bluuebunny/crossref_metadata_2025 huggingface dataset:
The effect of terrain topography on commonly used resistivity arrays
Authors: Tsourlos Panagiotis I. , Szymanski John E. , Tsokas Gregory N.
Published: October 2002

In this work a study of the effect of surface topographical variations on several dc resistivity arrays is presented. A 2.5-D finite‐element modeling scheme is used to examine the performance of several arrays over buried features that exist below a range of different topographical terrain contexts, such as valleys, hills, and steep slopes. A mesh‐generating algorithm allows a realistic representation of terrain topography. The results confirm that topographical variations can have a significant impact on the field resistivity data values for all resistivity arrays. Further, topographical variations can be treated flexibly using a realistic resistivity forward modeling process. Thus, topographic effects, to the degree they can be modeled, are predictable and should be taken into account when designing surveys and interpreting data.
A new vertical continuation procedure for airborne electromagnetic field data from the modified image method
Authors: Bergeron Clyde J. , Brusstar John R. , Yi Ningke , Wu Yan , Ioup Juliette W.
Published: October 2002

Airborne electromagnetic (AEM) data measured by equipment in a bird tethered to a helicopter have large variations caused by the unavoidable vertical excursions of the helicopter as it traverses its flight path. Such large changes tend to mask the smaller changes in field strength caused by lateral variations in the earth’s electrical conductivity along the flight path, which is the information that is the goal of AEM surveys. Signals produced by conductivity anomalies such as sea‐ice keels and pipelines in marshes or in the shallow ocean are enhanced and may be apparent directly in the continued fields. Furthermore, electronic or environmental noise is more easily detected in the continued fields and reduced by various methods of filtering and signal processing. In the modified image method (MIM) formalism for AEM fields, the algebraic expression for the secondary to primary field ratio [Formula: see text] is given in terms of R, where R is the total complex vertical separation of the primary and image dipoles [Formula: see text] scaled to the coil spacing ρ, [Formula: see text] is the complex effective skin depth, and h is the altitude of the bird. An inverse algebraic relation gives R as a function of [Formula: see text]. In this paper we present a simple and accurate method of continuing the field by way of continuing R. Because R is linear in h, the vertical continuation of R from h to h0is accomplished by a simple linear translation. This method is applied to a flight line of an AEM survey of Barataria Bay, Louisiana, which includes both the marsh and a near‐shore region of the Gulf of Mexico. The smoothnesss of the continued data over the Gulf implies that the variability of the continued data over the marsh is attributable to horizontal variation in salinity, soil porosity, and water depth rather than noise. To produce more accurate values for R, we have also included details of an extended half‐space renormalization function which, in effect, removes residual differences between the fields calculated from the MIM algebraic and the numerical evaluation of the exact Sommerfeld integral representations of the [Formula: see text] field.
Principles of probability tomography for natural‐source electromagnetic induction fields
Authors: Mauriello Paolo , Patella Domenico
Published: October 2002

The 3-D interpretation problem of natural‐source electromagnetic (EM) induction field data collected over a flat air‐earth boundary is dealt with using the concept of probability tomography. This paper presents a method to recognize the most probable localization of the induced electric charge accumulations across resistivity discontinuities and current channeling inside conductive bodies. We begin by writing the solutions for the electric (magnetic) ground surface EM field components in the frequency domain as a sum of elementary contributions, each resulting from a single induced‐charge (dipole) element. Then we express the total electric (magnetic) power associated with each EM field component as a sum of crosscorrelation integrals between the measured component and the homologous synthetic expression resulting from each causative induced‐charge (dipole) element. The synthetic component takes the key role of scanner function in the new imaging procedure. Moreover, using the crosscorrelation bounding inequality we introduce the concept of EM induction occurrence probability as a suitable parameter for the tomographic representation of the induced‐charge and dipole distributions underground. For each electric and magnetic surface component we define the corresponding occurrence probability function as the crosscorrelation product of the observed component and the relative scanning function, divided by the square root of the product of the respective variances. In the space domain, the 3-D tomographic procedure consists of scanning the half‐space below the survey area by the unit strength charge or dipole element, which is given a regular grid of space coordinates within the volume. At each node of the grid, the occurrence probability function is calculated. We use the complete set of calculated grid values to single out the zones of highest occurrence probability of electric charge accumulations and current channeling elements. The physical reliability of the proposed tomography is tested on synthetic and field examples.
Gravity inversion of a discontinuous relief stabilized by weighted smoothness constraints on depth
Authors: Barbosa Valéria C. F. , Silva João B. C. , Medeiros Walter E.
Published: October 2002

