in the laboratory and in the field. The data can be fit to a calculated geophone response with a single coupling resonant frequency and damping factor. For frequencies much lower than the coupling resonant frequency, the geophone accurately follows the ground motion, but for higher frequencies the coupling can alter both the amplitude and phase of the seismic signal. The normal planting of vertical geophones in the field results in coupling adequate for conventional recording that uses frequencies less than 100 Hz. However, for very loose soils or for high‐frequency seismic recording, I recommend that the geophones be buried to place the geophones in firmer soil. The coupling resonant frequency for vertical geophones is determined by the firmness of the soil, and I have measured resonant frequencies ranging from 100 to 500 Hz at different locations. The coupling resonant frequency is insensitive to changes in the mass or base diameter of the vertical geophones. Because the firmness of the soil increases with depth, the coupling resonant frequency can he increased by burial of the geophones or by the use of longer spikes. Adequate coupling is very important in shear‐wave recording because the rocking of horizontal geophones causes a low‐frequency coupling resonance. It is crucial that horizontal geophones be planted with their bases firmly contacting the soil. Geophones so planted have a resonance around 130 Hz, whereas those 1 cm off the ground can have a resonance of 30 Hz or lower. Soil conditions have little effect on the resonant frequency. Horizontal geophones with 1-inch spikes are as well coupled as those with longer spikes, but the best coupling is achieved by burial of the geophones.
The gravity profile and its role in positioning the edge of a two‐dimensional faulted structure having an arbitrary vertical variation of density
Authors: Paul M. K. , Goodacre A. K.
Published: October 2002

It is shown analytically that as long as there is no lateral variation of density within a two‐dimensional faulted structure the value of gravity at any point on the upward extension of the fault remains constant. Consequently, this result leads to a direct method of gravity interpretation, where it is possible to delineate the edge of such a faulted structure by continuing the surface gravity values upward to different levels and then, by using suitable numerical techniques, find the desired resulting straight‐line gravity contour. The method has been verified using several theoretical models and applied to two sets of real data to demonstrate its usefulness. On a local scale, the Gloucester fault near Ottawa, Canada appears to be nearly vertical. On a larger scale, the gravity signature along the Southern Appalachian COCORP seismic profile can be explained by density jumps across a steeply dipping, major crustal discontinuity.
The influence of a conductive host on two‐dimensional borehole transient electromagnetic responses
Authors: Eaton Perry A. , Hohmann Gerald W.
Published: October 2002

We have computed transient borehole electromagnetic (EM) responses of two‐dimensional (2-D) models using a direct and explicit finite‐difference algorithm. The program computes the secondary electric field which is defined as the difference between the total field and the primary (half‐space) field. The time derivative of the vertical magnetic field in a borehole is computed by numerical differentiation of the total electric field. These models consist of a thin horizontal conductor with a finite width, embedded in a conductive half‐space. Dual line sources energized by a step‐function current lie on the surface of the half‐space and simulate the long sides of a large rectangular loop. Numerical results substantiate several important features of the transient impulse response of such models. The peak response of the target is attenuated as the resistivity of the host decreases. A sign reversal in the secondary electric field occurs later in time as the resistivity of the host decreases. The peak response and the onset of late‐time behavior are delayed in time as well. Secondary responses for models with different host resistivities (10–1000 Ω-m) are approximately the same at late time. If the target is less conductive, the effects of the host, i.e., the attenuation and time delay, are less. It is readily apparent that there exists a time window within which the target’s response is at a maximum relative to the half‐space response. At late time the shape of the borehole anomaly due to a thin conductive 2-D target appears to be independent of the conductivity of the host. The late‐time secondary decay of the target is neither exponential nor power law, and a time constant computed from the slope of a log‐linear decay curve at late time may be much larger than the actual value for the same target in free space.
Megasource time‐domain electromagnetic sounding methods
Authors: Keller George V. , Pritchard James I. , Jacobson Jimmy Joe , Harthill Norman
Published: October 2002

