bluuebunny/crossref_metadata_2025 huggingface dataset:
Ground‐penetrating radar simulation in engineering and archaeology
Authors: Goodman Dean
Published: October 2002

Forward modeling of ground penetration radar is developed using exact ray‐tracing techniques. Structural boundaries for a ground model are incorporated via a discrete grid with interfaces described by splines, polynomials, and in the case of special structures such as circular objects, the boundaries are given in terms of their functional formula. In the synthetic radargram method, the waveform contributions of many different wave types are computed. Using a finely digitized antenna directional response function, the radar crosssection of buried targets and the effective area of the receiving antenna can be statistically modeled. Attenuation along the raypaths is also monitored. The forward models are used: “1” as a learning tool to avoid pitfalls in radargram interpretation, (2) to understand radar signatures measured across various engineering structures, and (3) to predict the response of cultural structures buried beneath important archaeological sites in Japan.
Laboratory velocities and attenuation of P‐waves in limestones during freeze‐thaw cycles
Authors: Remy Jean‐Michel , Bellanger Michel , Homand‐Etienne Françoise
Published: October 2002

The velocity and the attenuation of compressional P‐waves, measured in the laboratory at ultrasonic frequencies during a series of freezing and thawing cycles, are used as a method for predicting frost damage in a bedded limestone. Pulse transmission and spectral ratio techniques are used to determine the P‐wave velocities and the attenuation values relative to an aluminum reference sample with very low attenuation. Limestone samples were water saturated under vacuum conditions, jacketed with rubber sleeves, and immersed in an antifreeze bath (50 percent methanol solution). They were submitted to repeated 24-hour freezing and thawing cycles simulating natural environment conditions. During the freeze/thaw cycles, P‐wave velocities and quality factor Q diminished rapidly in thawed rock samples, indicating modification of the pore space. Measurements of crack porosity were conducted by hydrostatic compression tests on cubic rock samples that had been submitted to these freeze/thaw cycles. These measurements are used as an index of crack formation. The hydrostatic compression tests confirmed the phases of rock damage that were shown by changes in the value of Q. Furthermore, comparisons between Q values and crack porosity demonstrated that the variations of P‐wave attenuation are caused by the creation of new cracks and not by the enlargement of pre‐existing cracks.
Estimating seismic velocities at ultrasonic frequencies in partially saturated rocks
Authors: Mavko Gary , Nolen‐Hoeksema Richard
Published: October 2002

Seismic velocities in rocks at ultrasonic frequencies depend not only on the degree of saturation but also on the distribution of the fluid phase at various scales within the pore space. Two scales of saturation heterogeneity are important: (1) saturation differences between thin compliant pores and larger stiffer pores, and (2) differences between saturated patches and undersaturated patches at a scale much larger than any pore. We propose a formalism for predicting the range of velocities in partially saturated rocks that avoids assuming idealized pore shapes by using measured dry rock velocity versus pressure and dry rock porosity versus pressure. The pressure dependence contains all of the necessary information about the distribution of pore compliances for estimating effects of saturation at the finest scales where small amounts of fluid in the thinnest, most compliant parts of the pore space stiffen the rock in both compression and shear (increasing both P‐ and S‐wave velocities) in approximately the same way that confining pressure stiffens the rock by closing the compliant pores. Large‐scale saturation patches tend to increase only the high‐frequency bulk modulus by amounts roughly proportional to the saturation. The pore‐scale effects will be most important at laboratory and logging frequencies when pore‐scale pore pressure gradients are unrelaxed. The patchy‐saturation effects can persist even at seismic field frequencies if the patch sizes are sufficiently large and the diffusivities are sufficiently low for the larger‐scale pressure gradients to be unrelaxed.
Ambiguity analysis of well‐log data
Authors: Buoro Alvaro Bueno , C. Silva João B.
Published: October 2002

Ambiguity in a geophysical interpretation problem is the possibility of accepting more than one solution caused either by solution nonuniqueness or instability. Nonuniqueness is related to the existence of more than one solution regardless of the precision of observations. On the other hand, instability is related to the acceptance of different solutions producing data fittings within the expected observational errors. We studied the ambiguity in the inversion of well‐logging data using a method based on the analysis of a finite number of acceptable solutions, which are ordered, in the solution space, according to their contributions to the overall ambiguity. The analysis of the parameter variations along these ordered solutions provides an objective way to characterize the most ambiguous parameters. Because this analysis is based on the geometry of an ambiguity region, empirically estimated by a finite number of alternative solutions, it is possible to analyze the ambiguity due not only to errors in the observations, but also to discrepancies between the interpretation model and the true geology. Moreover, the analysis can be applied even in the case of a nonlinear interpretation model. The analysis was performed with recorded data, and compared with the analysis using singular value decomposition, leading to comparable results. Following the determination of the most ambiguous parameters, a reparameterization is possible by grouping these parameters into a single parameter leading to a simpler interpretation model and, therefore, to a drastic reduction in the ambiguity.
Anisotropy induction logging
Authors: Nekut Anthony G.
Published: October 2002

