bluuebunny/crossref_metadata_2025 huggingface dataset:
Interpretation of depositional facies from seismic data
Authors: Sangree J. B. , Widmier J. M.
Published: October 2002

Depositional environments can be predicted from seismic data through an orderly approach to the interpretation of seismic reflections. One keystone to this approach is an understanding of the effects of lithology and bed spacing on reflection parameters. Amplitude, frequency, and continuity are some of the parameters most useful for interpreting environments. Reflection amplitude contains information on the velocity and density contrasts at individual interfaces and on the extent of interbedding. Frequency is primarily a characteristic of the nature of the seismic pulse, but it is also related to such geologic factors as the spacing of reflectors or lateral changes in interval velocity. Continuity of reflections is closely associated with continuity of bedding (e.g., continuous reflections suggest widespread, layered deposits). A second keystone to this interpretive approach is the parallelism of reflection cycles to gross bedding and, therefore, to physical surfaces that separate older from younger sediments. Exceptions to this concept include (1) fluid contact reflections, (2) limitations imposed by seismic resolution, and (3) various non‐geologic coherent events. In spite of these exceptions, this concept provides a powerful tool for the analysis of reflection patterns. Reflection cycle patterns include the configuration of reflections (i.e., layered, chaotic, and reflection‐free) and the nature of cycle terminations at the depositional unit boundaries. The external form of the depositional unit can be analyzed from a grid of seismic lines and is valuable in interpreting the depositional processes responsible for the unit. Sheet, sheet drape, wedge, lens, fan, and other forms are described. The areal associations of these forms are often critical to environmental interpretation. Examples of facies interpretation from seismic sections are shown for depositional environments ranging from shelf to basin floor.
Attenuation of seismic waves in dry and saturated rocks: II. Mechanisms
Authors: Johnston D. H. , Toksöz M. N. , Timur A.
Published: October 2002

Theoretical models based on several hypothesized attenuation mechanisms are discussed in relation to published data on the effects of pressure and fluid saturation on attenuation. These mechanisms include friction, fluid flow, viscous relaxation, and scattering. The application of these models to the ultrasonic data of Toksöz et al (1979, this issue) indicates that friction on thin cracks and grain boundaries is the dominant attenuation mechanism for consolidated rocks under most conditions in the earth’s upper crust. Increasing pressure decreases the number of cracks contributing to attenuation by friction, thus decreasing the attenuation. Water wetting of cracks and pores reduces the friction coefficient, facilitating sliding and thus increasing the attenuation. In saturated rocks, fluid flow plays a secondary role relative to friction. At ultrasonic frequencies in porous and permeable rocks, however, Biot‐type flow may be important at moderately high pressures. “Squirting” type flow of pore fluids from cracks and thin pores to larger pores may be a viable mechanism for some rocks at lower frequencies. The extrapolation of ultrasonic data to seismic or sonic frequencies by theoretical models involves some assumptions, verification of which requires data at lower frequencies.
Complex seismic trace analysis
Authors: Taner M. T. , Koehler F. , Sheriff R. E.
Published: October 2002

The conventional seismic trace can be viewed as the real component of a complex trace which can be uniquely calculated under usual conditions. The complex trace permits the unique separation of envelope amplitude and phase information and the calculation of instantaneous frequency. These and other quantities can be displayed in a color‐encoded manner which helps an interpreter see their interrelationship and spatial changes. The significance of color patterns and their geological interpretation is illustrated by examples of seismic data from three areas.
Fields of view of airborne gamma‐ray detectors
Authors: Grasty R. L. , Kosanke K. L. , Foote R. S.
Published: October 2002

In planning and interpreting airborne gamma‐ray surveys, an important consideration is the relative contribution of surface areas of a homogeneous radioactive source to the detected radiation. Numerical calculations have shown that along a flight line the width of the strip that produces a fixed percentage of the detected radiation is significantly less than the diameter of the circle contributing the same percentage of the radiation detected by a stationary gamma‐ray detector. Experimental angular sensitivity measurements of typical sodium‐iodide detectors were incorporated into the calculations and showed that the results were not strongly dependent on the detector configuration. The results are shown to have applications in estimating the count rate from small sources and in the design of an optimum ground survey grid for test strips selected for the calibration of airborne gamma‐ray spectrometers.
Gravity interpretation of Raguba field, Sirte basin, Libya
Authors: El‐Batroukh Salah I. , Zentani Ahmed S.
Published: October 2002

