bluuebunny/crossref_metadata_2025 huggingface dataset:
Elastic wave velocities during evaporative drying
Authors: Goertz David , Knight Rosemary
Published: October 2002

Laboratory measurements of drying rates and elastic wave velocities are made on limestone, dolomite, and sandstone samples during evaporative drying. The drying rate data are very similar in form. There is a constant rate period at higher saturations and a falling rate period below saturation levels of approximately 0.2. The falling rate period marks the transition in the sample from hydraulically connected to disconnected water. There is a strong link between elastic wave velocities and the drying process because different pore geometries drain at different stages in drying. The drainage of these different geometries results in specific changes in the moduli and velocities. Simple models of the pore geometries and the drying process are used to model the velocity data. The velocity‐saturation relationship for each of the three samples is very different in form because of differences in pore‐space microgeometry. Of particular interest is the velocity response during the falling rate period of drying. In the limestone and the sandstone, there is a significant decrease in bulk and shear moduli and elastic wave velocities because of the drainage of crack‐like pores and grain contacts. In contrast, the absence of these pore geometries in the dolomite results in essentially no changes in the moduli at low saturations. An understanding of the drying process and resulting pore‐scale fluid distribution provides useful insights into the observed form of the velocity‐saturation relationship.
Error bars for sonic slowness measurements
Authors: Kimball Christopher V. , Scheibner David J.
Published: October 2002

Semblance provides a quality indicator for beam‐former array processing methods such as slowness/time coherence (STC) and dispersive STC (DSTC). Semblance does not indicate the slowness variance because the array design, the processing time‐bandwidth parameters, and the signal spectral content are not taken into account. For the classical case of a single propagating wave with added Gaussian noise, semblance is distributed according to the noncentral beta distribution. This distribution is parameterized by the number of receivers, M, the processing time‐bandwidth product, BT, and signal‐to‐noise ratio. Given M and BT, the measured semblance can be inverted to give a high‐quality estimate of the signal‐to‐noise ratio. The Cramer‐Rao lower bounds on slowness variance for the classical case of signals in added Gaussian noise depend on the array dimensions, the signal power spectrum, and the signal‐to‐noise ratio. Estimates of the Cramer‐Rao bounds can be calculated from the estimated signal‐to‐noise ratio and the measured signal power spectrum. Monte‐Carlo simulations show that these bounds approximate the actual slowness variance for high, but practical, signal‐to‐noise ratios. Preliminary results on field data show that error bars based on Cramer‐Rao bounds are credible, and that error bars yield slowness standard deviations comparable to those from repeat runs.
The interaction of tube waves with borehole fractures, Part II: Analytical models
Authors: Kostek Sergio , Johnson David Linton , Winkler Kenneth W. , Hornby Brian E.
Published: October 2002

We develop a series of analytical models that can be used to interpret Stoneley‐wave reflections from fractures intersecting a borehole, thus facilitating the determination of effective fracture apertures from logs. The first model considers the combined effects of borehole enlargements (e.g., washouts) and fractures on the reflection coefficient of Stoneley waves. The result is expressed in terms of the washout volume, which can be obtained from a caliper log, as well as the fracture width. The predictions of this model are in excellent agreement with finite‐difference calculations. Next we develop an analytical elastic model that generalizes the rigid formation model. It also agrees with finite‐difference calculations. Finally, we establish the equivalence between a closely spaced multiple fracture and a permeable medium. In all the examples, the generalization of the result for the single fracture in a rigid formation can be accomplished with standard logging measurements.
Seismic reflection and ground‐penetrating radar imaging of a shallow aquifer
Authors: Cardimona Steven J. , Clement William P. , Kadinsky‐Cade Katharine
Published: October 2002

In 1995 and 1996, researchers associated with the US Air Force’s Phillips and Armstrong Laboratories took part in an extensive geophysical site characterization of the Groundwater Remediation Field Laboratory located at Dover Air Force Base, Dover, Delaware. This field experiment offered an opportunity to compare shallow‐reflection profiling using seismic compressional sources and low‐frequency ground‐penetrating radar to image a shallow, unconfined aquifer. The main target within the aquifer was the sand‐clay interface defining the top of the underlying aquitard at 10 to 14 m depth. Although the water table in a well near the site was 8 m deep, cone penetration geotechnical data taken across the field do not reveal a distinct water table. Instead, cone penetration tests show a gradual change in electrical properties that we interpret as a thick zone of partial saturation. Comparing the seismic and radar data and using the geotechnical data as ground truth, we have associated the deepest coherent event in both reflection data sets with the sand‐clay aquitard boundary. Cone penetrometer data show the presence of a thin lens of clays and silts at about 4 m depth in the north part of the field. This shallow clay is not imaged clearly in the low‐frequency radar profiles. However, the seismic data do image the clay lens. Cone penetrometer data detail a clear change in the soil classification related to the underlying clay aquitard at the same position where the nonintrusive geophysical measurements show a change in image character. Corresponding features in the seismic and radar images are similar along profiles from common survey lines, and results of joint interpretation are consistent with information from geotechnical data across the site.
Seismic‐controlled nonlinear extrapolation of well parameters using neural networks
Authors: Liu Zhengping , Liu Jiaqi
Published: October 2002