The apparent tensor anisotropic conductivity of the formations around a borehole can be calculated from triaxial induction logging data. In particular, analysis of the late‐time transient coupling of triaxial coincident loops yields a simple method to determine the principle axes and conductivities.
Crosswell seismic reflection/diffraction tomography: A reservoir characterization application
Authors: Tura M. Ali C. , Greaves Robert J. , Beydoun Wafik B.
Published: October 2002

A crosswell seismic experiment at the San Emidio oil field in Bakersfield, California, is carried out to evaluate crosswell reflection/diffraction tomography and image the interwell region to locate a possible pinchout zone. In this experiment, the two wells used are 2500 ft (762 m) apart, and the zone to be imaged is 11 000 ft (3350 m) to 13 000 ft (3960 m) deep. With the considered distances, this experiment forms the first large scale reservoir characterization application of crosswell reflection/diffraction tomography. A subset of the intended data, formed of two common receiver gathers and one common shot gather, was collected at the San Emidio oil field. The crosswell data display a wide variety of wave modes including tube waves, singly and multiply reflected/diffracted waves, and refracted waves. The data are processed using frequency filters, median filters, and spatial muting filters to enhance the reflected/diffracted energy. A 2-D layered velocity model with gradients is built using zero‐offset VSPs and full‐waveform acoustic logs from the two wells. This model is used to generate synthetic finite‐difference data for the field data acquisition geometry. The synthetic data are processed and imaged using the elastic ray‐Born 𝓁2-migration/inversion (ERBMI) method. A smooth 2-D velocity model incorporating only gradients and a few layers is used as a background model for the imaging. Considering the limited data acquisition geometry, synthetic data images compare favorably with the initial velocity model. With the encouraging results obtained from synthetic data, the ERBMI method, with the smooth background velocity model is used next to image the processed field data. Images obtained from the crosswell data show a good match with the reflected field in the zero‐offset VSPs and with migrated surface seismic data. From the interpretation of these images, the potential of this crosswell seismic method for answering questions regarding reservoir continuity and existence of pinchout zones can be seen.
Synthetic seismograms for a long spread of seismometer stations
Authors: Rosenbaum J. H.
Published: October 2002

The assumption that the earth is made up of planeparallel, homogeneous, elastic layers, which can exhibit transverse isotropy and moderate constant‐Q attenuation, leads to an effective method of modeling the response from a point source into a long spread of seismometer stations. Most of the computations are carried out in the complex frequency and complex horizontal wavenumber domains. Minimum sampling criteria are based on an algorithm that suppresses time and distance aliasing at the expense of the large dynamic range available on the digital computer. Other artifacts can be identified and are removed by additional wavenumber filtering. Computational effort is almost independent of the number of detectors and their nature. The structure and symmetry properties of the propagator matrices describing the response are the same for isotropic and transversely isotropic layers. Synthetic seismic panels for a regional model of a west Texas well site exhibit strong, shot‐generated surface waves. A very simple model, based on a south Texas well site, shows the effects of transverse isotropy and the reverberatory nature of converted signals generated by a surface P‐wave.
A simple derivation of Cordell’s stripping filter
Authors: Aina Adebayo
Published: October 2002

The presence of a strong regional (deep source) anomaly often makes the identification of local (shallow source) anomalies difficult. It may become necessary to either separate the regional and the residual, or carry out an operation on the observed map that enhances the local anomalies at the expense of the regional background. The problem of separation of deep and shallow‐source potential field anomalies is one of the oldest problems in geophysics. Various authors (Griffin, 1949; Nettleton, 1954; Abdelrahman et. al., 1985, for example) have presented details and schemes for separating residual and background anomalies. More recently, Cordell (1985) through Green’s equivalent stratum theorem obtained a filter in the frequency domain that may be used for separating shallow and deep‐source potential field anomalies.
Estimation of phase velocities and Q‐factors from zero‐offset, vertical seismic profile data
Authors: Amundsen Lasse , Mittet Rune
Published: October 2002

