bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of induced‐polarization logging in a borehole
Authors: Freedman R. , Vogiatzis J. P.
Published: October 2002

Currently, there is interest by the petroleum well‐logging industry in the potential use of induced polarization (IP) measurements to improve formation evaluation in shaly sands. Shell Development Company has constructed an experimental four‐electrode IP and resistivity logging tool to obtain downhole measurements in shaly sands. This study contributes to the theoretical understanding and interpretation of the dynamic (i.e., time‐dependent) response of this type of downhole IP logging device. A low‐frequency (e.g., 32 Hz or less) electric current oscillating at a single fixed frequency is applied between a pair of current electrodes in a borehole. The resulting voltages induced between pairs of potential measuring electrodes in the borehole are calculated by solving the time‐dependent Maxwell’s equations. Inductive electromagnetic (EM) coupling contributions to apparent (e.g., measured) IP phase angles are automatically taken into account. The model is applied to the study of normal logging arrays for which the voltage measuring electrodes are interior to the current electrodes. The model responses are calculated for normal arrays in both infinitely thick noninvaded formations and infinitely thick invaded formations. EM coupling contributions to apparent IP phase angles have an approximately universal dependence on a scaling parameter defined here. The scaling relationship permits the quantitative estimate of EM coupling effects for specific tool parameters (i.e., electrode spacings and frequencies) and formation characteristics (i.e., apparent conductivities). Therefore, scaling relationships of this type should be useful in the design of IP tools. An inverse method, developed for determining true formation IP phase angles and resistivities from apparent values measured by an IP tool, utilizes data from multiple pairs of voltage‐measuring electrodes and exploits the fact that, for the systems of interest, the inverse resistivity and IP problems can be “decoupled.” The assumption that IP phase angles have a logarithmic dependence on frequency over a decade frequency interval leads to a nonlinear relationship between percent frequency effect (PFE) and IP phase angle. This nonlinear relationship agrees well with experimental data.
Two and one‐half dimensional Born inversion with an arbitrary reference
Authors: Bleistein Norman , Cohen Jack K. , Hagin Frank G.
Published: October 2002

Multidimensional inversion algorithms are presented for both prestack and poststack data gathered on a single line. These algorithms both image the subsurface (i.e., give a migrated section) and, given relative true amplitude data, estimate reflection strength or impedance on each reflector. The algorithms are “two and one‐half dimensional” (2.5-D) in that they incorporate three‐dimensional (3-D) wave propagation in a medium which varies in only two dimensions. The use of 3-D sources does not entail any computational penalty, and it avoids the serious degradation of amplitude incurred by using the 2-D wave equation. Our methods are based on the linearized inversion theory associated with the “Born inversion.” Thus, we assume that the sound speed profile is well approximated by a given background velocity, plus a perturbation. It is this perturbation that we seek to reconstruct. We are able to treat the case of an arbitrary continuous background profile. However, the cost of implementation increases as one seeks to honor, successively, constant background, depth‐only dependent background, and, ultimately, fully lateral and depth‐dependent background. For depth‐only dependent background, the increase in CPU time is quite modest when compared to the constant‐background case. We exploit the high‐frequency character of seismic data ab initio. Therefore, we use ray theory and WKBJ Green’s functions in deriving our inversion representations. Furthermore, our algorithms reduce to finding quantities by ray tracing with respect to a background medium. In the constant‐background case, the ray tracing can be eliminated and an explicit algorithm obtained. In the case of a depth‐only dependent background, the ray tracing can be done quite efficiently. Finally, in the general 2.5-D case, the ray‐tracing procedure becomes the principal issue. However, the robustness of the inversion allows for a sparse computation of rays and interpolation for intermediary values. The inversion techniques presented here cover the cases of common‐source gather, common‐receiver gather, and common‐offset gather. Zero offset is a special case of the last of these. For offset data, the reflection coefficient is angle‐dependent, so parameter extraction is more difficult than in the zero‐offset case. Nonetheless, we are able to determine the unknown angle pointwise and derive parameter estimates at the same time as we produce the image. For each reflector, this estimate of the output is based on the Kirchhoff approximation of the upward‐scattered data. Thus, it is constrained to neither small discontinuities in sound speed at the reflector nor to small offset angle as would be the case for a strict “Born approximation” of the reflection process. The prestack algorithms presented here are inversions of single gathers. The question of how best to composite or “stack” these inversions is analogous to the question for any migration scheme and is not treated here.
Borehole seismic profiles in the Ekofisk Field
Authors: Christie P. A. F. , Dangerfield J. A.
Published: October 2002