We present a data‐driven method of joint inversion of well‐log and seismic data, based on the power of adaptive mapping of artificial neural networks (ANNs). We use the ANN technique to find and approximate the inversion operator guided by the data set consisting of well data and seismic recordings near the wells. Then we directly map seismic recordings to well parameters, trace by trace, to extrapolate the wide‐band profiles of these parameters using the approximation operator. Compared to traditional inversions, which are based on a few prior theoretical operators, our inversion is novel because (1) it inverts for multiple parameters and (2) it is nonlinear with a high degree of complexity. We first test our algorithm with synthetic data and analyze its sensitivity and robustness. We then invert real data to obtain two extrapolation profiles of sonic log (DT) and shale content (SH), the latter a unique parameter of the inversion and significant for the detailed evaluation of stratigraphic traps. The high‐frequency components of the two profiles are significantly richer than those of the original seismic section.
Seismic signatures of permeability in heterogeneous porous media
Authors: Shapiro Sergei A. , Müller Tobias M.
Published: October 2002

In homogeneous poroelastic systems, the permeability tensor practically does not influence propagating seismic waves in the low frequency range (0–1000 Hz; see, e.g., Schmitt, 1989; Gelinsky and Shapiro, 1996). In this paper, we show that this situation changes in heterogeneous systems such as, layered or fractured sediments. Due to the heterogeneities of poroelastic structures, the attenuation of P-waves is influenced by the permeability in an enhanced way. We show, however, that such a “seismic permeability” can differ very strongly from the hydraulic permeability.
Multichannel analysis of surface waves
Authors: Park Choon B. , Miller Richard D. , Xia Jianghai
Published: October 2002

The frequency‐dependent properties of Rayleigh‐type surface waves can be utilized for imaging and characterizing the shallow subsurface. Most surface‐wave analysis relies on the accurate calculation of phase velocities for the horizontally traveling fundamental‐mode Rayleigh wave acquired by stepping out a pair of receivers at intervals based on calculated ground roll wavelengths. Interference by coherent source‐generated noise inhibits the reliability of shear‐wave velocities determined through inversion of the whole wave field. Among these nonplanar, nonfundamental‐mode Rayleigh waves (noise) are body waves, scattered and nonsource‐generated surface waves, and higher‐mode surface waves. The degree to which each of these types of noise contaminates the dispersion curve and, ultimately, the inverted shear‐wave velocity profile is dependent on frequency as well as distance from the source. Multichannel recording permits effective identification and isolation of noise according to distinctive trace‐to‐trace coherency in arrival time and amplitude. An added advantage is the speed and redundancy of the measurement process. Decomposition of a multichannel record into a time variable‐frequency format, similar to an uncorrelated Vibroseis record, permits analysis and display of each frequency component in a unique and continuous format. Coherent noise contamination can then be examined and its effects appraised in both frequency and offset space. Separation of frequency components permits real‐time maximization of the S/N ratio during acquisition and subsequent processing steps. Linear separation of each ground roll frequency component allows calculation of phase velocities by simply measuring the linear slope of each frequency component. Breaks in coherent surface‐wave arrivals, observable on the decomposed record, can be compensated for during acquisition and processing. Multichannel recording permits single‐measurement surveying of a broad depth range, high levels of redundancy with a single field configuration, and the ability to adjust the offset, effectively reducing random or nonlinear noise introduced during recording. A multichannel shot gather decomposed into a swept‐frequency record allows the fast generation of an accurate dispersion curve. The accuracy of dispersion curves determined using this method is proven through field comparisons of the inverted shear‐wave velocity ([Formula: see text]) profile with a downhole [Formula: see text] profile.
Reliability analysis in dynamic systems: Implications for positioning marine seismic networks
Authors: Gikas Vassilis , Cross Paul A. , Ridyard David
Published: October 2002

Real‐time integrated monitoring of reliability (the impact of undetected biases in positioning observations on the position of geophysical elements) during marine seismic surveys is essential to ensure there is no significant loss in the resolution of the processed seismic image. The B-method is currently the most rigorous testing procedure for bias identification. It has been used extensively in geodetic networks, but to date it has found very little application within a dynamic environment. Here the method is expanded for use in dynamic systems—especially in marine seismic positioning networks such as those represented by a multisource, multistreamer, multivessel acquisition system. With this approach, we have found completely rigorous reliability estimates based on an integrated navigation method. We conclude that although the reliability of a system is in general a consequence of the redundancy, its geometry, and the stochastic model of the observations, these factors affect internal and external reliability in rather different ways. A key contribution is the demonstration that simple, easily understandable, and explainable statistics can be used to give a more complete description of quality than that obtained from precision measures alone. Assessing precision alone may give a false sense of security regarding the quality of the location of a seismic spread. The exploration and production industry is now making increasing use of old and new marine streamer 3-D surveys as part of 4-D time lapse reservoir monitoring projects. A more rigorous approach to positioning data quality, such as the method described in this paper, may play a material role in the avoidance of interpretable artifacts in these projects.
Extended local Rytov Fourier migration method
Authors: Huang Lian‐Jie , Fehler Michael C. , Roberts Peter M. , Burch Charles C.
Published: October 2002