We present a new stable gravity inversion method applied to the mapping of an interface separating two homogeneous media. In contrast with previous similar methods, it does not impose an overall smoothness on the estimated interface to stabilize the solution. The density contrast between the media is assumed to be known. The interpretation model for the upper medium consists of rectangular juxtaposed prisms whose thicknesses represent the depths to the interface and are the parameters to be estimated. The true interface is assumed to be flat everywhere except at faults. To incorporate this attribute into the estimated relief, we developed an iterative process in which three kinds of constraints are imposed on parameters: (1) proximity between values of adjacent parameters, (2) lower and upper bounds to parameters, and (3) proximity between the values of parameters and fixed numerical values. Starting with an initial solution which presents an overall smooth relief, the method enhances initially estimated geometric features of the interface; that is, flat areas will tend to become flatter and steep areas will tend to become steeper. This is accomplished by weighting the constraints, which requires proximity between adjacent parameters. The weights are initialized with values equal to unity and are updated automatically to enhance any discrepancy between adjacent depths that have been detected at the initial solution. Constraints 2 and 3 are used both to compensate for the decrease in solution stability caused by the introduction of small weights and to reinforce flatness at the basin bottom. Constraint 2 imposes that any depth be nonnegative and smaller than an a priori known maximum depth value, whereas constraint 3 imposes that all depths be closest to a value greater than the maximum depth. The trade‐off between these conflicting constraints is attained with a final relief presenting flat bottom and steep borders. The method was tested with a synthetic gravity anomaly produced by a simulated sedimentary cratonic extensional basin whose basement consists of steep edges and a flat bottom. The results showed an improvement in the resolution of the relief, leading to a reliable mapping both of the sharp discontinuities at the borders and of the lateral extent of the base of the basin. Additionally, the method produced excellent estimates for the average dip angles of the basin edges (presumably controlled by normal faults), indicating, in this way, its potential in interpreting data produced by this kind of basin. The method was applied to the Bouguer anomaly from the northern portion of Steptoe Valley, Nevada, delineating an isolated basin with a wider, flat base and relatively straight borders as compared with the estimate imposing overall smoothness on the relief.
Inversion of controlled source audio‐frequency magnetotellurics data for a horizontally layered earth
Authors: Routh Partha S. , Oldenburg Douglas W.
Published: October 2002

We present a technique for inverting controlled source audio‐frequency magnetotelluric (CSAMT) data to recover a 1-D conductivity structure. The earth is modeled as a set of horizontal layers with constant conductivity, and the data are apparent resistivities and phases computed from orthogonal electric and magnetic fields due to a finite dipole source. The earth model has many layers compared to the number of data points, and therefore the solution is nonunique. Among the possible solutions, we seek a model with desired character by minimizing a particular model objective function. Traditionally, CSAMT data are inverted either by using the far‐field data where magnetotelluric (MT) equations are valid or by correcting the near‐field data to an equivalent plane‐wave approximation. Here, we invert both apparent resistivity and phase data from the near‐field transition zone and the far‐field regions in the full CSAMT inversion without any correction. Our inversion is compared with that obtained by inverting near‐field corrected data using an MT algorithm. Both synthetic and field data examples indicate that a full CSAMT inversion provides improved information about subsurface conductivity.
Identifying patchy saturation from well logs
Authors: Dvorkin Jack , Moos Daniel , Packwood James L. , Nur Amos M.
Published: October 2002

Gassmann’s (1951) equations relate the elastic bulk (Ksat) and shear (Gsat) moduli of a fully saturated rock to the elastic bulk and shear moduli of the dry‐rock frame (Kdryand Gdry, respectively), porosity ϕ, and the bulk moduli of the mineral phase [Formula: see text] and pore fluid [Formula: see text].
Wavelet spectrum estimation
Authors: Tan Tik Hing
Published: October 2002

A method for complex source signature estimation for offshore acquisition is presented. Although the wave‐propagation velocity in the water is assumed to be known, no assumptions about the material properties underwater are made. The inputs to the algorithm are the pressure field measured along an end of spread streamer cable, the coordinates of the sources, and the depth of the cable. The output is the complex spectrum of each individual source in the source array. This article discusses the physical interpretation of the equations. It is shown that the estimated wavelets depend on the direct field only. The sensitivities of the method are also discussed. Application of the algorithm on generated and measured data is presented.
Detection of seismic refraction signals using a variance fractal dimension technique
Authors: Jiao Lingxiu , Moon Wooil M.
Published: October 2002