In October 1983 a major borehole seismic survey was carried out in the Ekofisk oil field in the Norwegian sector of the North Sea on behalf of the Phillips Petroleum Licence 018 group of companies. A vertical seismic profile with the source vertically above the geophone in the highly deviated wellbore and three multilevel walk‐away borehole profiles were acquired in an area showing very poor surface seismic returns owing to gas charging in the overlying sediments. The processing of the data through to a series of conventional common‐midpoint sections has permitted detailed interpretation of the top of the Ekofisk formation and the top of the Tor formation apart from well control. Both formations are producers separated by a tight zone. The Tor formation is the primary horizon to waterflood. Information as to its lateral continuity is important in the location of proposed waterflood injector wells. Prior to the survey, the field was considered effectively unfaulted. An apparent graben lying subparallel to the borehole was detected by the surveys. Reflections from below the reservoir formations are evident. A byproduct of the survey is strong evidence for the existence of lateral velocity gradients or apparent transverse velocity isotropy associated with the overlying gas‐charged sediments. Subsequent to the oral presentation of this paper, the graben was drilled and found to have a 41 m fault throw, in its western flank.
A simple sensitivity analysis of time‐domain and frequency‐domain electromagnetic measurements
Authors: Gómez‐Treviño E.
Published: October 2002

Electromagnetic responses in both time and frequency domains can be looked upon as weighted spatial averages of the nearby conductivity distribution. Using this concept, I analyze the simplest possible situation: a uniform half‐space in the presence of uniform current sources. Both transient and harmonic measurements are equally effective in providing conductivity values for the half‐space; while the conductivity values are the same, the averaging processes are not. To different types of measurements, even within the same domain, there correspond different types of sensitivity or weighting functions. Some of these functions emphasize the near‐surface conductivity, while others emphasize the conductivity at depth. These and other features of the weighting functions explain the behavior of magnetotelluric sounding curves.
Ship Shoal Block 91 field, offshore Louisiana: Discovery and development of a stratigraphic field
Authors: Roux Rosanne M.
Published: October 2002

The Ship Shoal 91 field is a subtle stratigraphic play which consists of a seismic amplitude whose updip limits do not fit structure. The amplitude corresponds to both gas and oil in the Upper Miocene sand. To attempt to predict pay sand distribution accurately, several geologic models were defined as development drilling progressed. The models utilized self‐potential curve shape, core, and seismic character analyses, combined with computer modeling studies, engineering data, and scanning electron microscope studies. Four geologic models evolved, ranging from a structural trap formed by small faults, to a stratigraphic pinchout of the pay sand, to a channel sand, and finally to a complex channel sand, resulting in the current model: a faulted stratigraphic trap. The development strategy for determining well locations also evolved, due to unexpected well results. The initial strategy was based on the presence of seismic amplitudes, until it became apparent that the amplitude alone is not a reliable hydrocarbon indicator. The revised strategy is based on complex attribute analysis, including the amplitude envelope and the weighted frequency. The discovery and development of this stratigraphic field required thorough integration of geology, geophysics, and engineering disciplines. The Ship Shoal 91 field demonstrates that complex stratigraphic fields are still of exploration interest and can be successfully developed by using an integrated approach.
Leveling airborne gamma‐radiation data using between‐channel correlation information
Authors: Green A. A.
Published: October 2002

A procedure for estimating background‐correction terms for the uranium channel of an airborne gamma‐ray survey has been developed. The residuals obtained from a multiple linear regression of flight‐line means for the uranium channel on the means for thorium and potassium are used to correct the uranium channel for each line. The procedure assumes that, were it not for these background errors, the uranium flight‐line means would be a linear function of the means for potassium and thorium. It also assumes that the background correction is the same for the whole of each line. In spite of these limitations, the method produces good background estimates consistent with those found by more sophisticated methods.
Large‐offset seismic surveying using ocean‐bottom seismographs and air guns: Instrumentation and field technique
Authors: Nakamura Yosio , Donoho Paul L. , Roper Phillip H. , McPherson Paul M.
Published: October 2002

Repeatable, closely spaced signal sources from large‐capacity air guns and detection and recording of signals using highly flexible, microprocessor‐controlled, digital ocean‐bottom seismographs allow us to acquire high‐quality, large‐offset, marine seismic refraction and reflection data. The acquired data are readily adaptable to various processing techniques originally developed for seismic reflection data. There are several requirements and problems specific to the technique. For example, bubbly signals from one or two large‐capacity air guns are often preferable to bubble‐suppressed signals from tuned arrays in identifying weak arrivals at large offset distances. Recorded water‐wave signals at near ranges provide precise locations of detectors relative to shots.
Syntactic pattern recognition and Hough transformation for reconstruction of seismic patterns
Authors: Huang K. Y. , Fu K. S. , Cheng S. W. , Lin Z. S.
Published: October 2002

Hierarchical syntactic pattern recognition and the Hough transformation are proposed for automatic recognition and reconstruction of seismic patterns in seismograms. In the first step, the patterns are hierarchically decomposed or recognized into single patterns, straight‐line patterns, or hyperbolic patterns, using syntactic pattern recognition. In the second step, the Hough transformation technique is used for reconstruction, pattern by pattern. The system of syntactic seismic pattern recognition includes envelope generation, a linking process in the seismogram, segmentation, primitive recognition, grammatical inference, and syntax analysis. The seismic patterns are automatically recognized and reconstructed.
Reciprocity theorem applied to the geophone‐ground coupling problem
Authors: Tan T. H.
Published: October 2002

