bluuebunny/crossref_metadata_2025 huggingface dataset:
Instantaneous polarization attributes and directional filtering
Authors: Morozov Igor B. , Smithson Scott B.
Published: October 2002

We introduce a systematic definition of instantaneous attributes for an arbitrary multicomponent seismic signal. The definition is a natural generalization of known complex trace attributes of a one‐component signal. Instantaneous amplitude and all polarization parameters are defined as invariants of “instantaneous phase rotation.” The principal feature of our approach is the unique definition of the instantaneous phase for a signal with any number of components. Plots of subtle polarization parameters of multicomponent seismic data are easily obtained using conventional seismic plotting routines. We illustrate our approach on a synthetic example and apply it to real 3‐component, wide‐angle crustal data. Plots of polarization attributes provide evidence for shear‐wave splitting in an [Formula: see text] arrival. Having determined the instantaneous polarization vector, we design a new type of time‐domain spatial directional filter. The filter enhances linearly polarized events with specified instantaneous polarization. The filter can work with any number of components in the data, has no user‐specified parameters, and is controlled only by the signal. We conclude that rigorously defined instantaneous phase, amplitude, and polarization attributes provide new effective means for the visualization, analysis, and processing of multicomponent signal.
On: “A Case Study of Integrated Hydrocarbon Exploration Through Basalt,” by Robert Withers, Dwight Eggers, Thomas Fox, and Terry Crebs (November 1994 GEOPHYSICS 59, p 1666–1679).
Authors: Steenland N. C.
Published: October 2002

This paper describes geophysical exploration in northern Oregon, an area covered by the Columbia River Basalt Group (CRBG) on the surface. Obtaining geophysical results through these basalts is a major problem. Magnetic, gravity, seismic, MT and subsurface data are utilized.
Estimation of the discrete Fourier transform, a linear inversion approach
Authors: Sacchi Mauricio D. , Ulrych Tadeusz J.
Published: October 2002

Spatio‐temporal analysis of seismic records is of particular relevance in many geophysical applications, e.g., vertical seismic profiles, plane‐wave slowness estimation in seismographic array processing and in sonar array processing. The goal is to estimate from a limited number of receivers the 2-D spectral signature of a group of events that are recorded on a linear array of receivers. When the spatial coverage of the array is small, conventional f-k analysis based on Fourier transform leads to f-k panels that are dominated by sidelobes. An algorithm that uses a Bayesian approach to design an artifacts‐reduced Fourier transform has been developed to overcome this shortcoming. A by‐product of the method is a high‐resolution periodogram. This extrapolation gives the periodogram that would have been recorded with a longer array of receivers if the data were a limited superposition of monochromatic planes waves. The technique is useful in array processing for two reasons. First, it provides spatial extrapolation of the array (subject to the above data assumption) and second, missing receivers within and outside the aperture are treated as unknowns rather than as zeros. The performance of the technique is illustrated with synthetic examples for both broad‐band and narrow‐band data. Finally, the applicability of the procedure is assessed analyzing the f-k spectral signature of a vertical seismic profile (VSP).
Multicomponent VP/VS correlation analysis
Authors: Gaiser James E.
Published: October 2002

An important step in the simultaneous interpretation or inversion of multicomponent data sets is to quantitatively estimate the ratio of P‐wave velocity to S‐wave velocity [Formula: see text]. In this endeavor, I have developed correlation techniques to determine long‐wavelength components of [Formula: see text] that can lead to more accurate measurements of rock properties and processing parameters. P‐wave reflections are correlated with converted P‐ to S‐wave reflections (or S‐wave reflections) from the same location to determine which events are related to the same subsurface impedance contrasts. Shear waves are transformed (compressed) to P‐wave time via average [Formula: see text] conjugate operators before correlation. Aided by conventional P‐wave velocity information and petrophysical relationships, this technique provides optimal [Formula: see text] estimates in a similar manner that semblance analyses provide stacking velocities. These estimates can be used to transform the entire S‐wave trace to P‐wave time for short‐wavelength amplitude inversion. Also, a target‐oriented correlation analysis quantitatively determines interval [Formula: see text] at a specific horizon or group of horizons. Data from vertical seismic profile (VSP) stacked traces are used to evaluate these techniques. Long‐wavelength average and interval [Formula: see text] estimates obtained from the correlation analyses agree closely with [Formula: see text] results determined from VSP direct‐arrival traveltimes.
Spectral‐domain decomposition methods for the solution of acoustic and elastic wave equations
Authors: Faccioli Ezio , Maggio Fabio , Quarteroni Alfio , Taghan Aldo
Published: October 2002

A new spectral‐domain decomposition method is presented for acoustic and elastodynamic wave propagation in 2-D heterogeneous media. Starting from a variational formulation of the problem, two different approaches are proposed for the spatial discretization: a mixed Fourier‐Legendre and a full Legendre collocation. The matching conditions at subdomain interfaces are carefully analyzed, and the stability and efficiency of time‐advancing schemes are investigated. The numerical validation with some significant test cases illustrates the accuracy, flexibility, and robustness of our methods. These allow the treatment of complex geometries and heterogeneous media while retaining spectral accuracy.
Single station magnetotelluric impedance estimation: Coherence weighting and the regression M‐estimate
Authors: Egbert Gary D. , Livelybrooks Dean W.
Published: October 2002

