bluuebunny/crossref_metadata_2025 huggingface dataset:
Relationship between phase velocities and polarization in transversely isotropic media
Authors: de Parscau J.
Published: October 2002

Most techniques used to estimate anisotropy from multiple‐source offset VSP data assume angles measured from particle motion as an incidence angle. However, the difference between P‐wave polarization and the propagation direction for an anisotropic medium can be higher than 8 degrees. This difference provides a nonnegligible error in the estimation of anisotropy parameters from phase velocities. An exact model, proposed to describe P‐ and SV‐phase velocity variations for a transversely isotropic medium (TIM), takes into account the polarization angles. This model is a function of two anisotropy parameters (η and τ), of the vertical P‐ and SV‐wave phase velocities and of the polarization angle γ. However, η and τ can be used to express the polarization angle equation in a much simpler way. To quantify the error in estimated anisotropy parameters due to the assumption that the polarization angle is equal to the incidence angle, I study five TIMs. Each medium has an anisotropy that is representative of those observed in seismic surveying. The anisotropy parameters are recovered by inverting the P‐ and SV‐wave phase velocities for different incidence angles, and these incidence angles are assumed to be equal to the corresponding polarization angles. The mean error in estimated parameters is about 10 percent. This error is about the same as the one that would be obtained for velocities with uncertainties in their measurements. Unfortunately, the inversion of phase velocities measured from a real multiple‐source offset VSP to estimate anisotropy parameters needs, for calculating the misfit function, to add both errors in velocities due to hypothesis for angles and errors in velocity measurements due to uncertainties in data. In this case an exact model eliminates errors due to the assumption for the model and provides a more accurate estimation of anisotropy parameters.
Near‐real time reduction of shipboard gravity using Kalman‐filtered GPS measurements
Authors: Genrich J. F. , Minster J.-B.
Published: October 2002

We have developed a Kalman filter to estimate accurate Eötvös corrections and horizontal ship accelerations from Global Positioning System (GPS) fixes. High‐resolution shipboard gravity measurements are obtained with a newly designed, linear phase, Finite Impulse Response (FIR) low‐pass filter. Both filters are combined to yield accurate, near‐real time, Eötvös‐corrected underway gravity estimates. Error ranges that reflect uncertainty in navigation for these estimates are calculated from autocovariances of Kalman velocity estimates by means of variance propagation expressions for time‐invariant linear digital filters. Estimates of horizontal ship acceleration are combined with a simplified instrument impulse response model in an attempt to remove transient noise from the gravimeter output. We apply the technique to data collected by two shipboard gravimeters, a LaCoste & Romberg Model S Air‐Sea Gravity Meter and a Bell Aerospace BGM-3 Marine Gravity Meter System, operated side‐by‐side on the Scripps R/V Thomas Washington during Leg 1 of the Roundabout expedition. In the absence of significant horizontal accelerations due to course or speed changes, both instruments yield data with good repeatability, characterized by rms differences of less than 1 mGal. Horizontal accelerations generate transient signals that cannot be modeled at present to an accuracy of better than 5 mGal. Difficulties in removing these transients are primarily due to insufficient quantitative knowledge of the response of the instrument, including the gyro‐stabilized platform. This can be determined analytically or empirically.
Hysteresis in the electrical resistivity of partially saturated sandstones
Authors: Knight Rosemary
Published: October 2002

Laboratory measurements of the resistivity of three sandstone samples collected during imbibition (increasing Sw) and drainage (decreasing Sw) show pronounced hysteresis in resistivity throughout much of the saturation range. The variation in resistivity can be related to changes in pore‐scale fluid distribution caused by changes in saturation history. The form of the hysteresis is such that resistivity measured during imbibition is consistently less than that measured, at the same saturation, during drainage. This can be attributed to the presence of conduction at the air/water interface in partially saturated samples; an effect that is enhanced by fluid geometries associated with the imbibition process. The results of this study suggest that the dependence of geophysical data on saturation history should be considered when interpreting data from the unsaturated zone.
Controlled‐source audiofrequency magnetotelluric responses of three‐dimensional bodies
Authors: Boschetto N. B. , Hohmann G. W.
Published: October 2002

