bluuebunny/crossref_metadata_2025 huggingface dataset:
Controlled source tensor magnetotellurics
Authors: Li Xiaobo , Pedersen Laust B.
Published: October 2002

Impedance tensor and tipper vectors, known to connect the electromagnetic surface components for plane‐wave excitation, are shown to be uniquely defined for horizontal electric or horizontal magnetic dipole sources. Two independent source polarizations are needed for their estimation in practice. Analysis of impedance tensors and tipper vectors for a layered half‐space shows that the impedance tensor can be antidiagonalized by rotating the measurement system so that one of the measurement directions coincides with the direction to the transmitter dipole. The tipper vector points towards the transmitter dipole. In the static limit, all transfer functions are real, and simple analytic results for a homogeneous half‐space show that impedance elements are proportional to the inverse of the product of conductivity and distance between source and receiver, while the tipper vector is a unit vector pointing towards the transmitter dipole.
Depth filtering for one‐component seismic data
Authors: Sun Robert , McMechan George A.
Published: October 2002

The concept of downward continuation of a seismic wavefield recorded on the earth’s surface to remove near‐surface effects has previously been applied by a number of authors including Schultz and Sherwood (1980), Berryhill (1979, 1984), and McMechan and Chen (1990). Recently, McMechan and Sun (1991) demonstrated, using synthetic elastic data, that downward continuation of an elastic (two‐component) seismic wavefield separates various seismic waves, based on their depth of propagation. This was used to simultaneously remove direct waves and ground roll. The direct compressional and shear waves and the ground roll get left behind in the near surface during downward continuation; subsequent upward continuation reconstructs the surface‐recorded wavefield without the waves propagating in the shallow layers.
Characteristics and processing of seismic data collected on thick, floating ice: Results from the Ross Ice Shelf, Antarctica
Authors: Beaudoin Bruce C. , ten Brink Uri S. , Stern Tim A.
Published: October 2002

Coincident reflection and refraction data, collected in the austral summer of 1988/89 by Stanford University and the Geophysical Division of the Department of Scientific and Industrial Research, New Zealand, imaged the crust beneath the Ross Ice Shelf, Antarctica. The Ross Ice Shelf is a unique acquisition environment for seismic reflection profiling because of its thick, floating ice cover. The ice shelf velocity structure is multilayered with a high velocity‐gradient firn layer constituting the upper 50 to 100 m. This near surface firn layer influences the data character by amplifying and frequency modulating the incoming wavefield. In addition, the ice‐water column introduces pervasive, high energy seafloor, intra‐ice, and intra‐water multiples that have moveout velocities similar to the expected subseafloor primary velocities. Successful removal of these high energy multiples relies on predictive deconvolution, inverse velocity stack filtering, and frequency filtering. Removal of the multiples reveals a faulted, sedimentary wedge which is truncated at or near the seafloor. Beneath this wedge the reflection character is diffractive to a two‐way traveltime of ∼7.2 s. At this time, a prominent reflection is evident on the southeast end of the reflection profile. This reflection is interpreted as Moho indicating that the crust is ∼21-km thick beneath the profile. These results provide seismic evidence that the extensional features observed in the Ross Sea region of the Ross Embayment extend beneath the Ross Ice Shelf.
Normal moveout stretch mute on shallow‐reflection data
Authors: Miller Richard D.
Published: October 2002

This paper demonstrates the potential consequences of overlooking the significance of allowable stretch ratio when performing normal moveout corrections on shallow‐reflection data. Two shallow reflection data sets with drastically different near‐surface geologic settings conclusively show the significance of subtle changes in allowable stretch mute. An improper stretch mute can reduce the dominant frequency and bandwidth of a stacked reflection by as much as 50 Hz. The sensitivity of shallow reflections to offset may require allowable stretch selection accuracy to be within ± 1 percent. It may be necessary to reduce the fold of an individual stacked shallow reflection by as much as 60 percent to avoid excessive degradation of wavelet properties and consequent loss of resolution. A proper normal moveout stretch mute can reduce distortion of reflection wavelet spectra caused by nonvertical incidence recording to less than 10 percent. Stretched reflection wavelets improperly muted can be misinterpreted on CDP stacked sections as stacked refractions or subtle stratigraphic features.
Evaluation of terrain effects in AEM surveys using the boundary element method
Authors: Liu Guimin , Becker Alex
Published: October 2002

In mountainous areas, electromagnetic terrain effects are readily observed in the course of VLF (14–20 kHz) measurements made on the surface and constitute a serious source of geological noise that affects the collected data. One may, therefore, inquire whether similar effects will be observed during the course of conventional helicopter‐towed electromagnetic (HEM) surveys as the frequency of the newer systems is increased beyond the lower regions of the audio range. To answer the question, we have evaluated the terrain effects that would be observed with a conventional HEM system in a number of simple cases. The operating frequency chosen for most of the numerical simulations was 8 kHz, while the topographic features investigated were taken to be two‐dimensional. The calculations were done using the boundary element method of solving the appropriate integral equations. Accuracy of the numerical solutions was shown to vary from 1 percent for a half space to 10 percent for a shallow valley where the verification was done on a laboratory scale model. For the models investigated, the amplitude of the computed secondary fields shows a distinct correlation with the overflown topography. Surprisingly, however, the phase of the secondary field remains invariant and so may be reliably used to compute the resistivity of the terrain below the aircraft.
Direct detection using AVO, Central Graben, North Sea
Authors: Snyder Allan G. , Wrolstad Keith H.
Published: October 2002