The Colorado School of Mines time‐domain electromagnetic (EM) sounding system makes use of a grounded length of cable powered with high‐amplitude current square waves to generate an EM field for probing the earth. The vertical component of magnetic induction is detected at a sounding site located at a relatively large distance compared to the desired depth of investigation. With a source moment of a million ampere meters or greater, offset distances of several tens of kilometers can be achieved easily, providing depths of investigation of up to 10 km. The recorded induction field versus time curves are routinely interpreted by comparison with computer‐generated theoretical curves for a layered earth. Megasource EM surveys have been carried out at The Geysers in northern California and near Yakima in central Washington, providing apparently meaningful information on the electrical structure in these areas at depths as great as 10 km.
Estimation of static corrections for shear‐wave profiling using the dispersion properties of Love waves
Authors: Mari J. L.
Published: October 2002

When processing shear‐wave data it is often difficult to compute the static corrections using the same methods as for P-waves, because of the low velocities involved and of the interference with residual P-waves. A method especially tailored for static corrections of SH-waves is presented, which makes use of the dispersion property of Love waves. Assuming that the weathered zone of thickness H acts as a wave guide for the surface waves, their phase velocity ranges from [Formula: see text] at infinite frequency to [Formula: see text] at zero frequency, where [Formula: see text] and [Formula: see text] are the shear‐wave velocities into and below the weathered zone. For intermediate frequencies the phase velocity of Love waves is a known function of H, [Formula: see text] and [Formula: see text]. If the phase velocity of Love waves is measured on field records for a number of distinct frequencies, it is therefore possible to compute H, [Formula: see text], [Formula: see text], and the static corrections. In contrast with conventional techniques which require first arrivals to be picked manually, the computation of static corrections using the dispersion of Love waves is essentially based on velocity analysis efficiently performed by digital computers. Three field examples were conducted. The first of these was a noise analysis using 256 geophone positions ranging from 3 to 258 m. The two other examples are CDP lines obtained with polarized SH sources. The geophone group extension is 20 m and the distance between groups is 10 m. In all these examples, the upper layer thickness and velocity values resulting from the Love wave dispersion appeared to be in good agreement with those computed from the conventional methods.
The Radon transform and its properties
Authors: Durrani T. S. , Bisset D.
Published: October 2002

This paper presents a summary of the fundamental properties of the Radon transform, including delay effects, data shifting, rotation, scaling, windowing, bowtie events, energy conservation, etc., and includes representative examples on standard data sets such as 2-D delta functions, boxcar events, Gaussian bell, and conic sections which reinforce the basic concepts of the Radon transform.
Low‐altitude aeromagnetic reconnaissance for petroleum in the Arctic National Wildlife Refuge, Alaska
Authors: Donovan Terrence J. , Hendricks John D. , Roberts Alan A. , Eliason Patricia Termain
Published: October 2002

Variations in the Earth’s magnetic field arising from local concentrations of shallow subsurface magnetite were mapped in the Arctic National Wildlife Refuge and elsewhere in northern Alaska. The anomalies were delineated with a magnetic horizontal gradiometer mounted on a low‐flying (300 ft or ∼90 m above ground) fixed‐wing airplane. Limited data from stable carbon‐isotope and remanent magnetism measurements of rock cores from the Cape Simpson region strongly suggest that the magnetic anomalies result from the chemical reduction of iron oxides in the presence of seeping hydrocarbons. The magnetic contrast between sedimentary rocks of normally low magnetic susceptibility and those locally enriched with this epigenetic magnetite results in distinctive high‐wavenumber and low‐amplitude total‐field anomalies. Magnetometers extended from each wing tip and in a tail stinger permit calculation of the resultant horizontal gradient vector relative to the flight path. This calculation allows more meaningful interpolation of data for the unsurveyed area between adjacent flight lines spaced at 1.0 mile (1.6 km), thereby allowing generation of accurate computer‐enhanced images or maps. Problems related to diurnal variations and solar storms at high magnetic latitudes are largely overcome because changes in total magnetic field do not significantly affect the magnetic gradient. Analysis of an experimental survey, covering 2745 line‐miles (4418 line‐km), reveals numerous anomalies, the most prominent of which parallels the Marsh Creek anticline. The data provide further evidence that the Marsh Creek anticline is prospective for oil and/or gas. Although the effect of permafrost on epigenetic processes has not been investigated, the data suggest that special‐purpose aeromagnetic surveying may be a useful and inexpensive way to explore for oil and gas in this harsh environment.
On the effect of random timing errors on velocity estimates derived from normal moveout estimates
Authors: Powell J. A.
Published: October 2002

