bluuebunny/crossref_metadata_2025 huggingface dataset:
The common reflecting element (CRE) method revisited
Authors: Cruz J. C. R. , Hubral Peter , Tygel Martin , Schleicher Jörg , Höcht German
Published: October 2002

The common reflecting element (CRE) method is an interesting alternative to the familiar methods of common midpoint (CMP) stack or migration to zero offset (MZO). Like these two methods, the CRE method aims at constructing a stacked zero‐offset section from a set of constant‐offset sections. However, it requires no more knowledge about the generally laterally inhomogeneous subsurface model than the near‐surface values of the velocity field. In addition to being a tool to construct a stacked zero‐offset section, the CRE method simultaneously obtains information about the laterally inhomogeneous macrovelocity model. An important feature of the CRE method is that it does not suffer from pulse stretch. Moreover, it gives an alternative solution for conflicting dip problems. In the 1-D case, CRE is closely related to the optical stack. For the price of having to search for two data‐derived parameters instead of one, the CRE method provides important advantages over the conventional CMP stack. Its results are similar to those of the MZO process, which is commonly implemented as an NMO correction followed by a dip moveout (DMO) correction applied to the original constant‐offset section. The CRE method is based on 2-D kinematic considerations only and is not an amplitude‐preserving process.
A comparison of the correlation structure in GPR images of deltaic and barrier‐spit depositional environments
Authors: Tercier Paulette , Knight Rosemary , Jol Harry
Published: October 2002

We have used geostatistical analysis of radar reflections to quantify the correlation structures found in 2-D ground‐penetrating radar (GPR) images. We find that the experimental semivariogram, the product of the geostatistical analysis of the GPR data, is well‐defined and can be modeled using standard geostatistical models to obtain an estimate of the range or correlation length, and the maximum correlation direction, in the 2-D GPR image. When we compare the results from geostatistical analysis of GPR data from selected deltaic and barrier‐spit depositional environments we find different correlation structures in GPR images from different depositional environments. GPR images from braid deltas have near‐horizontal correlation directions and correlation lengths on the order of a few meters. In contrast, the GPR image of a fan‐foreset delta has a very long (>24 m) correlation length and a maximum correlation direction plunging 20°. In the GPR images from barrier spits, we find maximum correlation directions that are horizontal or plunging a few degrees. The correlation lengths range from 7 to 43 m, depending on the orientation of the GPR image relative to spit end growth, and on the specific radar facies that is analyzed.
Synthesis of multicomponent quasi-Pand converted quasi-P-Sseismograms for intersecting fracture systems
Authors: Ortega Andrey A. , McMechan George A.
Published: October 2002

Dynamic ray shooting with interpolation is an economical way of computing approximate Green’s functions in 3-D heterogeneous anisotropic media. The amplitudes, traveltimes, and polarizations of the reflected rays arriving at the surface are interpolated to synthesize three‐component seismograms at the desired recording points. The algorithm is applied to investigate kinematic quasi-P-wave propagation and converted quasi-P-S-wave splitting variations produced in reflections from the bottom of a layer containing two sets of intersecting dry vertical fractures as a function of the angle between the fracture sets and of the intensity of fracturing. An analytical expression is derived for the stiffness constant C16that extends Hudson’s second‐order scattering theory to include tetragonal-2 symmetry systems. At any offset, the amount of splitting in nonorthogonal (orthorhombic symmetry) intersecting fracture sets is larger than in orthogonal (tetragonal-1 symmetry) systems, and it increases nonlinearly as a function of the intensity of fracturing as offset increases. Such effects should be visible in field data, provided that the dominant frequency is sufficiently high and the offset is sufficiently large. The amount of shear‐wave splitting at vertical incidence increases nonlinearly as a function of the intensity of fracturing and increases nonlinearly from zero in the transition from tetragonal-1 anisotropy through orthorhombic to horizontal transverse isotropy; the latter corresponds to the two crack systems degenerating to one. The zero shear‐wave splitting corresponds to a singularity, at which the vertical velocities of the two quasi‐shear waves converge to a single value that is both predicted theoretically and illustrated numerically. For the particular case of vertical fractures, there is no P-to-S conversion of vertically propagating (zero‐offset) waves. If the fractures are not vertical, the normal incidence P-to-S reflection coefficient is not zero and thus is a potential diagnostic of fracture orientation.
Image appraisal for 2-D and 3-D electromagnetic inversion
Authors: Alumbaugh David L. , Newman Gregory A.
Published: October 2002