An amplitude versus offset (AVO) study was undertaken for Lines ECL-1 and -2 and Well A of the United Kingdom North Sea to determine if variations in fluid saturation can be detected in Eocene sand mounds. These features are a major play in this area. The Eocene sands contain gas and oil in Well A. Elastic modeling using the well data was done to match the seismic common midpoint (CMP) gather. Target zone fluid saturations in the model were then altered to investigate changes in AVO and stacked trace response. The model and field data were processed using the same processing steps and parameters. Seismic offset dependent amplitude stack (SODAS), a color display system, was used to display the AVO results for the field and model data. It was found that gas, oil, and water sands could be distinguished from each other, though full and partial gas saturation were indistinguishable. Clay content, porosity, and multiple reflections also had important effects on the AVO response. On the basis of the hydrocarbon indicators that were investigated we then interpreted the hydrocarbon limits on Line ECL-1 with well control and evaluated Line ECL-2, which has an undrilled prospect. From our analysis, we concluded that waterbearing sands were most likely present in the prospect area on Line ECL-2, though the data is rather inconclusive.
Suppression of multiple reflections using the Radon transform
Authors: Foster Douglas J. , Mosher Charles C.
Published: October 2002

Multiple suppression using a variant of the Radon transform is discussed. This transform differs from the classical Radon transform in that the integration surfaces are hyperbolic rather than planar. This specific hyperbolic surface is equivalent to parabolae in terms of computational expense but more accurately distinguishes multiples from primary reflections. The forward transform separates seismic arrivals by their differences in traveltime moveout. Multiples can be suppressed by an inverse transform of only part of the data. Examples show that multiples are effectively attenuated in prestack and stacked seismograms.
Anisotropy in sedimentary rocks modeled as random media
Authors: Kerner Claudia
Published: October 2002

The anisotropic behavior to be expected from various types of sediments is investigated by considering them as laminated media, with randomly varying velocity depth distributions. Two different stochastic processes are used to model transitional and cyclic layering. The kinematics of waves propagating through the laminated media is studied by evaluating overall elastic parameters of the transversely isotropic medium in the long wavelength limit using averaging techniques. Models with strong velocity fluctuations and high correlation between P‐ and S‐wave velocities exhibit significant anisotropy, comparable in magnitude to field and laboratory measurements. Elastic wavefields for the stochastic models were computed and the results were compared with analytical and numerical results for homogeneous anisotropic media computed with the derived overall parameters. The wavefield modeling shows that anisotropy and scattering are not simply effects influencing waves on the opposite ends of the wavelength scale but that there is an intermediate range where both effects profoundly influence wave propagation.
A scattered equivalent‐source method for interpolation and gridding of potential‐field data in three dimensions
Authors: Cordell Lindrith
Published: October 2002

Potential‐field geophysical data observed at scattered discrete points in three dimensions can be interpolated (gridded, for example, onto a level surface) by relating the point data to a continuous function of equivalent discrete point sources. The function used here is the inverse‐distance Newtonian potential. The sources, located beneath some of the data points at a depth proportional to distance to the nearest neighboring data point, are determined iteratively. Areas of no data are filled by minimum curvature. For two‐dimensional (2-D) data (all data points at the same elevation), grids calculated by minimum curvature and by equivalent sources are similar, but the equivalent‐source method can be tuned to reduce aliasing. Gravity data in an area of high topographic relief in southwest U.S.A. were gridded by minimum curvature (a 2-D algorithm) and also by equivalent sources (3-D). The minimum‐curvature grid shows strong correlation with topography, as expected, because variation in gravity effect due to variation in observation‐point elevation (topography) is ignored. However, the data gridded and reduced to a level surface at the mean observation‐point elevation, by means of the equivalent‐source method, also show strong correlation with topography even though variation in observation‐point elevation is accounted for. This can be attributed mostly to the inadequacy of constant‐density terrain correction or to data error. Three‐dimensional treatment in this example is required as a means of calculating the data onto a level surface, above regions where data and geologic sources overlap, as a necessary first step for making geologic correction, variable‐density terrain correction, and evaluating data error. Better spectral estimates are obtained by direct calculation of the Fourier transform of the equivalent‐source function than by the discrete fast Fourier transform computer algorithm.
Attenuation mechanisms in sands: Laboratory versus theoretical (Biot) data
Authors: Prasad Manika , Meissner Rolf
Published: October 2002