We develop a novel depth‐migration method termed the extended local Rytov Fourier (ELRF) migration method. It is based on the scalar wave equation and a local application of the Rytov approximation within each extrapolation interval. Wavefields are Fourier transformed back and forth between the frequency‐space and frequency‐wavenumber domains during wavefield extrapolation. The lateral slowness variations are taken into account in the frequency‐space domain. The method is efficient due to the use of a fast Fourier transform algorithm. Under the small angle approximation, the ELRF method leads to the split‐step Fourier (SSF) method that is unconditionally stable. The ELRF method and the extended local Born Fourier (ELBF) method that we previously developed can handle wider propagation angles than the SSF method and account for the phase and amplitude changes due to the lateral variations of slowness, whereas the SSF method only accounts for the phase changes. The stability of the ELRF method is controlled more easily than that of the ELBF method.
2-D ray‐based tomography for velocity, layer shape, and attenuation from GPR data
Authors: Cai Jun , McMechan George A.
Published: October 2002

Tomographic inversion for near‐surface structure from multioffset ground‐penetrating radar (GPR) reflection data is implemented. The model is parameterized as layers with constant velocity and constant attenuation. The solution is divided into two parts. First, the velocity and shape of each layer are estimated from the observed reflection traveltimes; then using this geometry and the corresponding ray paths, the attenuation of each layer is estimated from the observed relative amplitudes. Solution of both tomographic problems is performed by singular value decomposition of the sensitivity matrix (to compute information density, resolution, and covariance matrices as well as the solution). Tests on synthetic data, for which the solution is known, show that the algorithm converges efficiently to the neighborhood of the correct solution. Inversion of multichannel field data in a GPR line from a fluvial/eolian environment produces a model that is consistent with structural images and velocity estimates obtained independently by traditional common‐midpoint processing.
Synthesis of amplitude‐versus‐offset variations in ground‐penetrating radar data
Authors: Zeng Xiaoxian , McMechan George A. , Xu Tong
Published: October 2002

To evaluate the importance of amplitude‐versus‐offset information in the interpretation of ground‐penetrating radar (GPR) data, GPR reflections are synthesized as a function of antenna separation using a 2.5-D finite‐difference solution of Maxwell’s equations. The conductivity, the complex dielectric permittivity, and the complex magnetic permeability are varied systematically in nine suites of horizontally layered models. The source used is a horizontal transverse‐electric dipole situated at the air‐earth interface. Cole‐Cole relaxation mechanisms define the frequency dependence of the media. Reflection magnitudes and their variations with antenna separation differ substantially, depending on the contrast in electromagnetic properties that caused the reflection. The spectral character of the dielectric and magnetic relaxations produces only second‐order variations in reflection coefficients compared with those associated with contrasts in permittivity, conductivity, and permeability, so they may not be separable even when they are detected. In typical earth materials, attenuation of propagating GPR waves is influenced most strongly by conductivity, followed by dielectric relaxation, followed by magnetic relaxation. A pervasive feature of the simulated responses is a locally high amplitude associated with the critical incident angle at the air‐earth interface in the antenna radiation pattern. Full wavefield simulations of two field data sets from a fluvial/eolian environment are able to reproduce the main amplitude behaviors observed in the data.
Application of vertical radar profiling technique to Sendai Castle
Authors: Zhou Hui , Sato Motoyuki
Published: October 2002

The vertical radar profiling (VRP) technique is able to explore much deeper than conventional surface ground‐penetrating radar (GPR) because it uses boreholes. It has been successfully applied at the Sendai Castle site in Japan to investigate the extent of an old stone wall and strata buried by a more recent stone wall. The transmitter of a polarimetric radar system was moved within a borehole, and the receiver was fixed on the ground surface several meters away from the borehole head. Cross‐ and copolarization data were measured at a receiver position with a different orientation to the receiver. Ten data sets were acquired by placing the receiver in five directions. The depolarization is strong, indicating the subsurface contains a great amount of gravel. To get clear and intuitive images of the subsurface, we applied data processing techniques, including the separation of direct and reflected waves of raw VRP data using f-k filtering approach and Kirchhoff migration of separated reflected waves. By comparing the migrated images, we learned that cross‐ and copolarization data sets received at the same position give the same images of the subsurface, although the appearances of the original data sets are different. The degree of consistency of all data sets recorded in different directions is quite high, and the migrated images near the borehole fit the borehole core very well. The images reveal the distribution of the old stone wall and other layers.
Monitoring hydraulic fracture growth: Laboratory experiments
Authors: Groenenboom Jeroen , van Dam Dirkjan B.
Published: October 2002