Over the last 40 years the problem of geophone coupling has received regular attention. The problem of geophone coupling is the difference between the velocity as measured by the geophone and the velocity of the ground without the geophone. The papers published on this subject are based either on experimental work or on theoretical considerations. Washburn and Wiley (1941) show the remarkable differences between records from geophones laid on the surface and geophones placed on wooden posts driven about 1 ft into the ground. Similar observations were made by other experimentalists such as Fail et al. (1962), Hoover and O’Brien (1980), Omata (1983), and Krohn (1984).
Dip moveout in the frequency‐wavenumber domain
Authors: Notfors C. D. , Godfrey R. J.
Published: October 2002

In recent years dip moveout (DMO) has come into routine use in the seismic processing industry. The main benefits of including DMO in the processing sequence are that (1) stacking velocities after DMO are dip‐independent, and (2) “reflection point smear” associated with dipping events is eliminated by laterally shifting the reflection points to their zero‐offset position. These effects of DMO are also beneficial for estimation of stacking velocities by simplifying the interpretation of velocity analysis. Dip moveout also has an inherent dip filtering effect (Bolondi et al., 1984) by lowering the frequency content on the stacked section of steeply dipping aliased events, which leads to reduced migration noise.
Principle of prestack migration based on the full elastic two‐way wave equation
Authors: Wapenaar C. P. A. , Kinneging N. A. , Berkhout A. J.
Published: October 2002

The acoustic approximation in seismic migration is not allowed when the effects of wave conversion cannot be neglected, as is often the case in data with large offsets. Hence, seismic migration should ideally be founded on the full elastic wave equation, which describes compressional as well as shear waves in solid media (such as rock layers, in which shear stresses may play an important role). In order to cope with conversions between those wave types, the full elastic wave equation should be expressed in terms of the particle velocity and the traction, because these field quantities are continuous across layer boundaries where the main interaction takes place. Therefore, the full elastic wave equation should be expressed as a matrix differential equation, in which a matrix operator acts on a full wave vector which contains both the particle velocity and the traction. The solution of this equation yields another matrix operator. This full elastic two‐way wave field extrapolation operator describes the relation between the total (two‐way) wave fields (in terms of the particle velocity and the traction) at two different depth levels. Therefore it can be used in prestack migration to perform recursive downward extrapolation of the surface data into the subsurface (at a “traction‐free” surface, the total wave field can be described in terms of the detected particle velocity and the source traction). Results from synthetic data for a simplified subsurface configuration show that a multiple‐free image of the subsurface can be obtained, from which the angle‐dependent P-P and P-SV reflection functions can be recovered independently. For more complicated subsurface configurations, full elastic migration is possible in principle, but it becomes computationally complex. Nevertheless, particularly for the 3-D case, our proposal has improved the feasibility of full elastic migration significantly compared with other proposed full elastic migration or inversion schemes, because our method is carried out per shot record and per frequency component.
Wave‐field transformations of vertical seismic profiles
Authors: Hu Liang‐Zie , McMechan George A.
Published: October 2002

Vertical seismic profile (VSP) data may be partitioned in a variety of ways by application of wave‐field transformations. These transformations provide insights into the nature of the data and aid in the design of processing operations. Transformations are implemented in a reversible sequence that takes the observed VSP data from the depth‐time (z-t) domain through the slowness‐time intercept (p-τ) domain (by a slant stack), to the slowness‐frequency (p-ω) domain (by a 1-D Fourier transform over τ), to the wavenumber‐frequency (k-ω) domain (by resampling using the Fourier central‐slice theorem), and finally back to the z-t domain (by an inverse 2-D Fourier transform). Multidimensional wave‐field transformations, combined with k-ω, p-ω, and p-τ filtering, can be applied to wave‐field resampling, interpolation, and extrapolation; separation of P-waves and S-waves; separation of upgoing and downgoing waves; and wave‐field decomposition for isolation, identification, and analysis of arrivals.
Circumferential propagation of elastic waves on boreholes and cylindrical cavities
Authors: Siggins A. F. , Stokes A. N.
Published: October 2002

Elastic waves propagating circumferentially on boreholes in rock are potentially useful in many nondestructive testing applications, including measurement of stress‐induced anisotropy and remote detection of cracks on borehole walls. Although analyses of the far‐field response of the surrounding rock to forces applied within boreholes have been described by several authors, little attention has been focused on the response of the borehole wall to such forces. Fourier‐Bessel and finite‐element methods of analysis are used to derive dynamic compliance transfer functions for a borehole in an elastic rock subject to a localized internal force. The analyses are in overall agreement, and both predict that the borehole behaves as a lossy resonant system when excited by such steady‐state forces. The lossy resonances can be attributed to standing surface waves. For a borehole within a rock with Poisson’s ratio of 0.10, the first three dimensionless wavenumbers [Formula: see text] at which resonant amplifications occur are 0.288, 0.836, and 1.403. These wavenumbers are approximately equal to the real part of the first three Rayleigh eigenvalues of the frequency equation for cylindrical cavities in elastic media.
On the connection between the Herglotz‐Wiechert‐Bateman and tau‐sum inversions
Authors: Vera Emilio E.
Published: October 2002