A Bouguer anomaly map of Raguba oil field and the surrounding areas is presented and interpreted. The main features of the map are: (1) A belt of positive anomalies approaching Bouguer values of 3 mgal in the field area then increasing up to 9 mgal toward the northwest. (2) Negative Bouguer values of −20 mgal on the east and west sides and a negative value of −13 mgal on the northeast side of the field. (3) Steep anomaly gradients trending north‐south on both sides of the field. To the north, the trend takes a northwest direction. (4) All these anomalies are superimposed on a regional trend of 0.16 mgal/km negative toward the south. The positive belt is interpreted as a horst structure characterized by crystalline basement at shallow depths. The negative anomalies are due to the density contrast between the sediments and the basement. Structural sections along certain profiles are presented and used for constructing gravity models calculated by computer.
Analysis of dispersive waves by wave field transformation
Authors: McMechan George A. , Yedlin Mathew J.
Published: October 2002

The dispersive waves in a common‐shot wave field can be transformed into images of the dispersion curves of each mode in the data. The procedure consists of two linear transformations: a slant stack of the data produces a wave field in the phase slowness‐time intercept (p — τ) plane in which phase velocities are separated. The spectral peak of the one‐dimensional (1-D) Fourier transform of the p — τ wave field then gives the frequency associated with each phase velocity. Thus, the data wave field is linearly transformed from the time‐distance domain into the slowness‐frequency (p — ω) domain, where dispersion curves are imaged. All the data are present throughout the transformations. Dispersion curves for the mode overtones as well as the fundamental are directly observed in the transformed wave field. In the p — ω domain, each mode is separated from the others even when its presence is not visually detectable in the untransformed data. The resolution achieved in the result is indicated in the p — ω wave field by the width and coherence of the image. The method is applied to both synthetic and real data sets.
Seismic attenuation: Effects of pore fluids and frictional‐sliding
Authors: Winkler Kenneth W. , Nur Amos
Published: October 2002

Seismic wave attenuation in rocks was studied experimentally, with particular attention focused on frictional sliding and fluid flow mechanisms. Sandstone bars were resonated at frequencies from 500 to 9000 Hz, and the effects of confining pressure, pore pressure, degree of saturation, strain amplitude, and frequency were studied. Observed changes in attenuation and velocity with strain amplitude are interpreted as evidence for frictional sliding at grain contacts. Since this amplitude dependence disappears at strains and confining pressures typical of seismic wave propagation in the earth, we infer that frictional sliding is not a significant source of seismic attenuation in situ. Partial water saturation significantly increases the attenuation of both compressional (P) and shear (S) waves relative to that in dry rock, resulting in greater P‐wave than S‐wave attenuation. Complete saturation maximizes S‐wave attenuation but causes a reduction in P‐wave attenuation. These effects can be interpreted in terms of wave induced pore fluid flow. The ratio of compressional to shear attenuation is found to be a more sensitive and reliable indicator of partial gas saturation than is the corresponding velocity ratio. Potential applications may exist in exploration for natural gas and geothermal steam reservoirs.
A computer method for determining seismic first arrival times
Authors: Hatherly P. J.
Published: October 2002

Although the seismic first event is most often defined as the first kick on a trace, it is not commonly appreciated that the visual emergence of the kick is dependent upon the signal‐to‐noise ratio and the amplifier gain. Consistent picking can only be achieved by identifying constant phases on the pulse, such as the inflection points or points of peak amplitude. This computer technique identifies both the first kick and the first inflection point on each trace to determine accurate and consistent first arrival times. This is achieved for each shot by determining the mean difference between the times to the first inflection points and the corresponding first kick points and then subtracting this difference from the times to the first inflection points. The remaining constant errors in the arrival times caused by factors such as inaccurate uphole corrections or delays in the detonator or shot‐firing system are removed using a least‐squares procedure which minimizes the difference in the forward and reverse traveltimes between shotpoints.
Evidence for nondiffusive transport of 86Rn in the ground and a new physical model for the transport
Authors: Kristiansson Krister , Malmqvist Lennart
Published: October 2002

Concentration of radon has been measured in the soil near the ground surface with solid‐state, nuclear track detectors with the inverted cup technique. Measurements were made in the overburden at depth intervals 0.1–0.7 m, at 0.1–6 m, and at a constant depth of 0.2 m, in a narrow rectangular matrix. The results disagree with the hypothesis that radon concentration only depends upon local production and migration by diffusion with a diffusion length of about 1 m. A transport length of 0.1–0.2 m is observed near the ground surface and the transport is dominated by a flow component. Radon measurements in the ground surface over the Laisvall lead mine have given evidence of radon transport through rock exceeding a distance of 100 m, which is possible only if the migration is a flow transport with a characteristic transport length larger than about 10 m/day. To explain the radon transport in the overburden and through the rock with a common transport system, the existence of a general upward flow of geo‐gas is proposed. This geo‐gas works as a carrier mechanism for radon. The physical conditions for the existence of a flow transport of radon are discussed.
The limits of resolution of zero‐phase wavelets
Authors: Kallweit R. S. , Wood L. C.
Published: October 2002