In seismic processing, we face the problem of characterizing the effect of anelastic attenuation on the seismic signal during propagation. To solve this problem, approximate physical properties of the porous fluid‐filled rock are needed. We give a strategy for estimating effective parameters governing absorption and dispersion of waves in viscoelastic media by inverting zero‐offset vertical seismic profiling (VSP) data acquired in a medium with plane horizontal layers. The VSP data are filtered such that all energy except for the direct downgoing wave and the primary reflected wave from each interface is zeroed. This procedure requires a model with layer thicknesses greater than a minimum limit. A stack of thin layers must be replaced by a layer with average physical properties and with thickness exceeding the required minimum. The model parameter vector is partitioned into two vectors. The first contains the frequency‐dependent complex propagation velocity in each layer, and is evaluated over the frequency band where the signal‐to‐noise ratio is acceptable. In the second vector, the geophone‐to‐formation coupling factors, which are assumed to be frequency‐independent, are gathered. It is straightforward to determine both the frequency‐dependent phase velocities and the frequency‐dependent quality (Q‐) factors from the frequency‐dependent propagation velocities. We assume that layer boundaries and layer densities can be obtained from well logs. We give the equations for a simplified forwardmodeling scheme and the equations for the solution of the nonlinear inverse problem. The algorithm is applied to both synthetic and real data. Inversion of synthetic data shows that the phase velocities can be satisfactorily estimated, and that Q‐factors below approximately 50 are well‐resolved, even for large errors in the geophone‐to‐formation coupling factors. The estimated phase velocities from the real data behave fairly stable as a function of frequency. The results for the quality factors are less conclusive, but the low Q‐factors may be of correct size.
A study of downhole electromagnetic sources for mapping enhanced oil recovery processes
Authors: Newman Gregory A.
Published: October 2002

An evaluation of downhole electromagnetic (EM) sources has been made for mapping 3-D enhanced oil recovery (EOR) processes. Two types of sources considered were vertical magnetic dipoles and vertical to near‐vertical electric dipoles and bipoles. These sources were used to produce magnetic field responses expected of EOR processes for crosswell configurations. A borehole‐to‐surface configuration was also studied for the downhole electric source, since this configuration can be highly sensitive to shallow 3-D EOR targets. For the crosswell arrays, the criteria used to evaluate the sources were the magnitudes of the observed signals with and without the process and the amount these signals change because of a migrating process. Instrumental noise was considered in the evaluation. Findings show that either electric or magnetic sources can produce truly significant changes in the fields, provided the fields before and after the initiation of the process are compared. An order of magnitude change in the fields has been demonstrated. The key to measuring such changes is to use the highest frequency possible. This frequency will be limited by instrumental noise. A migrating process did not produce field changes that are as large as those observed with and without the process. Nevertheless, model simulations showed that changes in the fields caused by the migrating processes are significant and measurable. Calculated responses of a shallow process at the surface showed that they were extremely sensitive to small deviations in the orientation of the downhole electric source. Quantitative interpretation should proceed only with techniques that explicitly consider the source orientation.
3-D elastic prestack, reverse‐time depth migration
Authors: Chang Wen‐Fong , McMechan George A.
Published: October 2002

By combining and extending previous algorithms for 2-D prestack elastic migration and 3-D prestack acoustic migration, a full 3-D elastic prestack depth migration algorithm is developed. Reverse‐time extrapolation of the recorded data is by 3-D elastic finite differences; computation of the image time for each point in the 3-D volume is by 3-D acoustic finite differences. The algorithm operates on three‐component, vector‐wavefield common‐source data and produces three‐component vector reflectivity distributions. Converted P‐to‐S reflections are automatically imaged with the primary P‐wave reflections. There are no dip restrictions as the full wave equation is used. The algorithm is illustrated by application to synthetic data from three models; a flat reflector, a dipping truncated wedge overlying a flat reflector, and the classical French double dome and fault model.
Digital field tape format standards — SEG-D, REVISION 1
Authors: Allen Richard , Crews Gary , Guyton William , McLemore Clifford A. , Peterson Bob , Rapp C. S. , Walker Leon , Whigham Larry R. , White David A. , Wood George
Published: October 2002

A number of additions are being made to the SEG-D field tape format to make the format more useful in the current seismic data acquisition environment. These changes are intended to address the majority of short term needs in the industry today. A more extensive revision to the field tape format will be required within the next few years in order to address long term needs.
Near‐surface thermal prospecting: Review of processing and Interpretation
Authors: Khesin B. E. , Eppelbaum L. V.
Published: October 2002

Temperature measurements at shallow depths (up to 3 m) contain useful information about features of the geological structures in the areas under investigation; however, the noise caused by seasonal temperature variations and terrain relief effects may significantly distort the observed temperature field. Therefore, procedures are developed for the calculation and removal of these noise sources: (a) seasonal variations are first eliminated by a procedure using repeated observations; (b) terrain relief corrections are calculated by a correlation technique, which facilitates the identification of anomalies associated with concealed geological features. Essential similarities between thermal and magnetic prospecting make it possible to apply to thermal prospecting modifications of the rapid methods of characteristic points and tangents developed for magnetic prospecting. These methods are applicable to conditions of inclined relief, arbitrary magnetization polarization), and an unknown level of the normal field. The methods can be used to locate disturbing bodies by their associated temperature anomalies. Interpretation is made possible by approximating bodies by a dipping thin sheet or a horizontal circular cylinder. The interpretation results obtained both on models and polymetallic (Greater Caucasus) and oil and gas (Middle Kura Depression) deposits testify to the accuracy and reliability of these methods. These methods were also used successfully for interpretation of temperature anomaly over underground cavity in Cracov (Poland).
1-D seismic inversion of dual wavefield data: Part II, A Gulf of Mexico example
Authors: Hildebrand Steve T. , McMechan George A.
Published: October 2002

