bluuebunny/crossref_metadata_2025 huggingface dataset:
The borehole controlled‐source audiomagnetotelluric response of a three‐dimensional fracture zone
Authors: West Richard C. , Ward Stanley H.
Published: October 2002

Approximate theoretical borehole CSAMT profiles of the normalized vertical electric and magnetic fields [Formula: see text] and [Formula: see text] were computed for several models of practical interest using a 3-D MT computer program based on the method of integral equations. [Formula: see text] and [Formula: see text] are the horizontal fields measured at the surface. A conductive tabular prism in a layered and homogeneous half‐space was chosen to simulate a 3-D fracture zone composed of individual, interconnected fractures. Model parameters varied during this study were depth and dip of the tabular body, the conductivity and layering of the half‐space, the frequency of the plane‐wave source, and the separation between the borehole and target. In addition, a model composed of two horizontal prisms was investigated. Decreasing the host conductivity, the depth of the prism, or the separation between the borehole and prism increases the magnitude of the subsurface normalized vertical electric and magnetic fields. Depth to the top of a dipping prism in a half‐space can be determined from the crossover of the profile of the [Formula: see text] real component. Peak amplitudes of the [Formula: see text] profiles provide information about the location of the maximum current density within the prism, which is quite variable for the imaginary component. There is no simple relationship between a small borehole‐to‐prism separation and the separation between the antisymmetric peaks of the [Formula: see text] profiles. Without knowledge of the borehole‐to‐prism separation, the dip of a nonhorizontal prism cannot be determined accurately. However, the down‐dip side of a dipping prism is indicated by the larger peak anomaly in the real component of the profile. The normalized [Formula: see text] anomalies of the conductive prism seem more sensitive to body position and also to variation in the host resistivity than are the normalized [Formula: see text] anomalies, thus making the former parameter more susceptible to geologic noise. The resistivity of the half‐space and overburden and the frequency of the source significantly influence the amplitude and phase of the secondary electric fields. Current channeling is a significant contributor to the response of the prism within a half‐space of even moderate resistivity at frequencies of 10 to 100 Hz. The [Formula: see text] field is little influenced by horizontal layering, so the borehole profiles reflect mainly subsurface inhomogeneities. The two main advantages of this technique are that the signal is much larger than the level of natural‐field noise and data acquisition is rapid because of the high frequency of the source (10–1000 Hz). However, the borehole profiles can be significantly affected by nearby inhomogeneities and by the incident field when the borehole and sensor are not vertically aligned.
Removal of the detector‐ground coupling effect in the vertical seismic profiling environment
Authors: Wuenschel Paul C.
Published: October 2002

In a “controlled” experiment with the Gulf VSP tool, the detector‐ground coupling was measured and removed from the recorded signal using the Washburn‐Wiley algorithm. Repeat measurements were made at a common detector depth with two coupling configurations, the first to permit the true ground motion to be recorded and the second to ensure that a coupling resonance existed within the seismic frequency band. The algorithm removed the distortion of the body‐wave portion of the seismogram caused by the coupling resonance for the second configuration and recovered true ground motion. However, lowering the coupling resonance into the seismic band also caused the tool to become sensitive to tube waves. This observation is helpful in evaluating current VSP tools; it implies that any VSP tool that is sensitive to tube waves has a coupling resonance within the seismic frequency band, and that the signal recorded with such a tool does not measure true ground motion. This test also showed that a detector used to monitor source signature variations must have a bandwidth comparable to the VSP signal.
Verification of five magnetotelluric systems in the mini‐EMSLAB experiment
Authors: Young Charles T. , Booker John R. , Fernandez Ricardo , Jiracek George R. , Martinez Mario , Rogers James C. , Stodt John , Waff Harve S. , Wannamaker Phillip E.
Published: October 2002

Given the degree of complexity of modern magnetotelluric (MT) instrumentation, comparison of the total performance for two or more systems is an important verification test. This paper compares the processed data from five MT systems which were designed and constructed separately, and which employ different electrode types, electrode separations, magnetometers, and methods of signal processing. The comparison shows that there is a high degree of agreement among the data from the different systems. The study also demonstrates the compatibility and reliability of the MT systems employed as part of EMSLAB Juan de Fuca (Electromagnetic Sounding of the Lithosphere and Asthenosphere Beneath the Juan de Fuca Plate). This project, proposed by a consortium of institutions, involves not only magnetotellurics studies but also studies of magnetic variation, on land and on the sea bottom. The project calls for the real‐time MT systems to occupy stations along segments of a profile in Oregon. A composite profile will be created from the segments. Prior to commencing the main MT profiling phase, one week was set aside in August, 1984, for all groups to record and process MT data sequentially at six sites in diverse geologic terrains; this experiment was called mini‐EMSLAB.
The pseudospectral method: Accurate representation of interfaces in elastic wave calculations
Authors: Fornberg Bengt
Published: October 2002

When finite‐difference methods are used to solve the elastic wave equation in a discontinuous medium, the error has two dominant components. Dispersive errors lead to artificial wave trains. Errors from interfaces lead to circular wavefronts emanating from each location where the interface appears “jagged” to the rectangular grid. The pseudospectral method can be viewed as the limit of finite differences with infinite order of accuracy. With this method, dispersive errors are essentially eliminated. The mappings introduced in this paper also eliminate the other dominant error source. Test calculations confirm that these mappings significantly enhance the already highly competitive pseudospectral method with only a very small additional cost. Although the mapping method is described here in connection with the pseudospectral method, it can also be used with high‐order finite‐difference approximations.
Shear‐wave logging with dipole sources
Authors: Chen S. T.
Published: October 2002