When seismic traveltime data T(X) are mapped into the domain of intercept time τ and ray parameter p, the traveltime information acquires a geometrical organization that is not in either the T(X) or X(p) domains, and it allows easy discrimination between precritical and postcritical arrivals (Diebold and Stoffa, 1981). In theory, in a laterally homogeneous medium and in the absence of low‐velocity zones, the τ-p domain representation of the postcritical arrivals is a distinct, unbroken trajectory that can be easily recognized and used for velocity‐depth inversion. In practice, however, τ-p mapping can only be performed for a discrete set of p values and over a finite range of X and T. Hence, not only a discrete but an inherently incomplete set of postcritical (τ, p) pairs is obtained.
The dielectric constant of sandstones, 60 kHz to 4 MHz
Authors: Knight Rosemary J. , Nur Amos
Published: October 2002

Complex impedance data were collected for eight sandstones at various levels of water saturation [Formula: see text] in the frequency range of 5 Hz to 4 MHz. The measurements were made using a two‐electrode technique with platinum electrodes sputtered onto the flat faces of disk‐ shaped samples. Presentation of the data in the complex impedance plane shows clear separation of the response due to polarization at the sample‐electrode interface from the bulk sample response. Electrode polarization effects were limited to frequencies of less than 60 kHz, allowing us to study the dielectric constant κ′ of the sandstones in the frequency range of 60 kHz to 4 MHz. κ′ of all samples at all levels of saturation shows a clear power‐law dependence upon frequency. Comparing the data from the eight sandstones at [Formula: see text], the magnitude of the frequency dependence was found to be proportional to the surface area‐to‐volume ratio of the pore space of the sandstones. The surface area‐to‐volume ratio of the pore space of each sandstone was determined using a nitrogen gas adsorption technique and helium porosimetry. κ′ also exhibits a strong dependence on [Formula: see text]. κ′ increases rapidly with [Formula: see text] at low saturations, up to some critical saturation above which κ′ increases more gradually and linearly with [Formula: see text]. Using the surface area‐to‐volume ratios of the sandstones, the critical saturation in the dielectric response was found to correspond to water coverage of approximately 2 nm on the surface of the pore space. Our interpretation of the observed dependence of κ′ on both frequency and [Formula: see text] is that it is the ratio of surface water to bulk water in the pore space of a sandstone that controls the dielectric response through a Maxwell‐Wagner type of mechanism.
Combining attenuation by Q and spherical divergence
Authors: Iverson William P.
Published: October 2002

A general correlation of cross‐well seismic data and surface seismic data is attempted simply by examining a combination of the two major mechanisms of seismic wave attenuation, anelastic Q, and spherical divergence. High‐frequency cross‐well seismology can be hindered by the assumption that an order of magnitude increase in frequency is accompanied by an order of magnitude decrease in propagation distance such that the anelastic attenuation (described by quality factor Q) remains constant. Such a comparison, however, neglects the effects of geometrical spreading, which is independent of frequency. Through the consideration of both Q and spherical divergence, it is demonstrated that the total attenuation of 2 000 Hz energy at a distance of 613 m is equivalent to the total attenuation of 20 Hz energy at a distance of 7 000 m. Applications of kilohertz cross‐well seismic surveys between wells spaced by over 600 m could be possible with present dynamic‐range capabilities in seismic recording systems. Such applications would allow the use of high‐resolution cross‐well seismic surveys between wells drilled on a 40 acre spacing. One example of cross‐well seismic data is shown to demonstrate the high‐frequency content (kilohertz) which can be obtained. Theoretical calculations indicate that kilohertz energy could be recorded at distances up to 1 km. The assumptions are that present surface seismology measurements define the dynamic‐range capabilities of recording instruments, and that Q with spherical divergence accounts for all attenuation. All calculations neglect the problems of miniaturization required for downhole applications. The goal is that omission of extremely low Q regions near the surface and the noise‐free borehole environment can overcome the miniaturization problems.
On: “A simplified fast method of deriving weathering correction” by M. H. Morsi (GEOPHYSICS, 51, 1698–1690, August 1986).
Authors: Keith Colum M.
Published: October 2002

The results of the subject paper can be derived in a somewhat more straightforward fashion that admits of easy generalization. As in the subject paper, we take the weathering layer and elevation correction as composed of two parts, one that corrects from the surface to the base of that weathering layer, and the other that corrects from the base of the weathering layer to the desired datum.
Spectral reflectance of lichens and their effects on the reflectance of rock substrates
Authors: Ager C. M. , Milton N. M.
Published: October 2002