Linearized methods are presented for appraising resolution and parameter accuracy in images generated with 2-D and 3-D nonlinear electromagnetic (EM) inversion schemes. When direct matrix inversion is used, the model resolution and a posteriori model covariance matrices can be calculated readily. By analyzing individual columns of the model resolution matrix, the spatial variation of the resolution in the horizontal and vertical directions can be estimated empirically. Plotting the diagonal of the model covariance matrix provides an estimate of how errors in the inversion process, such as data noise and incorrect a priori assumptions, map into parameter error and thus provides valuable information about the uniqueness of the resulting image. Methods are also derived for image appraisal when the iterative conjugate gradient technique is applied to solve the inverse. An iterative statistical method yields accurate estimates of the model covariance matrix as long as enough iterations are used. Although determining the entire model resolution matrix in a similar manner is computationally prohibitive, individual columns of this matrix can be determined. Thus, the spatial variation in image resolution can be determined by calculating the columns of this matrix for key points in the image domain and then interpolating between. Examples of the image analysis techniques are provided on 2-D and 3-D synthetic cross‐well EM data sets as well as a field data set collected at Lost Hills oil field in central California.
Multicomponent georadar data: Some important implications for data acquisition and processing
Authors: Lehmann Frank , Boerner David E. , Holliger Klaus , Green Alan G.
Published: October 2002

Many seismic reflection processing techniques are applied routinely to ground‐penetrating radar (georadar or GPR) data. Although similarities exist between seismic (acoustic) and radar wave propagation there are some significant differences, some of the most important of which are associated with the dipole nature (1) of georadar sources and receivers and (2) of elemental sources used to represent scattering bodies. Neglecting the dipole character of electromagnetic surveys may result in incomplete or biased images of the subsurface. In an attempt to understand better the consequences of recording dipolar wavefields, we have simulated numerous multicomponent georadar data sets. These simulations are based on the weak scattering (Born) approximation, such that point heterogeneities in the subsurface can be represented by infinitesimal dipoles with moments parallel and proportional to the incident georadar wavefields. The effects of depolarization and dispersion are not included. Nevertheless, many subsurface structures can be modeled by suites of appropriately distributed infinitesimal dipoles. Georadar images of even the simplest subsurface structures are shown to depend strongly on the relative orientations and positions of the source and receiver antennas. A positive aspect of dipolar wavefields is that multicomponent georadar profiles contain information on the locations of both in‐plane and out‐of‐plane structures. Furthermore, “pseudoscalar” wavefields can be simulated from coincident georadar data sets acquired with two pairs of parallel source‐receiver antennas, one oriented perpendicular to the other. Pseudoscalar georadar data, which are characterized by low degrees of directionality, can be processed (including migration) confidently using standard seismic processing software (assuming that dispersion is not a major problem). To illustrate the advantages of multicomponent georadar data, two field examples are presented. One demonstrates the value of recording dual‐component georadar data along isolated profiles; the other shows the benefits of combining 3-D georadar data sets acquired with dual component source‐receiver antenna pairs to form pseudoscalar wavefield images.
A unified framework for the deconvolution of traces of nonwhite reflectivity
Authors: Saggaf M. M. , Robinson Enders A.
Published: October 2002

One of the fundamental assumptions of conventional deconvolution methods is that reflection coefficients follow the white‐noise model. However, analysis of well logs in various regions of the world confirms that in the majority of cases, reflectivity tends to depart from the white‐noise behavior. The assumption of white noise leads to a conventional deconvolution operator that can recover only the white component of reflectivity, thus yielding a distorted representation of the desired output. Various alternative processes have been suggested to model reflection coefficients. In this paper, we will examine some of these processes, apply them, contrast their stochastic properties, and critique their use for modeling reflectivity. These processes include autoregressive moving average (ARMA), scaling Gaussian noise, fractional Brownian motion, fractional Gaussian noise, and fractionally integrated noise. We then present a consistent framework to generalize the conventional deconvolution procedure to handle reflection coefficients that do not follow the white‐noise model. This framework represents a unified approach to the problem of deconvolving signals of nonwhite reflectivity and describes how higher‐order solutions to the deconvolution problem can be realized. We test generalized filters based on the various stochastic models and analyze their output. Because these models approximate the stochastic properties of reflection coefficients to a much better degree than white noise, they yield generalized deconvolution filters that deliver a significant improvement on the accuracy of seismic deconvolution over the conventional operator.
Potential field from a dc current source arbitrarily located in a nonuniform layered medium
Authors: Sato Hédison Kiuity
Published: October 2002

In this paper, I evaluate the potential field due to a dc current source located anywhere within a horizontally layered space, all layers possessing exponentially varying resistivities. The solution takes the form of Hankel transforms with their kernel expressions containing functions defined by recursion formulas. The resulting expressions can be used to model any kind of resistivity array. Specializing the general solution to full‐ or half‐space models possessing exponential dependence, I find terms I interpret as primary and secondary contributions. Analytic expressions for the secondary electrical field revealed an error in the Stoyer and Wait solution for a half‐space. To test the derived n-layer solution based on recursion formulas, I modeled resistivity logs for both homogeneous layers and intercalated homogeneous and heterogeneous layers. Curves from the theory reproduced asymmetries and ripples observed in real resistivity logs.
Mineral exploration in the Thompson nickel belt, Manitoba, Canada, using seismic and controlled‐source EM methods
Authors: White Don , Boerner David , Wu Jianjun , Lucas Steve , Berrer Eberhard , Hannila Jorma , Somerville Rick
Published: October 2002