We carry out small‐scale hydraulic fracture experiments to investigate the physics of hydraulic fracturing. The laboratory experiments are combined with time‐lapse ultrasonic measurements with active sources using both compressional and shear‐wave transducers. For the time‐lapse measurements we focus on ultrasonic measurement changes during fracture growth. As a consequence we can detect the hydraulic fracture and characterize its shape and geometry during growth. Hence, this paper deals with fracture characterization using time‐lapse acoustic data. During fracture growth the acoustic waves generate diffractions at the tip of the fracture. The direct compressional and shear diffractions are used to locate the position of the tip of the fracture. More detailed analysis of these diffractions can be used to obtain information on the geometry and configuration of the fracture tip, including the creation of a zone that is not penetrated by fluid. Furthermore, it appears that the acoustic diffraction is generated mainly at the fluid front and only weakly at the dry tip. In addition, the wavefield that has been transmitted through the hydraulic fracture is measured. Shear‐wave transmissions are shadowed because the shear modulus vanishes inside the fluid‐filled fracture. From this observation we conclude that the fracture is mechanically open. In other words, no friction occurs related to the movement of fracture faces that are in mechanical contact. Compressional transmissions show a distinctive dispersion relative to the measurement in the unfractured medium. This dispersion can be used to determine the width (or aperture) of the fracture by fitting the measured dispersion with the theoretical prediction as a function of the unknown fracture width. We show that the width profile of the fracture can be reconstructed by using a set of transmission records with different source and receiver locations. By performing a validation experiment, we show that the width determination method is reliable, although the estimated fracture width is only a few percent of the incident wavelength. The strength of the method relies on time‐lapse measurements combined with fitting the changes in the measured waveforms during the experiment. The combination of diffractions and transmissions helps us visualize the dynamic process of hydraulic fracture growth. Hence, acoustic measurements with active sources prove their usefulness for fracture characterization.
Numerical estimation of electrical conductivity in saturated porous media with a 2-D lattice gas
Authors: Küntz Michel , Mareschal Jean Claude , Lavallée Paul
Published: October 2002

A 2-D lattice gas is used to calculate the effective electrical conductivity of saturated porous media as a function of porosity and conductivity ratio [Formula: see text] between the pore‐filling fluid and the solid matrix for various microscopic structures of the pore space. The way the solid phase is introduced allows the porosity ϕ to take any value between 0 and 1 and the geometry of the pore structure to be as complex as desired. The results are presented in terms of the formation factor [Formula: see text], with [Formula: see text] the effective conductivity of the saturated rock and [Formula: see text] the conductivity of the fluid. It is shown that the formation factor F as a function of the porosity ϕ follows a power law [Formula: see text], equivalent to the empirical Archie’s law. The exponent m varies with the microgeometry of the pore space and could therefore reflect the microstructure at the macroscopic scale. The prefactor a of the power law, however, is close to 1 regardless of the microstructure. For a given microgeometry of the pore space, the variation of the residual electrical conductivity of the solid matrix induced by a finite conductivity ratio [Formula: see text] does not significantly influence the variation of the effective conductivity of the fluid‐solid binary mixture unless the porosity is low.
Shallow seismic reflection profiling over the Brevard zone, South Carolina
Authors: Hawman Robert B. , Prosser Cynthia L. , Clippard Jeffrey E.
Published: October 2002

Shallow reflection profiles over crystalline rocks of the Brevard zone show events that correlate with projections of mapped lithologic contacts, in spite of strong attenuation and statics effects associated with a zone of severe chemical weathering (saprolite). The profiles, which were collected in western South Carolina about 5 km from Appalachian ultra Deep Core Hole (ADCOH) regional lines 1 and 3, straddle the contact between the Brevard fault zone and the wider Brevard mylonitic shear zone to the southeast. The principal goal of these experiments was to test the feasibility of using a hammer source and 24-channel seismograph to image near‐surface brittle and ductile structures to a depth of several hundred meters. We recorded two dip lines and one strike line with a total common midpoint (CMP) coverage of 1 km. Processing focused on eliminating static shifts and separating reflections from overlapping refracted S-waves. A statics approach based on the alignment of first arrivals on common‐offset gathers effectively removed most of the statics variations. Suppressing S-waves by apparent velocity filtering of shot gathers caused considerable smearing in the stacked section because of the short receiver spreads. Moreover, for shot gathers with high levels of incoherent noise and/or residual statics, velocity filtering imposed a level of coherency on the output that was not present in the original gather. Better results were obtained by applying severe surgical mutes to isolate the most reliable portions of the velocity‐filtered shot gathers prior to CMP stacking. Final CMP stacked sections show distinct packages of moderately dipping reflections with a lateral continuity of 25 to 100 m over a depth range of 85 to 400 m. The reflections are attributed to compositional layering, variations in degree of anisotropy, near‐horizontal expansion joints, and variations in fracture density. Southeast‐dipping reflections imaged for a 450-m dip line straddling the southeast flank of the brittle Brevard fault zone correlate with projected lithologic contacts of thrust‐faulted units mapped at the surface. Reflections show poorer continuity for lines recorded over a thick sequence of more uniform lithology within the adjacent Brevard mylonite zone. Reflections with apparent dips to the northwest are consistent with images in the ADCOH regional profiles and may reflect late‐stage folding and normal faulting.
Shear‐wave velocity and density estimation from PS-wave AVO analysis: Application to an OBS dataset from the North Sea
Authors: Jin Side , Cambois G. , Vuillermoz C.
Published: October 2002