The first step in the interpretation of magnetotelluric (MT) data involves estimating [Formula: see text] frequency‐domain impedances, Z(w), from the raw electric and magnetic field time series e(t), h(t) (approx. [Formula: see text] real numbers/site) [e.g., Swift, 1967, Sims et al., 1971]. Superficially, this initial data reduction step is almost trivial. Making the usual MT assumptions that the external sources are spatially uniform, and allowing for noise in the simplest way, e and h are related in the frequency domain via the linear statistical model e = Zh + ε, (1) where ε represents noise. The impedance Z can then be estimated quite simply by Fourier transforming the time series, and using linear least squares (LS] to minimize the misfit to equation (1) [Sims et al., 1971]. Unfortunately, this simple approach can fail catastrophically for noisy data, producing estimates that are heavily biased or wildly oscillatory [e.g., Gamble et al., 1979a; Jones et al., 1989; Figure 1]. As a consequence a number of refinements to the simple LS approach have been proposed in an effort to guarantee impedance estimates that are useful for subsequent stages in the interpretation process.
Finite‐difference migration derived from the Kirchhoff‐Helmholtz integral
Authors: Rühl Thomas
Published: October 2002

Finite‐difference (FD) migration is one of the most often used standard migration methods in practice. The merit of FD migration is its ability to handle arbitrary laterally and vertically varying macro velocity fields. The well‐known disadvantage is that wave propagation is only performed accurately in a more or less narrow cone around the vertical. This shortcoming originates from the fact that the exact one‐way wave equation can be implemented only approximately in finite‐difference schemes because of economical reasons. The Taylor or continued fraction expansion of the square root operator in the one‐way wave equation must be truncated resulting in an approximate version of the one‐way wave equation valid only for a restricted angle range.
Measurement of the seismoelectric response from a shallow boundary
Authors: Butler Karl E. , Russell R. Don , Kepic Anton W. , Maxwell Michael
Published: October 2002

Field experiments carried out at a site near Vancouver, Canada have shown that a shallow lithologic boundary can be mapped on the basis of its seismoelectric response.As seismic waves cross the boundary between organic‐rich fill and impermeable glacial till, they induceelectric fields that can be measured at the surfacewith grounded dipole receivers. Sledgehammer and blasting cap seismic sources, positioned up to 7 m away from the interface, have produced clear seismoelectric conversions. Two types of seismoelectric signals are observed. The primary response is distinguished by near simultaneous arrivals at widely separated receivers. Its arrival time is equal to the time required for a seismic P‐wave to travel from the shotpoint to the fill/till boundary. On the surface, its maximum amplitude (about 1 mV/m) ismeasured by dipoles located within a few meters of the shotpoint. At greater distances, the amplitude of the primaryarrival decays rapidly with offset, and secondary seismoelectric arrivals become dominant. They differ from the primary response in that their arrival times increasewith dipole offset, and they appear to be generatedin the immediate vicinity of each dipole sensor. Our studies show that the responses cannot be attributedto piezoelectricity or to resistivity modulation in the presence of a uniform telluric current. We infer that seismically induced electrokinetic effects or streamingpotentials are responsible for the seismoelectric conversion,and a simple electrostatic model is proposed to account for the two types of arrivals. Although our experimentswere small in scale, the results are significant in that they suggest that the seismoelectric method may be used to map the boundaries of permeable formations.
Stress‐induced azimuthal anisotropy in borehole flexural waves
Authors: Sinha Bikash K. , Kostek Sergio
Published: October 2002

A nonlinear elastic model for acoustic waves in a stressed medium is used to calculate tectonic stress‐induced changes in borehole flexural dispersions. Our theoretical analysis shows that a horizontal uniaxial stress in the formation causes a crossover in flexural dispersions for the radial polarization aligned parallel and normal to the stress direction. This crossover in flexural dispersions is caused by stress‐induced radial heterogeneities in acoustic wave velocities that are different in the two principal stress directions. Other sources of borehole flexural anisotropy caused by finely layered dipping beds, aligned fractures, or microstructures found in shales, exhibit neither such radial heterogeneities nor flexural dispersion crossovers. Consequently, a crossover in flexural dispersion can be used as an indicator of stress‐induced anistropy. In this situation, the fast shear polarization direction coincides with the far‐field uniaxial stress direction. The analysis also yields an expression for the largest shear stress parameter in terms of the fast and slow seismic shear‐wave velocities with shear polarization parallel and perpendicular to the far‐field stress direction.
The use of 3-D prestack depth imaging to estimate layer velocities and reflector positions
Authors: Reshef Moshe
Published: October 2002

Experience gained in 2-D work shows that structural imaging can be used effectively as a tool for interval velocity analysis in complex areas. This study attempts to implement a similar procedure in 3-D.
Diffraction of seismic waves by cracks with application to hydraulic fracturing
Authors: Liu Enru , Crampin Stuart , Hudson John A.
Published: October 2002