Seismic signals in deep crustal surveys are often contaminated with various types of noise, mainly caused by the low signal‐to‐noise (S/N) earth environment. A variance fractal dimension (VFD) technique is investigated and tested with real data sets for detection of seismic refraction signals from background noise. The data tested in this study were collected during the 1992 Lithoprobe Abitibi‐Grenville Transect high‐resolution refraction and wide‐angle reflection seismic experiments. The sharpness of transition features on the VFD trajectory is used as a criterion for distinguishing specific seismic phases. The window size and window interval applied in the application of VFD technique were determined using synthetic seismic data for generation of the optimum VFD trajectory. The window size of 48 samples and the window interval of 8 sample intervals were chosen to calculate the fractal dimension values and create the trajectories for detecting phases Pg, Pn, PmP, and ground roll. The VFD technique was also tested and applied for automatic detection of the first breaks in the high‐resolution seismic reflection data collected during the 1990 Lithoprobe regional and high‐resolution seismic surveys. The sharp transition features corresponding to the first arrivals in the seismic reflection data are distinct and provide us with a robust and powerful tool for separating the seismic signals from noise.
Inversion of controlled‐source tensor magnetotelluric data for a layered earth with azimuthal anisotropy
Authors: Li Xiaobo , Oskooi Behrooz , Pedersen Laust B.
Published: October 2002

Fractures in the upper part of the crust are pervasive in crystalline areas. They often are aligned and nearly vertical. Magnetotelluric studies carried out in different parts of the world often indicate that the upper brittle part of the crust is electrically anisotropic. One model is that deep fluid‐filled fractures, when observed from the surface, mimic an anisotropic medium. The transfer functions of the controlled‐source tensor magnetotelluric (CSTMT) method contain information from current systems running both parallel and perpendicular to the principal horizontal axes of anisotropy, so that the CSTMT method is capable of detecting azimuthal anisotropy. For the CSTMT method, electric dipoles provide the source field, and hence fields are localized. Thus, the distortion generated by a major conductive anomaly lying outside the induction volume defined by the transmitter and receiver positions will be small. Therefore, one‐dimensional models often may be valid. We have developed a nonlinear least‐squares inversion approach to invert CSTMT data for azimuthal anisotropy in an one‐dimensional layered earth. Near‐surface distortion effects on the impedance tensor are parameterized as a real distortion matrix. The elements of the distortion matrix are incorporated into the model parameters. By using tipper functions which are less distorted by near‐surface structures, near‐surface distortion effects can be removed adequately. One striking feature of our inversion algorithm is that the partial derivatives of the response functions, with respect to model parameters, are given in analytical forms, which results in an efficient computation of the Jacobian matrix. Theoretical studies show good convergence and good resolution of the model parameters. We have applied the inversion scheme to a set of controlled‐source data from the Siljan impact structure in Sweden. The derived models give much better data fits than a corresponding isotropic model.
2-D electromagnetic modeling by finite‐element method with a dipole source and topography
Authors: Mitsuhata Yuji
Published: October 2002

I present a method for calculating frequency‐domain electromagnetic responses caused by a dipole source over a 2-D structure. In modeling controlled‐source electromagnetic data, it is usual to separate the electromagnetic field into a primary (background) and a secondary (scattered) field to avoid a source singularity, and only the secondary field caused by anomalous bodies is computed numerically. However, this conventional scheme is not effective for complex structures lacking a simple background structure. The present modeling method uses a pseudo‐delta function to distribute the dipole source current, and does not need the separation of the primary and the secondary field. In addition, the method employs an isoparametric finite‐element technique to represent realistic topography. Numerical experiments are used to validate the code. Finally, a simulation of a source overprint effect and the response of topography for the long‐offset transient electromagnetic and the controlled‐source magnetotelluric measurements is presented.
Sound velocity of drilling mud saturated with reservoir gas
Authors: Carcione José M. , Poletto Flavio
Published: October 2002

Knowledge of the in‐situ sound velocity of drilling mud can be used in mud‐pulse acoustic telemetry for evaluating the presence and amount of gas invasion in the drilling mud. We propose a model for calculating the in‐situ density and sound velocity of water‐based and oil‐based drilling muds containing formation gas. Drilling muds are modeled as a suspension of clay particles and high‐gravity solids in water or oil, with the acoustic properties of these fluids depending on pressure and temperature. Since mud at different depths experiences different pressures and temperatures, downhole mud weights can be significantly different from those measured at the surface. Taking this fact into consideration, we assume constant clay composition and obtained the fraction of high‐gravity solids to balance the formation pressure corresponding to a given drilling plan. This gives the in‐situ density of the drilling mud, which together with the bulk moduli of the single constituents allow us to compute the sound velocity using Reuss’s model. In the case of oil‐based muds, we take into account the gas solubility in oil. When gas goes into solution, the mud is composed of solid particles, live oil and, eventually, free gas. A phenomenological model based on a continuous spectrum of relaxation mechanisms is used to describe attenuation due to mud viscosity. The calculations for water‐based and oil‐based muds showed that the sound velocity is strongly dependent on gas saturation, fluid composition, and drilling depth.
Surface‐related multiple elimination on land seismic data—Strategies via case studies
Authors: Kelamis Panos G. , Verschuur D. J.
Published: October 2002