Modeling the controlled‐source audiofrequency magnetotelluric (CSAMT) responses of simple three‐dimensional (3-D) structures due to a grounded electric bipole confirms that the CSAMT technique accurately simulates plane‐wave results in the far‐field zone of the transmitter. However, at receiver sites located above large conductive or resistive bodies, the presence of the inhomogeneity extends or reduces, respectively, the frequency range of the far‐field zone. Measurements made on the surface beyond a large 3-D body display a small but spatially extensive effect due to decay of the artificial primary field. Situating a 3-D inhomogeneity beneath the source permits an evaluation of “source overprint” effects. When such a body is resistive, a slight shift in the near‐field response to higher frequencies occurs. When a body below the transmitter is conductive, it is possible to make far‐field measurements closer to the transmitter or lower in frequency. However, as the size of the conductor and its secondary‐field response increases, large transition‐zone responses distort the data. For both a plane‐wave source and a finite source, current channeling into a 3-D conductor from conductive overburden enhances the response of a target. The modeled response of a dike‐like conductor shows no better results for either the broadside or collinear configuration. The location and extent of such a body are better defined when measuring the electric field perpendicular to the strike of the prism, but resistivity estimates are better when using the electric field parallel to the strike of the prism, irrespective of transmitter orientation. Models designed from data collected at Marionoak, Tasmania, yield results which indicate that the thin, vertical graphitic unit intersected by drilling is detectable by the CSAMT method, but probably is not the sole source of the large anomaly seen in the CSAMT data.
A new calibration method for gravity meters
Authors: LaCoste Lucien
Published: October 2002

Current calibration methods give gravity meter accuracies of 3 to 5 μGal [Formula: see text] over 10 mGal intervals. The new calibration method should extend the accuracies to 100 mGal intervals. The new method nulls with both a micrometer screw and an electrostatic feedback nuller. The screw is calibrated and used only at discrete points and the electrostatic nuller completes the nulling to obtain gravity readings. Determining the calibration curve itself rather than measuring and integrating calibration (interval) factors avoids approximations. Circular errors (periods of the order of a turn of the screw) are completely taken into account at the discrete points. Calibration only at discrete points makes calibration time reasonable.
Relations among hydrocarbon reservoirs, epigenetic sulfidization, and rock magnetization: Examples from the south Texas coastal plain
Authors: Goldhaber M. B. , Reynolds R. L.
Published: October 2002

This paper focuses on the association between concentrations of iron disulfide [Formula: see text] minerals in the shallow subsurface and underlying hydrocarbon accumulations. Such [Formula: see text] concentrations are the result of migration of either [Formula: see text] or organic constituents from the underlying hydrocarbons. The [Formula: see text] from reservoirs is produced inorganically from sulfate in the reservoir rocks at high temperature (>90°C) and migrates to shallower beds to react inorganically with iron to form [Formula: see text]. Organic constituents from reservoirs, in contrast, provide nourishment for sulfate reducing bacteria in shallow relatively cool (<90°C) beds. Sandstone in the Ray Point uranium district in Live Oak County, Texas contains abundant [Formula: see text] which formed both from deep‐seated [Formula: see text] and from [Formula: see text] produced in the shallow subsurface by bacteria that utilized organic materials from depth. Deep petroleum reservoirs were physically connected to near‐surface (<100 m) beds containing epigenetic [Formula: see text] by the Oakville fault. Epigenetic iron sulfide formation occurred in at least four episodes over at least five million years. Evidence from the Ray Point district and elsewhere in Texas illustrates that sulfidization reactions have destroyed magnetic iron‐titanium oxide minerals in the vicinity of major growth faults, resulting in a systematic decrease in magnetic susceptibility and magnitude of remanent magnetization in the vicinity of such faults. Growth faults which tap hydrocarbon deposits may be detectable using aeromagnetic methods.
Syngenetic magnetic anomaly sources: Three examples
Authors: Gay S. Parker , Hawley Bronson W.
Published: October 2002

Aeromagnetic anomalies encountered in three areas, two in the western United States and one in Central America, are shown to arise from magnetic sedimentary formations. These examples are selected from a larger number of similar areas surveyed by Applied Geophysics, Inc. in various places in the U.S. Midcontinent and Rocky Mountain regions. The first area discussed is the northwest corner of Nebraska where the Miocene Arikaree formation, comprised of magnetic airfall and windblown tuffs, causes anomalies in areas of incised topography. The second area is located in south central Utah, where the Upper Cretaceous Kaiparowits sandstones contain detrital magnetite that causes large anomalies in tilted structures and over incised topography. The third area treated covers over half of southern Belize in Central America, including much of the offshore portion. Here, the Toledo formation of Paleocene‐Eocene age contains a thick section of clastic detritus rich in lithic grains of volcanic rocks that produce magnetic highs over thrusted and folded anticlinal axes. These three examples of magnetic anomalies due to syngenetic magnetite in widely scattered areas and from different types of source materials bring into question the assumption of so‐called “diagenetic magnetite” (or other magnetic minerals) as a cause of magnetic anomalies in other petroleum basins. It is necessary in all cases to determine the magnetic source from surface or subsurface geology, as was done here, rather than making assumptions strictly from magnetic profiles or mathematical models.
2-D resistivity modeling: An approach to arrays parallel to the strike direction
Authors: Queralt P. , Pous J. , Marcuello A.
Published: October 2002

An algorithm for two‐dimensional electrical resistivity modeling using the finite‐element method with mixed boundary conditions can calculate the electrical potential along any arbitrary direction. In the particular case of the direction parallel to the strike of the structure, a numerical singularity occurs. We resolved it by calculating the potential near the singularity and improving the involved interpolation of the transformed potential. Investigating the shape of potential in the Fourier transformed space, we conclude that this interpolation may combine logarithmic and exponential functions.
Steep‐dip v(z) imaging from an ensemble of Stolt‐like migrations
Authors: Mikulich Weston , Hale Dave
Published: October 2002

