bluuebunny/crossref_metadata_2025 huggingface dataset:
Resistivity imaging of controlled‐source audiofrequency magnetotelluric data
Authors: Sasaki Yutaka , Yoneda Yoshihiro , Matsuo Koichi
Published: October 2002

The controlled‐source audiofrequency magnetotelluric (CSAMT) method is an electromagnetic (EM) method where a transmitter (a grounded electric bipole or a current loop) is placed far away from the receiver sites. If the transmitter is located at distances greater than 3–5 skin depths, the plane-wave approximation is valid and the techniques used for (natural source) MT interpretation can be applied (Goldstein and Strangway, 1975; Sandberg and Hohmann, 1982). The CSAMT method can be employed in a detailed survey by closely spacing a number of receiver sites along a traverse. The borehole CSAMT technique is proposed in West and Ward (1988) to enhance the ability of the surface CSAMT method to detect subsurface inhomogeneities. In these cases, two‐dimensional (2-D) smooth‐model inversion (Rodi et al., 1984; Sasaki, 1989; deGroot‐Hedlin and Constable, 1990) would be particularly useful for deriving a resistivity model from the far‐field data and for presenting the survey results in the form of an image.
Electromagnetic modeling of 3-D sources over 2-D inhomogeneities in the time domain
Authors: Leppin Michael
Published: October 2002

A numerical method is presented by which the transient electromagnetic response of a two‐dimensional (2-D) conductor, embedded in a conductive host rock and excited by a rectangular current loop, can be modeled. This 2.5-D modeling problem has been formulated in the time domain in terms of a vector diffusion equation for the scattered magnetic induction, which is Fourier transformed into the spatial wavenumber domain in the strike direction of the conductor. To confine the region of solution of the diffusion equation to the conductive earth, boundary values for the components of the magnetic induction on the ground surface have been calculated by means of an integral transform of the vertical component of the magnetic induction at the air‐earth interface. The system of parabolic differential equations for the three magnetic components has been integrated for 9 to 15 discrete spatial wavenumbers ranging from [Formula: see text] to [Formula: see text] using an implicit homogeneous finite‐difference scheme. The discretization of the differential equations on a grid representing a cross‐section of the conductive earth results in a large, sparse system of linear equations, which is solved by the successive overrelaxation method. The three‐dimensional (3-D) response has been computed by an inverse Fourier transformation of the cubic spline interpolated scattered magnetic induction in the wavenumber domain using a digital filtering technique. To test the algorithm, responses have been computed for a two‐layered half‐space and a vertical prism embedded in a conductive host rock. These examples were then compared with results obtained analytically or numerically using frequency‐domain finite‐element and time‐domain integral equation methods. The new numerical procedure gives satisfactory results for a wide range of 2-D conductivity distributions with conductivity ratios exceeding 1:100, provided the grid is sufficiently refined at the corners of the conductivity anomalies.
A vibroseis reflection seismic survey at the Buchans Mine in central Newfoundland
Authors: Spencer Carl , Thurlow Geoffrey , Wright James , White Don , Carroll Peter , Milkereit Bernd , Reed Laurie
Published: October 2002

We present results from the first major vibroseis seismic reflection survey at a mine site in North America. It is demonstrated that conventional high‐stack fold reflection seismic techniques can image fault structures associated with volcanogenic massive sulfide bodies, despite the fact that these structures are locally steeply dipping and produce records with low signal‐to‐noise ratios. The new lines were recorded at the locations of two earlier experimental explosive surveys that failed to image many strong reflectors. The principal reasons for the success of the vibroseis experiment were the proper choice of sweep frequencies for maximum signal‐to‐noise ratio, the use of high‐stack fold, and the careful analysis of velocities and statics during processing. A comparison of the new seismic sections with borehole and other geophysical data indicates that the origin of reflections at Buchans can be attributed to various mechanisms including contrasts in lithologies and rock competence and intrusion of diabase sills into pre‐existing fault zones. The best reflections emanate from shallow‐dipping brittle‐ductile thrust fault zones characterized by fault gouge within broader fractured zones. The two seismic lines presented have proven to be a useful and cost‐effective supplement to existing borehole and geophysical data and have provided enough new information on the nature of thrust faulting at the mine to significantly influence current exploration plans.
Homomorphic processing of the tube wave generated during acoustic logging
Authors: Ellefsen Karl J. , Burns Daniel R. , Cheng C. H.
Published: October 2002