Three processing strategies for the estimation and subsequent elimination of surface‐related multiple energy on land seismic data are presented. They can be applied in a prestack mode (to shot and common‐midpoint gathers) or in a poststack mode. The algorithm for the multiple attenuation is based on wave theoretical principles in which the data are used as a prediction operator. The estimated multiples are then adaptively subtracted from the input data to obtain primary‐only data. A processing step prior to applying multiple elimination is an important component of these methodologies, particularly in the prestack analysis. Its aim is to regularize the data, improve the S/N ratio, and balance the seismic amplitudes. This results in smooth prediction operators. The effectiveness of these schemes in suppressing multiples is demonstrated with a number of case studies involving processing land seismic data.
Automatic 1-D inversion of magnetotelluric data: Finding the simplest possible model that fits the data
Authors: Pedersen Laust B. , Gharibi Mehran
Published: October 2002

We demonstrate that automatic layered inversion of plane‐wave electromagnetic data can be carried out by modifying standard least‐squares inversion schemes. The modifications include a logarithmic reparameterization of the unknown model parameters, whereby all layer parameters are forced to remain within given bounds. However, the most important modification to help the optimization process find the best model and to avoid local minima is to split the data into several subbands, starting from the highest frequencies. By this stripping procedure, the shallower part of the model becomes well estimated first. As more data are introduced, more layers may be required to improve the data fit. The new inversion procedure has been applied to many sets of theoretical data representing increasingly complicated layering that is often found in near‐surface studies. The main result of these simulations is that there is a very strong coupling between the resolution power of plane‐wave data and their random errors. Particularly at small error levels around 1–2%, the resolution power increases greatly. For larger error levels the fine layering of the models becomes smeared, and only the variations in thickness of the upper layer can be resolved. A comparison with the popular “Occam” procedure shows, not unexpectedly, that a blocky earth is best represented by a blocky earth model rather than by a smooth model.
Approximate dispersion relations forqP-qSV-waves in transversely isotropic media
Authors: Schoenberg Michael A. , de Hoop Maarten V.
Published: October 2002

To decouple qP and qSV sheets of the slowness surface of a transversely isotropic (TI) medium, a sequence of rational approximations to the solution of the dispersion relation of a TI medium is introduced. Originally conceived to allow isotropic P-wave processing schemes to be generalized to encompass the case of qP-waves in transverse isotropy, the sequence of approximations was found to be applicable to qSV-wave processing as well, although a higher order of approximation is necessary for qSV-waves than for qP-waves to yield the same accuracy. The zeroth‐order approximation, about which all other approximations are taken, is that of elliptical TI, which contains the correct values of slowness and its derivative along and perpendicular to the medium’s axis of symmetry. Successive orders of approximation yield the correct values of successive orders of derivatives in these directions, thereby forcing the approximation into increasingly better fit at the intervening oblique angles. Practically, the first‐order approximation for qP-wave propagation and the second‐order approximation for qSV-wave propagation yield sufficiently accurate results for the typical transverse isotropy found in geological settings. After only slight modification to existing programs, the rational approximation allows for ray tracing, (f-k) domain migration, and split‐step Fourier migration in TI media—with little more difficulty than that encountered presently with such algorithms in isotropic media.
Crustal modeling from spectrally correlated free‐air and terrain gravity data—A case study of Ohio
Authors: Kim Jeong Woo , von Frese Ralph R. B. , Kim Hyung Rae
Published: October 2002

We investigate the use of spectral correlation theory to analyze terrain gravity effects and free‐air gravity anomalies of Ohio for possible constraints on the thickness variations and neotectonics of the crust. Terrain gravity effects are computed from the topography by Gauss‐Legendre quadrature integration and are compared against independent free‐air gravity anomaly observations for their wavenumber correlation spectrum. Spectral correlation filters are designed accordingly to extract terrain‐correlated free‐air gravity anomalies that are subtracted from the terrain gravity effects for estimates of the compensated terrain gravity effects. These effects are used to model the Moho by inversion, assuming they predominantly reflect crustal thickness variations. Our results characterize the middle third of Ohio as a broad zone of thickened Precambrian crust, which also may include rifted regions where the thickness of the prerift crust has been reduced greatly. Furthermore, we find that about 83% of the instrumentally determined earthquake epicenters are located within the inferred thinner regions of Ohio’s crust or at their margins where compressional stresses may dominate. In general, these crustal thickness variations provide new constraints on modeling the tectonic evolution and geotechnical parameters of the crust—constraints that are important for evaluating earthquake hazards, the distribution and extraction of crustal resources, and the storage of hazardous waste and other crustal engineering applications.
A constrained parametric inversion for velocity analysis based on CFP technology
Authors: Kabir M. M. Nurul , Verschuur D. J.
Published: October 2002