Stable estimates for angle‐dependent acoustic impedance are obtained from a dual wavefield data set that consists of pressure and vertical particle velocity measurements. Using these acoustic impedance estimates, a nonlocal inversion procedure predicts sediment material impedance, interval velocity, density, and their associated relative errors as a function of depth. The method is successfully applied to data from a marine seismic experiment conducted by Halliburton Geophysical Services in the Gulf of Mexico in 1989. The inverted values for the seafloor sediment density and interval velocity lie close to those obtained independently by other methods, implying that the inversion illustrated is valid for use with field data.
3-D interpretation of reflected arrival times by finite‐difference techniques
Authors: Riahi M. Ali , Juhlin Christopher
Published: October 2002

Finite‐difference methods have generally been used to solve dynamic wave propagation problems over the last 25 years (Alterman and Karal, 1968; Boore, 1972; Kelly et al., 1976; and Levander, 1988). Recently, finite‐difference methods have been applied to the eikonal equation to calculate the kinematic solution to the wave equation (Vidale, 1988 and 1990; Podvin and Lecomte, 1991; Van Trier and Symes, 1991; Qin et al., 1992). The calculation of the first‐arrival times using this method has proven to be considerably faster than using classical ray tracing, and problems such as shadow zones, multipathing, and barrier penetration are easily handled. Podvin and Lecomte (1991) and Matsuoka and Ezaka (1992) extended and expanded upon Vidale’s (1988) algorithm to calculate traveltimes for reflected waves in two dimensions. Based on finite‐difference calculations for first‐arrival times, Hole et al. (1992) devised a scheme for inverting synthetic and real data to estimate the depth to refractors in the crust in three dimensions. The method of Hole et al. (1992) for inversion is computationally efficient since it avoids the matrix inversion of many of the published schemes for refraction and reflection traveltime data (Gjøystdal and Ursin, 1981).
On: “The electrical field in a borehole with a casing,” by A. A. Kaufman (January 1990 GEOPHYSICS, 55, p. 29–38) and “A transmission line model for electrical logging through casing,”
Authors: Kaufman A. A. , Wightman W. E.
Published: October 2002

In two interesting papers (Kaufman, 1990; and Kaufman and Wightman, 1993), it is proposed that D.C. (direct current) flow in a cased borehole can be described by a transmission‐line model. This important conjecture is given considerable support by these authors in their analysis. I have found a relatively simple and straightforward formulation, using a modal approach, that gives further insight.
Elastic‐wave stimulation of oil production: A review of methods and results
Authors: Beresnev Igor A. , Johnson Paul A.
Published: October 2002

Numerous observations accumulated principally during the last 40 years show that seismic waves generated from earthquakes and cultural noise may alter water and oil production. In some cases wave excitation may appreciably increase the mobility of fluids. The effect of elastic waves on the permeability of saturated rock has been confirmed in numerous laboratory experiments. Two related applications have arisen from these findings. In the first application, high‐power ultrasonic waves are applied for downhole cleaning of the near‐wellbore in producing formations that exhibit declining production as a result of the deposition of scales and precipitants, mud penetration, etc. In many cases, ultrasound effectively removes the barriers to oil flow into the well. The ultrasonic method is reported to be successful in 40–50 percent of the cases studied. In the case of successful treatment, the effect of improved permeability may last up to several months. Whereas this method has a very local effect, a second application is used to stimulate the reservoir as a whole. Here seismic frequency waves are applied at the earth’s surface by arrays of vibroseis‐type sources. This method has produced promising results; however, further testing and understanding of the mechanisms are necessary.
Correcting amplitude, time, and phase mis‐ties in seismic data
Authors: Bishop Thomas N. , Nunns Alan G.
Published: October 2002

Seismic lines from different vintages frequently mis‐tie where they intersect. The mis‐ties may stem from different amplitudes, a shift in time, or different wavelet character. We can correct these mis‐ties to a great extent by applying a scale factor, a static time shift, and a phase rotation to one of the lines. We describe algorithms to compute the amplitude, time, and phase differences at each intersection among a series of 2-D seismic lines. We can then use an iterative least‐squares technique to derive optimal mis‐tie corrections for each line. We include a necessary modification of the least‐squares technique for the nonlinear phase data. The various algorithms are stable, fast, and accurate. We have used them in conjunction with an interactive workstation seismic interpretation program for five years. The scalar mis‐tie corrections greatly enhance the consistency of the seismic data. We show the results of the mis‐tie package applied to a 20‐line survey consisting of five vintages of seismic data. The resulting mis‐tie corrections significantly improve the fit between the 20 lines.
Some influences of stratigraphy and structure on reservoir stress orientation
Authors: Bruno Michael S. , Winterstein Don F.
Published: October 2002