We have developed a new method to process the tube wave, which is generated during acoustic logging, to obtain estimates for its wavenumber, attenuation coefficient, amplitude, and phase at every frequency. To improve the accuracy of the estimates, the method can use data from multiple sources and data collected at successive depths in the borehole. This new method has several advantages over other methods that are currently used to process acoustic logging data: the new method can obtain accurate estimates of the wavenumber and amplitude from only a few receivers; the receivers can be irregularly spaced; and no spurious estimates are generated. Nonetheless, this new method has one disadvantage compared to others: it can only estimate the parameters for one, high‐amplitude wave like the tube wave. Also, like all other existing methods, the new method obtains only reasonable estimates for the attenuation coefficient when data from many receivers are processed.
Interpretive gridding by anisotropic kriging
Authors: Hansen R. O.
Published: October 2002

Most interpolation algorithms perform poorly on data sampled along profiles crossing features whose length scales are small along the profiles but large transverse to them, such as lineaments. Rather than reproducing the linear features, these algorithms create a series of closures around the profiles. By introducing additional information into the algorithm, in particular by using an anisotropic covariance model for kriging that contains a priori information about the lineations, more realistic results can be obtained. An algorithm of this type produces a much more reasonable map of aeromagnetic data from the Cobb Offset zone of the Juan de Fuca Ridge than either minimum curvature gridding or isotropic kriging.
A field test of an electrodeless arc discharge, borehole seismic source
Authors: Mahrer Kenneth D. , Zook Brian J.
Published: October 2002

Waveforms generated by an impulsive, 1.2 kJ, seven‐conductor wireline electrodeless arc discharge borehole seismic source or sparker at Texaco’s Humble, TX field test site were recorded by three borehole sensor arrays: two free‐hanging hydrophone streamers in in‐line boreholes at 82 m and 170 m from the source well and a grouted, three‐component geophone string in a borehole 110 m from the source well. A repeatability test of the source, consisting of single firings of the source at a rate of 1 firing per 5 s, showed very clean, very strong, Ricker‐like wavelets. Despite a high‐degree of attenuation (exact value of Q is not known), the useful frequency passband of the wavelets was from 200 Hz to 1200 Hz for the data recorded by the 82-m offset hydrophones and 200 Hz to 500 Hz for the 170-m hydrophones. Using 62 single‐firing wavelets recorded in the 82-m offset well gave mean and median crosscorrelations greater than 0.96 with standard deviations less than 0.02. A stack test, consisting of 1, 2, 4, 8, 16, and 32-stacked waveforms, confirmed the shape, strong S/N ratio, and high correlation of the sparker output. The 32-stack, which took less than 3 minutes to generate, was recorded by the noisy, near-surface geophones at a raypath distance of nearly 300m.
Elastic modeling in discontinuous media
Authors: Cunha Carlos A.
Published: October 2002

Results from elastic‐wave simulations in a simple model show that for models characterized by a set of layers with sharp boundaries (discontinuous stiffness tensor), traditional finite‐difference methods fail to correctly describe the dynamics of the propagation process. The failure comes from the lack of distinction between model and field variables; the same differential operator is applied to discontinuous (model) and continuous (wavefield) components. This problem is solved with a modified high‐order finite‐difference modeling scheme (dual‐operator method) that uses two distinct operators for evaluating the model and field derivatives, a modified version of Virieux‐staggered grid, and Shoenberg‐Muir averaging relations. The dual‐operator method generates a dynamic response that is more accurate than traditional finite‐difference schemes and comparable to Haskell‐Thomson propagator‐matrix schemes, with the advantage that it can be applied to structurally complex models. The method produces stable, accurate results for both solid and liquid layers.
Modulus decomposition of compressional and shear velocities in sand bodies
Authors: Murphy William , Reischer Andrew , Hsu Kai
Published: October 2002

The advent of borehole shear slowness measurements in sonically slow formations has lead to breakthroughs in the subsurface profiling of geological bodies. In sand bodies, compressional and shear velocities depend predictably on porosity, mineralogy, grain contacts, and fluid saturation. An interpretation is best performed by decomposing the velocities into moduli that are intrinsic measures of the rock frame and pore fluid compressibilities. Careful experiments on pure materials (i.e., pure quartz sandstones) demonstrate two simplifying constitutive relationships. First, the bulk and shear frame moduli are simple functions of the porosity. A comparison of the measured shear frame modulus to the prediction for the pure material distinguishes sand from shale. Second, the ratio of the bulk and shear frame moduli is a constant 0.9 independent of the porosity. The measured velocities are directly inverted to yield the bulk modulus of the pore fluid. The fluid saturation effects are so dramatic at high porosity that not only gas but oil may also be distinguished from water. The relationships are tested in several case studies where the results are encouraging. Finite‐frequency effects may complicate the interpretation where filtrate invasion is significant. Attenuation provides further information because compressional absorption is particularly sensitive to gas saturation. A potential application of the modulus decomposition may be to quantify, in amplitude versus offset seismics proximal to the well‐bore, the fractional change in shear frame modulus from the fractional change in pore fluid modulus.
Practical steps toward realizing the potential of Monte Carlo automatic statics
Authors: DuBose John B.
Published: October 2002