This investigation deals with resolving reflections from thin beds rather than the detection of events that may or may not be resolved. Resolution is approached by considering a thinning bed and how accurately measured times on a seismic trace represent actual, vertical two‐way traveltimes through the bed. Theoretical developments are in terms of frequency and time rather than wavelength and thickness because the latter two variables require knowledge of interval velocities. These results are compared with similar studies by Rayleigh, Ricker (1953), and Widess (1973, 1980). We show that the temporal resolution of a broadband wavelet with a white spectrum is controlled by its highest terminal frequency [Formula: see text], and the resolution limit approximates 1/(1.5 [Formula: see text]), provided the wavelet’s band ratio exceeds two octaves. The practical limit of resolution, however, occurs at a one‐quarter wavelength condition and approximates 1/(1.4 [Formula: see text]). The resolving power of zero‐phase wavelets can be compared quantitatively once a wavelet is known in the time domain.
Estimation of sulfide content from induced polarization data
Authors: Nelson P. H. , Van Voorhis G. D.
Published: October 2002

Using electrode spacings ranging from 2 to 10 m, we made 157 measurements of phase and resistivity at 99 sites in four open‐pit porphyry copper mines, thereby sampling a wide variety of rock and alteration types. Rock samples were collected from these sites and examined in the laboratory to characterize the alteration and mineralization and to quantify the sulfide concentration. Our experience has been that in‐situ electrical measurements give a better basis for quantitative correlations between electrical parameters and mineralization than do laboratory measurements on small samples. In general, resistivity declines and phase lag increases as the sulfide content increases, but the scatter in the trends precludes any useful empirical rules. However, the scatter is much reduced when the ratio of phase to resistivity is considered: the ratio increases with the square of the sulfide content. Thus it appears that in many cases the phase‐to‐resistivity ratio can be used to estimate sulfide content. With appropriate conversion factors, this ratio is equivalent to the quadrature component of electrical conductivity and to the metal conduction factor. This dependence of quadrature conductivity upon the square of sulfide content is substantiated by two other data sets taken from the literature. These three independent sets of measurements also seem consistent in showing that the dependence of phase upon sulfide content depends upon mineralization habit. The observed trends must be explained before a satisfactory model of electrical conduction in mineralized rock can be established. These results indicate that careful in‐situ studies have much to offer for further electrical property investigations.
Approximate computation of the acoustic impedance from seismic data
Authors: Berteussen K. A. , Ursin B.
Published: October 2002

The approximate computation of the acoustic impedance from seismic data is usually based on the recursive formula [Formula: see text] where [Formula: see text] is the acoustic impedance in layer number k and [Formula: see text] is the pressure reflection coefficient for the interface between layer k and [Formula: see text]. The above formula is derived from a discrete layered earth model. When we consider a continuous earth model and discretize the results, we obtain the recursive formula [Formula: see text] The two expressions give very similar numerical results. For [Formula: see text], the relative difference is less than 5 percent and this cannot be visually recognized on an acoustic impedance section. The expression for the continuous model is more suitable for understanding the result of the approximate computation of the acoustic impedance function from band‐limited seismic data. The calculated impedance minus the impedance in the top layer is approximately equal to the reflectivity function convolved with the integrated seismic pulse multiplied with twice the impedance in the top layer. For impedance values less than 0.2 in absolute value this is also equal to the acoustic impedance function (minus the acoustic impedance in the top layer) convolved with the seismic pulse. The computation of the acoustic impedance from band‐limited seismic data corresponds to an exponential transformation of the integrated seismic trace. On a band‐limited acoustic impedance section with well‐separated reflectors and low noise level the direction of change in the acoustic impedance can be correctly identified. The effect of additive noise in the seismic data is governed by a nonlinear transformation. Our data examples show that the computation of acoustic impedance becomes unstable when noise is added. In order to avoid the nonlinear transformation of the seismic data, it has been suggested to integrate the seismic data. This results in an estimate of the logarithm of the acoustic impedance. For band‐limited seismic data with noise this gives a band‐limited estimate of the logarithm of the acoustic impedance plus the integrated noise. A disadvantage of this method is that the variance of the integrated noise increases linearly with time.
Subhorizontal fractures in a granitic pluton: Their detection and implications for radioactive waste disposal
Authors: Green A. G. , Mair J. A.
Published: October 2002

Several countries are actively investigating the feasibility of using crystalline rock bodies for the ultimate disposal of radioactive nuclear waste. As part of the concept assessment phase in the Canadian nuclear fuel waste management program, a multidisciplinary research investigation is being conducted across the Lac du Bonnet batholith in southeastern Manitoba; no radioactive materials, other than sealed sources or tracers, are to be emplaced within this rock body. The results of a high‐resolution seismic reflection survey, together with information from a number of deep boreholes, demonstrates that major subhorizontal fractures occur at depths of up to 800 m within the batholith. An integrated interpretation of the seismic data with selected borehole logs shows that warm water is flowing up the major fractures from depth. A natural corollary of these results is that a three‐dimensional seismic reflection survey combined with an appropriate exploratory drilling program has the potential for delineating blocks of a rock body that may be relatively unfractured.
Generalized linear inversion of reflection seismic data
Authors: Cooke Dennis A. , Schneider William A.
Published: October 2002