The velocity and attenuation of compressional([Formula: see text], [Formula: see text] respectively) and shear waves ([Formula: see text], [Formula: see text], respectively), determined with the Pulse Transmission technique at a frequency of about 100 kHz, are compared with the grain size, shape, porosity, density, and static frame compressibility of dry and water‐saturated sands. Except for [Formula: see text], all the quantities [Formula: see text], [Formula: see text] and [Formula: see text] are dependent on grain size and are higher in coarser grains than in finer grains. [Formula: see text] decreases significantly with increasing differential pressure in coarse‐grained sediments, but the same sediments show an anomalous increase with differential pressure in [Formula: see text] at low pressures. We have also modeled the [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] of these samples to understand the mechanisms governing the observed changes. The Contact Radius model with surface force effects predicts both [Formula: see text] and [Formula: see text] to be dependent on grain size. Frictional losses in unconsolidated coarse‐grained sands must also be considered at small strains [Formula: see text]. Velocity and losses measured in saturated sands are higher than those predicted by the Biot model, which does not account for any grain size dependence of the seismic qualities.
The dynamics of viscous compressible fluid in a fracture
Authors: Dvorkin Jack , Mavko Gary , Nur Amos
Published: October 2002

We solve a problem of fluid dynamics in an arbitrarily shaped fracture containing proppant. The fracture is embedded in an absolutely rigid solid. The fluid flow is induced by normal harmonic oscillations of fracture walls that are permeable, allowing the fluid to filtrate into the surrounding formation. The amplitude of oscillations can be described as a function of a spatial coordinate along the fracture. Fluid flow inside the fracture is described by Biot’s modification of Darcy’s law that includes both inertia and filtration terms. Filtration inside the fracture may drastically change its acoustic characteristics. The acoustic resonance along the length of the fracture filled with proppant can be observed only if the permeability of the proppant is extremely high; otherwise filtration of the fluid in the fracture dampens the resonance. If the resonance is observed, its period differs from that in a fracture without internal filtration. The apparent acoustic velocity in the fracture is lower than sound velocity in the fluid because of coupling between the fluid and the proppant. In a fracture without proppant, fluid filtration into the surrounding rock acts to decrease resonance peaks and to increase resonance period. Resonance period changes significantly in fractures of varying shape, an effect which is important when estimating the size of a tapered fracture.
Surface‐consistent deconvolution in the log/Fourier domain
Authors: Cambois Guillaume , Stoffa Paul L.
Published: October 2002

In the surface‐consistent hypothesis, a seismic trace is the convolution of a source operator, a receiver operator, a reflectivity operator (representing the subsurface structure) and an offset‐related operator. In the log/Fourier domain, convolutions become sums and the log of signal amplitude at a given frequency is the sum of source, receiver, structural, and offset‐related terms. Recovering the amplitude of the reflectivity for a given frequency is then a linear problem (very similar to a surface‐consistent static correction problem). However, this linear system is underconstrained. Thus, among the infinite number of possible solutions, a particular one must be selected. Studies with real data support the choice of a spatially band‐limited solution. The surface‐consistent operators can then be calculated very efficiently using an inverse Hessian method. Applications to real seismic data show improvement compared with previous techniques. Surface‐consistent deconvolution is robust and fast in the log/Fourier domain. It allows the use of long operators, improves statics estimation, and removes the amplitude variations due to source or receiver coupling.
International Geomagnetic Reference Field, 1991 Revision: International Association of Geomagnetism and Aeronomy (IAGA) Division V, Working Group 8: Analysis of the main field and secular variation
Authors: Langel R. A.
Published: October 2002

The International Geomagnetic Reference Field (IGRF) is a series of mathematical models of the main geomagnetic field and its secular variation. Each model consists of a set of spherical harmonic (or Gauss) coefficients, g and h in a series expansion of the geomagnetic potential [Formula: see text], where a is the mean radius of the Earth (6371.2 km); r the radial distance from the center of the Earth; ϕ the east longitude measured from Greenwich; θ the geocentric colatitude; and [Formula: see text] the associated Legendre function of degree n and order m, normalized according to the convention of Schmidt [see, e.g., Langel (1987)]. In principle, N should be ∞ but the Working Group is of the opinion that in practice the available data for most epochs do not justify N greater than 10. This value is chosen to maintain consistency between models at different epochs. The coefficients are in units of nanotesla (nT).
Symmetry properties of the scattering matrix in 3-D electromagnetic modeling using the integral equation method
Authors: Xiong Zonghou
Published: October 2002

Modeling large three‐dimensional (3-D) earth conductivity structures continues to pose challenges. Although the theories of electromagnetic modeling are well understood, the basic computational problems are practical, involving the quadratically growing requirements on computer storage and cubically growing computation time with the number of cells required to discretize the modeling body.
Tensor CSAMT studies at the Buchans Mine in central Newfoundland
Authors: Boerner D. E. , Wright J. A. , Thurlow J. G. , Reed L. E.
Published: October 2002