The azimuth of maximum horizontal stress in a reservoir can vary significantly with depth and with position on a subsurface structure. We present and discuss evidence from field data for such variation and demonstrate both analytically and with finite‐element modeling how such changes might take place. Under boundary conditions of uniform far‐field displacement, changes in stratigraphic layering can reorient the principal stress direction if the formation is intrinsically anisotropic. If the formation stiffness is lower perpendicular to bedding than parallel to bedding (as is often the case in layered geologic media), an increase in dip will reduce the component of compressive stress in the dip azimuth direction. Folds can reorient principal stresses because flexural strain varies with depth and position. Compressive stress perpendicular to a fold axis increases with depth at the crest of an anticline and decreases with depth at the limb. When the regional stress anisotropy is weak, this change in stress magnitude can reorient the local principal stress directions. Numerical simulations of such effects gave results consistent with changes in stress orientation at the Cymric and Lost Hills oil fields in California as observed via shear‐wave polarization analyses and tiltmeter surveys of hydraulic fracturing. Knowledge of such variation of stress direction with depth and structural position is critical for drilling, completions, hydraulic fracture, and well pattern designs.
Discrete plane‐wave decomposition by least‐mean‐square‐error method
Authors: Yilmaz Orhan , Taner M. Turhan
Published: October 2002

The recording of a point source wavefield can be decomposed into a set of plane‐wave components, each corresponding to different angles of propagation. Such plane‐wave seismograms have a far simpler structure than the spherical waves of the point source records, which makes them desirable in many steps of seismic data processing such as predictive deconvolution, migration, inversion, etc. The implementation of the plane‐wave decomposition requires the computation of the Radon transform in the discrete data domain. A straightforward application of the integral solutions to geophysical problems fails to compensate for the sampled and limited aperture nature of the actual data. In this paper, we give a new method in which the x-t domain is shown to relate to the p-τ domain by a linear system of equations in the time‐space domain. An iterative least‐mean‐square‐error method is introduced to solve the set of equations. This method is combined with a unique method of alias suppression which uses the reasonable range of dips possible at a given (x, t) location and acts as interpolation of the x-t data. This combination improves the initial estimates and speeds up convergence. Our transform is independent of the number of plane‐waves and selected ray parameter range. We present synthetic and real data examples to demonstrate the accuracy and robustness of the method. The examples are compared against results using the generalized Radon transform approach used by Beylkin (1987) and against conventional slant stack.
Wavefield separation by 3-D filtering in crosshole seismic reflection processing
Authors: Rowbotham Peter S. , Goulty Neil R.
Published: October 2002

In processing crosshole seismic reflection data, it is necessary to separate the upgoing and downgoing primary reflections from each other, from the direct waves, and from other wave types in the data. We have implemented a 3-D f-k-k filter for wavefield separation that is applied in a single pass. The complete data set is treated as a data volume, with each sample defined by the three coordinates of source depth, receiver depth, and time. The filter works well because upgoing primaries, downgoing primaries, and direct waves lie in different quadrants in f-k-k space. The strongest multiples, including mode‐converted multiples, lie in the same quadrants in f-k-k space as the direct waves, so they are readily rejected together. Tube waves and mode‐converted primaries are also suppressed as most of the energy in these wave types lies outside the pass volume for P‐wave primaries. Some head wave and S‐wave primary energy will be passed by the filter; however, these waves tend to have low amplitudes and late arrival times, respectively, and will be smeared out on imaging with the P‐wave velocity field. We have processed a real crosshole data set using two different methods of wavefield separation: applying 2-D f-k filtering to common source gathers and applying a 3-D f-k-k filter to the whole data set. The migrated image produced after 3-D f-k-k filtering contains less coherent noise and consequently shows improved continuity of reflectors.
Leveling total‐field aeromagnetic data with measured horizontal gradients
Authors: Nelson J. Bradley
Published: October 2002

Temporal changes in the earth’s magnetic field occur in the frequency band of millihertz to a few Hertz. Amplitudes of variations above 0.1 Hz are usually much smaller than 1 nT, changes of 50–100 nT over periods of a few hours are not uncommon.
Constant Q attenuation of subsurface radar pulses
Authors: Turner Greg , Siggins Anthony F.
Published: October 2002

Q is a measure of the energy stored to the energy dissipated in a propagating wave and can be estimated from the ratio of attenuation and frequency. For seismic waves, Q has been found to be essentially independent of frequency. As a result, attenuation is an approximately linear function of frequency and the impulse response function of the earth. Hence, the distortion of a seismic pulse as it propagates can be described by a single parameter. Laboratory measurements show that the attenuation of radio waves in some geological materials can also be approximated by a linear function of frequency over the bandwidths of typical subsurface radar pulses. We define a new parameter Q* to describe the slope of this linear region. The impulse response of the transfer function for a given value of Q* differs from that of the same value of Q only in total amplitude. Thus the change of shape of a radar pulse as it travels through these materials can also be described by a single parameter. The constant Q* model successfully describes the distortion of radar pulses propagated through a laboratory water tank and through weathered granite in a borehole field study.
Seismic signatures of reservoir transport properties and pore fluid distribution
Authors: Akbar Nabil , Mavko Gary , Nur Amos , Dvorkin Jack
Published: October 2002