Generalized linear inversion, sometimes known as model perturbation, nonlinear regression, or inverse modeling, is applied to synthetic and real seismic data sets with the objective of obtaining an impedance profile as a function of time. The impedances solved for are parameterized in a manner that describes the unknown earth using fewer variables than previous seismic generalized linear inversion techniques. In this application only single traces of common‐midpoint (CMP) processed data will be inverted. The method of generalized linear inversion (GLI) presented here is designed to improve on the shortcomings of recursive inversion with respect to relative and absolute scale of the impedance results, resolution of impedance boundaries, and distortion from residual wavelet effects. In obtaining these goals other advantageous aspects of GLI were discovered. For example, it is insensitive to noise in many cases, and it will allow an interpreter to fix the impedance of any number of known lithologies in an interval being inverted. This last property is extremely useful when evaluating a prospect on an otherwise well‐understood seismic line. The GLI method is illustrated on a number of synthetic examples and one field data set from the Powder River basin of Wyoming.
A simple objective method for minimizing crossover errors in marine gravity data
Authors: Prince Roger A. , Forsyth Donald W.
Published: October 2002

A method is presented which does not require a model for the source of crossover errors in marine gravity data in order to minimize them. The cruises are divided up into straight line segments and the assumption is made that whatever the sources of error, their net effect will be constant over the length of the track segment. A least‐squares approach is used where the crossover differences in the original data are the observations which it is desired to match. The desired set of constant corrections, one for each segment, is that which will minimize the sum of the squares of the residual crossover errors. This method has the advantage of reducing the crossover errors while simultaneously preserving the relative gravity anomalies along individual ship’s profiles. A data set consisting of gravity measurements made on nine cruises in the region of the Vema fracture zone in the equatorial Atlantic is used as a case study. The resulting least squares solution reduces the root mean square (rms) of 298 crossover errors from 10.3 mGals in the original data to 2.9 mGals after the calculated segment corrections are made. An F‐test shows that the reduced rms deviation from the mean is statistically significant at the 99 percent confidence level. A least‐squares fit was also done to find the best single cruise corrections for each of the 9 cruises for the 204 crossings between cruises. The original rms error is reduced from 11.4 to 7.8 mGals and the improvement is again significant at the 99 percent confidence level. An analysis of the variances shows that 37.5 percent of the total variance can he explained by constant corrections to each of the 9 cruises, while an additional 49.5 percent of the total variance can be explained by individual segment corrections. A linear regression analysis of the segment corrections as a function of elapsed time in the cruise suggests that for two of the cruises, drift of the gravity meter was not properly corrected for in the original data. Analysis of the segment corrections as a function of ship’s heading suggests that for two other cruises, cross‐coupling effects were not properly corrected. Eötvös corrections caused by navigational errors are the most likely explanation for many of the remaining individual segment corrections. After the calculated corrections were made, a free‐air anomaly map of the region was drawn. A comparison with an earlier published, free‐air anomaly map of the right half of this region shows that the contours are similar, but that the new map is shifted by a few mGals relative to the older map. This discrepancy between the old and new maps is a consequence of matching the data between the left and right sides of the new map and does not arise if the right side is considered alone with the least‐squares technique.
Induced polarization of shaly sands
Authors: Vinegar H. J. , Waxman M. H.
Published: October 2002

Equations have been developed that relate induced polarization (IP) in shaly sands to measurable petrophysical parameters. The induced‐polarization process has been modeled in terms of two mechanisms: clay counterion displacement and membrane blockage. The resulting equations can be used to determine shaliness, brine conductivity, and oil saturation from in‐phase and out‐of‐phase conductivities. Laboratory measurements have confirmed the IP dependence on these variables, as well as on temperature.
Apparent resistivity and diffusion velocity
Authors: Raiche A. P. , Gallagher R. G.
Published: October 2002

Formulas are derived to calculate transient electromagnetic (TEM) vertical diffusion velocity as a function of resistivity, time, and transmitter loop size. For large loops on very conductive earths, this velocity depends strongly on loop size. Diffusion velocity is used to calculate apparent resistivity pseudodepth sections (ARPDS) from apparent resistivity time sections. However, ARPDS do not seem to bear an easily discernible relationship to the geoelectric structure. Furthermore, it is shown that the apparent resistivity is not directly related to the volume through which TEM diffusion has occurred.
Relationships between compressional‐wave and shear‐wave velocities in clastic silicate rocks
Authors: Castagna J. P. , Batzle M. L. , Eastwood R. L.
Published: October 2002