Seismic reflection and electromagnetic (EM) data were acquired near Thompson, Manitoba, Canada, to map the subsurface extent of the Paleoproterozoic, nickel ore‐bearing Ospwagan Group. These data are supplemented by surface and borehole geology and by laboratory measurements of density, seismic velocity, and electrical conductivity, which indicate that Ospwagan Group rocks are generally more seismically reflective and electrically conductive than the Archean basement rocks that envelop them. The combined seismic/EM interpretation suggests that the Thompson Nappe (cored by Ospwagan Group rocks) lies blind beneath the Archean basement gneisses, to the east of the subvertical Burntwood lineament, in a series of late recumbent folds and/or southeast‐dipping reverse faults. The EM data require that the shallowest of these fold/fault structures occur within the basement gneisses or perhaps less conductive Ospwagan Group rocks. The results of this study demonstrate how seismic and deep sounding EM methods might be utilized as regional exploration tools in the Thompson nickel belt.
Radio tomography and borehole radar delineation of the McConnell nickel sulfide deposit, Sudbury, Ontario, Canada
Authors: Fullagar Peter K. , Livelybrooks Dean W. , Zhang Ping , Calvert Andrew J. , Wu Yiren
Published: October 2002

In an effort to reduce costs and increase revenues at mines, there is a strong incentive to develop high‐resolution techniques both for near‐mine exploration and for delineation of known orebodies. To investigate the potential of high‐frequency EM techniques for exploration and delineation of massive sulfide orebodies, radio frequency electromagnetic (RFEM) and ground‐penetrating radar (GPR) surveys were conducted in boreholes through the McConnell massive nickel‐copper sulfide body near Sudbury, Ontario, from 1993–1996. Crosshole RFEM data were acquired with a JW-4 electric dipole system between two boreholes on section 2720W. Ten frequencies between 0.5 and 5.0 MHz were recorded. Radio signals propagated through the Sudbury Breccia over ranges of at least 150 m at all frequencies. The resulting radio absorption tomogram clearly imaged the McConnell deposit over 110 m downdip. Signal was extinguished when either antenna entered the sulfide body. However, the expected radio shadow did not eventuate when transmitter and receiver were on opposite sides of the deposit. Two‐dimensional modeling suggested that diffraction around the edges of the sulfide body could not account for the observed field amplitudes. It was concluded at the time that the sulfide body is discontinuous; according to modeling, a gap as small as 5 m could have explained the observations. Subsequent investigations by INCO established that pick‐up in the metal‐cored downhole cables was actually responsible for the elevated signal levels. Both single‐hole reflection profiles and crosshole measurements were acquired using RAMAC borehole radar systems, operating at 60 MHz. Detection of radar reflections from the sulfide contact was problematic. One coherent reflection was observed from the hanging‐wall contact in single‐hole reflection mode. This reflection could be traced about 25 m uphole from the contact. In addition to unfavorable survey geometry, factors which may have suppressed reflections included host rock heterogeneity, disseminated sulfides, and contact irregularity. Velocity and absorption tomograms were generated in the Sudbury Breccia host rock from the crosshole radar. Radar velocity was variable, averaging 125 m/μs, while absorption was typically 0.8 dB/m at 60 MHz. Kirchhoff‐style 2-D migration of later arrivals in the crosshole radargrams defined reflective zones that roughly parallel the inferred edge of the sulfide body. The McConnell high‐frequency EM surveys established that radio tomography and simple radio shadowing are potentially valuable for near‐ and in‐mine exploration and orebody delineation in the Sudbury Breccia. The effectiveness of borehole radar in this particular environment is less certain.
The use of airborne gamma‐ray spectrometry—A case study from the Mount Isa inlier, northwest Queensland, Australia
Authors: Jayawardhana Prasantha Michael , Sheard S. N.
Published: October 2002

An airborne survey was undertaken on the Mount Isa inlier in 1990–1992. During this survey, both airborne magnetic and gamma‐ray spectrometric data were recorded over 639 170 line-km. Because of perceived value of the radiometric data, stringent calibration procedures, including the creation of a test range, were adopted. In addition to the data from the newly‐flown areas, 76 760 line‐km of existing data were acquired from other companies, and were reprocessed and merged with the Mount Isa survey. The total area covered by the Mount Isa airborne survey was 151 300 km2. Over the last five years, several studies have been undertaken that seek to exploit the Mount Isa region gamma‐ray database and maximise the use of radiometrics for mineral exploration. This paper highlights the results of these studies by focussing on radiometric signatures of major mines in the Mount Isa Inlier, radioelement contour maps, geomagnetic/radiometric interpretation maps, lithological mapping, regolith mapping, geochemical sampling, and spatial modeling using geographical information systems (GIS). Due to the recent introduction of GIS technology and better techniques for handling high quality digital data, there has been a revived interest in making more use of image data sets. The integration of raster and vector data sets for both spectral and spatial modeling has maximized the potential of this approach.
Seismic imaging
Authors: Gray Samuel H.
Published: October 2002