We investigate the effects of permeability, frequency, and fluid distribution on the viscoelastic behavior of rock. The viscoelastic response of rock to seismic waves depends on the relative motion of pore fluid with respect to the solid phase. Fluid motion depends, in part, on the internal wave‐induced pore pressure distribution that relates to the pore micro‐structure of rock and the scales of saturation. We consider wave‐induced squirt fluid flow at two scales: (1) local microscopic flow at the smallest scale of saturation heterogeneity (e.g., within a single pore) and (2) macroscopic flow at a larger scale of fluid‐saturated and dry patches. We explore the circumstances under which each of these mechanisms prevails. We examine such flows under the conditions of uniform confining (bulk) compression and obtain the effective dynamic bulk modulus of rock. The solutions are formulated in terms of generalized frequencies that depend on frequency, saturation, fluid and gas properties, and on the macroscopic properties of rock such as permeability, porosity, and dry bulk modulus. The study includes the whole range of saturation and frequency; therefore, we provide the missing link between the low‐frequency limit (Gassmann’s formula) and the high‐frequency limit given by Mavko and Jizba. Further, we compare our model with Biot’s theory and introduce a geometrical factor whose numeric value gives an indication as to whether local fluid squirt or global (squirt and/or Biot’s) mechanisms dominate the viscoelastic properties of porous materials. The important results of our theoretical modeling are: (1) a hysteresis of acoustic velocity versus saturation resulting from variations in fluid distributions, and (2) two peaks of acoustic wave attenuation—one at low frequency (caused by global squirt‐flow) and another at higher frequency (caused by local flow). Both theoretical results are compared with experimental data.
An image of the Columbia Plateau from inversion of high‐resolution seismic data
Authors: Lutter William J. , Catchings Rufus D. , Jarchow Craig M.
Published: October 2002

We use a method of traveltime inversion of high‐resolution seismic data to provide the first reliable images of internal details of the Columbia River Basalt Group (CRBG), the subsurface basalt/sediment interface, and the deeper sediment/basement interface. Velocity structure within the basalts, delineated on the order of 1 km horizontally and 0.2 km vertically, is constrained to within ±0.1 km/s for most of the seismic profile. Over 5000 observed traveltimes fit our model with an rms error of 0.018 s. The maximum depth of penetration of the basalt diving waves (truncated by underlying low‐velocity sediments) provides a reliable estimate of the depth to the base of the basalt, which agrees with well‐log measurements to within 0.05 km (165 ft). We use image blurring, calculated from the resolution matrix, to estimate the aspect ratio of imaged velocity anomaly widths to true widths for velocity features within the basalt. From our calculations of image blurring, we interpret low velocity zones (LVZ) within the basalts at Boylston Mountain and the Whiskey Dick anticline to have widths of 4.5 and 3 km, respectively, within the upper 1.5 km of the model. At greater depth, the widths of these imaged LVZs thin to approximately 2 km or less. We interpret these linear, subparallel, low‐velocity zones imaged adjacent to anticlines of the Yakima Fold Belt to be brecciated fault zones. These fault zones dip to the south at angles between 15 to 45 degrees.
Shot gather DMO in the double log domain
Authors: Granser Harald
Published: October 2002

Dip moveout (DMO) correction is applied routinely now in seismic processing. Over the years, numerous DMO methods have been developed for the frequency‐wavenumber domain (Hale 1991).
APPLICATION OF THE FOURIER INTEGRAL TO SOME GEOPHYSICAL INSTRUMENT PROBLEMS
Authors: Born W. T. , Kendall J. M.
Published: October 2002

This paper considers only the simple part of Fourier Integral theory required for the work in hand. A special type of Henrici rolling sphere harmonic analyzer has been built which makes possible a large reduction in the amount of labor involved in dealing graphically with Fourier Integral operations. Examples of the use of this procedure as applied to several geophysical instrument problems are given.
THE RANGE OF AMPLITUDES IN SEISMIC REFLECTION RECORDS
Authors: Kendall J. M.
Published: October 2002

Curves are presented which show the amplitudes of some reflection records, plotted as a function of time, for several widely separated areas. It is shown that amplitude ratios of the order of 1000:1 occur on records in a time interval of two seconds. After correcting for divergence of the recorded waves, it is found that their amplitude decreases exponentially with time. The usable length of record as a function of the size of charge was investigated for one shot hole.
A REFRACTION THEORY ADAPTABLE TO SEISMIC WEATHERING PROBLEMS
Authors: Banta H. E.
Published: October 2002