Rock outcrops in arid and semiarid regions frequently have abundant lichen cover. We collected lichens and their granite, slate, and hornfels substrates in the Extremadura region of Spain and measured spectral reflectance in the 400–2500 nm region in the laboratory. Lichen cover as low as 30 percent masks the spectral features of spectrally flat rocks such as slate and hornfels. The sharp spectral features of granite are obscured at 60–80 percent lichen cover. The strong reflectance peak at 2220 nm in lichens interferes with recognition of the 2200 nm absorption band in hydroxyl‐bearing minerals that is used to locate hydrothermally altered and potentially mineral‐rich areas. However, lichen spectra are characterized by distinctive absorption features that occur in few minerals and can thus be used to identify lichen presence. From computer modeling of laboratory spectra, we conclude that Landsat Thematic Mapper (TM) band ratios of 3 : 4 and 5 : 2 best minimize lichen effects in TM image analysis.
Reverse‐time depth migration: Impedance imaging condition
Authors: Hildebrand Steve T.
Published: October 2002

Reverse‐time depth migration includes an imaging condition and a time extrapolation procedure where the wave field recorded on a surface is continued into a model earth. In this paper, both the pressure and the particle velocity are used in a reverse‐time depth‐migration technique. Using these two continued wave fields, one constructs a map of the earth’s impedance as a function of depth. This is also an exact inversion procedure where multiples and source wavelets are deconvolved.
Airborne electromagnetic bathymetry
Authors: Zollinger R. , Morrison H. F. , Lazenby P. G. , Becker A.
Published: October 2002

Airborne electromagnetic techniques provide an efficient means for determining the bathymetry of shallow seas and coastal regions. Offshore time‐domain data collected with the INPUT® system may be interpreted quickly and inexpensively using the simple table look‐up algorithm described here. This interpretation algorithm is based on two dimensionless parameters which depend on the six channel amplitudes of the recorded INPUT data. These amplitudes are computed and tabulated for a suite of simple two‐layer models in which seawater depth is the only variable quantity. The experimental data can then be interpreted by comparison with the tables of theoretical values to obtain the bathymetry beneath the flight path. The inversion algorithm has proven accurate and stable. In the range of 0–40 m of seawater, the representative difference between the interpreted depths and the charted depths is about 2 m. Bottom gradients lower than 5 percent were resolved accurately beneath 0–25 m of water. Independent interpretations of data sets collected at different altitudes and in different directions show excellent repeatability. The algorithm exhibits low sensitivity to changes in the transmitter‐receiver configuration and to variations in the underlying half‐space conductivities when conductivity values are in the range [Formula: see text]. The fractional error produced in the calculated depths by variations in the conductivity of the seawater is approximately equal to the fractional difference between the actual conductivity of the sea and the value of 4 S/m assumed in the model. ® Trademark of Barringer Research Ltd.
The application of diffraction tomography to cross‐hole seismic data
Authors: Pratt R. Gerhard , Worthington M. H.
Published: October 2002

Previously published equations for diffraction tomography do not solve the “two and one‐half dimensional problem” (point source illumination of two‐dimensional geology) if sources and receivers are confined to linear arrays. In spite of this lack of a formal solution, useful images can be formed by the application of two‐dimensional formulas to such problems. The estimation of difference fields, of crucial importance in diffraction tomography, reduces to the problem of estimating the source function. Using assumptions about the consistency of the source behavior, we extract the source function in a statistical fashion from cross‐hole data. Using this technique, the difference fields are computed directly from the recorded wave fields for two experiments and diffraction tomographic images are obtained. In the first experiment, the data are generated using a two‐dimensional finite‐difference modeling algorithm. In the second, a physical scale model of a crosshole experiment is performed in an ultrasonic modeling tank. Images are obtained within both the first Born and Rytov’s approximation. Our results indicate that Rytov’s approximation achieves good resolution of the lower wavenumber components of the object, whereas the first Born approximation is more successful where the object is discontinuous.
A theoretical study of acoustic S-wave and P-wave velocity logging with conventional and dipole sources in soft formations
Authors: Winbow Graham A.
Published: October 2002

This paper is focused on the special features of the wavetrains recorded by conventional and dipole sonic logging tools in soft formations defined to be those whose shar velocity is less than the sound velocity of drilling mud. Such formations are commonn in the Gulf Coast, the Canadian Arctic, the Bass Strait of Australia, and many other region. A conventional logging tool operating at normal frequencies [Formula: see text] records P waves, water waves, and Stoneley waves in soft formations. A dipole tool records modal waves and water waves at frequencies of order 15 kHz, but produces almost pure S-wave first arrivals at low frequencies [Formula: see text] since at 1 kHz, a mode which we refer to as a “dipole Stoneley wave” is efficiently excited. For very soft materials such as clays, where the formation P-wave velocity can be less than the fluid velocity, the formation P velocity can be logged by operating a conventional sonic tool at low frequencies [Formula: see text] so as to excite a leaky mode traveling at very close to the formation P-wave velocity. Water waves are not important for high‐velocity formations where they arrive at the trailing edge of the modal part of the wavetrain. However, in soft formalions they form a prominent part of the wavetrain at normal logging frequencies [Formula: see text] and disappear at low frequencies [Formula: see text]. Water waves are carried by leaky modes.
Formation temperature estimation by inversion of borehole measurements, Part II: Effects of fluid penetration on bottom‐hole temperature recovery
Authors: Cao Song , Lerche Ian , Hermanrud Christian
Published: October 2002