A novel application of the tensor controlled source audio‐magnetotelluric (CSAMT) method was part of a multidisciplinary geophysical study of an existing mine site at Buchans, Newfoundland. The orthogonal components of the horizontal electromagnetic fields used for magnetotelluric and CSAMT interpretation of the earth’s conductivity structure were found to be inappropriate at Buchans because of strong scattering in the electric fields. Instead, the length of the major axes of the electric and magnetic field polarization ellipses and the vertical magnetic field were used as data. The data from two bipole sources demonstrate that the bulk response of the earth in the vicinity of Buchans is predominantly one‐dimensional (1-D). These data were inverted to layered earth models with a first‐order correction for electric field distortions. The parameter space considered during the inversion was contracted substantially by incorporating the vertical magnetic field data and by using depths to interfaces as determined by reflection seismic data. The model resulting from the inversions is essentially a two‐layered earth with an increase in resistivity between 1000–1400 m depth. The contrast in the electrical properties is interpreted to be coincident with the Powerline Fault, a floor thrust of a duplex structure with significant out‐of‐sequence movement. Hence, the thrusting may have caused the emplacement of older fractured, and locally mineralized rocks over younger more competent (resistive) ones.
Mapping and depth ordering of residual gravity sources
Authors: Beltrão Jacira F. , Silva João B. C.
Published: October 2002

Apparent density maps are derived from observed residual Bouguer maps under the assumptions that the sources are restricted to a depth interval and that the density distribution is not a function of the vertical coordinate. If the depth interval containing the sources is known, the computed apparent density maps are very close to the true density distributions. Even when the depth interval of the sources is not known, but the sources have constant density, their horizontal extent may be delineated by the half‐maximum contours of the apparent densities. We developed an interpretation method using a family of curves based on source density, depth to top of sources, and source thickness. The main advantage in the use of these families of curves is the broad range of possible interpretations involving the above‐mentioned parameters. Rough semiquantitative interpretations are possible in the absence of any further a priori information; more refined semiquantitative interpretations require additional quantitative knowledge of one parameter and just a lower or upper bound for another parameter. Finally, quantitative interpretations are possible only with additional quantitative knowledge of two of the three parameters involved.
Magnetotelluric analysis using Mohr circles
Authors: Lilley F. E. M.
Published: October 2002

The Mohr circle, most commonly met in the analysis of mechanical stress, is used to depict magnetotelluric impedance information, taking the real and quadrature parts of magnetotelluric tensors separately. The magnetotelluric concepts of two‐dimensionality, three‐dimensionality, skew and anisotropy are then all given quantitative expression on a diagram, as are various magnetotelluric invariants. In particular, a new invariant, the “central impedance,” becomes evident in a discussion of effective impedances. Some insight is gained into impedance rotations, and an anisotropy angle is defined, analogous to skew angle. Mohr circles are also tested to depict the effects of the shear and twist operations on a regionally two‐dimensional structure. Generally, the application of shear or twist results in an impedance tensor with a Mohr circle of typical three‐dimensional form.
DMO velocity analysis with Jacubowicz’s dip‐decomposition method
Authors: Kessler David , Chan Wai‐Kin
Published: October 2002

Dip‐moveout (DMO) velocity analysis (VA) may be performed in several ways. Using the Fourier transform‐based DMO techniques (Hale 1984, Notfors and Godfrey 1987, Liner and Bleinstein 1988), VA is done iteratively where a sequence of VA, normal moveout (NMO), DMO, inverse NMO, and a second VA yields an estimate of the DMO velocities. Using an integral method for application of a DMO process, Forel and Gardner (1988) proposed a way for performing VA by transforming the data into the [Formula: see text] domain, where DMO velocities are obtained by any common VA technique. In another work, carried out by Chon and Gonzalez (1987) a velocity‐sensitivity analysis was added to a Kirchhoff integral DMO algorithm.
Radar signature of a 2.5-D tunnel
Authors: Moran Mark L. , Greenfield Roy J.
Published: October 2002

The effects of an infinitely long cylindrical void on short‐pulse cross‐borehole radar waveforms are modeled and analyzed. Pulsed electromagnetic sensing system (PEMSS) data are of particular interest. The PEMSS system developed by the Southwest Research Institute uses a vertically oriented electric dipole that emits a short electromagnetic pulse with peak power output centered around 30 MHz, which gives wavelengths of roughly 1.5 cavity diameters. The transmitter and receiver are typically located in boreholes separated by approximately 30 m. The model is based on field solutions for a vertically oriented point‐source electric dipole. A three‐dimensional (3-D) analytical frequency domain derivation of the Green’s function is found using a spatial Fourier transform over the cylinder axis. The resulting wavenumber integral is evaluated by a numerical integration over wavenumber. Time‐domain waveforms are produced by applying a Fourier transform to a 7-80 MHz band of frequencies in the Green’s function spectrum. Model results agree well with PEMSS field data sets. Further modeling examines the effects on waveforms for a wide variety of cases in which the raypath is not orthogonal to the tunnel axis, including the effect of tunnel dip. An air‐filled tunnel with a radius greater than 1.0 m produces a low amplitude shadow zone along its entire length. A low amplitude early arrival is observed in simulations with air‐filled tunnels in which the source to receiver path forms an acute angle larger than 45 degrees with the tunnel axis. This arrival is interpreted as propagation through the tunnel. When this angle is smaller than 45 degree the tunnel is effectively an opaque object and only the energy diffracted around the cylindrical void is observed. Waveform behavior gradually transitions from propagation through the tunnel in the vicinity of 45 degrees.
Geophysical evaluation of sandstone aquifers in the Recôncavo‐Tucano Basin, Bahia—Brazil
Authors: de Lima Olivar A. L.
Published: October 2002