A number of authors (e.g., Brown, 1969; Hubral and Krey, 1980) have discussed systematic velocity estimation errors caused by nonhyperbolic normal moveout. The primary concern of these authors is often the departure of (presumably perfectly accurate measurements of) normal moveout from that predicted by Dix’s (1955) equation. In this note the effect of measurement errors on velocity estimates is examined with the aid of standard statistical techniques. The concern is not deterministic errors, but those caused by random errors.
Comparison of P‐ and S‐wave seismic data: A new method for detecting gas reservoirs
Authors: Ensley Ross Alan
Published: October 2002

Compressional waves are sensitive to the type of pore fluid within rocks, but shear waves are only slightly affected by changes in fluid type. This suggests that a comparison of compressional‐ and shear‐wave seismic data recorded over a prospect may allow an interpreter to discriminate between gas‐related anomalies and those related to lithology. This case study documents that where a compressional‐wave “bright spot” or other direct hydrocarbon indicator is present, such a comparison can be used to verify the presence of gas. In practice, the technique can only be used for a qualitative evaluation. However, future improvement of shear‐wave data quality may enable the use of more quantitative methods as well.
On: “Wave propagation and sampling theory” by J. Morlet, G. Arens, E. Fourgean, and D. Giard (GEOPHYSICS, 47, parts I and II, 203–236).
Authors: Laski J. D.
Published: October 2002

Investigating waves for sedimentary series, Morlet et al. start from basic formulas for elastic waves. I have compared formulas used by Morlet et al. for: (a) compressional velocity of elastic waves, (b) stress‐strain relationship, and (c) energy flow vector (Poynting’s vector), with corresponding formulas in Koehler and Taner (1977). The results follow.
PROPAGATION OF ELASTIC WAVES IN THE EARTH
Authors: Howell L. G. , Kean C. H. , Thompson R. R.
Published: October 2002

An investigation of elastic‐wave propagation in near‐surface materials using single‐frequency continuous waves and pulses, over a range of 20 to 1400 cycles, is described. While only touching upon the diverse problems involved in a study of this kind the results indicate a complexity requiring considerable research if a proper understanding, commensurate with the importance of the problem, is to be attained.
ADDITIONAL EVIDENCE ON THE RELATION OF TEMPERATURE TO STRUCTURE IN THE SALT CREEK OIL FIELD, NATRONA COUNTY, WYOMING
Authors: Van Orstrand C. E.
Published: October 2002

Observations of bottom hole temperatures in approximately 100 deep wells in the Salt Creek field have been made during the past few years by the Stanolind Oil and Gas Company. These recent observations confirm and extend in a remarkable manner the results obtained in the summers of 1922 and 1923 when it was found from temperature surveys in 21 wells that the temperatures over a considerable portion of the field were definitely related to the structure.
REFLECTION AND TRANSMISSION COEFFICIENTS FOR PLANE WAVES IN ELASTIC MEDIA
Authors: Muskat M. , Meres M. W.
Published: October 2002

The results are given of a systematic series of calculations on the coefficients of reflection and transmission for plane waves incident on elastic interfaces. Tables are given for the amplitudes of the reflected and transmitted longitudinal and transverse waves, for the intensities of these components, and for the fractions of the incident energy carried away by them. For incident longitudinal waves calculations were carried out for angles of incidence between 0 and 30 with 5° intervals. For incident transverse waves polarized in the plane of incidence results are given for four angles of incidence up to approximately 16°. For incident transverse waves polarized normal to the plane of incidence the calculations were carried through for all angles of incidence—in steps of 5°—up to total reflection. All the calculations were carried through for interfacial density ratios of 0.7 to 1.3 in steps of 0.1, and interfacial velocity ratios between 0.5 and 2.0 in steps of 0.25.
THE TIME DELAY OF A WAVE GROUP IN THE WEATHERED LAYER
Authors: Wolf Alfred
Published: October 2002