An exponential relation [Formula: see text] between depth and velocity of propagation of seismic waves is postulated. The exponent is a number between zero and unity. Expressions for the time‐distance relation, the depth‐distance relation, the average vertical velocity, and the vertical time‐horizontal time relation are derived. Applications of the relations to seismic weathering problems are discussed.
THE RELATION BETWEEN DEPTH, LITHOLOGY, AND SEISMIC WAVE VELOCITY IN TERTIARY SANDSTONES AND SHALES
Authors: Haskell N. A.
Published: October 2002

Velocity data in Tertiary sands and shales, obtained at 62 wells in the southern San Joaquin Valley, California, are considered statistically. A mean vertical velocity gradient of 0.464 feet per second per foot is found, and evidence is presented which indicates that, on the average, 0.360 feet per second per foot, or 78% of the total vertical gradient, is due to the effect of the pressure exerted by the present over‐burden. It is found that in shales the effect of overburden is smaller than this average (0.240 and 0.231 feet per second per foot for two shale formations), and in sandier formations it is greater than the average (0.428 and 0.581 feet per second per foot for two moderately sandy formations). However, the velocity vs. depth curves for sands and for shales may intersect at any depth, and it does not appear to be possible, in general, to distinguish between the two solely on the basis of the velocity at a given depth. Comparison with similar data of Weatherby and Faust for older beds and with the experimental results of Birch and Bancroft for hard sandstone and slate suggests that these gradients cannot persist very far beyond the present maximum depth of observation of about 14,000 feet.
Ultrasonic velocity‐porosity relationships for sandstone analogs made from fused glass beads
Authors: Berge Patricia A. , Bonner Brian P. , Berryman James G.
Published: October 2002

Using fused glass beads, we have constructed a suite of clean sandstone analogs, with porosities ranging from about 1 to 43 percent, to test the applicability of various composite medium theories that model elastic properties. We measured P‐ and S‐wave velocities in dry and saturated cases for our synthetic sandstones and compared the observations to theoretical predictions of the Hashin‐Shtrikman bounds, a differential effective medium approach, and a self‐consistent theory known as the coherent potential approximation. The self‐consistent theory fits the observed velocities in these sandstone analogs because it allows both grains and pores to remain connected over a wide range of porosities. This behavior occurs because this theory treats grains and pores symmetrically without requiring a single background (host) material, and it also allows the composite medium to become disconnected at a finite porosity. In contrast, the differential effective medium theory and the Hashin‐Shtrikman upper bound overestimate the observed velocities of the sandstone analogs because these theories assume the microgeometry is represented by isolated pores embedded in a host material that remains continuous even for high porosities. We also demonstrate that the differential effective medium theory and the Hashin‐Shtrikman upper bound correctly estimate bulk moduli of porous glass foams, again because the microstructure of the samples is consistent with the implicit assumptions of these two theoretical approaches.
Ambiguities in AVO inversion of reflections from a gas‐sand
Authors: Drufuca Giuseppe , Mazzotti Alfredo
Published: October 2002

We examine the reflections from a thick sand layer embedded in shales deposited in an open marine environment of Miocene age. Borehole data indicate that the sand bed is gas saturated. Making the assumptions of single interface reflections, plane‐wave propagation in elastic and isotropic media, and correct amplitude recovery of the actual seismic data, we try to invert the amplitude variation with offset (AVO) response for the compressional velocity [Formula: see text], shear velocity [Formula: see text], and density [Formula: see text] of the gas‐sand layer, knowing the parameters of the upper layer and the calibration constant. The actual reflections reach incidence angles up to 54 degrees at the farthest offset. Notwithstanding the large range of incidence angles, the outcomes of the inversion are ambiguous for we find many solutions that fit equally well, in a least‐squares sense, the observed AVO response. We present the locus of the solutions as curves in compressional velocity [Formula: see text], shear velocity [Formula: see text], and density [Formula: see text] space. To gain a better understanding of the results, we also perform the same inversion experiment on synthetic AVO data derived from the borehole information. We find that when inverting the AVO response in the same range of incidence angles as in the real data case, the exact solution is found whichever starting point we choose; that is, we have no ambiguity. However, if we limit the incidence angle range, e.g., to 15 degrees, the invention is no longer able to find a unique solution and the set of admissible solutions defines regular curves in [Formula: see text], [Formula: see text], [Formula: see text] space. We infer that residual noise in the recorded data is responsible for the ambiguities of the solutions, and that because of numerical noise, a wide range of incidence angle is required to obtain a unique solution even in noise‐free synthetic data.
Prestack migration using arrival angle information
Authors: Takahashi Toru
Published: October 2002