New velocity data in addition to literature data derived from sonic log, seismic, and laboratory measurements are analyzed for clastic silicate rocks. These data demonstrate simple systematic relationships between compressional and shear wave velocities. For water‐saturated clastic silicate rocks, shear wave velocity is approximately linearly related to compressional wave velocity and the compressional‐to‐shear velocity ratio decreases with increasing compressional velocity. Laboratory data for dry sandstones indicate a nearly constant compressional‐to‐shear velocity ratio with rigidity approximately equal to bulk modulus. Ideal models for regular packings of spheres and cracked solids exhibit behavior similar to the observed water‐saturated and dry trends. For dry rigidity equal to dry bulk modulus, Gassmann’s equations predict velocities in close agreement with data from the water‐saturated rock.
Reverse‐time migration of offset vertical seismic profiling data using the excitation‐time imaging condition
Authors: Chang Wen‐Fong , McMechan George A.
Published: October 2002

To apply reverse‐time migration to prestack, finite‐offset data from variable‐velocity media, the standard (time zero) imaging condition must be generalized because each point in the image space has a different image time (or times). This generalization is the excitation‐time imaging condition, in which each point is imaged at the one‐way traveltime from the source to that point. Reverse‐time migration with the excitation‐time imaging condition consists of three elements: (1) computation of the imaging condition; (2) extrapolation of the recorder wave field; and (3) application of the imaging condition. Computation of the imaging condition for each point in the image is done by ray tracing from the source point; this is equivalent to extrapolation of the source wave field through the medium. Extrapolation of the recorded wave field is done by an acoustic finite‐difference algorithm. Imaging is performed at each step of the finite‐difference extrapolation by extracting, from the propagating wave field, the amplitude at each mesh point that is imaged at that time and adding these into the image space at the same spatial locations. The locus of all points imaged at one time step is a wavefront [a constant time (or phase) trajectory]. This prestack migration algorithm is very general. The excitation‐time imaging condition is applicable to all source‐receiver geometries and variable‐velocity media and reduces exactly to the usual time‐zero imaging condition when used with zero‐offset surface data. The algorithm is illustrated by application to both synthetic and real VSP data. The most interesting and potentially useful result in the processing of the synthetic data is imaging of the horizontal fluid interfaces within a reservoir even when the surrounding reservoir boundaries are not well imaged.
Acoustic relaxation in sedimentary rocks: Dependence on grain contacts and fluid saturation
Authors: Murphy William F. , Winkler Kenneth W. , Kleinberg Robert L.
Published: October 2002

Partial fluid saturation affects absorption and dispersion in sandstones. The proposed theoretical model describes acoustic relaxation due to local fluid flow. Previously proposed models of local flow were based on microgeometries not representative of sedimentary rocks; they were unable to describe the behavior of partially saturated sandstones. The new model is based upon observed microstructures in sandstones. A fraction of the grain contacts in sandstones are permeated by sheet‐like gaps. The incomplete solid‐solid contact allows an interconnected fluid film to exist between the grain surfaces. The model consists of a narrow gap connected to a finite annular pore. An acoustic stress wave drives the film out of the narrow contact region and into the adjacent pore. The viscous flow results in a dissipation of energy. The model predicts the real and imaginary parts of the complex frame moduli as a function of frequency and fluid saturation. The predictions agree well with experimental results.
Elastic reverse‐time migration
Authors: Chang Wen‐Fong , McMechan George A.
Published: October 2002

Elastic, prestack, reverse‐time, finite‐difference migration of two‐component seismic surface data requires data extrapolation and application of an imaging condition. Data extrapolation involves synchronous driving of the vertical‐component and horizontal‐component finite‐difference meshes with the time reverse of the recorded vertical and horizontal traces, respectively. Extrapolation uses the coupled elastic wave equation for variable velocity solved with a second‐order, explicit finite‐difference scheme. The imaging condition at any point in the grid is the one‐way traveltime from the source to that point. Elastic migrations of both synthetic test data and real two‐component common‐source gathers produce simpler images than acoustic migrations because of the coalescing of double reflections (compressional waves and shear waves) into single loci.
Imaging quasi‐layered conductive structures by simple processing of transient electromagnetic data
Authors: Macnae James , Lamontagne Yves
Published: October 2002