Stolt migration is often performed with many different velocities, creating a cube (or ensemble) of migrated constant‐velocity sections. With this ensemble, an interpreter can quickly, even interactively, carve out a single, variable‐velocity time migration. Unfortunately, the variable‐velocity section obtained in this way cannot image steep dips correctly. Phase‐shift (i.e., Gazdag) migration accurately images reflections from steep interfaces where velocity varies with depth. However, it is computationally slow compared to Stolt’s f-k migration, its constant‐velocity counterpart. Similarities between the Gazdag and Stolt methods allow a mapping between the two to be derived. This new mapping, which replaces the constant‐velocity Stolt mapping, defines a new Stolt‐like migration. Stolt‐like migration can then be performed to create an ensemble of migrated data, from which a Gazdag‐equivalent migration can be carved. All dips can be imaged properly, even where velocity varies significantly with depth, while the advantages of working with the ensemble of migrated sections are preserved.
A physical model study of microcrack‐induced anisotropy
Authors: Ass’ad Jamal M. , Tatham Robert H. , McDonald John A.
Published: October 2002

A laboratory study of the effects of oriented pennyshaped inclusions embedded in a solid matrix on the propagation of seismic shear waves shows good agreement with theoretical predictions for some polarizations and poor agreement for polarizations at large crack densities. The models are constructed of solid matrix of epoxy resin with inclusions of thin rubber discs of approximately equal cross-sectional areas. The theoretical basis for these experiments is the theory of Hudson, in which the wavelength is greater than the dimensions of the individual cracks and their separation distance, and the cracks are in dilute concentration. By a pulse transmission method, seismograms were gathered in models free of inclusions and models with inclusions. Seismic measurements of velocity anisotropy, for variations in both a polarization and propagation direction, were performed on physical models with inclusions (cracks) representing five different crack densities (1, 3, 5, 7, and 10 percent). Variations in velocity anisotropy at different crack densities have been evaluated by using Thomsen’s parameter (γ) which relates velocities to their elastic constants, [Formula: see text]. Comparisons between experimental and theoretical results indicate that with the waves polarized parallel to the aligned inclusions, [Formula: see text] agree well with the theoretical model. However, shear waves for the same propagation direction but polarized perpendicular to the plane containing the inclusions [Formula: see text] produced results that agree well with the theory for crack densities up to 7 percent, but disagree for higher crack densities. The deviation of γ at 10 percent crack density suggests that crack‐crack interaction and their coalescence may be observable and could lead to seismic techniques to differentiate between microcracks and larger macrocracks.
Seismic reflection investigations of a bedrock surface buried under alluvium
Authors: Goforth Tom , Hayward Chris
Published: October 2002

Seismic reflection techniques were used to characterize a bedrock surface buried under alluvium near a construction site on the campus of Baylor University in Waco, Texas. One of the objectives of the study was to determine if either compressional or shear seismic profiling could be used to reduce the number of engineering boreholes required to determine the bedrock depth and relief prior to building construction. The upper few meters of the alluvium is dry but the lower portion is below the water table, making the bedrock surface a difficult target for compressional waves. The compressional reflection coefficient at the water table is an order of magnitude greater than that at the bedrock surface, and the dry alluvium reduces the signal bandwidth such that the two reflections cannot be distinguished. Also, the multimode Rayleigh ground roll, traveling along the surface at about half the speed of the compressional wave, swamps the reflections. By using shear waves to profile the alluvium/bedrock interface, it was possible to avoid the water table and ground roll problems associated with compressional profiling. Walkaway survey results and analytical models presented demonstrate that shear waves do not “see” the water table, and masking of the bedrock target by the reflection at the dry/wet alluvium interface does not occur. Nor was ground roll a problem because the Love “ground roll,” traveling at a velocity almost as fast as the shear body wave, moves across the geophone spread before the return of the shallow reflections. Common depth point (CDP) and optimum offset shear profiles are presented. Uncertainty in determining the depth to bedrock from the seismic data was estimated to be 3 ft (0.9 m), which is sufficiently accurate to be useful in reducing the number of preconstruction boreholes required in the Brazos floodplain.
An integrated surface and borehole seismic case study: Fort St. John Graben area, Alberta, Canada
Authors: Hinds Ronald C. , Kuzmiski Richard , Anderson Neil L. , Richards Barry R.
Published: October 2002