S-wave velocity and density information is crucial for hydrocarbon detection, because they help in the discrimination of pore filling fluids. Unfortunately, these two parameters cannot be accurately resolved from conventional P-wave marine data. Recent developments in ocean‐bottom seismic (OBS) technology make it possible to acquire high quality S-wave data in marine environments. The use of (S)-waves for amplitude variation with offset (AVO) analysis can give better estimates of S-wave velocity and density contrasts. Like P-wave AVO, S-wave AVO is sensitive to various types of noise. We investigate numerically and analytically the sensitivity of AVO inversion to random noise and errors in angles of incidence. Synthetic examples show that random noise and angle errors can strongly bias the parameter estimation. The use of singular value decomposition offers a simple stabilization scheme to solve for the elastic parameters. The AVO inversion is applied to an OBS data set from the North Sea. Special prestack processing techniques are required for the success of S-wave AVO inversion. The derived S-wave velocity and density contrasts help in detecting the fluid contacts and delineating the extent of the reservoir sand.
Marine magnetotellurics for base‐of‐salt mapping: Gulf of Mexico field test at the Gemini structure
Authors: Hoversten G. Michael , Constable Steven C. , Morrison H. Frank
Published: October 2002

A sea‐floor magnetotelluric (MT) survey was conducted over the Gemini subsalt petroleum prospect in the Gulf of Mexico (GOM) to demonstrate that the base of salt can be mapped using marine magnetotelluric (MMT) methods. The high contrast in electrical resistivity between the salt and the surrounding sediments provides an excellent target for MMT. The Gemini salt body, located at 28° 46′ N 88° 36′ W, is a relatively complex shape buried 2–5 km below the sea floor in 1-km-deep water. Its geometry has been previously determined using 3-D seismic prestack depth migration with well log control. In order not to confuse limitations in interpretation technique with limitations in data acquisition, numerical forward and inverse modeling guided the survey design to locate a profile that would be amenable to 2-D inversion, even though the body was clearly 3-D. The seismic imaging of the base of salt along the chosen profile is considered good, thus providing a good control for testing the MT method. In many other areas of the GOM, and indeed other portions of the Gemini structure itself, seismic imaging of the base of salt is problematic. Data were collected using autonomous sea‐floor data loggers equipped with induction coil magnetic sensors and electric field sensors consisting of silver‐silver chloride electrodes connected to an ac‐coupled amplifier originally designed for sea‐floor controlled‐source studies. Nine sites of excellent quality MT responses were obtained. Smooth 2-D inversion of the data produce a confined resistive anomaly at the correct location and depth, and recently developed sharp‐boundary 2-D inversion recovers base of salt in excellent agreement with the seismic models. Simple perturbation analysis shows that base of salt has been resolved to within 5–10% of burial depth.
The application of geophysics during evaluation of the Century zinc deposit
Authors: Mutton Andrew J.
Published: October 2002

During the period 1990 to 1995, experimental programs using high‐resolution geophysics at several Australian operating mines and advanced evaluation projects were undertaken. The primary aim of those programs was to investigate the application of geophysical technology to improving the precision and economics of the ore evaluation and extraction processes. Geophysical methods used for this purpose include: 1) borehole geophysical logging to characterize ore and rock properties more accurately for improved correlations between drill holes, quantification of resource quality, and geotechnical information. 2) imaging techniques between drill holes to map structure directly or to locate geotechnical problems ahead of mining. 3) high‐resolution surface methods to map ore contacts and variations in ore quality, or for geotechnical requirements. In particular, the use of geophysics during evaluation of the Century zinc deposit in northern Australia demonstrated the potential value of these methods to the problems of defining the lateral and vertical extent of ore, quantitative density determination, prediction of structure between drill holes, and geotechnical characterization of the deposit. An analysis of the potential benefit of using a combination of borehole geophysical logging and imaging suggested that a more precise structural evaluation of the deposit could be achieved at a cost of several million dollars less than the conventional evaluation approach based on analysis from diamond drill‐hole logging and interpolation alone. The use of geophysics for the Century evaluation also provided substance to the possibility of using systematic geophysical logging of blast holes as an integral part of the ore extraction process. Preliminary tests indicate that ore boundaries can be determined to a resolution of several centimeters, and ore grade can be estimated directly to a usable accuracy. Applying this approach routinely to production blast holes would yield potential benefits of millions of dollars annually through improved timeliness and accuracy of ore boundary and quality data, decreased dilution, and improved mill performance. Although the indications of substantial benefits resulting from the appropriate and timely use of geophysics at Rio Tinto’s mining operations are positive, some challenges remain. These relate largely to the appropriate integration of the technology with the mining process, and acceptance by the mine operators of the economic value of such work. Until the benefits are demonstrated clearly over time, the use of geophysics as a routine component of evaluation and mining is likely to remain at a low level.
Systematic error analysis of demagnetization and implications for magnetic interpretation
Authors: Guo Wanwu , Dentith Michael C. , Bird Robert T. , Clark David A.
Published: October 2002