This work develops a number of innovations to the simulated annealing Monte Carlo automatic statics technique. Frequency‐domain processing reduces the need for computer memory and helps provide rich sampling. Realistic search limits drastically reduce the computational load and prevent wasted processing. A modification to the objective function provides more stability. Finally, a fast and simple algorithm is introduced to remove null space and long wavelength components from successive solutions. This algorithm is used in an automatic temperature control process to improve convergence. Together, these innovations make Monte Carlo automatic statics more efficient and easier to use.
Spectral analysis and inversion of experimental codas
Authors: Jannaud L. R. , Adler P. M. , Jacquin C. G.
Published: October 2002

A method developed for the determination of the characteristic lengths of an heterogeneous medium from the spectral analysis of codas is based on an extension of Aki’s theory to anisotropic elastic media. An equivalent Gaussian model is obtained and seems to be in good agreement with the two experimental data sets that illustrate the method. The first set was obtained in a laboratory experiment with an isotropic marble sample. This sample is characterized by a submillimetric length scale that can be directly observed on a thin section. The spectral analysis of codas and their inversion yields an equivalent correlation length that is in good agreement with the observed one. The second data set is obtained in a crosshole experiment at the usual scale of a seismic survey. The codas are recorded, analysed, and inverted. The analysis yields a vertical characteristic length for the studied subsurface that compares well with the characteristic length measured by seismic and stratigraphic logs.
A simple method for resolving large converted‐wave (P-SV) statics
Authors: Cary Peter W. , Eaton David W. S.
Published: October 2002

The processing of converted‐wave (P-SV) seismic data requires certain special considerations, such as commonconversion‐point (CCP) binning techniques (Tessmer and Behle, 1988) and a modified normal moveout formula (Slotboom, 1990), that makes it different for processing conventional P-P data. However, from the processor’s perspective, the most problematic step is often the determination of residual S‐wave statics, which are commonly two to ten times greater than the P‐wave statics for the same location (Tatham and McCormack, 1991). Conventional residualstatics algorithms often produce numerous cycle skips when attempting to resolve very large statics. Unlike P‐waves, the velocity of S‐waves is virtually unaffected by near‐surface fluctuations in the water table (Figure 1). Hence, the P‐wave and S‐wave static solutions are largely unrelated to each other, so it is generally not feasible to approximate the S‐wave statics by simply scaling the known P‐wave static values (Anno, 1986).
Quantitative determination of uncertainties in seismic refraction prospecting
Authors: Liu Chuanhai , Stock Joann M.
Published: October 2002

We present a model of the propagation of refracted seismic waves in planar (horizontal or dipping) layered structures in which we quantify the errors from various sources. The model, called the (mixed) variance component model, separates the errors originating on the surface from those due to inhomogeneities of subsurface layers. The model starts with the assumption of homogeneous (constant‐velocity) layers, but by taking the principal errors into account, variations from this model (including degree of velocity inhomogeneity, vertical velocity gradients, and gradational interfaces) can be identified. A complete solution to the variance component model by Bayesian methods relies on the Gibbs sampler, a recently well‐developed statistical technique. Using the Gibbs sampler and Monte Carlo methods, we can estimate the posterior distributions of any parameter of interest. Thus, in addition to estimating the various errors, we can obtain the velocity‐versus‐depth curve with its confidence intervals at any relevant point along the line. We analyze data from a crustal‐scale refraction line to illustrate both features of this method. The results indicate that the conventional linear regression model for the first arrivals is inappropriate for this data set. As might be expected, geophone spacing strongly affects our ability to resolve the heterogeneities. Differences in the amount of velocity heterogeneity in different layers can be resolved, and may be useful for lithologic characterization. For this crustal‐scale problem, a velocity profile derived from this method is an improvement over simple linear interpretations, but it could be further refined by more comprehensive methods attempting to match later arrivals and wave amplitudes as well as first arrivals. The method could also be applied to smaller‐scale refraction problems, such as determination of refraction statics, or constraints on the degree of probable lateral variations in velocity of shallow layers, for improved processing of reflection data.
Streamer compass validation and verification
Authors: Court Ian N.
Published: October 2002

During the acquisition of seismic data in a marine environment, the positions of the hydrophone groups are derived from data obtained using compass units deployed at intervals along the length of a streamer. Interpretation of the compass data to derive the streamer position has always proved difficult. The data is a set of measurements of spatial gradients at known distances along the streamer. The integration method used to derive the streamer shape requires additional data to resolve the undetermined constant. Thus the derived positions of the hydrophone groups have a translational indeterminacy. The compass measurements are independent and all measured azimuths can he expected to have similar error estimates. However streamer shapes, derived using a least‐squares polynomial fitting algorithm are critically dependent on the accuracy of data obtained from the first and last units. These features have lead to innovative schemes to position the two ends of the streamers. Lasers, acoustics, and radio positioning, have all been used to locate accessible points closest to the streamer ends viz the towpoints and the tailbuoy. While this additional data has improved the quality of streamer positioning, these methods still leave the compasses as the only means of generating position information in the region where the hydrophone groups are actually situated.
Modeling complexly magnetized two‐dimensional bodies of arbitrary shape
Authors: Mariano John , Hinze William J.
Published: October 2002