The upper clastic sediments in the Recôncavo‐Tucano basin comprise a multilayer aquifer system of Jurassic age. Its groundwater is normally fresh down to depths of more than 1000 m. Locally, however, there are zones producing high salinity or sulfur geothermal waters. In other areas, the system is being exploited heavily to support a petrochemical center and related industries. Analysis of electrical logs of more than 150 wells enabled the identification of the most typical sedimentary structures and the gross geometries for the sandstone units in selected areas of the basin. Based on this information, the thick sands are interpreted as coalescent point bars and the shales as flood plain deposits of a large fluvial environment. The resistivity logs and core laboratory data are combined to develop empirical equations relating aquifer porosity and permeability to log‐derived parameters such as formation factor and cementation exponent. Temperature logs of 15 wells were useful to quantify the water leakage through semiconfining shales. The groundwater quality was inferred from spontaneous potential (SP) log deflections under control of chemical analysis of water samples. An empirical chart is developed that relates the SP‐derived water resistivity to the true water resistivity within the formations. The patterns of salinity variation with depth inferred from SP logs were helpful in identifying subsurface flows along major fault zones, where extensive mixing of water is taking place. A total of 49 vertical Schlumberger resistivity soundings aid in defining aquifer structures and in extrapolating the log derived results. Transition zones between fresh and saline waters have also been detected based on a combination of logging and surface sounding data. Ionic filtering by water leakage across regional shales, local convection and mixing along major faults and hydrodynamic dispersion away from lateral permeability contrasts are the main mechanisms controlling the observed distributions of salinity and temperature within the basin.
Computing the Fourier transform in geophysics with the transform decomposition DFT
Authors: Reed Michael J. , Nguyen Hung V. , Chambers Ronald E.
Published: October 2002

The Fourier transform and its computationally efficient discrete implementation, the fast Fourier transform (FFT), are omnipresent in geophysical processing. While a general implementation of the discrete Fourier transform (DFT) will take on the order [Formula: see text] operations to compute the transform of an N point sequence, the FFT algorithm accomplishes the DFT with an operation count proportional to [Formula: see text] When a large percentage of the output coefficients of the transform are not desired, or a majority of the inputs to the transform are zero, it is possible to further reduce the computation required to perform the DFT. Here, we review one possible approach to accomplishing this reduction and indicate its application to phase‐shift migration.
Tomographic reconstruction of near‐borehole slowness using refracted borehole sonic arrivals
Authors: Hornby Brian E.
Published: October 2002

Two‐dimensional (2-D) reconstructions of the near‐borehole slowness field are computed using arrival times of refracted borehole sonic arrivals. First‐arrival traveltimes, derived from both computer simulations and field data from full‐waveform sonic tools, were inverted for the near‐borehole formation slowness both axially along the borehole and radially away from the borehole. The inversion is nonlinear; the solution is obtained by means of a series of linear inversions followed by provisional ray tracings. Each iteration involves the application of a tomographic reconstruction algorithm similar to those used in seismic crosswell tomography or medical imaging applications. The technique was demonstrated using ray‐theoretic examples to simulate radial variations in slowness. In addition, full‐waveforms were generated using two‐and‐a‐half‐dimensional (2.5-D) FDM computer models. The finite‐difference method (FDM) computer models were used to test the validity of the ray‐theoretic approximation used in the inversion scheme and to simulate the full‐waveform sonic tool response for both radial and axial changes in formation properties. Field data examples highlighted radial changes in formation slowness caused by two separate mechanisms: water take up by swelling shales and the mechanical breakdown of the near‐borehole rock resulting from stress relief caused by the drilling process. Finally, refracted sonic arrivals from near‐borehole bed boundaries were identified in a horizontal well setting. Using refractions arriving beyond the headwave, a 2-D map of formation slowness was computed in the reservoir away from the borehole. Interpretation of the slowness map resulted in an estimation of the stand‐off of the horizontal borehole from the reservoir boundary.
The effects of source polarization in CSAMT data over two massive sulfide deposits in Australia
Authors: Kellett Richard , Bishop John , Van Reed Emmett
Published: October 2002