An “imaged” conductivity section of a layered earth can be obtained by simple transformation of step‐response electromagnetic data measured in the quasi‐static zone. This method of data transformation is presented as an alternative to conventional apparent conductivity transformations. At each delay time, the variation of the step response as a function of geometry (transmitter and receiver location) is transformed to an equivalent reference depth h, which can be related to the depth of electromagnetic field diffusion. The behavior of h as a function of delay time is nearly independent of the source‐receiver geometry. The slowness dt/dh divided by the magnetic permeability is almost exactly proportional to the cumulative conductance measured from the surface down to a depth h. Thus we can estimate an apparent conductivity, which we call the “imaged conductivity,” at depth to be [Formula: see text]. The cost of this transformation is a fraction of the cost of conventional data inversion, and it does not require an a priori constraint on the number of parameters used in the inversion. The empirically developed technique was used successfully to process UTEM field data measured over a quasi‐layered earth.
A terracing operator for physical property mapping with potential field data
Authors: Cordell Lindrith , McCafferty A. E.
Published: October 2002

The terracing operator works iteratively on gravity or magnetic data, using the sense of the measured field’s local curvature, to produce a field comprised of uniform domains separated by abrupt domain boundaries. The result is crudely proportional to a physical‐property function defined in one (profile case) or two (map case) horizontal dimensions. This result can be extended to a physical‐property model if its behavior in the third (vertical) dimension is defined, either arbitrarily or on the basis of the local geologic situation. The terracing algorithm is computationally fast and appropriate to use with very large digital data sets. Where gravity and magnetic data are both available, terracing provides an effective means by which the two data sets can be compared directly. Results of the terracing operation somewhat resemble those of conventional susceptibility (or density) mapping. In contrast with conventional susceptibility mapping, however, the terraced function is a true step function, which cannot be depicted by means of contour lines. Magnetic or gravity fields calculated from the physical‐property model do not, in general, produce an exact fit to the observed data. By intent, the terraced map is more closely analogous to a geologic map in that domains are separated by hard‐edged domain boundaries and minor within‐domain variation is neglected. The terracing operator was applied separately to aeromagnetic and gravity data from a 136 km × 123 km area in eastern Kansas. Results provide a reasonably good physical representation of both the gravity and the aeromagnetic data. Superposition of the results from the two data sets shows many areas of agreement that can be referenced to geologic features within the buried Precambrian crystalline basement. The emerging picture of basement geology is much better resolved than that obtained either from the scanty available drill data or from interpretation of the geophysical data by inspection.
Gridding with continuous curvature splines in tension
Authors: Smith W. H. F. , Wessel P.
Published: October 2002

A gridding method commonly called minimum curvature is widely used in the earth sciences. The method interpolates the data to be gridded with a surface having continuous second derivatives and minimal total squared curvature. The minimum‐curvature surface has an analogy in elastic plate flexure and approximates the shape adopted by a thin plate flexed to pass through the data points. Minimum‐curvature surfaces may have large oscillations and extraneous inflection points which make them unsuitable for gridding in many of the applications where they are commonly used. These extraneous inflection points can be eliminated by adding tension to the elastic‐plate flexure equation. It is straightforward to generalize minimum‐curvature gridding algorithms to include a tension parameter; the same system of equations must be solved in either case and only the relative weights of the coefficients change. Therefore, solutions under tension require no more computational effort than minimum‐curvature solutions, and any algorithm which can solve the minimum‐curvature equations can solve the more general system. We give common geologic examples where minimum‐curvature gridding produces erroneous results but gridding with tension yields a good solution. We also outline how to improve the convergence of an iterative method of solution for the gridding equations.
Applications of acoustics to source‐array and streamer tow‐point positioning
Authors: Court I. N.
Published: October 2002

Marine 3-D acquisition techniques commonly use multiple sources and streamers towed behind the seismic vessel. The full potential of modern 3-D processing can only be realized if accurate positioning of these sources and streamers is achieved. A multipath acoustic ranging technique has been used to solve this positioning problem. Ranges are determined from a network of measurements of the propagation times for ultrasonic waves traveling between pairs of transducers located at strategic points on the hull of the vessel, the source subarrays, and the streamers. The system falls naturally into two subsystems: a front‐end subsystem and a streamer shape‐positioning subsystem. The front‐end subsystem described is used to determine the position of the source subarrays and the streamer tow points. This subsystem has been employed on surveys in the North Sea, and examples of the measurements are given. Factors such as the ship’s velocity and heading affect the translation of the timing measurements to ranges. Applying the technique of variation of coordinates to these distances, all the positions in the front‐end system have been calculated giving real‐time displays which are updated every shotpoint. From the results, the positioning is estimated to have an accuracy of ±1.5 m cross‐course and 7±0.5 m along‐course relative to the ship’s navigation reference points.
Electrokinetic dissipation induced by seismic waves
Authors: Pride S. R. , Morgan F. D.
Published: October 2002