Using nonlinear inverse techniques, we show that the change in borehole temperature with time, after mud circulation has stopped, can be used to provide very precise estimates of true formation temperatures and of mud temperature at the time circulation stopped. In addition, ruggedly stable estimates can also be made of the thermal invasion distance, the formation thermal conductivity, and the efficiency of heating the mud by the thermal recovery wave. It is well known that convective heat flow into (or out of) the formation influences the thermal recovery. We show that the flow rate at the borehole can also be obtained approximately from the borehole temperature measurements using the inverse method. Fidelity and reproducibility of the inverse procedure arc examined using synthetic data. Applications to field data from three wells in the Cooper basin of Australia and four wells in the North Sea confirm the accuracy of the method in satisfying the observed data, in determining true formation temperatures, and in assessing the magnitude of fluid penetration into the formation.
Marine vibrators and the Doppler effect
Authors: Dragoset William H.
Published: October 2002

Marine seismic data acquired with a moving vibrator suffer phase dispersion caused by Doppler shifting of the source sweep function. The dispersion for a particular reflection event depends upon frequency, the type of sweep function, and the Doppler factor associated with that event. Synthetic vibrator data show that, at typical ship speeds, the Doppler factors for steeply dipping events are big enough to cause phase dispersion as large as several hundred degrees. If unaccounted for, such dispersive effects could make a moving marine vibrator unacceptable for imaging steep dips. In a constant‐offset section, the Doppler factor for a reflection event is the product of ship speed and the event’s time dip. That key, simple relationship allows a two‐dimensional f-k filter to remove the phase dispersion caused by the Doppler effect. Comparisons of both synthetic data and Gulf of Mexico field data, before and after application of the phase‐correcting filter, show that the filter improves steep‐dip imaging in marine vibrator data. For the Gulf of Mexico line, steep dips are imaged just as well in the phase‐corrected vibrator data as in air‐gun data.
Model studies of electrical leak detection surveys in geomembrane‐lined impoundments
Authors: Parra J. O. , Owen T. E.
Published: October 2002

Pole‐dipole array electrical potential distributions are calculated for a geomembrane‐lined liquid impoundment having single or multiple leaks. A three‐dimensional numerical model is employed to represent a small circular leak in the highly resistive plastic liner. The liquid waste material, the liner, and the soil under the impoundment are simulated by infinite horizontal layers, with approximate corrections for the finite size of the impoundment. Parametric curves for a single leak show that with optimum selection of electrode spacing and positioning and other field survey parameters, leaks can be detected effectively. To identify and resolve the presence of a cluster of leaks, the potential measurements must be made close to the liner and the detector dipole spacing must be smaller than the separation of the leaks. The results also indicate that the survey speed may be increased when portable leak detection equipment employing a vertical dipole detector is used.
Radar sounding in potash mines, Saskatchewan, Canada
Authors: Annan A. P. , Davis J. L. , Gendzwill D.
Published: October 2002

Dry salt has a very low electrical conductivity which permits radio signals to be transmitted through substantial thicknesses of salt with little attenuation. Radar sounding experiments conducted in several potash mines in Saskatchewan, Canada, confirm that radar is an effective method for probing salt environments. The experiments had two main objectives: namely, to assess the utility of impulse radar for mapping stratigraphy in the salt beds above, below, and beside mining tunnels and to determine the electrical character of the potash beds in situ, since little information was available on the radio‐frequency properties of potash ore. The experiments were conducted with an impulse radar system using antennas which radiated pulses having center frequencies between 100 and 1000 MHz. The higher frequency antennas detected thin clay beds and stress‐relief cracks to depths of several meters. Sounding for deeper structure known to exist at ranges of several tens of meters was not always successful. The lower frequency antenna systems were found most useful for deeper sounding; in some instances known geologic horizons were detected at depths of up to 20 m. Deeper structure was frequently masked by shallow, high‐reflectivity stress‐relief cracks. The potash deposits were found to have a bulk dielectric constant in the range 5 to 6. In many areas, the evaporites have a significant clay content which governed the signal attenuation. Depth of penetration of the radar was generally greater in areas of lower clay content.
Charge density and permeability in clay‐bearing sandstones
Authors: Goode Peter , Sen Pabitra N.
Published: October 2002

Electrical conduction in sandstones containing clay, i.e., shaly sands, is affected by change per unit pore volume [Formula: see text] (Waxman and Smits, 1968). Motivated by the facts that [Formula: see text] is proportional to the surface‐to‐volume ratio (Patchett, 1975), which in shaly samples is dominated by that of the clay particles, and that permeability k is inversely proportional to the square of the surface‐to‐volume ratio, we explore an empirical relation between k and [Formula: see text]. That k and [Formula: see text] might be related was first noted by Street (1961), but this is the first time a relation between k and [Formula: see text] has been tested on a large set of laboratory data. Although the data used here have been available in the literature for quite some time, the present use is novel.
An algebraic reconstruction technique for weakly anisotropic velocity
Authors: Stewart R. R.
Published: October 2002