Since the advent of the controlled‐source audio‐magnetotelluric method it has been recognized that the location and orientation of the bipole source is important in determining the response of the earth at the receiver. In this study, two‐dimensional (2-D) far‐field modeling has been used to illustrate the frequency-domain electromagnetic response of a simple conductive dike for two orthogonal polarizations of the source field. The current gathered from the surrounding media by the dike, when the electric field is parallel to the strike direction (E‐polarization), produces a strong anomaly not seen in the perpendicular H‐polarization. This model response has been identified in real data sets over the Rosebery and Flying Doctor orebodies of eastern Australia. In the case of Rosebery the E‐polarization data yielded little structural information because the penetration of the signal was reduced by strong current channeling in the orebody and adjacent black shales. At the Flying Doctor prospect the model predictions held but changes in the extent of the near‐field zone, for the two bipole locations, dominate the data. The changes in the source field observed over the Flying Doctor prospect are interpreted as evidence for anisotropy in the regional resistivity structure. The controlled‐source is a fundamental component of the CSAMT system, and the choice of the bipole location and orientation must be made considering the geology of the target region and the surrounding regional resistivity structure.
A least‐squares minimization approach to depth determination from moving average residual gravity anomalies
Authors: Abdelrahman El‐Sayed M. , El‐Araby Tarek M.
Published: October 2002

We have developed a least‐squares minimization method to estimate the depth of a buried structure from moving average residual gravity anomalies. The method involves fitting simple models convolved with the same moving average filter as applied to the observed gravity data. As a result, our method can be applied not only to residuals but also to the Bouguer gravity data of a short profile length. The method is applied to synthetic data with and without random errors. The validity of the method is tested in detail on two field examples from the United States and Senegal.
Shape and depth solutions from gravity data using correlation factors between successive least‐squares residuals
Authors: Abdelrahman El‐Sayed M. , El‐Araby Hesham M.
Published: October 2002

The gravity anomaly expression produced by most geologic structures can be represented by a continuous function in both shape (shape factor) and depth variables with an amplitude coefficient related to the mass. Correlation factors between successive least‐squares residual gravity anomalies from a buried vertical cylinder, horizontal cylinder, and sphere are used to determine the shape and depth of the buried geologic structure. For each shape factor value, the depth is determined automatically from the correlation value. The computed depths are plotted against the shape factor representing a continuous correlation curve. The solution for the shape and depth of the buried structure is read at the common intersection of correlation curves. This method can be applied to a Bouguer anomaly profile consisting of a residual component caused by local structure and a regional component. This is a powerful technique for automatically separating the Bouguer data into residual and regional polynomial components. This method is tested on theoretical examples and a field example. In both cases, the results obtained are in good agreement with drilling results.
Static image principle for anisotropic‐conducting half‐space problems: PEC and PMC boundaries
Authors: Lindell Ismo V. , Ermutlu Murat E. , Nikoskinen Keijo I. , Eloranta Esko H.
Published: October 2002

The image principle for an isotropic half‐space bounded by perfect electric conductor (PEC) or perfect magnetic conductor (PMC) plane is presented in most elementary textbooks on electromagnetics. It is perhaps not so well known that this principle can also be generalized to anisotropic media in the static case, because it is not covered in leading monographs of geoelectromagnetics (Wait, 1982; Negi and Saraf, 1989; Eskola, 1992). The anisotropic image method can be applied to geologic media that exhibit anisotropic electrical conductivity caused by the fractures and fissures in the rock. Such structures are important in the sites for disposal of nuclear waste. The characterization of these structures by electrical geophysical methods is very essential because they form the main paths for groundwater flow. The air‐ground boundary can be treated as a PMC plane representing the nonconducting medium. Otherwise the medium is assumed to be linear (ohmic) and homogeneous in terms of electrical conductivity. The image method presented is also relevant to problems arising in the traditional ore prospecting where a conducting ore body buried in an electrically anisotropic host rock generates secondary electric fields (Asten, 1974; Eloranta, 1988).
Stability versus accuracy for an explicit wavefield extrapolation operator
Authors: Nautiyal Atul , Gray Samuel H. , Whitmore N. D. , Garing John D.
Published: October 2002

Wavefield extrapolation by recursive (depth‐by‐ depth) application of a convolutional operator in the frequency‐space domain, commonly used for depth migration in a laterally‐varying earth, has interesting accuracy and stability properties. We analyze these properties by investigating the operator and its spatial Fourier transform. In particular, we show that the instability caused by spatially truncating the operator can be remedied unconditionally by applying an appropriately chosen spatial taper. However, unconditional stability is gained only at the expense of accuracy. We also identify frequencies and depth extrapolation step sizes for which the problems of accuracy or stability are the most pronounced.
Time‐domain magnetization of soils (VRM), experimental relationship to quadrature susceptibility
Authors: Dabas Michel , Skinner John R.
Published: October 2002