A method has been devised for the forward computation of magnetic anomalies due to two‐dimensional (2-D) polygonal bodies with heterogeneously directed magnetization. The calculations are based on the equivalent line source approach wherein the source is subdivided into discrete elements that vary spatially in their magnetic properties. This equivalent dipole line method provides a fast and convenient means of representing and computing magnetic anomalies for bodies possessing complexly varying magnitude and direction of magnetization. The algorithm has been tested and applied to several generalized cases to verify the accuracy of the computation. The technique has also been used to model observed aeromagnetic anomalies associated with the structurally deformed, remanently magnetized Keweenawan volcanic rocks in eastern Lake Superior. This method is also easily adapted to the calculation of anomalies due to two and one‐half‐dimensional (2.5-D) and three‐dimensional (3-D) heterogeneously magnetized sources.
Eigenvalues and eigenvectors of linearized elastic inversion
Authors: de Nicolao Andrea , Drufuca Giuseppe , Rocca Fabio
Published: October 2002

Small contrasts in the parameters allow the linearization of elastic modeling and inversion. Although this assumption simplifies parameter estimation, it also impoverishes information: a linearized model, besides being an approximation, requires shrinking the data space to precritical angles. This work investigates the two basic questions regarding the type of information that can be retrieved and how to retrieve it. The method is the singular value decomposition of the transfer function between data and parameters. This tool allows a simple interpretation of the information contained in reflections, and it outlines a general estimation procedure. A medium characterized by a uniform background is considered; reflections are linearized according to the Born approximation. P-P reflections are analyzed. The simplicity of the model allows a theoretical study of the effects of velocity errors in the overburden. The analysis is performed for two different sets of elastic parameters. In both cases the conclusion is that only the estimate of one parameter can be accurate while the quality of the estimate of a second parameter critically depends upon the velocity uncertainty and the maximum incident angle; estimation of a third parameter is very difficult.
On: “Effect of temporal and spatial variations of the primary signal on VLF total‐field surveys” by M. A. Vallee, M. Chouteau, and G. J. Palacky (January 1992 GEOPHYSICS, p. 97‐105)
Authors: Roy Jean
Published: October 2002

Vallee et al. (1992) remark on the sensitivity of airborne ratio measuring VLF instruments to platform attitude stability. The authors also remind the users of VLF total field amplitude data, as produced by instruments such as the Herz TOTEM, of two problems associated with this type of data: spatial and temporal fluctuations of the VLF primary field. They recommend the use of a dense network of VLF monitoring stations and numerical modeling of field propagation to cope with these problems. These two recommendations are briefly discussed here and one alternative recommendation is made.
A new approach to imaging with low‐frequency electromagnetic fields
Authors: Lee Ki Ha , Xie Ganquan
Published: October 2002

We present a new method for interpreting electromagnetic (EM) data using ray tomography. Direct application of ray tomography to low‐frequency EM data is difficult because of the diffusive nature of the field. Diffusive EM fields can, however, be mathematically transformed to wavefields defined in a time‐like variable. The transform uniquely relates a field satisfying a diffusion equation in time, or in frequency, to an integral of the corresponding wavefield. If the corresponding wavefields can be computed from low‐frequency EM data, one should be able to interpret these data using techniques developed for the wavefields. To test the idea, numerically calculated transient magnetic fields were first transformed to wavefields. The typical window of the time‐domain data required for the transform is 1.5 decades. Traveltimes from a source to the receivers were estimated from the reconstructed wavefields. Time‐domain data with a Gaussian noise of 3 percent gave a traveltime resolution of better than one percent. For the tomographic inversion, the cross‐section between the transmitter and receiver boreholes is divided into a number of rectangular elements, and a continuous slowness is assigned to each of these elements. A functional is formulated by invoking Fermat’s principle for the traveltime data. Imposing a stationary condition on the functional gives an iterative procedure for the slowness model. Rays are allowed to bend smoothly within each cell. Incorporating smoothly bending rays is extremely important when the velocity contrast is large. A model with a conductivity contrast of ten (10) has been successfully imaged in 120 iterations with 5 CPU hours on a SUN SPARCstation 2.
Processing remote sensing images using the 2-D FFT—Noise reduction and other applications
Authors: Watson Ken
Published: October 2002