We describe a method of modeling seismic waves interacting with single liquid‐filled large cracks based on the Kirchhoff approximation and then apply it to field data in an attempt to estimate the size of a hydraulic fracture. We first present the theory of diffraction of seismic waves by fractures using a Green's function representation and then compute the scattered radiation patterns and synthetic seismograms for fractures with elliptical and rectangular shapes of various dimensions. It is shown that the characteristics of the diffracted wavefield from single cracks are sensitive to both crack size and crack shape. Finally, we compare synthetic waveforms to observed waveforms recorded during a hydraulic fracturing experiment and are able to predict successfully the size of a hydraulically induced fracture (length and height). In contrast to previously published work based on the Born approximation, we model both phases and amplitudes of observed diffracted waves. Our modeling has resulted in an estimation of a crack length 1.1 to 1.5 times larger than previously predicted, whereas the height remains essentially the same as that derived using other techniques. This example demonstrates that it is possible to estimate fracture dimensions by analyzing diffracted waves.
The transversely isotropic poroelastic wave equation including the Biot and the squirt mechanisms: Theory and application
Authors: Parra Jorge O.
Published: October 2002

The transversely isotropic poroelastic wave equation can be formulated to include the Biot and the squirt‐flow mechanisms to yield a new analytical solution in terms of the elements of the squirt‐flow tensor. The new model gives estimates of the vertical and the horizontal permeabilities, as well as other measurable rock and fluid properties. In particular, the model estimates phase velocity and attenuation of waves traveling at different angles of incidence with respect to the principal axis of anisotropy. The attenuation and dispersion of the fast quasi P‐wave and the quasi SV‐wave are related to the vertical and the horizontal permeabilities. Modeling suggests that the attenuation of both the quasi P‐wave and quasi SV‐wave depend on the direction of permeability. For frequencies from 500 to 4500 Hz, the quasi P‐wave attenuation will be of maximum permeability. To test the theory, interwell seismic waveforms, well logs, and hydraulic conductivity measurements (recorded in the fluvial Gypsy sandstone reservoir, Oklahoma) provide the material and fluid property parameters. For example, the analysis of petrophysical data suggests that the vertical permeability (1 md) is affected by the presence of mudstone and siltstone bodies, which are barriers to vertical fluid movement, and the horizontal permeability (1640 md) is controlled by cross‐bedded and planar‐laminated sandstones. The theoretical dispersion curves based on measurable rock and fluid properties, and the phase velocity curve obtained from seismic signatures, give the ingredients to evaluate the model. Theoretical predictions show the influence of the permeability anisotropy on the dispersion of seismic waves. These dispersion values derived from interwell seismic signatures are consistent with the theoretical model and with the direction of propagation of the seismic waves that travel parallel to the maximum permeability. This analysis with the new analytical solution is the first step toward a quantitative evaluation of the preferential directions of fluid flow in reservoir formation containing hydrocarbons. The results of the present work may lead to the development of algorithms to extract the permeability anisotropy from attenuation and dispersion data (derived from sonic logs and crosswell seismics) to map the fluid flow distribution in a reservoir.
Spectral correlation of magnetic and gravity anomalies of Ohio
Authors: von Frese Ralph R. B. , Jones Michael B. , Kim Jeong Woo , Li Wen Sheng
Published: October 2002

Geologic interpretation of Ohio's magnetic or gravity anomalies is hindered by the effects of anomaly superposition and source ambiguity inherent to potential field analysis. A common approach to minimizing interpretational ambiguities is to consider analyses of anomaly correlations. A spectral procedure is adapted which correlates anomaly fields in the frequency domain to produce filters separating positively and negatively correlated, as well as null correlated features. The correlation filter passes or rejects wavenumbers between coregistered fields based on the correlation coefficient between common wavenumbers as given by the cosine of their phase difference. This procedure is applied to reduced‐to‐pole magnetic and first vertical derivative gravity anomalies of Ohio for mapping correlative magnetization and density contrasts within the basement rocks. The analysis reveals predominantly positive correlations between anomaly maxima and minima. Correlative anomaly maxima may be generally modeled as mafic bodies of the upper crust. They map out a possible dike complex in northwestern Ohio, a batholith as a possible source of volcanic rocks in southwestern Ohio, and numerous mafic bodies related presumably to Keweenawan rifting and Grenville tectonics. Correlative anomaly minima include several isolated features that may define felsic terranes of the upper crust, and ringed features around some of the larger mafic bodies which also may contain significant edge‐effect components. A large circular feature in south‐central Ohio involves correlative minima of a possible anorthosite body that is ringed by an inversely correlative zone of positive density and negative magnetization contrasts. Another prominent negative correlation involves an extensive area of possible extrusive rocks with positive magnetization and negative density contrasts just north of the batholith in southwestern Ohio.
On: “Fresnel zones for broadband data” by M. Brühl, G. J. O. Vermeer, and M. Kiehn (March‐April 1996 GEOPHYSICS, 61, p. 600–604)
Authors: Knapp Ralph W.
Published: October 2002