Ground motion observed in seismic prospecting can be regarded as being composed of groups of waves of some predominant frequency. The velocity of propagation of such wave groups in the earth is the group velocity. A detailed application is made to the calculation of the time delay of a reflection wave group passing through the weathered layer. It appears that the weathering time of a reflection is a function of its frequency, and that it may differ from the weathering time of first arrival waves.
Synthetic vertical seismic profiles for nonnormal incidence plane waves
Authors: Aminzadeh F. , Mendel J. M.
Published: October 2002

Vertical seismic profiles (VSPs) are, by definition, recordings of seismic signals (total upgoing and downgoing seismic wave fields) at different depth points, usually at equally spaced intervals [Formula: see text], i = 1, 2, …, I. In a nonnormal incidence (NNI) elastic model, where each layer is described by thickness, density, and P- and S-wave velocities, the mapping between time and depth needed to generate synthetic VSPs is not usually straightforward. In this paper we develop a relatively simple procedure for generating synthetic vertical and horizontal direction plane wave NNI VSPs. No spatial discretization is necessary. We (1) compute two surface seismograms, one vertical and the other horizontal, exactly as described in Aminzadeh and Mendel (1982); and (2) downward continue the surface seismograms to fixed VSP depth points. This paper demonstrates an algorithm for downward continuation of an elastic wave field using state‐space representation and gives simulations which illustrate both z- and x-direction primaries and complete VSPs for different geologic models and different incident angles.
On: “Thin‐layer response and spectral bandwidth” by N. de Voogd and H. den Rooijen (GEOPHYSICS, 48, 12–18).
Authors: Saha J. G.
Published: October 2002

In their paper de Voogd and den Rooijen investigated the effects of spectral bandwidths of zero‐phase signals on interface resolution and also the response of a thin layer when the bandwidth is inadequate. They suggested a type of wavelet manipulation in modeling experiments by a simple frequency scaling and a shift [equation (19)] [Formula: see text] where p is the scaling factor and s is the shift along the frequency axis. The increase in spectral bandwidth which results from this transformation was also derived by the authors in equation (20).
Syntactic pattern recognition for the classification of Ricker wavelets
Authors: Huang Kou‐Yuan , Fu King‐sun
Published: October 2002

Syntactic pattern recognition techniques are applied to the analysis of one‐dimensional seismic traces for classification of Ricker wavelets. The system for one‐dimensional seismic analysis includes a likelihood ratio test, optimal amplitude‐dependent encoding, probability of detecting the signal involved in the global and local detection, plus minimum‐distance and nearest‐neighbor classification rules. The relation between error probability and Levenshtein distance is proposed.
Seismic wave propagation in a porous medium
Authors: de la Cruz V. , Spanos T. J. T.
Published: October 2002

A complete set of equations to describe low‐frequency seismic wave phenomena in fluid‐filled porous media is presented. The approach is based on the mathematics of volume‐averaging, aided by order‐of‐magnitude and physical arguments. The results are immediately utilizable by practicing seismologists. Our equations and those of Biot (1956a) are found to be largely consistent in form, and we suggest how Biot’s parameters may be defined in terms of basic physical parameters. The theory predicts two dilatational waves and two rotational waves. Under certain conditions these behave differently than would be expected on the basis of Biot’s theory.
Optimization of one‐way wave equations
Authors: Lee Myung W. , Suh Sang Y.
Published: October 2002

The theory of wave extrapolation is based on the square‐root equation or one‐way equation. The full wave equation represents waves which propagate in both directions. On the contrary, the square‐root equation represents waves propagating in one direction only.
Correction of systematic error in magnetic surveys: An application of ridge regression and sparse matrix theory
Authors: Ray Richard D.
Published: October 2002