The deltaic sandstones of the basal Kiskatinaw Formation (Stoddard Group, upper Mississippian) were preferentially deposited within structural lows in a regime characterized by faulting and structural subsidence. In the Fort St. John Graben area, northwest Alberta, Canada, these sandstone facies can form reservoirs where they are laterally sealed against the flanks of upthrown fault blocks. Exploration for basal Kiskatinaw reservoirs generally entails the acquisition and interpretation of surface seismic data prior to drilling. These data are used to map the grabens in which these sandstones were deposited, and the horst blocks which act as lateral seals. Subsequent to drilling, vertical seismic profile (VSP) surveys can be run. These data supplement the surface seismic and well log control in that: 1) VSP data can be directly correlated to surface seismic data. As a result, the surface seismic control can be accurately tied to the subsurface geology; 2) Multiples, identified on VSP data, can be deconvolved out of the surface seismic data; and 3) The subsurface, in the vicinity of the borehole, is more clearly resolved on the VSP data than on surface seismic control. On the Fort St. John Graben data set incorporated into this paper, faults which are not well resolved on the surface seismic data, are better delineated on VSP data. The interpretive processing of these data illustrate the use of the seismic profiling technique in the search for hydrocarbons in structurally complex areas.
An integrated surface seismic/seismic profile case study: Simonette area, Alberta
Authors: Hinds Ronald C. , Anderson Neil L. , Kuzmiski Richard
Published: October 2002

On the basis of conventional surface seismic data, the 13–15–63–25W5M exploratory well was drilled into a low‐relief Leduc Formation reef (Devonian Woodbend Group) in the Simonette area, west‐central Alberta, Canada. The well was expected to intersect the crest of the reef and encounter about 50–60 m of pay; unfortunately it was drilled into a flank position and abandoned. The decision to abandon the well, as opposed to whipstocking in the direction of the reef crest, was made after the acquisition and interpretive processing of both near( and far‐offset (252 and 524 m, respectively) vertical seismic profile (VSP) data, and after the reanalysis of existing surface seismic data. The near‐ and far‐offset VSPs were run and interpreted while the drill rig remained on‐site, with the immediate objectives of: (1) determining an accurate tie between the surface seismic data and the subsurface geology; and (2) mapping relief along the top of the reef over a distance of 150 m from the 13–15 well location in the direction of the adjacent productive 16–16 well (with a view to whipstocking). These surveys proved to be cost‐effective in that the operators were able to determine that the crest of the reef was out of the target area, and that whipstocking was not a viable alternative. The use of VSP surveys in this situation allowed the operators to avoid the costs associated with whipstocking, and to feel confident with their decision to abandon the well.
Ultrasonic velocities in sands—revisited
Authors: Blangy J. P. , Strandenes S. , Moos D. , Nur A.
Published: October 2002

Ultrasonic compressional and shear‐wave velocities of isotropic sands are shown to be dependent on their mineralogy, their porosity, their fluid content, and their state of consolidation, under fixed temperature and pressure conditions. This leads to a distinction between two broad classes of sands: those that are well consolidated, and those that are loosely consolidated. Changes in elastic velocities reflect changes in the ratio of bulk and shear moduli to density in response to lithologic variations. We decouple the two effects by examining changes in elastic moduli with respect to changes in lithology, and we observe three main points: (1) For consolidated sandstones, the effects of mineralogy and porosity can be approximated both empirically and theoretically by a modified isostrain theory: the dry bulk and shear moduli of the rock aggregate follow a “mixing law,” being linear combinations of the respective moduli of the individual constituents. The dry elastic moduli of families of clean sands and shaley sands are linear functions of porosity, with decreasing y‐axis intercepts as their clay‐to‐sand ratio increases. (2) Loosely consolidated sands and sandy shales appear to follow a behavior closer to that of the isostress theory for suspensions: the reciprocals of the bulk and shear moduli of the rock aggregate are linear combinations of the reciprocal moduli of their individual constituents. In general, the elastic moduli of poorly lithified sands are less sensitive to changes in mineralogy and porosity than those of consolidated sandstones. (3) For high permeability sands like the loosely consolidated sands of Troll, the Biot‐Gassman theory is a good approximation to the effects of fluids on seismic velocities. With our understanding of elastic moduli, we then show that dry ratios [Formula: see text] increase with porosity and clay content.
Wavenumber‐based filtering of marine point‐source data
Authors: Amundsen Lasse
Published: October 2002

In seismic processing, plane‐wave decomposition has played a fundamental role, serving as a basis for developing sophisticated processing techniques valid for depth‐dependent models. By comparing analytical expressions for the decomposed wavefields, we review several processing algorithms of interest for the geophysicist. The algorithms may be applied to marine point‐source data acquired over a horizontally layered viscoelastic and anisotropic medium. The plane‐wave decomposition is based on the Fourier transform integral for line‐source data and the Hankel transform integral for point‐source data. The involved wavenumber integrals of the cosine or Bessel‐function type are difficult to evaluate accurately and efficiently. However, a number of the processing techniques can easily be run as a filtering operation in the spatial domain without transforming to the wavenumber domain. The mathematical expressions for the spatial filters are derived using plane wave analysis. With numerical examples, we demonstrate the separation of upgoing and downgoing waves from the pressure, the removal of the source ghost from the pressure, and the transformation of point‐source pressure data to the corresponding line‐source data. The filters for these three processes work satisfactorily. Limited spatial aperture is discussed both for point‐source and line‐source data. The resolution kernels relating finite‐aperture decomposed data to infinite‐aperture decomposed data are given. The kernels are approximately equal in the asymptotic limit when the minimum offset is zero.
Do traveltimes in pulse‐transmission experiments yield anisotropic group or phase velocities?
Authors: Dellinger Joe , Vernik Lev
Published: October 2002