With the development of faster and less expensive computers, it is now practical to employ algorithms for processing remote sensing images that were only feasible on large mainframe computers a short time ago. The two‐dimensional (2-D) fast Fourier transform (FFT) is a powerful means for removing noise because it can be used to design and implement efficient filters based on the observed spatial or frequency patterns of the noise. Illustrations of transforms of image points and lines in various configurations and periodic patterns are used as a tutorial to identify and reduce a variety of noise patterns of increasing complexity from an assortment of spacecraft and aircraft systems. A general strategy for filtering developed from this study involves: filtering only derived products (not original images), initially removing noise evident in the transform, and applying a “minimum” filter to reduce residual noise. The 2-D FFT is also applied to mosaicking, enlargement, registration, and extraction of albedo and slope information.
Propagation of a nonlinear seismic pulse in an anelastic homogeneous medium
Authors: Kosik Dan W.
Published: October 2002

Seismic surveys are often conducted using dynamite charges buried near the surface in unconsolidated material. In such material a large zone near the source should exist wherein nonlinear anelastic wave propagation, can be expected to take place, and have a significant impact on the way in which a seismic pulse forms and how its energy gets distributed into the surrounding medium. To obtain a solution for a propagating pulse in this zone, the equations of motion for nonlinear anelastic wave propagation, good to second order in the displacements, are solved numerically for the problem of a Gaussian pressure pulse acting on the interior cavity of a cylindrically symmetric hole in the medium. An implicit finite‐difference algorithm is used for the solution to the equations of motion for this problem. The anelastic medium is characterized by multivalued stress‐strain relations that exhibit hysteresis, and therefore a loss of energy per cycle, corresponding to a medium with a constant Q factor. Several numerical examples are calculated contrasting the nonlinear anelastic, linear anelastic, and linear elastic propagating pulses to one another. The nonlinear anelastic propagating pulse is found to have an amplitude that is several times larger than would be expected for a pulse in a linear medium and has a peak propagation velocity that is slightly less than that for a linear pulse. Dispersive effects are also evident for the nonlinear pulse.
Reasonable choices for material constants in seismic modeling
Authors: Rosenbaum Joseph H.
Published: October 2002

In exploration geophysics, the seismic response is often modeled with the assumption that the earth consists of homogeneous formations that may exhibit transverse isotropy and moderate, “constant Q” attenuation. The geophysicist usually has only incomplete knowledge of the formation characteristics and must make educated guesses at the unknown elastic and absorption parameters. An extension of the long‐wavelength‐equivalent‐medium theory to the anelastic case has been used to derive default options and stability criteria, which require minimal information from the geophysicist, appear reasonable for many earth materials and reduce to a proper description in the case of isotropy. A very simple earth model is used to demonstrate that parasitic modes are excited and the computations can become unstable when the assumed anelastic model parameters violate some stability condition or are inconsistent.
Weak elastic anisotropy and the tube wave
Authors: Norris Andrew N. , Sinha Bikash K.
Published: October 2002

Tube‐wave speed in the presence of a weakly anisotropic formation can be expressed in terms of an effective shear modulus for an equivalent isotropic formation. When combined with expressions for the speeds of the SH‐ and quasi‐SV‐waves along the borehole axis, a simple inversion procedure can be obtained to determine three of the five elasticities of a transversely isotropic (TI) formation tilted at some known angle with respect to the borehole axis. Subsequently, a fourth combination of elastic moduli can be estimated from the expression for the qP‐wave speed along the borehole axis. The possibility of determining all five elasticities of a TI formation based on an assumed correlation between two anisotropy parameters is discussed.
On: “Magnetic interpretation using the 3-D analytic signal” by W. R. Roest, J. Verhoef, and M. Pilkington (GEOPHYSICS, 1, 116–125, January 1992).
Authors: Mohan N. L.
Published: October 2002

It is quite interesting to learn the 3-D analytic signal interpretation of Roest et al. (1992), using the vector addition. However, I am quite skeptical about their objective of determining the depth under the assumption that the magnetic anomalies are caused by vertical contacts from gridded magnetic data which, it appears to me, is nothing but an oversimplification of interpretation.
Multidomain analysis and wavefield separation of cross‐well seismic data
Authors: Rector James W. , Lazaratos Spyros K. , Harris Jerry M. , Schaack Mark Van
Published: October 2002