Foster, Jines, and van der Weg showed that systematic error in potential field surveys could be corrected by simultaneously adjusting all survey lines so that the root‐mean‐square mis‐tie at survey intersections was minimized. Their somewhat restrictive method is extended here for maximum flexibility, thereby allowing for irregular survey shapes, diagonal lines, constrained lines, varying degrees of adjustment for different lines, and weighting of intersections. By casting the adjustment as a large, sparse, linear least‐squares problem, the survey corrections may be efficiently computed using a block orthogonal decomposition scheme. This method is based on recent work in numerical linear algebra by Golub and Plemmons, Heath, and others. Foster et al. showed that the computed adjustments are not unique; for our extension we show the adjustments are also likely unstable. This nonuniqueness and instability of adjustments may be effectively handled by the use of ridge regression.
Directional depth migration
Authors: Higginbotham J. H. , Shin Y. , Sukup D. V.
Published: October 2002

Complicated geologic structures such as folds, overthrusts, and salt domes can produce reflectors with dips as great as 90 degrees. Because oil and gas accumulations are often associated with these steeply dipping interfaces, accurate processing of reflection seismic information from such areas becomes an important though challenging task. The proper imaging of steeply dipping reflectors requires accurate knowledge of the velocity field through which the wavefronts propagate. Thus, velocity analysis becomes extremely important. In addition to this problem, most migration algorithms have serious difficulties when dip is greater than about 50 degrees. In this discussion, we assume the velocity field is known, the data may be stacked correctly before migration, and the chief concern is migration accuracy. We describe a method for depth migration of very steeply inclined wavefronts through inhomogeneous velocity fields. The extension of the proposed technique to migration before stack is obvious.
The present state of seismic data acquisition: One view
Authors: Laster Stanley J.
Published: October 2002

Seismic data acquisition in the mid‐1980s is briefly reviewed. In terms of hardware, the trend has been toward an increased number of data channels in both land and marine applications. This has led to the development of digital telemetry systems. Positioning systems, particularly for marine work, have made use of artificial satellites. The perceived need for S‐wave information has led to development of S‐wave sources such as the horizontal vibrator. S‐waves in a few cases have been used to validate hydrocarbon indicators on seismic records. There has been a distinct trend toward three‐dimensional (3-D) seismic recording, both on land and at sea, and for both exploration and production applications.
Geophysics in Australian mineral exploration
Authors: Smith Robert J.
Published: October 2002

I review a variety of recent case histories illustrating the application of geophysics in mineral exploration in Australia. Geophysics is now an integral part of most programs. Examples are given of contributions by geophysics to all stages of mineral exploration, from regional area selection through to mine planning and development. Specific case histories summarized are as follows: (a) Olympic Dam copper‐uranium‐gold deposit, discovered using a conceptual genetic model and regional geophysical data; (b) Ellendale diamondiferous kimberlites, illustrating the use of low level, detailed airborne magnetics; (c) Ranger uranium orebodies, discovered by detailed airborne radiometric surveys; (d) geologic mapping near Mary Kathleen with color displays of airborne radiometric data; (e) mapping of lignite in basement depressions of the Bremer Basin, near Esperance, with INPUT; (f) White Leads, a lead‐zinc sulfide deposit discovered with induced polarization (IP) and TEM, near Broken Hill; (g) Hellyer, a lead‐zinc‐silver‐gold deposit discovered with UTEM; (h) application of geophysical logging near Kanmantoo; (i) Cowla Peak, a subbituminous steaming coal deposit mapped with ground TEM; and (j) Cook Colliery, where high‐resolution seismic reflection methods have successfully increased the workable reserves.
State‐of‐the‐art geophysical exploration for geothermal resources
Authors: Wright Phillip. M. , Ward Stanley H. , Ross Howard P. , West Richard C.
Published: October 2002