A method of velocity analysis based on the common focusing point (CFP) method is presented. The two important aspects of the method are the use of the CFP domain and the use of a new parameterization—a vertical velocity gradient to describe the lateral velocity variation within a layer. The layer velocity is defined with only two parameters: an average velocity [Formula: see text]and a vertical velocity gradient (β). Layer velocity parameterization using [Formula: see text] and β assumes that the lithology of the layer is constant and that the overburden and fluid pressure increase linearly with depth. This type of parameterization is suitable for areas with gross changes in lithology (clastic‐carbonate‐salt) and for rock in hydrostatic equilibrium. A layer‐based model is required for these areas. The salt dome data example presented belongs to this type of area, so the layer‐based model with the defined parameterization produced a very good subsurface velocity model. The method is based on the principle of equal traveltime between the focusing operator and the corresponding focus point response. The velocity estimation problem is formulated as a constrained parametric inversion process. The method of perturbation is applied where linear assumptions are made; the velocity inversion, however, is a nonlinear problem, and the model parameter updates are computed iteratively using Newton’s method. The velocity model is built by layers in a top‐down approach, which makes the problem quasi‐linear.
An acoustic wave equation for anisotropic media
Authors: Alkhalifah Tariq
Published: October 2002

A wave equation, derived using the acoustic medium assumption for P-waves in transversely isotropic (TI) media with a vertical symmetry axis (VTI media), yields a good kinematic approximation to the familiar elastic wave equation for VTI media. The wavefield solutions obtained using this VTI acoustic wave equation are free of shear waves, which significantly reduces the computation time compared to the elastic wavefield solutions for exploding‐reflector type applications. From this VTI acoustic wave equation, the eikonal and transport equations that describe the ray theoretical aspects of wave propagation in a TI medium are derived. These equations, based on the acoustic assumption (shear wave velocity = 0), are much simpler than their elastic counterparts, yet they yield an accurate description of traveltimes and geometrical amplitudes. Numerical examples prove the usefulness of this acoustic equation in simulating the kinematic aspects of wave propagation in complex TI models.
Seismic modeling by demigration
Authors: Santos Lúcio T. , Schleicher Jörg , Tygel Martin , Hubral Peter
Published: October 2002

Kirchhoff‐type, isochron‐stack demigration is the natural asymptotic inverse to classical Kirchhoff or diffraction‐stack migration. Both stacking operations can be performed in true amplitude by an appropriate selection of weight functions. Isochron‐stack demigration is closely related to seismic modeling with the Kirchhoff integral. The principal objective of this paper is to show how demigration can be used to compute synthetic seismograms. The idea is to attach to each reflector in the model an appropriately stretched (i.e., frequency‐shifted) spatial wavelet. Its amplitude is proportional to the reflection coefficient, transforming the original reflector model into an artificially constructed true‐amplitude, depth‐migrated section. The seismic modeling is then realized by a true‐amplitude demigration operation applied to this artificially constructed migrated section. A simple but typical synthetic data example indicates that modeling by demigration yields results superior to conventional zero‐order ray theory or classical Kirchhoff modeling.
Seismic finite‐difference modeling with spatially varying time steps
Authors: Tessmer Ekkehart
Published: October 2002

Numerical seismic modeling by finite‐difference methods usually works with a global time‐step size. Because of stability considerations, the time‐step size is determined essentially by the highest seismic velocity, i.e., the higher the highest velocity, the smaller the time step needs to be. Therefore, if large velocity contrasts exist within the numerical grid, domains of low velocity are oversampled temporally. Using different time‐step sizes in different parts of the numerical grid can reduce computational costs considerably.
Physical and numerical modeling of tuning and diffraction in azimuthally anisotropic media
Authors: Gibson Richard L. , Theophanis Stephen , Toksöz M. Nafi
Published: October 2002