A comparison is made between measurements with a multichannel bipolar pulsed‐induction meter (PIM) and measurements made with a phase‐sensitive susceptibility bridge on more than 211 soil samples from a variety of archaeological sites in Europe. Eddy currents originating from the decay of viscous remanent magnetization (VRM) were measured with the PIM. The in‐phase and out‐of‐phase [Formula: see text] magnetic mass susceptibilies were measured with the susceptibility bridge. Viscous remanent magnetization was measured through eight time sampling intervals on the pulsed‐induction meter, and the linearity of VRM for selected samples was tested at various field strengths (200 to 2300 μT). It is believed that this extension of the reversible range indicates a high coercive strength of the magnetic grains in the soil samples involved. Whatever the origin of the soil sample, a constant decay of VRM with time was observed between 56 and 417 μs, after switching off the inducing field. This relation is shown to be a power law with exponent −1.4. This roughly agrees with a theoretical model that takes into account a finite‐energizing time of the transmitter pulse and single‐domain (SD) behavior of particles. Finally, a comparison is made between VRM and quadrature susceptibility [Formula: see text]. This comparison is in accordance with well dispersed single‐domain theory, and was found to be a linear relationship.
“Single bubble” air‐gun array for deep exploration
Authors: Avedik F. , Renard V. , Allenou J. P. , Morvan B.
Published: October 2002

Large tuned air‐gun arrays operated in off‐shore petroleum exploration are also used for deep penetration marine seismic reflection surveys conducted to define structures in the earth’s crust. Because of the attenuation of higher frequencies, the useful upper frequency limit of these records is usually about 50–60 Hz. The aim of this paper is to report on a method of seismic pulse generation that preferentially concentrates the air gun’s energy in the low range of the seismic frequency band by centering the output on the first “bubble pulse” instead of the initial (primary) pulse. Experimental results show that, due to the increased low‐frequency energy content of this “single bubble” pulse, air‐gun arrays considerably reduced both in size and volume can generate the necessary acoustic energy for deep seismic exploration.
Wave propagation effects on amplitude variation with offset measurements: A modeling study
Authors: Martinez Ruben D.
Published: October 2002

Wave propagation effects can significantly affect amplitude variation with offset (AVO) measurements. These effects include spreading losses, transmission losses, interbed multiples, surface multiple reflections, P‐SV mode converted waves and inelastic attenuation. Examination of prestack elastic synthetic seismograms suggests that spreading losses and the transmission losses plus compressional interbed multiples are manifest mainly as a time and offset effect on the primary reflections. The surface related multiples and the P‐SV mode‐converted waves interfere with prestack amplitudes inducing distortions in the AVO pattern. Such distortions cause large variances in AVO model fitting. Prestack viscoelastic synthetic seismograms also suggest that inelastic attenuation further complicates the AVO response because of the offset and time variant amplitude decay effects and the phase change due to dispersion. Together, all these effects severely alter AVO behavior and result in serious errors in AVO parameter estimates being made from inadequately corrected seismograms. This modeling study suggests that time and offset dependent data processing prior to AVO analysis would be necessary to correct for the wave propagation effects, via either inverse filtering or model based approaches. Comparisons between acoustic and elastic synthetic seismograms show that corrections for the wave propagation effects derived using acoustic approximations are inadequate. Corrections need to be calculated based on elastic approximations provided that the inelastic attenuation effects have been previously removed.
The straight‐slope method for basement depth determination revisited
Authors: Skilbrei Jan Reidar
Published: October 2002

The straight‐slope method is still popular for depth to magnetic source estimation due to its simplicity and general reliability in manual interpretation (e.g., Nettleton, 1976). Other commonly used manual slope methods are Peters rule (Peters, 1949) and Sokolov rule (Åm, 1972). The straight‐slope method uses the horizontal projection of the straightline part of the magnetic anomaly curve at the inflection point as the depth estimator (see Figure 1). Because no straight line exists mathematically, the rule is purely empirical, even though visually a certain part of a curve will appear to be straight.
Traveltime tomography with a spatial‐coherency filter
Authors: Zhou Hau‐wei
Published: October 2002

Spatially incoherent small wavelengths in a velocity tomogram are usually noise that could destabilize an iterative traveltime tomographic process. By defining the number of connected cells of similar values in an image as the spatial‐coherency (SC) value of these cells, an SC filtering is introduced in this paper to suppress incoherent features in an image. An SC filter first quantifies the SC value of all image cells and then conducts a discriminative moving‐average over cells of low SC values. It is demonstrated that such an SC filtering of the intermediate‐stage tomograms may enhance the robustness of the iterative tomographic process to noise. The SC values can be used in general to deconvolve an image of any dimension into components of different lengths in spatial connectivity.
Effects of transverse isotropy on P‐wave AVO for gas sands
Authors: Kim Ki Young , Wrolstad Keith H. , Aminzadeh Fred
Published: October 2002