Laboratory measurements and field experiments indicate that many sedimentary rocks display velocity anisotropy (Robertson and Corrigan, 1983; White et al., 1983; Thomsen, 1986; Winterstein, 1986; Gaiser et al., 1987). Accounting for anisotropy is proving important in several areas including the processing and interpretation of shear‐wave data (Lynn and Thomsen, 1986; Bush and Crampin, 1987) and the integration of vertical seismic profile (VSP) data and surface seismic data (Chiu and Stewart, 1987a).
Ray bending due to strong velocity anomalies
Authors: Singh R. P. , Nyland E.
Published: October 2002

Tomographic methods may be useful in determining localized changes in rock masses if accurate methods of inverting the data can be found. However, in the presence of strong localized anomalies, the underlying assumptions of many tomographic inversion methods break down, In fact, velocity contrasts as low as 15 percent can produce quite distorted raypaths. Even though the raypaths are distorted, the traveltimes on these paths are quite close to those which would arise from the straight‐ray assumption. The longer path length is compensated for by higher velocity. As a consequence, blind application of tomographic inversion to data obtained in the presence of strong anomalies can result in an incorrect and distorted but consistent solution.
Models of the Bouguer gravity and geologic structure at Yucca Flat, Nevada
Authors: Ferguson John F. , Felch Roger N. , Aiken Carlos L. V. , Oldow John S. , Dockery Holly
Published: October 2002

The Bouguer gravity anomaly at Yucca Flat, Nevada, has been modeled by two different techniques: the Cordell‐Henderson and Parker‐Oldenburg methods. The three‐dimensional model has incorporated known density and structural information where possible. These models predict the structural relief on the Cenozoic‐Paleozoic contact to within 150 m or about 15 percent of the actual depth. The three‐dimensional Parker‐Oldenburg method has been found to be efficient in an application involving a large (9000 sample) data base. Numerical stability was ensured by the application of a consistent regularization (a low‐pass filter tuned to suppress the noise‐dominated portion of the data spectrum) of the downward continuation operator. The use of a single regularizing filter for the entire model is not completely satisfactory due to the oversmoothing of shallow regions of the basin. The model is useful in the delineation of the geologic history of the area. Structural features in the model support the hypothesis that regional stress fields rotated significantly during the Tertiary. Major structural elements of the basin are well defined on the Cenozoic‐Paleozoic interface. The principal basin‐bounding fault is the large‐throw Carpetbag fault on the west. This fault was most active during the earliest phases of subsidence. The Yucca fault is seen to be a much smaller feature in the model presented here. The basin is rotated down to the west, with normal hinge faults on the eastern margin.
An airborne gravity study of eastern North Carolina
Authors: Brozena John M. , Peters Mary F.
Published: October 2002

The Naval Research Laboratory (NRL) has developed a prototype airborne gravity measurement system. The core of the system is a LaCoste and Romberg air‐sea gravity meter mounted on a three‐axis stable platform. Corrections to the gravimeter data for altitude and variations in altitude are determined from a combination of highly precise radar and pressure altimeters. The original prototype system was designed for use over oceanic areas. We recently incorporated the pressure measurement to extend use of the airborne system to terrestrial regions where occasional radar altitudes over points of known topographic height can be obtained. The radar heights are used to relate the pressure altitudes to absolute altitudes and to determine the slopes of the isobaric surfaces. Vertical accelerations due to horizontal velocity over a curved, rotating earth (the Eötvös correction) and precise two‐dimensional positions are determined from a Texas Instrument P-code global positioning system. The updated system was tested over eastern North Carolina and the Outer Banks, an area that is difficult to survey by conventional means. Over one‐third of the region consists of low lying swampy terrain and another one‐third is the shallow water of the Pamlico and Albemarle Sounds. Neither the land method nor the shipboard gravity surveying method is well suited for these types of areas. Flying at an altitude of 600 m at 375 km/hr, we were able to cover an area over [Formula: see text] with a nominal track spacing of 9 km by 9 km in less than 18 hours of flying time. A comparison by the Defense Mapping Agency showed a 2.8 mGal rms and a −0.2 mGal mean difference between ground truth data and the airborne data at grid points when both data sets were interpolated to a common 9 km grid.
Reflection maxima for reflections from single interfaces
Authors: Rendleman C. A. , Levin F. K.
Published: October 2002

At a workshop on refraction and wide‐angle reflections, Hilterman (1985) pointed out that, in contrast to the plane‐wave case, when there is a point source, a P-wave reflected from a plane interface attains its maximum amplitude at an offset greater than that corresponding to the critical angle (Figure 1). The same conclusion had been drawn earlier by Červený (1967). However, neither Červený’s results, which were based on very complicated mathematical expressions derived by Brekhovskikh (1960), nor Hilterman’s computer‐generated data shed light on the physics implied by the shifted maximum.
Seismic expression of the Upper Morrow sands, western Anadarko Basin
Authors: Halverson Jens R.
Published: October 2002