I am very pleased to see the elaboration of my paper “Fresnel zones in the light of broadband data” (Knapp, 1990) done by Brühl et al. (1996).
Depth sounding by means of a coincident coil frequency‐domain electromagnetic system
Authors: Duckworth Kenneth , Krebes Edward S.
Published: October 2002

The concept of electromagnetic depth sounding by means of a coincident‐coil frequency‐domain electromagnetic system is developed in theory and demonstrated by means of physical scale modeling. The concept is based on the use of distance from the target as the sounding variable. The theoretical developments are confined to soundings conducted in free‐space with respect to either a homogeneous half‐space or a thin sheet conductor in conditions that approach the resistive limit. The use of distance from the target as the sounding variable becomes practical when the sounding system is a single compact unit of the type that a coincident coil concept inherently provides. In this method of sounding, the distance from the target is determined by taking the ratios of the fields measured at a variety of distances from the target conductor. This permits not only the distance to the target to be determined but also the direction to that target as may be of interest in soundings conducted in mines.
Velocity anisotropy in shales: A petrophysical study
Authors: Vernik Lev , Liu Xingzhou
Published: October 2002

Using ultrasonic velocity and anisotropy measurements on a variety of shales with different clay and kerogen content, clay mineralogy, and porosity at a wide range of effective pressure, we find that elastic anisotropy of shales increases substantially with compaction. The effect is attributed to both porosity reduction and smectite‐ to‐illite transformation with diagenesis. A means of kerogen content mapping using velocity versus porosity crossplot for shales is shown. Matrix anisotropy of shales dramatically increases with kerogen reaching the maximum values of about 0.4 at total organic carbon (TOC)=15–20%. A strong chemical softening effect was found in shales containing even minor amounts of swelling (smectite) clay when saturated with aqueous solution. This effect results in a significant P‐wave anisotropy reduction as compared to dry and oil‐saturated shales. Since mature black shales are normally oil wet, this effect can only have a local significance restricted to the wellbore wall. Accurate measurements of phase velocities, including velocities at a 45° direction to the bedding plane, allow us to immediately calculate elastic stiffnesses and anisotropic parameters. Intrinsic (high pressure) properties of shales display an ε > δ > 0 relation. Introduction of the bedding‐parallel microcracks in overpressured shales results in a δ decrease when fully fluid saturated and a δ increase when partially gas saturated, with a characteristic effect on the shape of the P‐wave velocity surface at small angles of incidence. Filtering the contribution of the intrinsic anisotropy of shales, it is possible to estimate the pore fluid phase, microcrack density, and aspect ratio parameters using seismic anisotropy measurements.
Damping of secondary bubble oscillations for towed air guns with a screen
Authors: Landrø Martin , Langhammer Jan , Martin James
Published: October 2002

A method for damping unwanted bubble oscillations from a horizontally towed seismic air gun is presented. The air bubble is destroyed by a perforated screen mounted at an optimal radius about the gun. Once the primary pressure peak has been generated by the emerging bubble, the bubble continues to expand and is destroyed by the screen, leading to a corresponding decrease in the measured pressure amplitude of the secondary bubble oscillations. For a stationary gun fired first without, and then with, the screen fitted, the primary‐to‐ bubble ratio improves in the near field from 1.7 to 5.2, respectively, at a firing depth of 3 m and from 1.5 to 5.5, respectively, at 5 m depth. The primary‐to‐bubble ratio for a towed air gun in the quasi‐far‐field improves from 2.0 to 11.0 at 4 m depth and from 1.5 to 8.7 at 7 m depth when the screen is fitted. The boat speed was 1.6 knots and the signatures were filtered with an out‐128 Hz (72 dB/Oct) DFS V filter.
Seismic constant‐velocity remigration
Authors: Schleicher Jörg , Hubral Peter , Höcht German , Liptow Frank
Published: October 2002

When a seismic common midpoint (CMP) stack or zero‐offset (ZO) section is depth or time migrated with different (constant) migration velocities, different reflector images of the subsurface are obtained. If the migration velocity is changed continuously, the (kinematically) migrated image of a single point on the reflector, constructed for one particular seismic ZO reflection signal, moves along a circle at depth, which we call the Thales circle. It degenerates to a vertical line for a nondipping event. For all other dips, the dislocation as a function of migration velocity depends on the reflector dip. In particular for reflectors with dips larger than 45°, the reflection point moves upward for increasing velocity. The corresponding curves in a Time‐migrated section are parabolas. These formulas will provide the seismic interpreter with a better understanding of where a reflector image might move when the velocity model is changed. Moreover, in that case, the reflector image as a whole behaves to some extent like an ensemble of body waves, which we therefore call remigration image waves. In the same way as physical waves propagate as a function of time, these image waves propagate as a function of migration velocity. Different migrated images can thus be considered as snapshots of image waves at different instants of migration velocity. By some simple plane‐wave considerations, image‐wave equations can be derived that describe the propagation of image waves as a function of the migration velocity. The Thales circles and parabolas then turn out to be the characteristics or ray trajectories for these image‐wave equations.
Processing of a nine‐component near‐offset VSP for seismic anisotropy
Authors: MacBeth Colin , Li Xiang‐Yang , Zeng Xinwu , Cox Dale , Queen John
Published: October 2002