When a fluid electrolyte moves relative to a solid, an electric field is generated that migrates ions and thus dissipates energy. This “electrokinetic” dissipation is theoretically compared to the viscous shear dissipation for fluid flow generated by a fixed time harmonic pressure gradient in a planar quartz duct. It is shown that, regardless of frequency, it is safe to ignore the electrokinetic losses when the electrolyte molarity is on the order of 0.1 M or greater. For low molarity electrolytes [Formula: see text], however, the electrokinetic losses do become significant compared to the viscous losses for flow in sufficiently tight pores. The ratio of electrokinetic dissipation [Formula: see text] to viscous dissipation [Formula: see text] is always a maximum when the electrokinetic radius (the duct haft‐width divided by the Debye length) is ≈1.5. The maximum value of [Formula: see text] does not exceed 0.5 for the NaCl, KCl, and quartz systems considered. The generated electric field pushes on the excess ions in the duct in a direction opposite to the applied pressure gradient, thus giving rise to an apparent viscosity enhancement. This enhancement is ⩽45 percent for the systems considered here, and can be directly obtained from the [Formula: see text] ratio. Indeed, the central effect of a large [Formula: see text] ratio is that the amount of relative fluid flow is reduced, and thus, the amount of wave attenuation is reduced.
Influence of temperature on electrical conductivity on shaly sands
Authors: Sen Pabitra N. , Goode Peter A.
Published: October 2002

In boreholes, temperatures vary and to extract hydrocarbon saturation from conductivity measurements, the influence of temperature on water and rock conductivities must be accounted for. The mobility [Formula: see text] of the counter‐ions due to clays and the electrical conductivity of pore‐filling brine show large changes with variation in temperature, whereas the microgeometry of the pore space exhibits negligible change. Using this idea, the temperature dependence of [Formula: see text] is extracted using data on dc electrical conductivity of shaly sands (σ) containing varying amounts of clay. The mobility of [Formula: see text] counter‐ions is found to vary approximately linearly with temperature. This explicit relationship is tested by comparing the predicted temperature dependence against the measured temperature dependence of conductivity of a set of rocks with high and low clay content. While the rock conductivity shows a large temperature dependence, the resistivity index is less sensitive to temperature. An approximate formula, which is superior to Arps’s formula, for water conductivity as a function of temperature is obtained.
Field comparison of shallow seismic sources near Chino, California
Authors: Miller Richard D. , Pullan Susan E. , Steeples Don W. , Hunter James A.
Published: October 2002

Data from a shallow seismic‐source comparison test conducted in an area with a water‐table depth in excess of 30 m and near‐surface velocities less than 330 m/s were acquired from 13 different sources at a single site near Chino, California. The sources included sledgehammer, explosives, weight drop, projectile impacts, and various buffalo guns. A possible reflecting event can be interpreted at about 70 ms. At this particular test site, the lowly sledgehammer is among the best sources to provide data to see the possible reflection. Our previous work and that of our colleagues suggests that any source could dominate the comparison categories addressed here, given the appropriate set of site characteristics.
Detecting organic chemical contaminants by spectral‐induced polarization method in glacial till environment
Authors: Vanhala Heikki , Soininen Heikki , Kukkonen Ilmo
Published: October 2002

We present results that considerably extend the range of applications of spectral induced polarization (IP) measurements in surveying soil contaminated by organic chemicals. Soil/organic mixtures are polarizable even with very low contents of clay minerals, even though the use of spectral IP has so far been restricted to soils consisting predominantly of clay minerals. Laboratory measurements made on contaminated (toluene, heptane, ethylene glycol) and uncontaminated glacial till samples show that organic chemicals have a distinct effect on the phase spectra. Theoretical modeling further indicates that the phenomena measured in the laboratory are also relevant and detectable in field investigations.
A 3-D seismic case history evaluating fluvially deposited thin‐bed reservoirs in a gas‐producing property
Authors: Hardage Bob A. , Levey Raymond A. , Pendleton Virginia , Simmons James , Edson Rick
Published: October 2002

We conducted a study at Stratton Field, a large Frio gas‐producing property in Kleberg and Nueces Counties in South Texas, to determine how to best integrate geophysics, geology, and reservoir engineering technologies to detect thin‐bed compartmented reservoirs in a fluvially deposited reservoir system. This study documents that narrow, meandering, channel‐fill reservoirs as thin as 10 ft (3 m) and as narrow as 200 ft (61 m) can be detected with 3-D seismic imaging at depths exceeding 6000 ft (1800 m) if the 3-D data are carefully calibrated using vertical seismic profile (VSP) control. Even though the 3-D seismic images show considerable stratigraphic detail in the interwell spaces and indicate where numerous thin‐bed compartment boundaries could exist, the seismic images cannot by themselves specify which stratigraphic features are the flow barriers that create the reservoir compartmentalization. However, when well production histories, reservoir pressure histories, and pressure interference tests are incorporated into the 3-D seismic interpretation, a compartmentalized model of the reservoir system can be constructed that allows improved development drilling and reservoir management to be implemented. This case history illustrates how realistic, thin‐bed, compartmented reservoir models result when geologists, engineers, and geophysicists work together to develop a unified model of a stratigraphically complex reservoir system.
Borehole flexural modes in anisotropic formations
Authors: Sinha Bikash K. , Norris Andrew N. , Chang Shu‐Kong
Published: October 2002