Fractured reservoirs are an important target for exploration and production geophysics, and the azimuthal anisotropy often associated with these reservoirs can strongly influence seismic wave propagation. We created a physical model of a fractured reservoir to simulate some of these propagation effects. The reservoir is represented by a phenolite disk that is thin with respect to the elastic wavelengths in the experiment, creating model dimensions that are representative of realistic reservoirs. Phenolite is strongly anisotropic with orthorhombic symmetry, which suggests that azimuthal amplitude versus offset (AVO) effects should be obvious in data. We acquired both SH- and P-wave data in common‐offset gathers with a near offset and a far offset and found that although the SH-wave data show clear azimuthal variations in AVO, the P-wave signals show no apparent changes with azimuth. We then applied numerical modeling to analyze the data. Because ray methods cannot model diffractions from the disk edge, we first used a ray‐Born technique to simulate variations in waveforms associated with such scattering. The synthetic seismograms reproduced variations in the SH-wave waveforms accurately, though the amplitude contrast between acquisition azimuths was overestimated. Assuming a laterally homogeneous model, we then applied ray methods to simulate tuning effects in SH- and P-wave data and confirmed that in spite of the large contrasts in elastic properties, the tuning of the P-wave reflections from the thin disk changed so there was negligible contrast in AVO with azimuth. Models of field scale reservoirs showed that the same effects could be expected for field applications.
Estimation of fracture parameters from reflection seismic data—Part III: Fractured models with monoclinic symmetry
Authors: Bakulin Andrey , Grechka Vladimir , Tsvankin Ilya
Published: October 2002

Geophysical and geological data acquired over naturally fractured reservoirs often reveal the presence of multiple vertical fracture sets. Here, we discuss modeling and inversion of the effective anisotropic parameters of two types of fractured media with monoclinic symmetry. The first model is formed by two different nonorthogonal sets of rotationally invariant vertical fractures in an isotropic host rock; the other contains a single set of fractures with microcorrugated faces. In monoclinic media with two fracture sets, the shear‐wave polarizations at vertical incidence and the orientation of the NMO ellipses of pure modes in a horizontal layer are controlled by the fracture azimuths as well as by their compliances. While theS-wave polarization directions depend only on the tangential compliances, the axes of the P-wave NMO ellipse are also influenced by the normal compliances and therefore have a different orientation. This yields an apparent discrepancy between the principal anisotropy directions obtained using P and S data that does not exist in orthorhombic media. By first using the weak‐anisotropy approximation for the effective anisotropic parameters and then inverting the exact equations, we devise a complete fracture characterization procedure based on the vertical velocities of theP- and two split S-waves (or converted PS-waves) and their NMO ellipses from a horizontal reflector. Our algorithm yields the azimuths and compliances of both fracture systems as well as the P- and S-wave velocities in the isotropic background medium. In the model with a single set of microcorrugated fractures, monoclinic symmetry stems from the coupling between the normal and tangential (to the fracture faces) slips, or jumps in displacement. We demonstrate that for this model the shear‐wave splitting coefficient at vertical incidence varies with the fluid content of the fractures. Although conventional fracture models that ignore microcorrugation predict no such dependence, our conclusions are supported by experimental observations showing that shear‐wave splitting for dry cracks may be substantially greater than that for fluid‐filled ones.
In‐mine seismic delineation of mineralization and rock structure
Authors: Greenhalgh Stewart A. , Mason Iain M. , Sinadinovski Cvetan
Published: October 2002

Significant progress has been made towards the goal of generating detailed seismic images as an aid to mine planning and exploration at the Kambalda nickel mines of Western Australia. Crosshole and vertical‐seismic‐profiling instrumentation, including a slimline multi‐element hydrophone array, three‐component geophone sensors, and a multishot detonator sound source, have been developed along with special seismic imaging software to map rock structure. Seismic trials at the Hunt underground mine established that high frequency (> 1 kHz) signals can be propagated over distances of tens of meters. Tomographic as well as novel 3-D multicomponent reflection imaging procedures have been applied to the data to produce useful pictures of the ore‐stope geometry and host rock. Tomogram interpretation remains problematic because velocity changes not only relate to differing rock types and/or the presence of mineralisation, but can also be caused by alteration/weathering and other rock condition variations. Ultrasonic measurements on rock core samples help in assigning velocity values to lithology, but geological assessment of tomograms remains ambiguous. Reflection imaging is complicated by the presence of strong tube‐wave to body‐wave mode conversion events present in the records, which obscure the weak reflection signatures. Three‐dimensional reflection data processing, especially three‐component analysis, is time consuming and difficult to perform. Notwithstanding the difficulties, the seismic migrations at the Hunt mine show a striking correlation with the known geology. Combined seismic and radar surveying from available underground boreholes and mine drivages is probably needed in the future to more confidently delineate mineralisation.
Exploration geophysics at the Pyhäsalmi mine and grade control work of the Outokumpu Group
Authors: Hattula Aimo , Rekola Timo
Published: October 2002