Unmigrated reflection seismic energy recorded at the earth’s surface, for example in the form of common‐offset records, often provides a remarkably coherent picture of subsurface structure. Coherent as it might be, however, this picture is not correct, suffering from several distorting effects, most notably those of diffraction from geologic bed truncations and lateral movement of the energy between the reflection points on dipping beds and the surface locations. Seismic imaging is the process that corrects these distortions. In the next paragraphs, I discuss this important technology.
On velocity/depth ambiguity in 3-D migration velocity analysis
Authors: Meng Zhaobo , Bleistein Norman
Published: October 2002

Velocity/depth ambiguity in the conventional premigration (time domain) velocity analysis has been studied by Bickel (1990) and Lines (1993) for the horizontal reflector case. Rathor (1997) extended this analysis to the dipping reflector case. These studies answered some important questions that arose in traditional common midpoint (CMP) time‐domain velocity analysis. However, it is not clear that their conclusions apply to migration velocity analysis (MVA) in the prestack‐migrated depth domain.
Amplitude variations with offset of pressure‐seal reflections
Authors: Carcione José M.
Published: October 2002

Knowledge of pressure compartments is essential for identifying potential risks in advance of drilling and in understanding the mechanism of hydrocarbon trapping and migration. These compartments are bounded by pressure seals which, under certain conditions, can be detected and characterized with seismic reflection data. Use of prestack seismic data requires the analysis of reflections generated at the pressure seals, such as the top of a reservoir, rather than the properties of the rock volume. In this sense, amplitude variations with offset (AVO) techniques can be a good tool to obtain information from such pressure seals or pressure transition zones. In this work, I investigate the AVO response of compartment seals by analyzing the plane‐wave reflection coefficients as a function of pore pressure (above and below the seal), incidence angle, and seal thickness. For the case investigated in this work (Berea sandstone with high gas saturation), the AVO effects are important at low effective pressures, that is, when the pore pressure approaches the confining pressure. In shale‐free transition zones, the anomaly is, in general, negative, whereas in shale/sandstone sequences, the anomaly can be positive or negative depending on seal thickness and formation pressure. In terms of the four‐category classification for AVO crossplotting, the PP anomalies are mainly class IV and class I.
Global optimization to maximize stack energy
Authors: Reister David B. , Barhen J. , DuBose John B. , Oblow E. M.
Published: October 2002

We have developed a new method for global optimization, designed to maximize stack energy. Our method includes upper bounds for the stack energy and maximization of the stack energy for each common midpoint. The method quickly identifies ranges of parameter values where the maximum cannot be located. We have used the approach to reach an excellent solution for a difficult sample problem that was created by adding static disruptions to field data. Our new method should apply in many areas of geophysics.
Adaptive filtering of random noise in 2-D geophysical data
Authors: Ristau J. P. , Moon Wooil M.
Published: October 2002

Random noise is often a problem in geophysical data visualization because it obscures fine details and complicates identification of image features. Adaptive filters have recently been used to suppress speckle (random) noise in synthetic aperture radar (SAR) images. SAR data are similar to seismic reflection data, both in their data acquisition approach and in their final data processed format. The nature of the random noise associated is also very similar, and adaptive filters can be applied to reduce random noise in both types of data sets. In this paper several popular adaptive filters—the Lee filter, the Frost filter, and the Kuan filter, which have been used frequently for speckle reduction in SAR data—are tested on Lithoprobe (AGT) deep seismic reflection data and on one set of oil industry shallow seismic reflection data. In addition, a standard band‐pass filter, which is common in many seismic data processing packages, is tested with the oil industry test data. Performance of the adaptive filters is also tested on Radarsat SAR data. The random (speckle) noise in both the Lithoprobe and the Radarsat (SAR) data sets is statistically very similar, and the adaptive filters tested successfully suppressed random noise while minimizing blurring. Among the tested filters the enhanced Lee filter performed best, closely followed by the enhanced Frost filter and the Kuan filter. The background noise in the oil industry seismic data is statistically quite different from the above two data sets; the results obtained were less than satifactory, although they were still encouraging. With the oil industry data, the enhanced Frost and Kuan filters performed better than the enhanced Lee filter. The commonly used band‐pass filter successfully removed the background (random) noise, but it also suppressed the reflection events, making the final result less desirable.
Characterization of dispersion, attenuation, and anisotropy at the Buena Vista Hills field, California
Authors: Hackert Chris L. , Parra Jorge O. , Brown Raymon L. , Collier Hughbert A.
Published: October 2002