At the present stage of development, use of geothermal energy saves about 77 million barrels of oil per year worldwide that would otherwise be required for electrical power generation and direct heat applications. More than a dozen countries are involved in development of geothermal resources. Currently, only the moderate‐ and high‐temperature hydrothermal convective type of geothermal system can be economically used for generating electric power. Lower‐temperature resources of several types are being tapped for space heating and industrial processing. Geophysics plays important roles both in exploration for geothermal systems and in delineating, evaluating, and monitoring production from them. The thermal methods, which detect anomalous temperatures directly, and the electrical methods are probably the most useful and widely used in terms of siting drilling targets, but gravity, magnetics, seismic methods, and geophysical well logging all have important application. Advances in geophysical methods are needed to improve cost effectiveness and to enhance solutions of geologic problems. There is no wholly satisfactory electrical system from the standpoint of resolution of subsurface resistivity configuration at the required scale, depth of penetration, portability of equipment, and survey cost. The resolution of microseismic and microearthquake techniques needs improvement, and the reflection seismic technique needs substantial improvement to be cost effective in many hard‐rock environments. Well‐logging tools need to be developed and calibrated for use in corrosive wells at temperatures exceeding 200°C. Well‐log interpretation techniques need to be developed for the hard‐rock environment. Borehole geophysical techniques and geotomography are just beginning to be applied and show promise with future development.
Offset‐amplitude variation and controlled‐amplitude processing
Authors: Yu Gary
Published: October 2002

The partition of plane seismic waves at plane interfaces introduces changes in seismic amplitude which vary with angle of incidence. These amplitude variations are a function of the elastic parameters of rocks on either side of the interface. Controlled‐amplitude processing is designed to obtain the true amplitude information which is geologic in origin. The offset‐amplitude information may be successfully used to predict the fluid type in reservoir sands. Various tests were carried out on a seismic profile from the Gulf Coast. The processing comparison emphasized the effects and pitfalls of trace equalization, coherent noise, offset, and surface‐related problems. Two wells drilled at amplitude anomaly locations confirmed the prediction of hydrocarbons from offset‐amplitude analysis. Furthermore, controlled‐amplitude processing provided clues in evaluating reservoir quality, which was not evident on the conventional relative amplitude data.
Seismic time‐invariant convolutional model
Authors: Robinson Enders A.
Published: October 2002

A layered‐earth seismic model is subdivided into two subsystems. The upper subsystem can have any sequence of reflection coefficients but the lower subsystem has a sequence of reflection coefficients which are small in magnitude and have the characteristics of random white noise. It is shown that if an arbitrary wavelet is the input to the lower lithologic section, the same wavelet convolved with the white sequence of reflection coefficients will be the reflected output. That is, a white sedimentary system passes a wavelet in reflection as a linear time‐invariant filter with impulse response given by the reflection coefficients. Thus, the small white lithologic section acts as an ideal reflecting window, producing perfect primary reflections with no multiple reflections and no transmission losses. The upper subsystem produces a minimum‐delay multiple‐reflection waveform. The seismic wavelet is the convolution of the source wavelet, the absorption effect, this multiple‐reflection waveform, and the instrument effect. Therefore, the seismic trace within the time gate corresponding to the lower subsystem is given by the convolution of the seismic wavelet with the white reflection coefficients of the lower subsystem. The linear time‐invariant seismic model used in predictive deconvolution has been derived. Furthermore, it is shown that any layered subsystem which has small reflection coefficients acts as a linear time‐invariant filter. This explains why time‐invariant deconvolution filters can be used within various time gates on a seismic trace which at first appearance might look like a continually time‐varying phenomenon.
Stratigraphic filtering, Part I: Derivation of the O’Doherty‐Anstey formula
Authors: Banik N. C. , Lerche l. , Shuey R. T.
Published: October 2002

We present a new and more detailed derivation of the formula due to O’Doherty and Anstey (1971) for filtering a transmitted wavelet by short‐period multiples. We use a continuous rather than discrete formulation and regard the impedance as a random variable. The mean pressure represents the downgoing wavelet as progressively modified by short‐period multiples, while the deviations from the mean field are essentially the upcoming reflections. Standard procedures and approximations lead to the dispersion relation of the mean pressure field. To describe the stratigraphic filtering, we introduce a dimensionless complex quantity F such that a wavelet which has traveled a time ΔT is modified by the filter [Formula: see text]. From the Kramers‐Kronig relation appropriate for a causal earth, F has real and imaginary parts [Formula: see text] and [Formula: see text] Where Q and δt define the apparent attenuation and time delay, both of which may depend upon frequency, R is the spectrum of reflection coefficients, and [Formula: see text] is the spectrum of the impedance fluctuations. The first equation means that the apparent attenuation depends only on the impedance fluctuations with spatial period half the seismic wavelength; the second means that the stratigraphic filter is minimum‐phase. We also show that changes of impedance on a spatial scale much larger than a seismic wavelength modify the amplitude so as to conserve energy, but they do not filter the waveform.
Non‐Gaussian reflectivity, entropy, and deconvolution
Authors: Walden A. T.
Published: October 2002