A convolutional sequence of matrix operators is offered as a convenient deterministic scheme for processing a multicomponent vertical seismic profile (VSP). This sequence is applied to a nine‐component near‐offset VSP recorded at the Conoco borehole test facility, Kay County, Oklahoma. These data are corrected for tool spin and near‐surface anisotropy together with source coupling or imbalance. After wave‐field separation using a standard f‐k filter, each source and receiver pair for the upgoing waves is adjusted to a common reference depth using a matrix operator based on the downgoing wave‐field. The up‐ and downgoing waves are then processed for anisotropy by a similarity transformation, to separate the [Formula: see text] and [Formula: see text] waves, from which the anisotropic properties are estimated. These estimates reveal a strong (apparent) vertical birefringence in the near‐surface, but weak or moderate values for the majority of the subsurface. The target zone consists of a thin sandstone and deeper shale layer, both of which possess a strong vertical birefringence. The sandstone corresponds to a zone of known fluid flow. An observed [Formula: see text] attenuation and polarization change in the shale suggest it contains large fractures.
To: Reply by the author to B. N. P. Argarwal (March‐April 1997 GEOPHYSICS, 62, p. 690–691) in the Discussion on “Interpretation of two dimensional magnetic bodies using Hilbert transforms,” by N. L. Mohan, N. Sundararajan, and S. V. Seshagiri Rao (March 1982 GEOPHYSICS, 52, p. 376–387)
Authors: Mohan N. L. , Sundararajan N. , Seshagiri Rao S. V.
Published: October 2002

This reply ran incomplete with the Discussion. It should have read “N. L. Mohan feels that he adequately addressed points raised earlier by Nagendra and Babu regarding the same paper.”
P‐wave reflection coefficients for transversely isotropic models with vertical and horizontal axis of symmetry
Authors: Rüger Andreas
Published: October 2002

The study of P‐wave reflection coefficients in anisotropic media is important for amplitude variation with offset (AVO) analysis. While numerical evaluation of the reflection coefficient is straightforward, numerical solutions do not provide analytic insight into the influence of anisotropy on the AVO signature. To overcome this difficulty, I present an improved approximation for P‐wave reflection coefficients at a horizontal boundary in transversely isotropic media with vertical axis of symmetry (VTI media). This solution has the same AVO‐gradient term describing the low‐order angular variation of the reflection coefficient as the equations published previously, but is more accurate for large incidence angles. The refined approximation is then extended to transverse isotropy with a horizontal axis of symmetry (HTI), which is caused typically by a system of vertical cracks. Comparison of the approximate reflection coefficients for P‐waves incident in the two vertical symmetry planes of HTI media indicates that the azimuthal variation of the AVO gradient is a function of the shear‐wave splitting parameter γ, and the anisotropy parameter describing P‐wave anisotropy for nearvertical propagation in the vertical plane containing the symmetry axis.
Delineating mineralization and imaging the regional structure with magnetotellurics in the region of Chibougamau (Canada)
Authors: Chouteau Michel , Zhang Ping , Dion Denis J. , Giroux Bernard , Morin Rémy , Krivochieva Stefka
Published: October 2002

Tensor magnetotelluric (MT) soundings were carried out to delineate the geometry of an extended shallow conductor detected by airborne electromagnetic method in the mining camp of Chibougamau (Canada). Careful distortion analysis of the impedance tensors from eleven sites indicate that the subsurface is reasonably 2-D within the 0.0025–10s range. The recovered regional impedances are corrected for static shifts employing 2-D inversion for minimum structure. The modeled resistivity cross‐sections show a far more complex conductivity structure than anticipated. The sheet‐like shallow conductor lies near the boundary between the conductive southern section located in the Abitibi subprovince and the resistive northern section located in the Opatica subprovince. It extends to a depth of 250 m. Two other limited conductive zones are imaged respectively between 0.4 km and 3 km and between 1.5 km and 4 km. We propose that the conductors are associated with sulfide mineralization and graphite in shear zones. They are delimited by steeply south dipping thrust faults. The northern resistive section does not show clear resistivity changes up to a depth of 15 km. Gravity data collected along the MT profile have provided additional constraints. Both MT and gravity models are consistent with one another and with the known regional geologic structure. They indicate that the Abitibi greenstone belt is 3.5–4.4 km thick in the survey area and that the steeply dipping geology at the surface is flattening at depths of 4 km approximately.
Comparison of sea‐ice thickness measurements under summer and winter conditions in the Arctic using a small electromagnetic induction device
Authors: Haas Christian , Gerland Sebastian , Eicken Hajo , Miller Heinz
Published: October 2002