We create a log of intrinsic dispersion and attenuation for the Antelope Shale formation of the Buena Vista Hills field, San Joaquin Valley, California. High dispersion (or low Q) values correlate with thin sand and carbonate beds within the Antelope Shale. These beds are at least ten times as permeable as the host shale formation, so this effect provides a possible avenue for seismic prediction of permeability. The dispersion log is formed through comparison of crosswell seismic velocities (measured at approximately 1 kHz) and sonic log velocities (measured at approximately 10 kHz). In order to provide a proper basis for comparison, the sonic log must first be adjusted for field anisotropy, scaling effects, and resolution of measurement. We estimate a local shale anisotropy of about 20% based on correlations generated from published measurements of other shale fields. We apply resolution enhancement to capture the thin sand and carbonate beds, and windowed Backus averaging to match the measurement scales. A modeling study verifies the technique, and shows that beds of thickness greater than 30 cm have a measurable signature. The actual resolution is on the order of the crosswell Fresnel length, or about 7 m for the model study.
Analysis of seismic wave dynamics by means of integral representations and the method of discontinuities
Authors: Duchkov Anton A. , Goldin Sergey V.
Published: October 2002

We analyze the dynamics (amplitudes and phase distortions) of seismic waves as they propagate along the ray. Our analysis is performed via a ray series approximation in the time domain. That is, we concentrate on characterizing the sharp changes (discontinuities) of the signal that are localized near the wavefront. After convolution of the terms of such a series with a proper temporally short (high‐frequency) wavelet, one obtains a synthetic seismic signal at a given point of interest. We present an outline of the proposed technique that yields integrals describing the wavefield. These integrals are similar to oscillatory integrals in the frequency domain. This description is uniformly valid near caustics, allowing the calculation of higher order terms of the ray series approximation. Practical use of the technique is illustrated by several examples which show two possible uses of the technique: general understanding of what is happening during wave propagation and practical calculations. First, we show how the structure of the ray decomposition changes near the simple caustic, and then we calculate a synthetic signal near the cusp caustic. The advantage of the technique is that the problem of seismic wave calculation is technically reduced to a problem of double integration of a Dirac δ-function; thus, it is computationally effective.
Time‐domain simulation ofSH-wave‐induced electromagnetic field in heterogeneous porous media: A fast finite‐element algorithm
Authors: Han Qiyu , Wang Zhijing (Zee)
Published: October 2002

When a horizontally polarized rotational mechanical wave (SH-wave) travels through a porous rock, acceleration of the rock frame induces a streaming current in the SH particle motion plane. This streaming current is parallel to the particle displacement and has an associated electromagnetic (EM) field. This phenomenon is often described as the electroseismic (EOS) conversion. Numerically, the EOS phenomenon can be simulated in either the frequency or the time domain. Frequency‐domain numerical simulation has huge memory and computational requirements. Traditional time‐domain simulation, on the other hand, must restrict the time steps to be very small to satisfy stability conditions, resulting in large workload. In this paper, we present a fast finite‐element (FE) method simulating the EOS conversion in the time domain. In our method, we decompose the large 2-D FE matrix equations into a set of 1-D matrix equations and solve the problem using the approximate 1-D multistep process. We present numerical examples of 1-D and 2-D models to illustrate the coevolution of the seismic and electromagnetic fields. Our simulation results show that the diffusive electrical field is induced from the spatial variations of mechanical and electrical properties of the porous media due to the imbalance of the induced electric current. Besides the direct SH-wave itself, the transmitted waves, multiple waves, reflected waves, and diffracted waves also induce diffusive electrical fields. The EOS conversion is potentially useful for reservoir characterization, but the EOS data may be difficult to interpret due to the complexity of the superposed wave fields. The diffusive nature of the induced EM fields suggests that antennas should be positioned close to the target of interest in in‐situ measurements. As a result, borehole EOS surveys are likely to be more practical than surface surveys.
Ground‐penetrating radar: Analysis of point diffractors for modeling and inversion
Authors: Saintenoy Albane C. , Tarantola Albert
Published: October 2002

The three electromagnetic properties appearing in Maxwell’s equations are dielectric permittivity, electrical conductivity and magnetic permeability. The study of point diffractors in a homogeneous, isotropic, linear medium suggests the use of logarithms to describe the variations of electromagnetic properties in the earth. A small anomaly in electrical properties (permittivity and conductivity) responds to an incident electromagnetic field as an electric dipole, whereas a small anomaly in the magnetic property responds as a magnetic dipole. Neither property variation can be neglected without justification. Considering radiation patterns of the different diffracting points, diagnostic interpretation of electric and magnetic variations is theoretically feasible but is not an easy task using ground‐penetrating radar. However, using an effective electromagnetic impedance and an effective electromagnetic velocity to describe a medium, the radiation patterns of a small anomaly behave completely differently with source‐receiver offset. Zero‐offset reflection data give a direct image of impedance variations while large‐offset reflection data contain information on velocity variations.
Reflection waveform inversion using local descent methods: Estimating attenuation and velocity over a gas‐sand deposit
Authors: Hicks Graham J. , Pratt R. Gerhard
Published: October 2002