The elastic properties of layered rocks are often measured using the pulse through‐transmission technique on sets of cylindrical cores cut at angles of 0, 90, and 45 degrees to the layering normal (e.g., Vernik and Nur, 1992; Lo et al., 1986; Jones and Wang, 1981). In this method transducers are attached to the flat ends of the three cores (see Figure 1), the first‐break traveltimes of P, SV, and SH‐waves down the axes are measured, and a set of transversely isotropic elastic constants are fit to the results. The usual assumption is that frequency dispersion, boundary reflections, and near‐field effects can all be safely ignored, and that the traveltimes measure either vertical anisotropic group velocity (if the transducers are very small compared to their separation) or phase velocity (if the transducers are relatively wide compared to their separation) (Auld, 1973).
Geophysical and geochemical logs from a copper oxide deposit, Santa Cruz project, Casa Grande, Arizona
Authors: Nelson Philip H. , Johnston David
Published: October 2002

In support of an in‐situ leaching experiment, five holes drilled into a copper oxide deposit have been logged with geophysical and geochemical tools developed for use in the petroleum industry. When combined with geological description, chemical analyses, and mineralogical data from core and cuttings, the logs provide information regarding the alteration, fracturing, copper distribution, porosity, and permeable zones. Correlations among sonic velocity, rock strength from mechanical tests on core, and alteration indicators from neutron and potassium logs demonstrate a close link between the state of alteration and the mechanical state of the rock. Neutron activation, natural gamma‐ray, and density logs, in combination, correlate so well with copper assays that log‐based prediction of copper content is possible; in addition, an estimate of whole‐rock mineralogy is presented in log format. Based on comparisons of flow logs and acoustic logs obtained in the same holes, reductions in acoustic velocity appear to be the best indicators of permeability increases.
RELATION OF GRAVITY TO STRUCTURE IN THE NORTHERN APPALACHIAN AREA
Authors: Nettleton L. L.
Published: October 2002

A series of gravity stations at approximately one mile intervals has been established eastward from Pittsburgh extending across the Appalachian Mountains and onto the pre‐Cambrian outcrops of the Piedmont. The large folds and faults of the steeply folded region have small or negligible gravity anomalies. This is attributed to a uniform density of the rocks involved which is verified by “density profiles.” There has been much speculation as to the relation of gravity to tectonics and present geology in the northern Appalachian area. This speculation is reviewed briefly and some relations not previously mentioned are pointed out. The major and more continuous gravity features correspond closely with the trends of Appalachian folding, as shown by a map of gravity contours superposed on the relief map of the northern Appalachian area. However, it seems probable that the source of the primary gravity features is in mass distributions rather deep within the earth’s crust, which must have their origin in the tectonic forces which caused the Appalachian uplift but are not superficially evident from the surface geology of the area. Some smaller features may be related to structural or lithologic units indicated by the surface geology.
CURRENT PENETRATION IN DIRECT CURRENT PROSPECTING
Authors: Muskat M. , Evinger H. H.
Published: October 2002

Using previously developed methods for calculating the potential about an electrode on the surface of a stratified earth, the fractions of total current, due to two current electrodes, flowing in a given layer and across the vertical plane mid‐way between the electrodes, are calculated. The method employed is applicable in principle to any number of layers, and numerical values are given for two‐layer and three‐layer earths. Curves are given showing the variation of these current fractions with electrode spacing for the following types of conductivity stratification: two‐layer earths with complete range of conductivity ratios; three‐layer earths which are homogeneous except for the middle layer, the latter having thicknesses equal to or twice that of the top stratum; and three‐layer earths having either infinitely conducting or resisting bottom layers and with middle stratum thicknesses equal to, twice, or three times that of the uppermost stratum.
Acoustic wave‐equation traveltime and waveform inversion of crosshole seismic data
Authors: Zhou Changxi , Cai Wenying , Luo Yi , Schuster Gerard T. , Hassanzadeh Sia
Published: October 2002