Demagnetization can affect the interpretation of magnetic data significantly. However, little attempt has been made to understand its effects by analyzing systematically the differences between demagnetization‐ corrected and uncorrected magnetic properties. A systematic error analysis is made in this paper using a 2-D elliptic cylinder model. Generally, demagnetization changes the effective susceptibility and remanence or the effective magnetization in both magnitude and direction. Error analyses show that demagnetization causes the magnitude of effective magnetization of a magnetic body to be less than its intrinsic magnetization. This implies that a theoretical anomaly computed without accounting for demagnetization will overestimate the amplitude of the anomaly associated with the body. The decrease in magnetization magnitude depends on the intrinsic magnetic susceptibility of a body as well as on the body’s geometry (flattening ratio) and its relative orientation (magnetic dip) in the geomagnetic field. The magnitude of the effective magnetization, relative to the intrinsic magnetization, decreases with increasing intrinsic magnetic susceptibility. This factor dominates the body’s effective magnetization. When intrinsic magnetic susceptibility is less than 0.1 SI, the demagnetization effects are generally insignificant and may be ignored in magnetic anomaly modeling. The magnetic dip and flattening ratio only cause minor fluctuations in the effective magnetization. Demagnetization also changes the direction of the effective magnetization vector by making it approach the plane of flattening of any flattened body. The difference between the inclinations of the effective and intrinsic magnetization changes the horizontal positions of extreme values of an anomaly, which may affect the precision of magnetic interpretations. Generally, the inclination difference is significant for magnetic dips of 30° to 70° and increases with increasing susceptibility and decreasing flattening ratio. In particular, for large flat‐lying magnetic geological units located at middle magnetic latitudes (30° to 70°), significant magnetic inclination deflections are expected because of demagnetization effects. Theoretical, experimental, and practical examples of magnetic interpretation are presented to illustrate these demagnetization effects.
Elastic properties of gas hydrate‐bearing sediments
Authors: Lee Myung W. , Collett Timothy S.
Published: October 2002

Downhole‐measured compressional- and shear‐wave velocities acquired in the Mallik 2L-38 gas hydrate research well, northwestern Canada, reveal that the dominant effect of gas hydrate on the elastic properties of gas hydrate‐bearing sediments is as a pore‐filling constituent. As opposed to high elastic velocities predicted from a cementation theory, whereby a small amount of gas hydrate in the pore space significantly increases the elastic velocities, the velocity increase from gas hydrate saturation in the sediment pore space is small. Both the effective medium theory and a weighted equation predict a slight increase of velocities from gas hydrate concentration, similar to the field‐observed velocities; however, the weighted equation more accurately describes the compressional- and shear‐wave velocities of gas hydrate‐bearing sediments. A decrease of Poisson’s ratio with an increase in the gas hydrate concentration is similar to a decrease of Poisson’s ratio with a decrease in the sediment porosity. Poisson’s ratios greater than 0.33 for gas hydrate‐bearing sediments imply the unconsolidated nature of gas hydrate‐bearing sediments at this well site. The seismic characteristics of gas hydrate‐bearing sediments at this site can be used to compare and evaluate other gas hydrate‐bearing sediments in the Arctic.
3‐D imaging using higher order fast marching traveltimes
Authors: Popovici Alexander Mihai , Sethian James A.
Published: October 2002

Recently, fast marching methods (FMM) beyond first order have been developed for producing rapid solutions to the eikonal equation. In this paper, we present imaging results for 3‐D prestack Kirchhoff migration using traveltimes computed using the first‐order and second‐order FMM on several 3‐D prestack synthetic and real data sets. The second order traveltimes produce a much better image of the structure. Moreover, insufficiently sampled first order traveltimes can introduce consistent errors in the common reflection point gathers that affect velocity analysis. First‐order traveltimes tend to be smaller than analytic traveltimes, which in turn affects the migration velocity analysis, falsely indicating that the interval velocity was too low.
Finite‐difference modeling of wave propagation in a fluid–solid configuration
Authors: van Vossen Robbert , Robertsson Johan O. A. , Chapman Chris H.
Published: October 2002