When prestack depth migration is applied to field data, migration artifacts or false images resulting from limited observation geometry or aperture often make it difficult to interpret the migrated images. The occurrence of artifacts is especially a problem on VSP data that usually have poor geometry for migration. To reduce such unwanted artifacts in migration images, I propose here that information about incident angles or polarization directions of scattered waves at a receiving point obtained from multicomponent data be incorporated into the extrapolation of wavefields in migration. In this method, a given arrival of a wave from a subsurface scatterer is extrapolated in the direction focused on its incident direction at a receiving point to improve the migration image of the scatterer. This focusing is achieved by a projection of observed multicomponent scattered data onto the expected polarization direction at a receiving point for a subsurface scatterer. Numerical examples using simple 2-D and 3-D homogeneous models show that the multicomponent migration presented here can largely decrease migration artifacts or false images. Numerical experiments with more realistic models reveal that multicomponent migration can provide interpretable migration images even for very limited observation geometry such as a single source offset VSP, where single‐component migration fails. Additional tests indicate that this multicomponent migration method can be useful even in case where the observed data contain noise and a background velocity model deviates from the true velocity structure.
Focused resolution of thin conducting layers by various dipole EM systems
Authors: Verma Saurabh Kumar , Sharma S. P.
Published: October 2002

Electromagnetic sounding in the frequency domain can be performed in two ways—either by changing frequency at a location (frequency sounding) or by changing the transmitter‐receiver (T-R) separation using a fixed frequency (geometric sounding). These changes in frequency or separation parameters effect vertical scanning of conductivity distributions below the earth’s surface. In case of thin conducting layers, there could be an optimum range of frequencies or T-R separations that provide maximum resolution of the layer parameters. Thus, for a given buried target layer, it should be possible to find ranges of frequencies or separations that yield the best focusing. The present study deals with the focused resolution of a thin conducting layer in frequency sounding with variable T-R separation for four different dipole configurations. It is observed from the inversion of the data from various dipole electromagnetic (EM) systems that different T-R separations have different resolutions for the same target layer. It is also observed that for a particular loop system, the best resolution is observed at an optimum T-R separation. The resolution becomes poorer when the T-R separation is either increased or decreased from this particular separation. Thus it has been possible to propose a “zone of focusing” for various dipole EM configurations. The study reveals that this zone is broadest for the horizontal coplanar loops system, implying that this system yields good resolution over a wide range of T-R separations. Compared to this, the perpendicular loops system yields a very sharp peak implying that it resolves the target over a very narrow range of separations. However, the perpendicular loops system provides resolution most parsimoniously requiring the least T-R separation. This is followed by the vertical coplanar, vertical coaxial, and horizontal coplanar loops systems.
Exploration of a lignite‐bearing basin in Northern Ireland using ground magnetic and VLF-EM methods
Authors: McCaffrey Robert J. , McElroy W. John , Leslie A. Graham
Published: October 2002

In an exploration technique feasibility study, a detailed magnetic and VLF-EM survey was carried out on the poorly exposed, lignite‐bearing Crumlin subbasin within the Lough Neagh Basin, Co. Antrim, Northern Ireland. The faulted and onlapped margins of the basin, as well as lithological units and structures within the basin, were delineated by simple processing techniques applied to the data. The combination of the two methods overcomes the limitations of each method when it is used alone. These techniques could be successfully applied to other lignite‐bearing basins sited on strongly magnetic basement worldwide.
Limited‐view diffraction tomography in a nonuniform background
Authors: Gelius Leiv‐J.
Published: October 2002

The main problems in geophysical diffraction tomography are (1) complicated media and (2) rather limited acquisition geometries. Existing algorithms solve the limited‐view problem in an iterative manner, but are valid only for line sources and 2-D homogeneous background models. In this paper, we derive an iterative algorithm based on asymptotic wave theory that can compensate for a limited acquisition geometry. The method is valid for a 2-D nonuniform background model and point‐source illumination (i.e., a 2.5-D geometry). Paraxial ray tracing is employed to model the arbitrary background wave response, and the general structure of the algorithm has a strong resemblance to the iterative ART‐algorithm used in straight ray tomography. Our method is shown to be stable in the presence of moderate white noise and gives reasonable results, both geometrically and quantitatively, when applied to synthetic crosshole data involving a nonhomogeneous background model and limited view.
High‐resolution crosswell imaging of a west Texas carbonate reservoir: Part 4—Reflection imaging
Authors: Lazaratos Spyros K. , Harris Jerry M. , Rector James W. , Van Schaack Mark
Published: October 2002

Reliable crosswell reflection imaging is a challenging task, even after the data have been wavefield‐separated in the time domain. Residual, strong coherent noise is still present in the data. Stacking is complicated by the wide range of reflection incidence angles available for imaging. With wavelengths of a few feet, small misalignments as a result of velocity or geometric errors produce destructive interference and degrade the quality of the stacked image. We present an imaging sequence that addressed these complications and allowed us to produce high‐quality stacked images for both P‐ and S‐waves from a large‐volume crosswell data set. A very good tie was achieved at both wells. Heterogeneities imaged from well to well included very thin beds [less than 5 ft (1.5 m) thick] within the reservoir, pinchouts, and a major angular unconformity—the Grayburg/San Andres—that could not be observed reliably with any other technique (log correlation, surface seismic imaging, or tomography). In fact, the produced crosswell reflection images exhibit dramatically higher resolution and continuity than the P‐wave traveltime tomogram.