A hybrid wave‐equation traveltime and waveform inversion method is presented that reconstructs the interwell velocity distribution from crosshole seismic data. This inversion method, designated as WTW, retains the advantages of both full wave inversion and traveltime inversion; i.e., it is characterized by reasonably fast convergence which is somewhat independent of the initial model, and it can resolve detailed features of the velocity model. In principle, no traveltime picking is required and the computational cost of the WTW method is about the same as that for full wave inversion. We apply the WTW method to synthetic data and field crosshole data collected by Exxon at their Friendswood, Texas, test site. Results show that the WTW tomograms are much richer in structural information relative to the traveltime tomograms. Subtle structural features in the WTW Friendswood tomogram are resolved to a spatial resolution of about 1.5 m, yet are smeared or completely absent in the traveltime tomogram. This suggests that it might be better to obtain high quality (distinct reflections) crosshole data at intermediate frequencies, compared to intermediate quality data (good quality first arrivals, but the reflections are buried in noise) at high frequencies. Comparison of the reconstructed velocity profile with a log in the source well shows very good agreement within the 0–200 m interval. The 200–300 m interval shows acceptable agreement in the velocity fluctuations, but the tomogram’s velocity profile differs from the sonic log velocities by a DC shift. This highlights both the promise and the difficulty with the WTW method; it can reconstruct both the and high wavenumber parts of the model, but it can have difficulty recovering the very low wavenumber parts of the model.
Application of aeromagnetic data to mineral potential evaluation in Minnesota
Authors: Spector Allan , Lawler Thomas L.
Published: October 2002

Aeromagnetic, ground magnetic, and gravity data, together with all available drillhole data and physical property measurements, were used to map the Precambrian geology of an area in Minnesota that is virtually devoid of outcrop. The work was done for purposes of land use planning and to encourage minerals exploration and mostly consisted of the analysis of profiles of aeromagnetic data to map magnetic/lithologic contacts, to infer structure, and to determine thickness of overburden cover. Two greenstone belts were resolved. They comprise higher density rocks separated by nonmagnetic metasedimentary intervals. The belts are deformed into synclinal structures that, according to modeling, range from 1 km to as much as 5 km in depth. Lithologic predictions were confirmed in five out of six holes drilled on completion of the magnetic interpretation. In over 40% of the area, Precambrian rocks are apparently mantled by less than 50 m of overburden, and in 50% of the area there is between 50 and 100 m of overburden cover. In the remaining 10%, the magnetic basement is overlain by a thick blanket of nonmagnetic Precambrian sedimentary rocks, over 200 m thick. Basement depth determinations were subsequently tested at six holes. Depth determinations at all drill sites were found to lie within the 20% error expectation of the method of depth determination. Thirty‐seven sites were resolved from the aeromagnetic data as targets for basemetal sulfide (copper, zinc) as well as precious metal (gold) mineralization. Thirteen magnetic anomalies were identified as possible kimberlite pipes.
Velocity dispersion and upscaling in a laboratory‐simulated VSP
Authors: Rio Philippe , Mukerji Tapan , Mavko Gary , Marion Dominique
Published: October 2002

A laboratory and numerical study was conducted to investigate the impact of scale‐dependent seismic wave propagation in randomly layered media, as applied to sonic logs, surface seismic, and vertical seismic profiles (VSPs). Analysis of the laboratory results (1) confirmed the wavelength dependence of velocities inferred from traveltimes, (2) indicated that scale effects can introduce traveltime errors when upscaling from logs to surface seismic and VSPs, and (3) illustrated that erroneous VSP interval velocities can result when layer thicknesses are smaller than about one‐tenth of the wavelength. A simple approximate recipe is presented for estimating these traveltimes by successively filtering the medium using a running Backus average and ray theory. The scale‐dependent dispersion was also predicted well using a more rigorous invariant imbedding formulation. The predicted traveltimes, using the approximate recipe, compare well with the times observed in the laboratory stack of steel and plastic layers and in numerical studies of stratified media. The dispersion curves predicted by the approximate method also show the overall behavior computed with the more rigorous invariant imbedding formulation.
Simulated annealing wavelet estimation via fourth‐order cumulant matching
Authors: Velis Danilo R. , Ulrych Tadeusz J.
Published: October 2002

The fourth‐order cumulant matching method has been developed recently for estimating a mixed‐phase wavelet from a convolutional process. Matching between the trace cumulant and the wavelet moment is done in a minimum mean‐squared error sense under the assumption of a non‐Gaussian, stationary, and statistically independent reflectivity series. This leads to a highly nonlinear optimization problem, usually solved by techniques that require a certain degree of linearization, and that invariably converge to the minimum closest to the initial model. Alternatively, we propose a hybrid strategy that makes use of a simulated annealing algorithm to provide reliability of the numerical solutions by reducing the risk of being trapped in local minima. Beyond the numerical aspect, the reliability of the derived wavelets depends strongly on the amount of data available. However, by using a multidimensional taper to smooth the trace cumulant, we show that the method can be used even in a trace‐by‐trace implementation, which is very important from the point of view of stationarity and consistency. We demonstrate the viability of the method under several reflectivity models. Finally, we illustrate the hybrid strategy using marine and field real data examples. The consistency of the results is very encouraging because the improved cumulant matching strategy we describe can be effectively used with a limited amount of data.
Sensitivity of the dispersion correction to Q error
Authors: Duren Richard E. , Trantham E. Clark
Published: October 2002