Prestack seismic reflection data contain amplitudes, traveltimes, and moveout information; waveform inversion of such data has the potential to estimate attenuation levels, reflector depths and geometry, and background velocities. However, when inverting reflection data, strong nonlinearities can cause reflectors to be incorrectly imaged and can prevent background velocities from being updated. To successfully recover background velocities, previous authors have resorted to nonlinear, global search inversion techniques. We propose a two‐step inversion procedure using local descent methods in which we perform alternate inversions for the reflectors and the background velocities. For our reflector inversion we exploit the efficiency of the back‐propagation method when inverting for a large parameter set. For our background velocity inversion we use Newton inverse methods. During the background velocity inversions it is crucial to adaptively depth‐stretch the model to preserve the vertical traveltimes. This reduces nonlinearity by largely decoupling the effects of the background velocities and reflectors on the data. Nonlinearity is further reduced by choosing to invert for slownesses and by inverting for a sparse parameter set which is partially defined using geological reflector picks. Applying our approach to shallow seismic data from the North Sea collected over a gas‐sand deposit, we demonstrate that the proposed method is able to estimate both the geometry and internal velocity of a significant velocity structure not present in the initial model. Over successive iterations, the use of adaptive depth stretching corrects the pull‐down of the base of the gas sand. Vertical background velocity gradients are also resolved. For an insignificant extra cost the acoustic attenuation parameter Q is included in the inversion scheme. The final attenuation tomogram contains realistic values of Q for the expected lithologies and for the effect of partial fluid saturation associated with a shallow bright spot. The attenuation image may also indicate the presence of fracturing.
Nonlinear 3-D traveltime inversion of crosshole data with an application in the area of the Middle Ural Mountains
Authors: Soupios Pantelis M. , Papazachos Constantinos B. , Juhlin Christopher , Tsokas Gregory N.
Published: October 2002

This paper deals with the problem of nonlinear seismic velocity estimation from first‐arrival traveltimes obtained from crosshole and downhole experiments in three dimensions. A standard tomographic procedure is applied, based on the representation of the crosshole area into a number of cells which have an initial slowness assigned. For the forward modeling, the raypath matrix is computed using the revisited ray bending method, supplemented by an approximate computation of the first Fresnel zone at each point of the ray, hence using physical and not only mathematical rays. Since 3-D ray tracing is incorporated, the inversion technique is nonlinear. Velocity images are obtained by a constrained least‐squares inversion scheme using both “damping” and “smoothing” factors. The appropriate choice of these factors is defined by the use of appropriate criteria such as the L-curve. The tomographic approach is improved by incorporating a priori information about the media to be imaged into our inversion scheme. This improvement in imaging is achieved by projecting a desirable solution onto the null space of the inversion, and including this null‐space contribution with the standard non‐null‐space inversion solution. The efficiency of the inversion scheme is tested through a series of tests with synthetic data. Moreover, application in the area of the Ural Mountains using real data demonstrates that the proposed technique produces more realistic velocity models than those obtained by other standard approaches.
Rapid inversion of 2-D geoelectrical data by multichannel deconvolution
Authors: Møller Ingelise , Jacobsen Bo H. , Christensen Niels B.
Published: October 2002

Modern geoelectrical data acquisition systems can record more than 100 000 data values per field day. Despite the growth in computer power and the development of more efficient numerical algorithms, interpreting such data volumes remains a nontrivial computational task. We present a 2-D one‐pass inversion procedure formulated as a multichannel deconvolution. It is based on the equation for the electrical potential linearized under the Born approximation, and it makes use of the 2-D form of the Fréchet derivatives evaluated for the homogeneous half‐space. The inversion is formulated in the wavenumber domain so that the 2-D spatial problem decouples into many small 1-D problems. The resulting multichannel deconvolution algorithm is very fast and memory efficient. The inversion scheme is stabilized through covariance matrices representing the stochastic properties of the earth resistivity and data errors. The earth resistivity distribution is assumed to have the statistical characteristics of a two‐parameter, self‐affine fractal. The local apparent amplitude and fractal dimension of the earth resistivity are estimated directly from geoelectrical observations. A nonlinearity error covariance matrix is added to the conventional measurement error covariance matrix. The stochastic model for the dependence of nonlinearity error on electrode configuration as well as resistivity amplitude and fractal dimension is determined pragmatically through nonlinear simulation experiments. Tests on synthetic examples and field cases including well control support the conclusion that for long data profiles this method automatically produces linearized resistivity estimates which faithfully resolve the main model features.
Viscoacoustic crosswell imaging using asymptotic waveforms
Authors: Keers Henk , Vasco Don W. , Johnson Lane R.
Published: October 2002

We present a method to invert viscoacoustic waveforms. The waveform inversion is based on a new forward algorithm for waves propagating through viscoacoustic media. The algorithm represents the wavefield in terms of a summation over nonstationary raypaths. The amplitudes at the caustics are computed correctly. The inclusion of nonstationary raypaths makes it possible to compute sensitivity functions for both velocity and attenuation. The sensitivity functions lead to a linearized formulation of the waveform inversion. The inversion is applied to two 2-D data sets that previously had been matched using only velocity. The first data set consists of crosswell data obtained from a tank filled with a water‐saturated sand. The second data set was obtained after injecting a nonaqueous‐phase liquid into the tank. The inclusion of attenuation significantly improves the fit to the data, especially in the case of the second experiment. A misfit reduction of 80% was obtained, whereas the misfit reduction after inversion for velocity only was 40%. The Q values obtained for the two experiments (Q = 60–100 before the injection and Q = 10–20 in the second experiment) fall within the range predicted by other experiments.
Reply by the authors to the discussion by A. T. de Hoop
Authors: Arntsen Børge , Carcione José M.
Published: October 2002