While cross‐well traveltime tomography can be used to image the subsurface between well pairs, the use of cross‐well reflections is necessary to image at or below the base of wells, where the reservoir unit is often located. One approach to imaging cross‐well reflections is to treat each cross‐well gather as an offset VSP and perform wavefield separation of direct and reflected arrivals prior to stacking or migration. Wavefield separation of direct and reflected arrivals in VSP is accomplished by separating the total wavefield into up and downgoing components. Since reflectors can exist both above and below the borehole wavefield, separation of cross‐well data into up‐ and downgoing components does not achieve separation of direct and reflected arrivals. In our technique, we use moveout filters applied in the domain of common vertical source/receiver offset to extract reflected arrivals from the complex total wavefield of a cross‐well seismic data set. The multiple domains available for filtering and analysis make cross‐well data more akin to multifold surface seismic data, which can also be filtered in multiple domains, rather than typical VSP data, where there is only one domain (common source) in which to filter. Wavefield separation of cross‐well data is shown to be particularly effective against multiples when moveout filters are applied in common‐offset space.
Field and/or resistivity mapping in MT-VLF and implications for data processing
Authors: Guerin Roger , Tabbagh Alain , Andrieux Pierre
Published: October 2002

The ratio of the horizontal electric and magnetic components of the electromagnetic field generated by a radio transmitter in the very low frequency (VLF) range is used in MT-VLF resistivity mapping to determine the apparent resistivity of the ground. A theoretical calculation of the responses of simple 2-D and 3-D prismatic bodies shows that the measurable lateral variations in both fields are independent, uncoupled, and correspond to frequency‐independent, static variations. It is advantageous therefore to process and map the fields separately. Processing methods used in gravimetry and magnetics are especially appropriate in correcting the apparent anisotropy related to the horizontal polarization of the primary field and in integrating (upward continuation) the high‐frequency spatial variations. The different processes tested on a synthetic case and on data obtained at the Centre de Recherches Géophysiques (CRG) test site showed how to eliminate shallow features and a current channeling conductor to favor the anomaly of a fault. A third example with a hydrogeological application shows the advantages of this method over Wenner direct‐current resistivity measurements.
Field comparison of shallow P‐wave seismic sources near Houston, Texas
Authors: Miller Richard D. , Pullan Susan E. , Steeples Don W. , Hunter James A.
Published: October 2002

A shallow P‐wave seismic source comparison was conducted at a site near Houston, Texas where the depth to the water table was approximately 7 m, and near‐surface materials consisted of clays, sands, and gravels. Data from twelve different sources during this November 1991 comparison are displayed and analyzed. Reflection events are interpretable at about 40 ms on some 220-Hz analog low‐cut filtered field files, and at 60 ms on most 110‐ and 220-Hz analog low‐cut filtered field files. Calculations and local water well information suggest the 40-ms event is from the top of the water table. Subsurface explosive sources seem to possess the highest dominant frequency, broadest bandwidth, and recorded amplitudes and, therefore, have the greatest resolution potential at this site. Our previous work and that of our colleagues suggests that, given a specific set of site characteristics, any source could dominate the comparison categories addressed here.
Inversion of wide‐angle seismic reflection times with damped least squares
Authors: Sain Kalachand , Kaila K. L.
Published: October 2002

The wide‐angle seismic reflection times appearing in the postcritical range are used extensively to image the crustal structure in deep seismic sounding investigations. The most commonly used method to calculate interval velocities and thicknesses of a stack of horizontal layers is based on Dix’s hyperbolic equation that requires traveltimes at zero offset and a prior estimate of root‐mean‐square (rms) velocity. Since the wide‐angle reflection times are represented by the nonhyperbolic Taner and Koehler series, a forced fit of such a data set by a hyperbolic equation causes large errors in the estimation of interval velocities. We propose a fast and simple method to determine the interval velocities from wide‐angle reflection times by minimizing the squared errors between the observed traveltimes and the forward response using a damped least‐squares technique. The forward response is calculated, including higher order terms of the reflection series, through Chebychev (orthogonal) polynomial approximations. Inversion of synthetic reflection times by this method, for a given velocity model contaminated by some random errors, and starting with various initial models, produces a final model that matches the true model. This modeling serves as a good indicator of the reliability of the estimated parameters. Besides, the method provides a measure of uncertainty and resolution of estimated parameters.
Transformation from 2-D to 3-D wave propagation for horizontally layered media
Authors: Amundsen Lasse , Reitan Arne
Published: October 2002

The relationship between 2-D and 3-D wave propagation in horizontally layered media was first investigated by Dampney (1971). In the last few years the usefulness and feasibility of transforming point‐source responses with 3-D geometric spreading to equivalent line‐source responses with 2-D geometric spreading have been thoroughly discussed (see Helgesen, 1990; Wapenaar et al., 1990, 1992; Herrmann, 1992; Helgesen and Kolb, 1993; Amundsen, 1993). In the case of cylindrical symmetry this transformation constitutes a required preprocessing step for several seismic processing algorithms based on 2-D wave propagation. The work of Dampney (1971) has apparently been missed by the authors discussing the 3-D to 2-D geometric spreading transform.
Interpreting data matrix asymmetry in near‐offset, shear‐wave VSP data
Authors: MacBeth Colin , Zeng Xinwu , Yardley Gareth S. , Crampin Stuart
Published: October 2002