A controlled‐phase acquisition and processing methodology for our company has been described by Trantham (1994). He pointed out that it is careful attention to wavelet phase that leads to improved well ties and a more geologically accurate seismic image. In addition, we prefer zero‐phase wavelets on our seismic sections. For a given amplitude spectrum they have the simplest shape and the highest peak; further, the peak occurs at the reflection time of the event. This alignment is important since the seismic wavelet generally broadens with increasing depth with a zero‐phase wavelet remaining symmetrical about the event time. Our experience has been that a true zero‐phase section can be tied over the entire length of a synthetic trace without having to slide the synthetic trace to tie different time zones.
Mapping groundwater contamination using dc resistivity and VLF geophysical methods–A case study
Authors: Benson Alvin K. , Payne Kelly L. , Stubben Melissa A.
Published: October 2002

Geophysical methods can be helpful in mapping areas of contaminated soil and groundwater. Electrical resistivity and very low‐frequency electromagnetic induction (VLF) surveys were carried out at a site of shallow hydrocarbon contamination in Utah County, Utah. Previously installed monitoring wells facilitated analysis of water chemistry to enhance interpretation of the geophysical data. The electrical resistivity and VLF data correlate well, and vertical cross‐sections and contour maps generated from these data helped map the contaminant plume, which was delineated as an area of high interpreted resistivities.
Electroseismic investigation of the shallow subsurface: Field measurements and numerical modeling
Authors: Mikhailov Oleg V. , Haartsen Matthijs W. , Toksöz M. Nafi
Published: October 2002

Recent studies have demonstrated that electroseismic phenomena in porous media have the potential to detect zones of high fluid mobility and fluid chemistry contrasts in the subsurface. However, there have only been a few field studies of these phenomena since they were first observed 60 years ago. None of these studies were able to support observations with an explicit comparison to results of full waveform modeling. In this paper, we demonstrate that the electroseismic phenomena in porous media can be observed in the field, explained, and modeled numerically, yielding a good agreement between the field and the synthetic data. We first outline the design of our field experiment and describe the procedure used to reduce noise in the electroseismic data. After that, we present and interpret the field data, demonstrating how and where different electroseismic signals originated in the subsurface. Finally, we model our field experiment numerically and demonstrate that the numerical results correctly simulate arrival times, polarity, and amplitude variation with offset behavior of the electroseismic signals measured in the field.
Seismic data processing in vertically inhomogeneous TI media
Authors: Alkhalifah Tariq
Published: October 2002

The first and most important step in processing data in transversely isotropic (TI) media for which velocities vary with depth is parameter estimation. The multilayer normal‐moveout (NMO) equation for a dipping reflector provides the basis for extending the TI velocity analysis of Alkhalifah and Tsvankin to vertically inhomogeneous media. This NMO equation is based on a root‐mean‐square (rms) average of interval NMO velocities that correspond to a single ray parameter, that of the dipping event. Therefore, interval NMO velocities [including the normal‐moveout velocity for horizontal events, [Formula: see text]] can be extracted from the stacking velocities using a Dix‐type differentiation procedure. On the other hand, η, which is a key combination of Thomsen's parameters that time‐related processing relies on, is extracted from the interval NMO velocities using a homogeneous inversion within each layer. Time migration, like dip moveout, depends on the same two parameters in vertically inhomogeneous media, namely [Formula: see text] and η, both of which can vary with depth. Therefore, [Formula: see text] and ε estimated using the dip dependency of P‐wave moveout velocity can be used for TI time migration. An application of anisotropic processing to seismic data from offshore Africa demonstrates the importance of considering anisotropy, especially as it pertains to focusing and imaging of dipping events.
Comparison of induced polarization and controlled‐source audio‐magnetotellurics methods for massive chalcopyrite exploration in a volcanic area
Authors: Basokur A. T. , Rasmussen T. M. , Kaya C. , Altun Y. , Aktas K.
Published: October 2002