The discussion by A. T. de Hoop is highly appreciated, and we regret to have omitted the reference to de Hoop (1995). Part of the developments in our paper are similar to those of de Hoop and Stam (1988). Mittet and Hokstad (1995) have also presented similar results for the purely elastic case. These are both properly referenced in the Introduction.
A DECIMAL CLASSIFICATION SYSTEM FOR GEOPHYSICAL EXPLORATION
Authors: Heiland C. A.
Published: October 2002

It is quite probable that many members of this Society, particularly those with reasonably permanent headquarters, have started to collect geophysical data of some kind or other. If one keeps adding to such a collection over a period of years, the necessity arises sooner or later to classify this material in some way. The writer’s own collection, started some twenty‐two years ago, would be of little value to him or to anybody else unless it were arranged in systematic form. It is very annoying not to be able to produce a certain article or data although one remembers perfectly well having seen and filed it. Without an orderly classification system, much time is lost searching for it, and needless expense is often incurred in duplicating material already in the files.
THE ANALYTIC BASIS OF GRAVITY INTERPRETATION
Authors: Hughes D. S.
Published: October 2002

One method of gravity interpretation involves the use of analytic continuation processes. In this discussion the resolving power of this method is tested numerically. Using hypothetical structures comprising single and double blocks, a surface‐gravity profile is derived. Using these values as an “observed gravity” profile, the “continuation” method is applied to compute the gravity at intermediate depths. Comparing these computed values with the actual (directly computed) gravity profile at these depth‐planes, the resolving power of the continuation method is demonstrated. It is shown that a very high precision in the observed data is necessary for very accurate resolution of structures.
THE PRODUCTION OF ELASTIC WAVES BY EXPLOSION PRESSURES. II. RESULTS OF OBSERVATIONS NEAR AN EXPLODING CHARGE
Authors: Sharpe Joseph A.
Published: October 2002

A high fidelity recording system has been used to observe the elastic wave motion at vertical separations from an exploding charge ranging from 15 to 300 feet. The motion near the charge has a predominant frequency of about 1000 cps for rigid material and a duration of a few milliseconds. The motion becomes increasingly complex and low frequency components become dominant as the point of observation recedes from the source. A secondary wave which may be a bound wave associated with the fluid in the drill hole was observed.
THE VALUE OF QUANTITATIVE INTERPRETATION OF GRAVITY DATA
Authors: Skeels D. C.
Published: October 2002

Although there is no unique interpretation of a given set of gravity data, there are many cases in which quantitative interpretation is decidedly worthwhile. This is especially true in cases where the gravity data are supplemented by a certain amount of geological data, or where the gravity anomaly is of such a shape that the range of possible solutions can be rather closely limited. Three examples are given of interpretations of actual data.
A MECHANICAL INTEGRATOR FOR THE COMPUTATION OF GRAVITY ANOMALIES
Authors: Siegert Arnold J. F.
Published: October 2002

With the instrument here described the gravity anomaly caused by an arbitrarily given finite body can be computed. The integrator is operated by tracing the contour lines representing the body in a manner similar to the operation of a planimeter. The instrument is useful mainly in the indirect method of interpretation of observed gravity anomalies, in the computation of terrain corrections, and in the evaluation of magnetic anomalies. Its design is based on a formula which expresses the gravity anomaly caused by an infinitely thin slab, bounded by a closed curve, as a line integral over this curve.
A RESISTIVITY SURVEY OF THE MONUMENT OIL FIELD
Authors: England C. M.
Published: October 2002

This paper describes an electrical resistivity survey made in 1935 of an area in New Mexico now known as the Monument Field. From the data obtained a map showing the structure at the base of the Red Beds was prepared which is in good agreement with structure disclosed by wells later drilled.
A PROPOSED METHOD OF MEASURING THE DERIVATIVES OF THE EARTH’S MAGNETIC FIELD
Authors: Jones J. H.
Published: October 2002

The method of measuring the derivatives of the earth’s magnetic field which is proposed depends on the fact that a small steady magnetic field will modify the e.m.f. induced in the secondary coil of a detector with a core of magnetizable material possessing high initial permeability and a high rate of increase of the permeability with the strength of the magnetizing field. It is shown that a detector of this type is equivalent to an inductor coil rotating with angular speed equal to the periodicity of the alternating magnetizing field.
GRAVITY AND MAGNETIC OBSERVATIONS ON IRON MOUNTAIN MAGNETITE DEPOSIT, LLANO COUNTY, TEXAS
Authors: Barnes Virgil E. , Romberg Frederick
Published: October 2002