In the western Anadarko Basin, the Lower Pennsylvanian Upper Morrow sands are both a prolific and an elusive exploration target. Initial production from some of these sands can reach well over 1000 barrels of oil per day, and yet an offset well just 350 m away from a good producer can miss the Morrow sand entirely and result in a dry hole. One‐dimensional merged log modeling, two‐ dimensional log interpolation modeling, color seismic inversion processing, and seismic facies mapping techniques have been applied to the Lear and Darden fields, two Upper Morrow sand fields in the Texas Panhandle. Here the Morrow sands reach an isopach thickness of 10 to 15 m at a depth of 2500 to 3000 m. These Morrow sands are within the thin‐bed regime (below the tuning point) so that there is a correlation between the amplitude of the reflection and the thickness of the sand. The velocity and density contrasts of the shales and sands are sufficient to produce a good acoustic impedance contrast, making the sands detectable on seismic data with good signal‐to‐noise ratios. The comparison of geologic isopach mapping and geophysical seismic facies mapping shows an excellent correlation in the delineation of the Upper Morrow sands.
An analytic generalized inverse for common‐depth‐point and vertical seismic profile traveltime equations
Authors: Schuster Gerard T.
Published: October 2002

This paper presents an analytic formula for the generalized inverse matrix associated with vertical seismic profile (VSP) and common‐depth‐point (CDP) traveltime equations. Its functional form explicitly depends upon the source‐receiver geometry and its validity is restricted to horizontally layered earth models where there is negligible ray bending. The importance of the generalized inverse is that it can be used to derive closed‐form expressions for the covariance matrix, condition number, resolution matrix, and average inverse eigenvalues for the traveltime equations; these formulas are useful in designing source‐receiver geometries which optimize velocity reconstruction by traveltime inversion. These formulas also provide a fundamental understanding of some resolution limitations associated with earth tomography experiments. Previously, resolution characteristics of tomographic inversions were usually estimated by empirical computer tests on specialized earth models. Some specific conclusions drawn from these formulas include the following: (1) The elements of the slowness covariance matrix for uncorrelated traveltime errors depend only upon the length and number of ray segments which terminate in a layer, not on the number of rays which intersect a layer. (2) Better velocity resolution in a layer is achieved by increasing the length (larger source offset) and number (more sources) of rays which terminate in that layer. (3) Decreasing the layer thickness will result in a poorly conditioned set of equations; in fact, the [Formula: see text] condition number of the covariance matrix will increase faster than [Formula: see text] as the number of layers N increases. (4) A tradeoff exists between better depth resolution and an increase in the variance of the slowness estimate. (5) The discrete generalized inverse is a back‐projection operator followed by a directional second‐order difference operator. The latter operator is a local and numerically tractable operator, while the equivalent filtering operation for the classical inverse Radon transform is a global Hilbert transform preceded by a first‐order derivative.
Modeling and processing of scattered waves in seismic reflection surveys
Authors: Gibson Bruce S. , Levander Alan R.
Published: October 2002

Various wave‐scattering mechanisms are known to degrade reflection signals by producing noise in seismic reflection data. Synthetic 2-D acoustic‐wave finite‐ difference data sets illustrate the effects of two such mechanisms. Twenty‐five shot gathers were generated for each of two models and the data were processed as standard CMP surveys. In one model, an irregular low‐ velocity surface layer produced multiply scattered surface waves that appear as linear noise trains in common‐shot gathers and stacked sections. The scattering of upcoming reflections at the lower interface of the layer also produced a significant amount of noise. When predictive deconvolution was applied before stack to reduce reverberations, the spectral character of the scattered surface waves seriously inhibited the action of that process. In the second model, a zone of smooth, random velocity variation was imposed between two reflectors deeper in the model. The heterogeneous zone (±5 percent rms velocity variation) substantially degraded the signal reflected from below it; events produced by body‐wave scattering are characterized by higher phase velocities than those seen in the first model. Conventional CMP stacking produced discontinuous subhorizontal events from the disturbed zone. The limited bandwidth of the propagating signal and spatial filtering attributable to CMP stacking cause these events to bear no simple relation to the velocity anomalies of the model, even after migration.
An iterative inversion of back‐scattered acoustic waves
Authors: LeBras Ronan , Clayton Robert W.
Published: October 2002

The application of the Born approximation to the scattered wave field, followed by a WKBJ and far‐field approximation on the propagation Green’s function for a slowly varying background medium, leads to a simple integral relation between the density and bulk‐modulus anomalies superimposed on the background medium and the scattered wave field. An iterative inversion scheme based on successive back‐projections of the wave field is used to reconstruct the two acoustic parameters. The scheme, when applied to data generated using the direct integral relation, shows that the variations of the parameters can be reconstructed. The procedure is readily applicable to actual data, since every iterative step is essentially a prestack Kirchhoff migration followed by the application of the direct Born approximation and far‐field operator.