In this paper, we compare and contrast the results of field experiments with the dipole‐dipole‐induced polarization (IP) and controlled‐source audio‐magnetotellurics (CSAMT) methods, along the same survey profiles, at a test area that was subject to extensive drilling and detailed geological investigation. The ore bodies are interbedded between two series of dacitic tuff. The depth and thickness of the massive chalcopyrite‐pyrite‐sphalerite body vary between 25 and 100 m and 0.5 and 16 m, respectively. Resistivity and IP phase measurements on the core samples collected from the test area provide some idea of the relative differences between the background rock units and the target. The resistivity of the chalcopyrite samples varies between 0.6 to 2 ohm-m and provides sufficient resistivity contrast with surrounding volcanic rock units for target detection. The results of dipole‐dipole IP profiling with a 50-m dipole length conducted along two profiles are presented in the form of apparent resistivity and phase pseudosections. CSAMT measurements were made at 13 frequencies from 2 Hz to 8192 Hz along three profiles. The receiver dipole length was 25 m. The CSAMT data are presented in the form of pseudosections using conventional and new definitions of apparent resistivity. The elliptical contours of low apparent resistivity generated by the transition‐field notch can be misleading with respect to the real anomaly of the ore body. These artificial anomaly patterns are suppressed by making use of an alternative apparent resistivity definition derived from the frequency‐normalized impedance. The qualitative interpretation based on the IP and CSAMT pseudosections shows that the location and the extension of the ore body are indicated better in the CSAMT apparent resistivity data computed from the alternative definition. The qualitative interpretation of the IP data is difficult because of the 3-D effect arising from the neighboring thicker parts of the ore body and pyrite particles within the basement. At the final stage, the far‐field range of the CSAMT apparent resistivity and phase data is identified by the visual inspection of the individual sounding diagrams. The models derived from the 2-D Occam inversion carried out on the far‐field data are compared with the drill‐hole information and are found to describe the actual geological situation.
A towed marine gravity meter for near‐bottom surveys
Authors: Zumberge Mark A. , Ridgway Jeff R. , Hildebrand John A.
Published: October 2002

Gravity is measured presently on the sea surface and on the sea floor. Surface gravity suffers from loss of resolution over the deep ocean because the perturbing source masses are far from the observer. Bottom measurements recover this resolution, but suffer from poor coverage because of the time needed for each measurement. We have constructed a gravimetry system that combines the rapid data collection capability of a moving platform with the high resolution gained by locating the observations near the bottom. This gravity sensor is tethered to a ship and towed just above the sea floor. The instrument consists of a LaCoste and Romberg shipboard gravity meter modified to fit inside a pressure case that is mounted on a platform designed for towing stability. We have tested it in a survey in the San Diego Trough, a 1000-m—deep sedimented valley in the Pacific Ocean in the California continental borderlands. Multiple gravity tracklines collected there at a depth of 935 m show a resolution of a few tenths of a mGal. The new instrument will be useful for surveys of features whose lateral extent is equal to or less than the ocean depth.
Inversion of geophysical data over a copper gold porphyry deposit: A case history for Mt. Milligan
Authors: Oldenburg Douglas W. , Li Yaoguo , Ellis Robert G.
Published: October 2002

In this paper, we invert magnetic, DC resistivity, induced polarization (IP), and airborne electromagnetic (EM) data from the Mt. Milligan copper‐gold porphyry deposit and jointly interpret the inversion results with available geological and mineralization data. The inversions are carried out over an area that encloses an intrusive stock, known as the MBX Stock, and the resulting mineralization zone surrounding it. The earth model is discretized into a large number of cells having constant physical properties, and distributions of magnetic susceptibility, conductivity, and chargeability are obtained by minimizing a model objective function subject to adequately fitting the corresponding data. A 3-D magnetic susceptibility model is obtained directly by inverting surface total field anomaly data. 3-D conductivity and chargeability models are formed by compositing 2-D sections recovered by inverting DC/IP pole‐dipole data. The airborne EM data are inverted with a 1-D algorithm and composited into a 3-D conductivity model. The physical property models are compared with a rock model constructed from geologic information from 600 drill holes and with a 3-D model of gold concentration. The physical property models have features that correlate with various geologic boundaries and rock units. More notably, the recovered chargeability and susceptibility seem to reflect the distribution of mineralization: chargeability highs coincide with the greatest gold concentration, while the susceptibility displays an anticorrelation with it. Overall, our inversion results are consistent with the geology and mineralization models for the Cu-Au porphyry deposit, while the anticorrelation between gold concentration and susceptibility provides an important constraint that helps define the distribution and geochemical control of the orebody.
Seismic monitoring of the growth of a hydraulic fracture zone at Fenton Hill, New Mexico
Authors: Li Yingping , Cheng Chuen H. , Toksöz M. Nafi
Published: October 2002

The hydraulic fracturing technique is an important method for enhancing hydrocarbon recovery, geothermal energy extraction, and solid waste disposal. Determination of the geometry and growth process of a hydraulic fracture zone is important for monitoring and assessing subsurface fractures. A relative‐source‐location approach, based on a waveform correlation and a grid search method, has been developed to estimate relative hypocenter locations for a cluster of 157 microearthquakes induced by hydraulic fracturing at the Los Alamos Hot Dry Rock (HDR) geothermal site. Among the 157 events, 147 microearthquakes occurred in a tight cluster with a dimension of 40 m, roughly defining a vertical hydraulic fracture zone with an orientation of N40°W. The length, height, and width of the hydraulic fracture zone are estimated to be 40, 35, and 5 m, respectively. Analysis of the spatial‐temporal pattern of the induced microearthquakes reveals that the fracture zone grew significantly, averaging 0.2 m/minute in a two‐hour period toward the northwest along the fracture zone strike.