A perturbation method of solution is an efficient way of analyzing elastic wave propagation along a borehole in anisotropic formations. The perturbation model allows us to calculate changes in the modal dispersion curves caused by the differences in elastic constants between the anisotropic formation of interest and a reference, or unperturbed, isotropic formation. The equivalent isotropic constants in the reference formation are obtained from the appropriate compressional‐and shear‐wave velocities for the selected propagation and polarization directions of the flexural mode. This choice of the unperturbed solution means that the required perturbation is minimal, resulting in enhanced accuracy of the perturbed solution. Computational results are presented for the dispersion curves of borehole flexural waves in a transversely isotropic (TI) formation as a function of borehole deviation from the TI symmetry axis. In addition, radial distributions of displacement and stress fields associated with the flexural wave are obtained as a function of frequency. These provide qualitative information on the radial depth of investigation with flexural wave logging. The flexural wave excitation function is a measure of the energy that a source converts to flexural motion. We deduce an expression for the flexural wave excitation and show that its bandlimited characteristic is influenced by both the borehole diameter and formation parameters. From the dispersion curves and excitation functions, we can compute the flexural waveforms caused by a dipole source with arbitrary orientation in the borehole. In the numerical computations, we have used the unperturbed mode shapes for an equivalent isotropic medium together with the perturbed dispersion relations caused by the formation anisotropy.
A field investigation of source parameters for the sledgehammer
Authors: Keiswetter Dean A. , Steeples Don W.
Published: October 2002

We examined amplitude and frequency changes in shallow seismic‐reflection data associated with simple source‐parameter modifications for the sledgehammer. Seismic data acquired at three sites with different near‐surface geology show the potential effects of varying the hammer mass, the hammer velocity, the plate mass, and the plate area. At these study sites, seismic amplitudes depend on plate‐surface area and on hammer mass but not heavily on hammer velocity or plate mass. Furthermore, although the total bandwidth of the recorded data was independent of source parameter changes, the peak frequency at one site was increased approximately 40 Hz by increasing the area of the plate. The results indicate that the effects of modifying the source parameters for the sledgehammer are site‐dependent. The experiments described are quick, cheap, and simple, and can be duplicated by others at prospective sites to answer site‐specific questions.
Velocity variations and water content estimated from multi‐offset, ground‐penetrating radar
Authors: Greaves Robert J. , Lesmes David P. , Lee Jung Mo , Toksöz M. Nafi
Published: October 2002

The common midpoint (CMP) processing technique has been shown to be effective in improving the results of ground‐penetrating radar (GPR) profiling. When radar data are collected with the CMP multioffset geometry, stacking increases the signal‐to‐noise ratio of subsurface radar reflections and results in an improved subsurface image. An important aspect of CMP processing is normal‐moveout velocity analysis. Our objectives are to show the effect of multiple velocity analyses on the stacked radar image and particularly, to demonstrate that this velocity information can also be used to determine subsurface water content. Most GPR surveys are very limited in spatial extent and assume that within the survey range, radar velocity structure in the shallow subsurface can be adequately approximated by a single velocity function in data processing. In this study, we show that variation in radar velocity can be quite significant and that the stacked profile improves as the number of velocity analysis locations is increased. Interval velocities can be calculated from the normal moveout velocities derived in the CMP velocity analysis. With some reasonable assumptions about subsurface conditions necessary for radar propagation, interval velocity can be converted to an estimate of volumetric water content. Therefore, by collecting GPR data in the multioffset CMP geometry, not only is the radar profile improved but it also allows for an interpretation of subsurface variation in water content. We show the application of these techniques to multioffset GPR data from the Chalk River test area operated by Atomic Energy of Canada Limited.
A fractal‐based algorithm for detecting first arrivals on seismic traces
Authors: Boschetti Fabio , Dentith Mike D. , List Ron D.
Published: October 2002

A new algorithm is proposed for the automatic picking of seismic first arrivals that detects the presence of a signal by analyzing the variation in fractal dimension along the trace. The “divider‐method” is found to be the most suitable method for calculating the fractal dimension. A change in dimension is found to occur close to the transition from noise to signal plus noise, that is the first arrival. The nature of this change varies from trace to trace, but a detectable change is always found to occur. The algorithm has been tested on real data sets with varying S/N ratios and the results compared to those obtained using previously published algorithms. With an appropriate tuning of its parameters, the fractal‐based algorithm proved more accurate than all these other algorithms, especially in the presence of significant noise. The fractal method proved able to tolerate noise up to 80% of the average signal amplitude. However, the fractal‐based algorithm is considerably slower than the other methods and hence is intended for use only on data sets with low S/N ratios.