Poor experimental control in shear‐wave VSPs may contribute to unreliable estimates of shear‐wave splitting and possible misinterpretation of the medium anisotropy. To avoid this, the acquisition and processing of multicomponent shear‐wave data needs special care and attention. Measurement of asymmetry in the recorded data matrix using singular‐value decomposition (SVD) provides a useful way of examining possible acquisition inaccuracies and may help guide data conditioning and interpretation to ensure more reliable estimates of shear‐wave polarization azimuth. Three examples demonstrate how variations in shear‐wave polarization and acquisition inaccuracies affect the SVD results in different ways. In the first example, analysis of synthetic seismograms with known depth changes in the polarization azimuth show how these may be detected. In the second example, a known source re‐orientation and polarity reversal is detected by applying SVD to near‐offset, shear‐wave VSP data, recorded in the Romashkino field, Tatar Republic. Additional information on a polarization change in the overburden is also obtained by comparing the SVD results with those for full‐wave synthetic seismograms. The polarization azimuth changes from N160°E in the overburden to N117°E within the VSP depth range. Most of the shear‐wave splitting is built up over the VSP depth range. The final example is a near‐offset, shear‐wave VSP data set from Lost Hills, California. Here, most of the shear‐wave splitting is in the shallow layers before the VSP depth range. SVD revealed a known correction for horizontal reorientation of the sources, but also exhibited results with a distinct oscillatory behavior. Stripping the overburden effects reduces but does not eliminate these oscillations. There appears to be a polarization change from N45°E in the overburden to N125°E in the VSP section. The details in these examples would be difficult to detect by visual inspection of the seismograms or polarization diagrams. Results from these preliminary analyses are encouraging and suggest that it may be possible to routinely use this, or a similar technique, to resolve changes in the subsurface anisotropy from multicomponent experiments where acquisition has not been carefully controlled.
Subsurface radar propagation deconvolution
Authors: Turner Greg
Published: October 2002

The frequency dependence of attenuation is typically far greater for subsurface radar waves than for seismic waves. Since this frequency dependence causes the subsurface radar pulse shape to change as it propagates through the earth, conventional wavelet deconvolution techniques are often inadequate to reconstruct the earth’s reflectivity series. The application of a time‐variant filter that undoes the effect of frequency‐dependent attenuation can substantially improve the resolution of subsurface radar data. This filter is minimum‐phase with amplitude characteristics equal to the inverse of the attenuation function of the medium investigated. Preliminary tests on data from surveys over a water tank, an underground mine stope, and fractured granite suggest that good results can also be achieved by assuming attenuation is linearly proportional to frequency, similar to a constant Q model.
Large‐explosive source, wide‐recording aperture, seismic profiling on the Columbia Plateau, Washington
Authors: Jarchow Craig M. , Catchings Rufus D. , Lutter William J.
Published: October 2002

Clear subsurface seismic images have been obtained at low cost on the Columbia Plateau, Washington. The Columbia Plateau is perhaps the most notorious of all “bad‐data” areas because large impedance contrasts in surface flood basalts severely degrade the seismic wavefield. This degradation was mitigated in this study via a large‐explosive source, wide‐recording aperture shooting method. The shooting method emphasizes the wide‐angle portion of the wavefield, where Fermat’s principle guarantees reverberation will not interfere with the seismic manifestations of crucial geologic interfaces. The basalt diving wave, normally discarded in standard common midpoint (CMP) seismic profiling, can be used to image basalt velocity structure via traveltime inversion. Maximum depth‐penetration of the diving wave tightly constrains basalt‐sediment interface depth. An arrival observed only at shot‐receiver offsets greater than 15 km can be used to determine the velocity and geometry of basement via simultaneous inversion. The results from this study suggest that previous geologic hypotheses and hydrocarbon play concepts for the Columbia Plateau may have been in error.
Traveltime computation in transversely isotropic media
Authors: Faria Eduardo L. , Stoffa Paul L.
Published: October 2002

An approach for calculating first‐arrival traveltimes in a transversely isotropic medium is developed and has the advantage of avoiding shadow zones while still being computationally fast. Also, it works with an arbitrary velocity grid that may have discontinuities. The method is based on Fermat’s principle. The traveltime for each point in the grid is calculated several times using previously calculated traveltimes at surrounding grid points until the minimum time is found. Different ranges of propagation angle are covered in each traveltime calculation such that at the end of the process all propagation angles are covered. This guarantees that the first‐arrival traveltime for a specific grid point is correctly calculated. The resulting algorithm is fully vectorizable. The method is robust and can accurately determine first‐arrival traveltimes in heterogeneous media. Traveltimes are compared to finite‐difference modeling of transversely isotropic media and are found to be in excellent agreement. An application to prestack migration is used to illustrate the usefulness of the method.
Finite‐difference modeling in transversely isotropic media
Authors: Faria Eduardo L. , Stoffa Paul L.
Published: October 2002