Velocity anisotropy should be taken into account when analyzing the amplitude variation with offset (AVO) response of gas sands encased in shales. The anisotropic effects on the AVO of gas sands in transversely isotropic (TI) media are reviewed. Reflection coefficients in TI media are computed using a planewave formula based on ray theory. We present results of modeling special cases of exploration interest having positive reflectivity, near‐zero reflectivity, and negative reflectivity. The AVO reflectivity in anisotropic media can be decomposed into two parts; one for isotropy and the other for anisotropy. Zero‐offset reflectivity and Poisson’s ratio contrast are the most significant parameters for the isotropic component while the δ difference (Δδ) between shale and gas sand is the most important factor for the anisotropic component. For typical values of Tl anisotropy in shale (positive δ and ε), both δ difference (Δδ) and ε difference (Δε) amplify AVO effects. For small angles of incidence, Δδ plays an important role in AVO while Δε dominates for large angles of incidence. For typical values of δ and ε, the effects of anisotropy in shale are: (1) a more rapid increase in AVO for Class 3 and Class 2 gas sands, (2) a more rapid decrease in AVO for Class 1 gas sands, and (3) a shift in the offset of polarity reversal for some Class 1 and Class 2 gas sands.
Estimation of near‐surface elastic parameters using multicomponent seismic data
Authors: Ata E. Z. , Corrigan D. , McMechan G. A. , Gaiser J. E.
Published: October 2002

In multicomponent seismic reflection surveys, surface and near‐surface effects can severely deteriorate the quality of reflection data. Such effects are more pronounced on shear‐wave than on compressional‐wave profiles. Amplitude anomalies, statics, and strong coherent source‐generated noise (i.e., surface waves) are often associated with inhomogeneous, poorly compacted near‐surface sediments of the weathering layer. The magnitude of such effects increases when sources and receivers are deployed at or near the surface in proximity to such inhomogeneities. Near‐surface effects can be investigated with respect to their depth of occurrence by burying seismic sources and/or receivers at various depths below the inhomogeneous weathering layer. In this context, an experiment was conducted to collect multicomponent seismic field data on a borehole‐controlled test site in east Texas. Data were recorded on three‐component surface and buried receivers using a full‐vector wavefield surface source. Although the geology appears simple, results of modeling one or two components of the field data with synthetics yields nonunique estimates of elastic parameters. Effects of anisotropy and heterogeneities are better identified and estimated with full‐wavefield surface and buried receiver observations. Single fold data from buried receivers yield reflection events with higher signal‐to‐noise ratio than 30‐fold CDP surface data previously acquired in the same area.
Mixed‐phase wavelet estimation using fourth‐order cumulants
Authors: Lazear Gregory D.
Published: October 2002

In recent years methods have been developed in the field of high‐order statistics that can reliably estimate a mixed‐phase wavelet from the noisy output of a convolutional process. These methods use high‐order covariance functions of the data called cumulants, which retain phase information and allow recovery of the wavelet. The assumption is that the reflection coefficient series is a non‐Gaussian, stationary, and statistically independent random process. The method described in this paper uses the fourth‐order cumulant of the data, and a moving‐average, noncausal parametric model for the wavelet. The fourth‐order moment function of this wavelet matches the fourth‐order cumulant of the data in a minimum mean‐squared error sense. Numerical simulations demonstrate that the method can accurately estimate mixed‐phase wavelets, even when the reflectivity has a distribution close to Gaussian. Three seismic data examples demonstrate possible uses of the method. In the first two, an average source wavelet is estimated from marine shot records acquired with air gun and water gun sources, respectively. These wavelets compare favorably with recorded far‐field signatures modified for receiver ghosting. In the third example, a wavelet estimate from marine stacked data is used to correct the phase of the stacked section, resulting in zero‐phase water bottom and salt top reflections.
Orthogonality in CSAMT and MT measurements
Authors: Boerner David E. , Kurtz Ron D. , Jones Alan G.
Published: October 2002

The electric and magnetic fields from a single plane‐wave source on a one dimensional (1-D) earth, or a plane‐wave source polarized parallel or perpendicular to strike on a two-dimensional (2-D) earth, are orthogonal. On a layered earth and in the far‐field of a controlled source, the electric and magnetic fields are also orthogonal. Therefore, orthogonality of E and H data is a necessary condition to justify the application of 1-D or 2-D modeling algorithms having a plane wave source. A strict criterion to prove orthogonality, and thus provide a rationale for the choice of interpretation methods, can be defined directly in terms of field data. However, field data acquired in the intermediate and near‐field of any electromagnetic (EM) source are generally not orthogonal, even on a plane‐layered earth. Representing these nonorthogonal data in an orthogonal coordinate system can be misleading, particularly for the minor axis components of the polarization ellipses. Nonorthogonality also arises because of 3-D scattering, with one common example being the electric field response of near surface structure. An example of field data illustrates the nonorthogonality in CSAMT measurements caused by the response of surficial geology. In these EM data, the angle between E and H is a sensitive indicator of geological contacts and faults. Quantitative analysis of these data can be performed with the assumptions of a “bulk” 1-D earth (i.e., orthogonal E and H in the far‐field) and purely galvanic scattering of the EM fields.