Standard deconvolution techniques assume that the wavelet is minimum phase but generally make no assumptions about the amplitude distribution of the primary reflection coefficient sequence. For a white reflection sequence the assumption of a Gaussian distribution means that recovery of the true phase of the wavelet is impossible; however, a non‐Gaussian distribution in theory allows recovery of the phase. It is generally recognized that primary reflection coefficients typically have a non‐Gaussian amplitude distribution. Deconvolution techniques that assume whiteness but seek to exploit the non‐Gaussianity include Wiggins’ minimum entropy deconvolution (MED), Claerbout’s parsimonious deconvolution, and Gray’s variable norm deconvolution. These methods do not assume minimum phase. The deconvolution filter is defined by the maximization of a function called the objective. I examine these and other MED‐type deconvolution techniques. Maximizing the objective by setting derivatives to zero results in most cases in a deconvolution filter which is the solution of a highly nonlinear Toeplitz matrix equation. Wiggins’ original iterative approach to the solution is suitable for some methods, while for other methods straightforward iterative perturbation approaches may be used instead. The likely effects on noise of the nonlinearities involved are demonstrated as extremely varied. When the form of an objective remains constant with iteration, the most general description of the method is likelihood ratio maximization; when the form changes, a method seeks to maximize relative entropy at each iteration. I emphasize simple and useful link between three methods and the use of M-estimators in robust statistics. In attempting to assess the accuracy of the techniques, the choice between different families of distributions for modeling the distribution of reflection coefficients is important. The results provide important insights into methods of constructing and understanding the statistical implications and behavior of a chosen nonlinearity. A new objective is introduced to illustrate this, and a few particular preferences expressed. The methods are compared with the zero‐memory nonlinear deconvolution approach of Godfrey and Rocca (1981); for their approach, two distinctly different yet statistically comparable models for reflection coefficients are seen to give surprisingly similarly shaped nonlinearities. Finally, it is shown that each MED‐type method can be viewed as the minimization of a particular configurational entropy expression, where some suitable ratio plays the role of a probability.
Letters to the Editor from former SEG Editors
Authors: Hubbert M. King
Published: October 2002

I have been informed by our present Editor of Geophysics, Stanley H. Ward, that on SEG’s 50th anniversary a special Golden Anniversary volume pertaining to the Society’s history is to be published. Among the contributions to this issue, past Editors have been invited to submit brief statements regarding their respective editorial experiences. This is in response to that request.
Single water gun far‐field pressure signatures estimated from near‐field measurements
Authors: Safar M. H.
Published: October 2002

An important recent development in marine seismic data acquisition is the introduction of the Gemini technique (Newman, 1983, Haskey et al., 1983). The technique involves the use of a single Sodera water gun as a reference source together with the conventional air gun or water gun array which is fired a second or two after firing the reference source. The near‐field pressure signature radiated by the reference source is monitored continuously. The main advantage of the Gemini technique is that a shallow high;resolution section is recorded simultaneously with that obtained from the main array.
A layer‐stripping solution of the inverse problem for a one‐dimensional elastic medium
Authors: Yagle Andrew E. , Levy Bernard C.
Published: October 2002

A fast algorithm for recovering profiles of density and Lamé parameters as functions of depth for the inverse seismic problem in an elastic medium is obtained. The medium is probed with planar impulsive P- and SV-waves at oblique incidence, and the medium velocity components are measured at the surface. The interconversion of P- and SV-waves defines reflection coefficients from which the medium parameter profiles are obtained recursively. The algorithm works on a layer‐stripping principle, and it is specified in both differential and recursive forms. A physical interpretation of this procedure is given in terms of a lattice filter, where the first reflections of the downgoing waves in each layer yield the various reflection coefficients for that layer. A computer run of the algorithm on the synthetic impulsive plane‐wave responses of a twenty‐layer medium shows that the algorithm works satisfactorily.