We developed a modeling algorithm for transversely isotropic media that uses finite‐difference operators in a staggered grid. Staggered grid schemes are more stable than the conventional finite‐difference methods because the differences are actually based on half the grid spacing. This modeling algorithm uses the full elastic wave equation that makes possible the modeling of all kinds of waves propagating in transversely isotropic media. The spatial derivatives are represented by fourth‐order, finite‐difference operators while the time derivative is represented by a secondorder, finite‐difference operator. The algorithm has no limitation on the acquisition geometry or on the heterogeneity of the media. The program is currently formulated to work in a 2-D transversely isotropic medium but can readily be extended to 3-D. Snapshots can be obtained at any time with no additional computational cost. A four‐layer model is used to show the usefulness of the method. Horizontal and vertical component seismograms are modeled in transversely isotropic media and compared with seismograms modeled in the corresponding isotropic media.
Inversion of the power spectrum from magnetic anomalies
Authors: Garca‐Abdeslem Juan , Ness Gordon E.
Published: October 2002

We develop methods for the modeling and inversion of the power‐density spectrum from magnetic anomaly data assuming that the crustal magnetic field is caused by an ensemble of vertical‐sided and uniformly magnetized prisms. The solution of the forward problem is achieved in the wavenumber domain, where a synthetic spectrum is given by the product of the mathematical expectations of single‐valued functions that describe depth, thickness, and horizontal dimensions of prisms in the ensemble. We use Gaussian and uniform distributions to describe the ensemble and provide a variety of functions from which different statistical models can be obtained. The solution of the inverse problem is achieved iteratively, starting from an initial set of model parameters. It is based on the ridge‐regression algorithm, and its usefulness is assessed in a number of examples with numeric, synthetic and real data spectra. The methods are first tested on the spectrum obtained from a simple artificial magnetic anomaly and on the artificial spectrum caused by an ensemble of source bodies and are found to be capable of recovering the source parameters. Next, the methods are applied to marine magnetic data from a survey offshore of the Yucatán Peninsula, Mexico. The results of this last application are consistent with the crustal structure observed at Chicxulub hole.
The squirt‐flow mechanism: Macroscopic description
Authors: Dvorkin Jack , Nolen‐Hoeksema Richard , Nur Amos
Published: October 2002

We introduce a poroelasticity model that incorporates the two most important mechanisms of solid/fluid interaction in rocks: the Biot mechanism and the squirt‐flow mechanism. This combined Biot/squirt (BISQ) model relates compressional velocity and attenuation to the elastic constants of the drained skeleton and of the solid phase, porosity, permeability, saturation, fluid viscosity and compressibility, and the characteristic squirt‐flow length. Squirt‐flow length is a fundamental rock property that does not depend on frequency, fluid viscosity, or compressibility and is determined experimentally. We find that the viscoelastic response of many sandstones is dominated by the squirt‐flow component of the BISQ mechanism and that the viscoelastic properties of these rocks can be expressed through a single dimensionless parameter [Formula: see text] where ω is angular frequency, R is the characteristic squirt‐flow length, and κ is hydraulic diffusivity. The Biot mechanism alone does not give an adequate explanation of the observed velocity dispersion and attenuation, and the viscoelastic behavior of many sandstones.
Time‐variant statics corrections during interpretation
Authors: Musgrove Frank W.
Published: October 2002

Even though statics processing is well advanced and is routinely applied, interpreters are still required to evaluate seismic data that has either no statics corrections or poor statics corrections. It is a critical interpretation skill to recognize uncorrected statics and to correct for their effects. Poor decisions and dry holes are often the result of failure to do this. Since multifold seismic data has become the norm, statics are no longer time invariant. The method of flattening stacked data on a shallow reflector that worked so well for single fold data will show anomalous structure and isochron variation caused by the time‐variant effects of uncorrected statics. Uncorrected statics can be identified by anomalous undulations in a shallow reflector, by the time‐variant effects with increasing reflection time, and by the variations in optimal stacking velocities, all of which have been documented in the literature. Knowledge of these properties from stacked data observations can lead to robust estimates of the surface position delay profile necessary for all statics corrections. Although prestack analysis and correction provide the best solutions, full spatial and temporal corrections can be calculated easily and applied to the interpretation of stacked data on most work stations to enable quicker, less expensive decision making. Prestack reprocessing is only one action that may result from a quality poststack analysis. Stacked data time picks can be fully corrected with a statics term and a velocity term. Partial corrections, applied only to the center trace of a static‐causing body can be made without knowledge of the exact surface position delay model. The ratio of central trace delays of two reflectors is approximately equal to the ratio of effective spread lengths used to stack the two reflectors. This method is applied to a real data example published last year in Geophysics where the anomalous isochron thinning is accurately predicted.