Drillhole‐determined sea‐ice thickness was compared with values derived remotely using a portable small‐offset loop‐loop steady state electromagnetic (EM) induction device during expeditions to Fram Strait and the Siberian Arctic, under typical winter and summer conditions. Simple empirical transformation equations are derived to convert measured apparent conductivity into ice thickness. Despite the extreme seasonal differences in sea‐ice properties as revealed by ice core analysis, the transformation equations vary little for winter and summer. Thus, the EM induction technique operated on the ice surface in the horizontal dipole mode yields accurate results within 5 to 10% of the drillhole determined thickness over level ice in both seasons. The robustness of the induction method with respect to seasonal extremes is attributed to the low salinity of brine or meltwater filling the extensive pore space in summer. Thus, the average bulk ice conductivity for summer multiyear sea ice derived according to Archie's law amounts to 23 mS/m compared to 3 mS/m for winter conditions. These mean conductivities cause only minor differences in the EM response, as is shown by means of 1-D modeling. However, under summer conditions the range of ice conductivities is wider. Along with the widespread occurrence of surface melt ponds and freshwater lenses underneath the ice, this causes greater scatter in the apparent conductivity/ice thickness relation. This can result in higher deviations between EM‐derived and drillhole determined thicknesses in summer than in winter.
GPR characterization of buried tanks and pipes
Authors: Zeng Xiaoxian , McMechan George A.
Published: October 2002

Ray‐based numerical simulations of monostatic and bistatic GPR responses for several tank and pipe configurations reveal the potential for noninvasive diagnostic evaluations. Examples include discrimination of the material from which a tank is constructed, its size, contents, fluid levels, and shape changes. Ambiguities occur when different configurations give similar responses, and evaluations become less reliable as noise increases. Simulations are able to reproduce the salient features of field GPR data recorded over a metal pipe, and over plastic pipes filled with air, fresh water, and salt water.
Estimation of density, magnetization, and depth to source: A nonlinear and noniterative 3-D potential‐field method
Authors: Fedi Maurizio
Published: October 2002

The depth to the top, or bottom, and the density of a 3-D homogeneous source can be estimated from its gravity or magnetic anomalies by using a priori information on the maximum and minimum source depths. For the magnetic case, the magnetization direction is assumed to be constant and known. The source is assumed to be within a layer of known depth to the top h and thickness t. A depth model, satisfying both the data and the a priori information is found, together with its associated density/magnetization contrast. The methodology first derives, from the measured data, a set of apparent densities [Formula: see text] (or magnetizations), which do not depend on the layer parameters h and t, but only on source thickness. A nonlinear system of equations based on [Formula: see text], with source thicknesses as unknowns, is constructed. To simplify the solution, a more practical system of equations is formed. Each equation depends on only one value of thickness. Solving for the thicknesses, taking into account the above a priori information, the source depth to the top (or to the bottom) is determined uniquely. Finally, the depth solutions allow a unit‐density gravity model to be computed, which is compared to the observed gravity to determine the density contrast. A similar procedure can be used for magnetic data. Tests on synthetic anomalies and on real data demonstrate the good performance of this method.
Inherent power‐law behavior of magnetic field power spectra from a Spector and Grant ensemble
Authors: Fedi Maurizio , Quarta Tatiana , De Santis Angelo
Published: October 2002

The Spector and Grant method, which has been in use for 25 years, relates average depths to source to rate of decay of the magnetic power spectra. This method, which assumes a uniform distribution of parameters for an ensemble of magnetized blocks, leads to a depth‐dependent exponential rate of decay. We show that also inherent in this model is a power‐law rate of decay that is independent of depth. For most cases, except for extreme depths and small block sizes, the observed power spectrum should be corrected for a power law decay rate of β∼3. If the depth distribution of the magnetic blocks is Gaussian, then the observed power spectrum should be corrected for both a depth independent power law and exponential decay. This power‐law decay is very similar to the scaling behavior, supposed as a fractal character, of observed magnetic fields in North America. As a general rule, when β∼3, further information is needed to discriminate between a fractal or Spector and Grant model. However, it is becoming quite clear that magnetic power spectra should be corrected for a power law decay before applying the Spector and Grant method for depth determination.
Effects of pore and differential pressure on compressional wave velocity and quality factor in Berea and Michigan sandstones
Authors: Prasad Manika , Manghnani Murli H.
Published: October 2002

Compressional‐wave velocity [Formula: see text] and quality factor [Formula: see text] have been measured in Berea and Michigan sandstones as a function of confining pressure [Formula: see text] to 55 MPa and pore pressure [Formula: see text] to 35 MPa. [Formula: see text] values are lower in the poorly cemented, finer grained, and microcracked Berea sandstone. [Formula: see text] values are affected to a lesser extent by the microstructural differences. A directional dependence of [Formula: see text] is observed in both sandstones and can be related to pore alignment with pressure. [Formula: see text] anisotropy is observed only in Berea sandstone. [Formula: see text] and [Formula: see text] increase with both increasing differential pressure [Formula: see text] and increasing [Formula: see text]. The effect of [Formula: see text] on [Formula: see text] is greater at higher [Formula: see text]. The results suggest that the effective stress coefficient, a measure of pore space deformation, for both [Formula: see text] and [Formula: see text] is less than 1 and decreases with increasing [Formula: see text].
Hybrid ℓ1/ℓ2 minimization with applications to tomography
Authors: Bube Kenneth P. , Langan Robert T.
Published: October 2002