Finite‐difference (FD) techniques are widely used to model wave propagation through complex structures. Two main sources of error can be identified: (1) from numerical dispersion and numerical anisotropy and (2) by modeling the response of internal grid boundaries. Conventional discretization criteria to reduce the effects of numerical dispersion and numerical anisotropy have long been established (5‐8 gridpoints per wavelength for a fourth‐order accurate FD scheme). We analyze the second source of errors, comparing different staggered‐grid FD solutions to the Cagniard‐de Hoop solution in models with fluid–solid contacts. Our results confirm that it is sufficient to rely on conventional discretization criteria if the fluid–solid interface is aligned with the grid. If accurate modeling of the Scholte wave is required, then a new imaging method we propose should be used to allow for conventional sampling of the wavefield to minimize numerical dispersion. However, for an interface not aligned with the grid (irregular interfaces), a spatial sampling of at least 15 gridpoints per minimum wavelength is required to obtain acceptable results, particularly in seismic seabed applications where Scholte waves may need to be modeled more accurately.
On: “Magnetic Interpretation” by D. Wantland
Authors: Peters Leo J.
Published: October 2002

The second paragraph of the introduction to Mr. Wantland’s paper, if amplified by a second one, would give the key to a real understanding of the problems of magnetic interpretation. This second statement should be that any interpretation of magnetic data should fit the observed distribution of magnetic intensity quantitatively as well as qualitatively. It is not until we ask quantitative questions that much progress can be made.
Reply to discussion by H. D. Outmans
Authors: Biot M. A. , Odé H.
Published: October 2002

In his criticism of our paper, Dr. Outmans objects to the assumption that salt flows as a Newtonian viscous material, holding that in reality it flows as a “solid.” This objection is based on the reasoning that in several cases shale material around salt domes has been pierced and brecciated by the rising salt. The faulting involved in the creation of the shale breccia is stated by Dr. Outmans to be “typically a property of a solid,” and according to him does not take place in a Newtonian viscous material. In consequence, any analysis of salt dome structures based on the assumption of Newtonian viscosity must be considered void.
Reply by author to the discussion by William A. Schneider
Authors: French William S.
Published: October 2002

I appreciate Schneider’s comments on the subject paper, and it would appear that we are in complete agreement concerning the relative importance of diffraction effects in the recordings of reradiated wave fields. The point in question, however, is the importance of this diffracted energy in the inverse problem as applied in petroleum exploration. In other words, in the wave field reconstruction step described in Appendix B of the paper, do those hydrophones which record specular energy from a given subsurface point contribute more or less information to the final geologic picture than those hydrophones recording possible diffraction energy from the same subsurface point? A related question is whether the diffracted energy alone is sufficient for imaging moderately complex geology. Schneider suggests that diftraction energy alone is sufficient. I would like to discuss some observations which have led me to the opinion that diffraction energy alone is insufficient.
Reply by the author to Nelson C. Steenland
Authors: Jacobsen Bo Holm
Published: October 2002

I fail to see where this comment of Dr. Steenland affects the validity of the contents of my paper. On the other hand, several of the misunderstandings in Dr. Steenland’s comment are not trivial and may be shared by other readers. Therefore I find it relevant to discuss in some detail the issues he brings up.
Reply by author to A. I. Bayoumi and E. M. Abdelrahman
Authors: Riad Samir
Published: October 2002

Bouguer anomalies in the area tested are mainly found to be due to structures on the basement and within the sedimentary cover. Lithologic variations in the basement are the cause of Bouguer anomalies in too few areas. Detailed quantitative studies of these areas showed that lithologic variations only cannot explain the given anomalies. So, even in such areas anomalies were found to be due to both effects, structures, and lithologic variations (Othman, 1976; Sayed, 1977; Moustafa et al, 1977). The effect of lithologic variations and basic intrusions within the sedimentary cover was found to be very small (less than 1 mgal in all cases) (Tealeb, 1973) and consequently will not be reflected on a Bouguer map of the studied scale. Thus we still believe that most lineaments traced from the Bouguer anomalies are due to structures (faults).
Reply by the author to M. J. Hall
Authors: Hammer Sigmund
Published: October 2002

The definitive article “Airborne gravity is here!,” as well as my responses to published “Discussions” of it, were consciously directed to all readers of Geophysics. After 55 years of associations with geologists, geophysicists, exploration managers, university students, and editors (with more than 50 publications), I believe that I may have some competence to judge this.
Reply by the authors to the discussion by Gildas Omnes
Authors: Gupta P. K. , Niwas Sri , Gaur V. K.
Published: October 2002

We are intrigued by Dr. Gildas Omnes’ reference to “misleading statements” in our above mentioned paper. It is a fact universally acknowledged that most inverse problems in geophysics are underdetermined and ill posed, possessing no unique solution. One is therefore left with the only option of seeking the “best” solution by heuristically minimizing certain norms or their weighted combinations. The “goodness” of the solution is qualified by the “closeness” of the inverse solution to the actual model parameters which can be tested using “difficult to invert” standard models. This is the basic premise on which the paper is based with claim to “goodness of the solution” and not uniqueness.
Seismoelectric wave conversions in porous media: Field measurements and transfer function analysis
Authors: Garambois Stéphane , Dietrich Michel
Published: October 2002