The power of geophysics is often realized while surveying barren exploration holes. Integrated interpretation of borehole electromagnetic (EM) and lithogeochemical data led to the discovery of a new volcanogenic massive sulfide (VMS) ore deposit at 500 m depth in the Pyhäsalmi area, which belongs to the Main Sulfide ore belt in Finland. In the deep exploration program, wide‐band multifrequency EM ground surveys were successfully used to detect both new ore lenses and geological structures. Mise‐a‐la‐masse (MAM) borehole and ground surveys as well as borehole EM surveys were effectively used to correlate intersections between drill holes and to locate new orebodies. The latest modeling of MAM data resulted in an exploration target at 700 m depth. The use of geophysics for exploration has been extended to mine production at Outokumpu. Geophysical logging detects ore‐waste boundaries, reduces expensive core drilling, and obtains physical property information quickly on ore intersections. Depending on ore type, geophysical borehole logging can also be applied to classify mineralization, interpret lithology, and sometimes to transform physical responses to metal grades in ore. At the Pyhäsalmi zinc‐copper‐sulfur mine, density logging in percussion boreholes is used to locate mineable ore boundaries and to classify drillhole intersections as massive or semimassive sulfide ore types. Pyrrhotite‐bearing zones are separated from other sulfides by inductive conductivity logs. The use of geophysical logging for grade estimation and control has been most effective in the nickel mines at Enonkoski, Finland, and Namew Lake, Canada (using conductivity logs), and in the Kemi chromium mine, Finland (using gamma‐gamma density logs).
Ultralow‐radiation‐intensity spectrometric probe for orebody delineation and grade control of zinc‐lead ore
Authors: Charbucinski Jacek , Borsaru Mihai , Gladwin Michael
Published: October 2002

A fully spectrometric gamma—gamma probe using an ultralow‐activity gamma‐ray source (i.e., <2 MBq) has been developed. The prototype low‐radiation‐intensity probe was tested at a zinc‐lead deposit in Queensland, Australia. The probe showed an excellent capability for orebody delineation. Lithological profiles derived from logging data showed sharp anomalies both in selected spectral regions and in spectral ratios during probe transition from orebody to barren rock, or vice versa. In comparison to standard geochemical analysis, the instrument demonstrated good potential for quantitative determination of lead and zinc content in ore with rms error of 0.3% and 2.4%, respectively. Delineation of nonlithological boundaries, through an application of a cutoff‐grade algorithm, has been demonstrated as a practical stand‐alone mine control tool.
Nonlinear inversion of the electrode logging measurements in a deviated well
Authors: Abubakar Aria , van den Berg Peter M.
Published: October 2002

We develop a nonlinear inversion method for reconstructing the 3-D electrical conductivity in a highly deviated well with invasion using electrode logging (static field) measurements. The integral equation is formulated in an oblique polar coordinate system so that the dipping‐bed environment can be modeled without approximations. The computation time is dramatically reduced by using a nonuniform discretization grid in the direction away from the borehole. Moreover, the contrast source inversion method, which has been successfully applied to the electrode logging problem using a homogeneous background, has been modified so that the inhomogeneous background can be used without introducing a borehole Green’s function. Numerical examples have shown that the present inversion method formulated in an oblique polar coordinate system can be used as an interpretation technique for the electrode logging measurements.
Mapping of induced polarization using natural fields
Authors: Gasperikova Erika , Morrison H. Frank
Published: October 2002

The observed electromagnetic response of a finite body is caused by induction and polarization currents in the body and by the distortion of the induction currents in the surrounding medium. At a sufficiently low frequency, there is negligible induction and the measured response is that of the body distorting the background currents just as it would distort a direct current (dc). Because this dc response is not inherently frequency dependent, any observed change in response of the body for frequencies low enough to be in this dc limit must result from frequency‐dependent conductivity. Profiles of low‐frequency natural electric (telluric) fields have spatial anomalies over finite bodies of fixed conductivity that are independent of frequency and have no associated phase anomaly. If the body is polarizable, the electric field profile over the body becomes frequency dependent and phase shifted with respect to a reference field. The technique was tested on data acquired in a standard continuous profiling magnetotelluric (MT) survey over a strong induced polarization (IP) anomaly previously mapped with a conventional pole‐dipole IP survey. The extracted IP response appears in both the apparent resistivity and the normalized electric field profiles.
Nonlinear conjugate gradients algorithm for 2-D magnetotelluric inversion
Authors: Rodi William , Mackie Randall L.
Published: October 2002