Least squares or [Formula: see text] solutions of seismic inversion and tomography problems tend to be very sensitive to data points with large errors. The [Formula: see text] minimization for 1 ≤ p < 2 gives more robust solutions, but usually with higher computational cost. Iteratively reweighted least squares (IRLS) gives efficient approximate solutions to these [Formula: see text] problems. We apply IRLS to a hybrid [Formula: see text] minimization problem that behaves like an [Formula: see text] fit for small residuals and like an [Formula: see text] fit for large residuals. The smooth transition from [Formula: see text] to [Formula: see text] behavior is controlled by a parameter that we choose using an estimate of the standard deviation of the data error. For linear problems of full rank, the hybrid objective function has a unique minimum, and IRLS can be proven to converge to it. We obtain a robust efficient method. For nonlinear problems, a version of the Gauss‐Newton algorithm can be applied. Synthetic crosswell tomography examples and a field‐data VSP tomography example demonstrate the improvement of the hybrid method over least squares when there are outliers in the data.
Modeling fault‐zone guided waves of microearthquakes in a geothermal reservoir
Authors: Lou Min , Rial José A. , Malin P. E.
Published: October 2002

Fault‐zone guided waves have been identified in microearthquake seismograms recorded at the Coso Geothermal Field, California. The observed guided waves have particle motions and propagation group velocities similar to Rayleigh wave modes. A numerical method has been employed to simulate the guided‐wave propagation through the fault zone. By comparing observed and synthetic waveforms the fault‐zone width and its P‐ and S‐wave velocity structure have been estimated. It is suggested here that the identification and modeling of such guided waves is an effective tool to locate fracture‐induced, low‐velocity fault‐zone structures in geothermal fields.
Use of postcritical reflections in solving the hidden‐layer problem of seismic refraction work
Authors: Sain Kalachand , Reddy P. R.
Published: October 2002

In a multilayered earth system, when the thickness of a layer compared to the overlying layer is small, refraction signal from that layer may not appear as a first arrival. In such a case, the analysis of first‐arrival refraction data cannot detect the layer and this leads to errors—overestimation of the thickness of the overlying layer and underestimation of depths to all underlying layers. This is known as the hidden‐layer problem. In a field situation, hidden layer(s) can be identified with the help of high‐energy postcritical reflections, which appear as strong later arrivals. In this paper, we describe an approach to calculate the thickness of the overlying layer and the thickness and velocity of the hidden layer based on the traveltime inversion of postcritical reflections from the top and bottom of the hidden layer. The blind‐zone thickness is also calculated using the estimated velocity of the hidden layer and the thickness of the overlying layer. The applicability of the method is illustrated with the help of both synthetic and field data.
A note on digital filtering with the second moment norm
Authors: Selvi Oguz
Published: October 2002

The linear inverse method developed by Backus and Gilbert (1968) relates model estimates to actual earth models by use of a resolving kernel. Seismic source wavelet deconvolution can be treated within the framework of the Backus and Gilbert (1968) inverse theory as presented in Oldenburg (1981) and Treitel and Lines (1982). The model of the Backus and Gilbert theory is the ground impulse response, the mapping kernel is the source wavelet, and the resolving kernel is the convolution between the source wavelet and the shaping filter. Backus and Gilbert formalism introduces several measures for the resolving kernel.
Seismic transient deconvolution with model‐based signal processing
Authors: Tague John A. , Schutz Kerry D.
Published: October 2002

Short duration seismic disturbances, obscured by earth noise and distorted by the seismometers used to measure them, can be reconstructed using model‐based signal processing. “Model based” means that mathematical models of the seismic transient, earth noise, and seismometer dynamics are infused into the signal processor that estimates the disturbance. The processor imposes no predetermined structure on the transient and the earth noise need not be white. Model‐based processors produce good quality estimates for a broad class of transient waveforms.
The measurement of velocity dispersion and frequency‐dependent intrinsic attenuation in sedimentary rocks
Authors: Sams M. S. , Neep J. P. , Worthington M. H. , King M. S.
Published: October 2002

A series of experiments to determine the elastic properties of a sequence of saturated sedimentary rocks over as wide a frequency range as possible was carried out at the Imperial College borehole test site. These experiments fall into four categories: vertical seismic profiles (VSPs) within the frequency range 30–280 Hz, crosshole surveys (0.2–2.3 kHz), sonic logging (8–24 kHz), and laboratory measurements (300–900 kHz). The intrinsic attenuation and velocity of compressional and shear waves were measured whenever possible. Velocity dispersion is observed for both compressional and shear waves. The intrinsic attenuation of compressional waves is frequency dependent with a peak in the attenuation in the sonic frequency band. The data were modeled assuming the attenuation is caused by local fluid flow in pores of small aspect ratio. The modeling indicates that the intrinsic attenuation may be dominated by cracks with aspect ratios of around [Formula: see text] to [Formula: see text].
Integrating 3-D seismic imaging and seismic attribute analysis with genetic stratigraphy: Implications for infield reserve growth and field extension, Budare Field, Venezuela
Authors: Raeuchle Sandra K. , Hamilton Douglas S. , Uzcátegui M.
Published: October 2002