An outcrop of magnetite in the Central Mineral region of Texas is explored experimentally with a dip needle and a gravity meter. Two anomalies are observed, one due to the outcropping body and one probably due to a previously unknown body close to the outcrop. The possible attitudes of the bodies are discussed, and their masses and probable dimensions are computed from the gravitational observations.
CONFERENCE—MAINTAINING GEOPHYSICAL OPERATIONS DURING THE WAR: REPORT OF THE WAR EFFORTS ADVISORY COMMITTEE
Authors: Weatherby B. B.
Published: October 2002

Members of the committee were widely scattered geographically and since several of them have been devoting a substantial part of their time to war work they were unavailable for meetings. Consequently most of the work has been carried on by the Chairman with the very able assistance of the President of the S. E. G., Mr. Goldstone, with whom numerous conferences have been held. I wish to apologize to all members for not having consulted them more frequently and I also wish to relieve them of any responsibility in connection with the activities of the committee. That responsibility is mine.
TRANSPORTATION
Authors: Patrick H. G.
Published: October 2002

During the past two or three weeks the writer has contacted a number of geophysics operators and contract shot‐hole drillers and discussed with them the transportation difficulties encountered in wartime operations. The inquiries were confined to the operators in the southwest and midwest. It is felt that we have a fairly good cross‐section of the experiences of the exploration group in these regions. Unfortunately, time did not permit our contacting any of the West Coast operators; consequently, we have no information on the subject from that section of the country. Perhaps representatives of the West Coast operators can supply us with information concerning their transportation problems. We shall attempt in this discussion to give a composite view of the transportation problem based on the experience of those who were contacted. Naturally, it may differ somewhat from the actual experiences of any particular operator. For the purpose of this discussion the general subject of transportation has been divided into three sub‐heads, namely 1) Tires, 2) New Automotive Equipment, and 3) Spare Parts.
SEISMOGRAPH EXPLOSIVES AND THE WAR
Authors: Seyffert W. A.
Published: October 2002

The effective prosecution of the war requires that all essential materials be made available first, to the armed forces, and second, to vital private industries. If this is not done the entire war effort will bog down. We understand that certain persons, evidently laymen, have recently asked whether the seismograph industry might not be handicapping the war effort by its use of one essential material, namely, explosives. These questions were, of course, brought about by the rather widespread idea that the terms “explosives” and “ammunition” are synonymous. Even so, we are surprised that such questions were raised.
GEOPHYSICAL HISTORY OF THE IOWA FIELD, CALCASIEU AND JEFFERSON DAVIS PARISHES, LOUISIANA
Authors: Eby J. Brian
Published: October 2002

The discovery well of the Iowa field was drilled in October of 1931, and the field has produced 48,000,000 barrels of oil to date. The history of the early refraction seismograph, gravity, and reflection seismograph exploration of this important field is presented.
THE EQUATION OF MOTION OF A GEOPHONE ON THE SURFACE OF AN ELASTIC EARTH
Authors: Wolf Alfred
Published: October 2002

The motion of a geophone case placed on the surface of an elastic earth does not follow faithfully the motion of the earth at high frequencies. In effect, a weight placed on the surface of an elastic solid constitutes a damped oscillating system. The elastic restoring forces are determined by the area of contact between the weight and the surface of the solid and by the elastic moduli of the solid. The damping force is due to emission of elastic waves by the oscillating weight. The motion of the solid also contributes to the inertia of the system. Equations are developed for these forces on the assumption that the wave length is long compared to the linear dimensions of the area of contact between the weight and the elastic solid. This leads to a determination of the frequency of oscillation and of the decrement of such a system.
A METHOD OF DETERMINING THE TIME BREAK ON DEEP SEA SEISMIC RECORDS FROM THE WATER SOUND ARRIVALS
Authors: Agocs W. B.
Published: October 2002

A method is presented whereby, through the use of the direct water wave first arrival from a shot fired on the ocean bottom, and the first and second reflections of sound from the surface of the sea, the following data may be obtained: (1) the ratio of the depth of the shot below the surface of the sea to the lateral distance between the shot and detector; (2) the lateral distance between the shot and the detector at the bottom of the sea; and finally, (3) the time of detonation or the time break in terms of the time scale of the detecting recorder. The velocity of sound in sea water shows a linear variation of the velocity with depth in definite zones with rather abrupt changes in slope of the velocity‐depth curve from zone to zone. Curvature due to refraction is found to be negligible in the calculations made for true oceanic depths. Sample computations are given.
WAVELET FUNCTIONS AND THEIR POLYNOMIALS
Authors: Ricker Norman
Published: October 2002

This paper is a further extension of the author’s wavelet theory of seismogram structure. The wavelet functions used in the previous paper are extended, a modified notation is adopted, and twenty four of the polynomials are given, together with their roots. Expressions are given for the displacement, velocity, and acceleration type wavelets.