We investigate a new algorithm for computing regularized solutions of the 2-D magnetotelluric inverse problem. The algorithm employs a nonlinear conjugate gradients (NLCG) scheme to minimize an objective function that penalizes data residuals and second spatial derivatives of resistivity. We compare this algorithm theoretically and numerically to two previous algorithms for constructing such “minimum‐structure” models: the Gauss‐Newton method, which solves a sequence of linearized inverse problems and has been the standard approach to nonlinear inversion in geophysics, and an algorithm due to Mackie and Madden, which solves a sequence of linearized inverse problems incompletely using a (linear) conjugate gradients technique. Numerical experiments involving synthetic and field data indicate that the two algorithms based on conjugate gradients (NLCG and Mackie‐Madden) are more efficient than the Gauss‐Newton algorithm in terms of both computer memory requirements and CPU time needed to find accurate solutions to problems of realistic size. This owes largely to the fact that the conjugate gradients‐based algorithms avoid two computationally intensive tasks that are performed at each step of a Gauss‐Newton iteration: calculation of the full Jacobian matrix of the forward modeling operator, and complete solution of a linear system on the model space. The numerical tests also show that the Mackie‐Madden algorithm reduces the objective function more quickly than our new NLCG algorithm in the early stages of minimization, but NLCG is more effective in the later computations. To help understand these results, we describe the Mackie‐Madden and new NLCG algorithms in detail and couch each as a special case of a more general conjugate gradients scheme for nonlinear inversion.
Seismic anisotropy
Authors: Thomsen Leon
Published: October 2002

The topic of seismic anisotropy in exploration and exploitation has seen a great deal of progress in the past decade‐and‐a‐half. The principal reason for this is the increased (and increasing) quality of seismic data, of the processing done to it, and of the interpretation expected from it. No longer an academic subject of little practical interest, it is now often viewed as one of the crucial factors which, if not taken into account, severely hampers our effective use of the data. The following brief overview is not intended to be exhaustive, since any such attempt would surely be incomplete. However, it does provide a high‐level survey of the advances seen (at the end of this period) to be important by one who was closely involved, and it directly extrapolates this history to predict the future development of the topic.
Prior information, sampling distributions, and the curse of dimensionality
Authors: Curtis Andrew , Lomax Anthony
Published: October 2002

This tutorial addresses geometrical issues that concern the specification of high‐dimensional sampling distributions in Bayesian inversion. We illustrate that simple, low‐dimensional geometrical concepts that are sometimes used to construct such distributions may become completely distorted (and even untrue) in higher dimensional problems. This has important implications for Bayesian inversion: if a convenient sampling distribution is constructed using low dimensional geometrical concepts which cause it to differ from the distribution representing our prior information, these differences can become extremely expensive to correct in higher dimensions. Indeed, they may make a nonlinear inversion computationally intractable when this need not be the case. A crucial factor in Bayesian inversion is, therefore, whether one firmly believes in a particular prior distribution. If so, this distribution may constitute the most efficient sampling distribution, even in cases where it is not straight‐forward to draw samples from that prior distribution. The sampling artifacts described above then become irrelevant since they represent true prior beliefs.
Prior information and uncertainty in inverse problems
Authors: Scales John A. , Tenorio Luis
Published: October 2002

Solving any inverse problem requires understanding the uncertainties in the data to know what it means to fit the data. We also need methods to incorporate data‐independent prior information to eliminate unreasonable models that fit the data. Both of these issues involve subtle choices that may significantly influence the results of inverse calculations. The specification of prior information is especially controversial. How does one quantify information? What does it mean to know something about a parameter a priori? In this tutorial we discuss Bayesian and frequentist methodologies that can be used to incorporate information into inverse calculations. In particular we show that apparently conservative Bayesian choices, such as representing interval constraints by uniform probabilities (as is commonly done when using genetic algorithms, for example) may lead to artificially small uncertainties. We also describe tools from statistical decision theory that can be used to characterize the performance of inversion algorithms.
AVO effects of a hydrocarbon source‐rock layer
Authors: Carcione José M.
Published: October 2002

The organic content of petroleum source‐rock layers can be determined from seismic data with a suitable AVO technique. Since the source rock has a layered structure, the best forward model is a transversely isotropic layer between two isotropic half‐spaces, parameterized by the layer thickness and the amount of organic material. This limited number of parameters is essential to obtain a robust inversion method. Here, I compute the PP- and PS-reflection coefficients of the source bed as a function of layer thickness and organic content, including attenuation mechanisms related to different stages of maturation. The source rock is modeled as a viscoelastic medium composed of illite/smectite, solid organic matter (kerogen), and fluids (oil and water). The properties of the kerogen/oil/water mixture are obtained with the Kuster and Toksöz model, assuming that oil and water are the inclusion in a kerogen matrix. Then, Backus averaging gives the complex stiffnesses of the layer. To derive the reflection coefficients, I relate the particle velocities and stresses on the top and bottom interfaces between the layer and the substrates. The results indicate that there is ambiguity regarding the layer thickness, since two or more values can have similar reflection coefficients. For instance, for moderate offsets and 25% kerogen content, the reflection‐coefficient curves oscillate with a period of 70 m for PP-waves and 40 m for PS-waves. On the other hand, the reflection coefficients have a mimimum at approximately 5% kerogen content and increase monotonically above this value. Therefore, the inversion may provide reliable values above approximately 10% kerogen content.