Despite being a mature oil producer, the Budare Field in the Eastern Venezuela Basin offers considerable reserve growth potential because of stratigraphic and structural complexity. Our ability to resolve these complexities was enhanced following acquisition in 1995 of a 3-D seismic data set over a large part of the field. The seismic data were tied by synthetic to well‐log data by several wells having sonic and density information and then integrated with the high‐resolution genetic stratigraphic framework established from well‐log correlations. Two key surfaces identified on the seismic data correlated directly to two stratigraphically defined sequence boundaries, maximum flooding surfaces (MFS) 80 and 100. A third seismic surface correlated approximately with the stratigraphically defined MFS 62. Collectively, these surfaces form fundamental control surfaces from which seismic attribute analysis and imaging from inverse modeling were undertaken. Four depositional trends detected by the seismic imaging and attribute analysis have important implications for reserve growth potential, guiding future field development. An incised valley, filled primarily with thick fluvial sandstones, was detected by mapping average seismic amplitudes between the MFS 62 and 80 markers, and several step‐out drilling locations were identified where the sandstones intersect structurally high positions. The distribution of thick distributary‐mouth‐bar facies, and moreover, the boundary with adjacent thin‐bedded strandplain facies, were similarly detected by mapping average seismic amplitudes in a 35-ms time window below MFS 80. The mouth‐bar facies coincide with the crestal position of a potentially large, structurally defined field extension supporting multiple potential infill wells. Several high‐negative‐amplitude anomalies coinciding with thick fluvial sandstones overlying MFS 62 display faulted boundaries and are interpreted as direct hydrocarbon indicators, providing obvious infill drilling locations, and finally, a marine ravinement surface separating the key oil‐producing reservoirs below MFS 80 was identified by seismic inversion.
Observation of the Biot slow wave in water‐saturated Nivelsteiner sandstone
Authors: Kelder Oscar , Smeulders David M. J.
Published: October 2002

Acoustic signals are used extensively in the oil industry to determine the physical properties of reservoir rock. In the interpretation of these signals empirical laws play a major role. To obtain a more fundamental interpretation of the recorded wavetrains, the need for a comprehensive theory for acoustic wave propagation and damping in rocks is obvious. In this respect, Biot's (1956a,b) theory is a straightforward and effective two‐phase theory. In contrast to Biot, who derived the macroscopic equations for wave propagation in saturated poro‐elastic material by postulating definite positive‐energy density functions, Burridge and Keller (1981), Whitaker (1986a,b,c), and Pride et al. (1992) applied rigorous averaging techniques to derive the poro‐elastic equations from a microscale. De Vries (1989) and Geerits (1996) used the averaging techniques to derive macroscopic poro‐elastic equations for the nonviscous case. The fundamental feature of all these theoretical descriptions is the existence of both a fast and slow compressional wave, as well as a shear wave. For the fast compressional wave, the pore fluid and the porous matrix are compressed simultaneously, but for the slow compressional wave, the porous matrix relaxes while the pore fluid is compressed. The attenuation mechanism for these waves is based on viscous dissipation generated by the flow of the pore fluid relative to the porous matrix. For the slow wave, the viscous dissipation results in a strong, frequency‐dependent attenuation, which makes this wave very difficult to observe in fluid‐saturated rocks. However, because the slow compressional wave is especially sensitive to certain interesting properties of the permeable material, the detection of this slow compressional wave has been one of the major issues in the acoustics of fluid‐saturated permeable solids.
Tomographic imaging of velocity and Q, with application to crosswell seismic data from the Gypsy Pilot Site, Oklahoma
Authors: Liao Qingbo , McMechan George A.
Published: October 2002

The centroid frequency shift method is implemented, tested with synthetic data, and applied to field data from three contiguous crosswell seismic experiments at the Gypsy Pilot in northern Oklahoma. The similtaneous iterative reconstruction technique is used for tomographic estimations of both P‐wave velocity and Q. No amplitude corrections or spreading loss corrections are needed for the Q estimation. The estimated in‐situ velocity and Q distributions correlate well with log data and local lithology. The Q/velocity ratio appears to correlate with the sand/shale ratio (ranging from an average of ∼15 s/km for the sand‐dominated lithologies to an average of ∼8.5 s/km for the shale‐dominated ones), with the result that new information is provided on interwell connectivity.
One‐pass 3-D seismic extrapolation with the 45° wave equation
Authors: Zhou Hongbo , McMechan George A.
Published: October 2002

One‐pass 3-D modeling and migration for poststack seismic data may be implemented by replacing the traditional 45° one‐way wave equation (a third‐order partial differential equation) with a pair of second‐ and first‐order partial differential equations. Except for an extra correction term, the resulting second‐order equation has a form similar to the Claerbout 15° one‐way wave equation, which is known to have a nearly circular impulse response. In this approach, there is no need to compensate for splitting errors. Numerical tests on synthetic data show that this algorithm has the desirable attributes of being second order in accuracy and economical to solve. A modification of the Crank‐Nicholson implementation maintains stability.