We present a series of field experiments showing the transient electric fields generated by a seismic excitation of the subsurface. After removing the powerline noise by adaptive filtering, the most prominent feature of the seismoelectric recordings is the presence of electric signals very similar to conventional seismic recordings. In one instance, we identified small‐amplitude precursory electromagnetic disturbances showing a polarity reversal on either side of the shotpoint. Concentrating on the dominant seismoelectric effect, we theoretically show that the electric field accompanying the compressional waves is approximately proportional to the grain acceleration. We also demonstrate that the magnetic field moving along with shear waves is roughly proportional to the grain velocity. These relationships hold true as long as the displacement currents are much smaller than the conduction currents (diffusive regime), which is normally the case in the low‐frequency range used in seismic prospecting. Furthermore, the analytical transfer functions thus obtained indicate that the electric field is mainly sensitive to the salt concentration and dielectric constant of the fluid, whereas the magnetic field principally depends on the shear modulus of the framework of grains and on the fluid’s viscosity and dielectric constant. Both transfer functions are essentially independent of the permeability. Our results suggest that the simultaneous recording of seismic, electric, and magnetic wavefields can be useful for characterizing porous layers at two different levels of investigation: near the receivers and at greater depth.
Traveltime inversion for 2‐D anomaly structures
Authors: Velis Danilo R.
Published: October 2002

This work presents a traveltime inversion method that uses parametric functions to represent 2‐D anomaly structures. These functions are described by a small set of unknown parameters which in turn are obtained after solving a highly nonlinear optimization problem via simulated annealing (SA). The procedure favors neither smooth nor high contrasting anomalies and keeps the number of unknowns very small so as to make the problem tractable using SA. Yet the strategy allows one to accommodate a large class of velocity models. Results indicate that this new approach typically yields better images than a standard linearized inversion based on a cell parameterization scheme.
Ellipsoid, geoid, gravity, geodesy, and geophysics
Authors: Li Xiong , Götze Hans‐Jürgen
Published: October 2002

Geophysics uses gravity to learn about the density variations of the Earth’s interior, whereas classical geodesy uses gravity to define the geoid. This difference in purpose has led to some confusion among geophysicists, and this tutorial attempts to clarify two points of the confusion. First, it is well known now that gravity anomalies after the “free‐air” correction are still located at their original positions. However, the “free‐air” reduction was thought historically to relocate gravity from its observation position to the geoid (mean sea level). Such an understanding is a geodetic fiction, invalid and unacceptable in geophysics. Second, in gravity corrections and gravity anomalies, the elevation has been used routinely. The main reason is that, before the emergence and widespread use of the Global Positioning System (GPS), height above the geoid was the only height measurement we could make accurately (i.e., by leveling). The GPS delivers a measurement of height above the ellipsoid. In principle, in the geophysical use of gravity, the ellipsoid height rather than the elevation should be used throughout because a combination of the latitude correction estimated by the International Gravity Formula and the height correction is designed to remove the gravity effects due to an ellipsoid of revolution. In practice, for minerals and petroleum exploration, use of the elevation rather than the ellipsoid height hardly introduces significant errors across the region of investigation because the geoid is very smooth. Furthermore, the gravity effects due to an ellipsoid actually can be calculated by a closed‐form expression. However, its approximation, by the International Gravity Formula and the height correction including the second‐order terms, is typically accurate enough worldwide.
Repeatability study of seismic source signatures
Authors: Aritman Bibi C.
Published: October 2002

This study discusses the repeatability of source signature using the instantaneous phase, as derived from the complex trace attributes. The study is part of a very large 2‐D seismic survey using sources of vibroseis and surface dynamite. The field procedure consisted of recording the production record, retaining the positions, then repeat‐recording the same shake or shot. The instantaneous phase was found to be the best measure for the difference between the first and the repeat records. In addition to the instantaneous phase, other analyses were used to evaluate changes in source signature. Results were tabulated for statistical comparisons and graded for quality. Excluding erroneous cases, the remainder of poor repeatabilities were studied. The analyses of near‐offset data seem to indicate that nonrepeatability of source signature relates mostly to changes in absorption and cohesion induced by elastic saturation at the near surface. In general, by time shifting and phase rotating the repeat record, the difference in instantaneous phase tends to diminish. The new idea of using instantaneous phase difference plots to evaluate repeatability offers improved evaluation of source signatures and can also be used to detect time‐lapsed changes in reservoir monitoring. By evaluating repeatability and avoiding elastic saturation near the surface, source signatures can be made more consistent, thus increasing the resolution of stacked data for 2‐D, 3‐D, and 4‐D seismic surveys.