Laboratory measurements have verified a novel technique for direct shear‐wave logging in hard and soft formations with a dipole source, as recently suggested in theoretical studies. Conventional monopole logging tools are not capable of measuring shear waves directly. In particular, no S waves are recorded in a soft formation with a conventional monopole sonic tool because there are no critically refracted S rays when the S-wave velocity of the rock is less than the acoustic velocity of the borehole fluid. The present studies were conducted in the laboratory with scale models representative of sonic logging conditions in the field. We have used a concrete model to represent hard formations and a plastic model to simulate a soft formation. The dipole source, operating at frequencies lower than those conventionally used in logging, substantially suppressed the P wave and excited a wave train whose first arrival traveled at the S-wave velocity. As a result, one can use a dipole source to log S-wave velocity directly on‐line by picking the first arrival of the full wave train, in a process similar to that used in conventional P-wave logging. Laboratory experiments with a conventional monopole source in a soft formation did not produce S waves. However, the S-wave velocity was accurately estimated by using Biot’s theory, which required measuring the Stoneley‐wave velocity and knowing other borehole parameters.
Shear‐wave processing of sonic log waveforms in a limestone reservoir
Authors: Goldberg David , Gant William T.
Published: October 2002

Sonic full waveforms were recorded in a shale‐limestone interval with an experimental 12-channel acoustic logging device. Well-defined shear (pseudo‐Rayleigh) waves were observed throughout most of the 142 m interval and were used to distinguish lithologic boundaries and zones of fracturing as interpreted by logs. The high signal‐to‐noise ratio of the waveforms permitted meaningful shear velocities to be obtained by intervals in this well. A correlation of changes in the [Formula: see text] ratio with depth aided in identifying some compositional changes in the well, but such a correlation could not be used to distinguish fracturing in the limestone. Shear‐wave amplitudes, however, were effective for identifying lithologic changes, as well as fractures, from the sonic log. Shear amplitudes were lower in the shale than in the limestone, and shear‐wave attenuation increased in fractured zones. There was no strong correlation between the degree of fracturing and the attenuation of Stoneley waves. In general, shear‐wave processing of sonic logs is recommended for interpretation of lithology and fracturing in any well in which full waveforms are recorded and shear waves propagate.
Uniform norm minimization for two‐signed solutions
Authors: Huestis Stephen P.
Published: October 2002

In an earlier paper (Huestis, 1986), I gave an iterative algorithm for efficient linear programming construction of the minimum uniform‐norm solution of a linear inverse problem. In such a problem, d data values [Formula: see text] are linear functionals of the unknown solution m(r), usually expressible as integrals over an appropriate domain V:[Formula: see text] The [Formula: see text] are known functions, and [Formula: see text] allows for measurement error.
Seismic contouring: A unique skill
Authors: Tucker Paul M.
Published: October 2002

Seismic contouring is a unique skill that requires combining the variable nature of the seismic signal with the varying structural styles of rock deformation. The computer is very helpful in producing suites of maps quickly, but it has often failed to incorporate structural styles and faults into meaningful geologic contours. This integration requires a visual image of the evolving structures that can best be obtained by an interpreter’s hand contouring or with an interactive work station guided by an interpreter’s hand. Each of the following situations requires a different approach to contouring and mapping the seismic data: (1) Three major styles of rock folding— (a) concentric folding, requiring smooth, hyperbolic contours: (b) plastic flow, requiring parallel contours and rounded reversals; and (c) topographic folding, requiring equal‐spaced contours and closed highs and lows. (2) Faults—May be difficult to visualize unless they are properly annotated and integrated into the contours. (3) Isotime maps—Should be contoured in the shape of depositional bodies. (4) Phantom horizon and dip maps—Needed when there are no continuous horizons. These maps can be useful as reconnaissance maps for planning programs in frontier areas. (5) Prospect (manager’s) maps—The summation of all previous maps. They must be precise, clear, and simple. They must say, “Here’s the oil!”
Ray‐Kirchhoff migration in inhomogeneous media
Authors: Zhu Tianfei
Published: October 2002

A Kirchhoff method that avoids possible singularities on the surface of integration and is more accurate than previous Kirchhoff methods has been developed for seismic migration in laterally inhomogeneous media. It is based on a newly derived integral solution to the acoustic wave equation. This solution indicates that wave fields in an inhomogeneous medium can be expressed as a summation of ray solutions determined by the transport and extended eikonal equations. The extended eikonal equation is, in turn, solved by an asymptotic series. For implementation, a perturbation scheme is developed for solving the ray and transport equations. In addition to computing higher‐order terms of the asymptotic series, this scheme can be used to avoid most of the ray tracing required for wave extrapolation in a medium where vertical variations may be large but lateral variations are small compared to velocity itself. Where an analytic ray solution for the reference velocity used in the perturbation scheme exists, ray‐Kirchhoff migration in such a medium can be carried out without numerical ray tracing.
Application of singular value decomposition to vertical seismic profiling
Authors: Freire Sergio L. M. , Ulrych Tad J.
Published: October 2002

An essential part of the interpretation of vertical seismic profiles (VSP) is the separation of the upgoing and downgoing waves. This paper presents a new approach which is based on the decomposition of time‐shifted VSP sections into eigenimages, using singular value decomposition (SVD). The first few eigenimages of the time‐shifted VSP section contain the contributions of the horizontally aligned downgoing waves. The last few eigenimages contain the contribution of uncorrelated noise components. The separated upgoing waves are recovered as a partial sum of the eigenimages. Important aspects of this approach are that regular sampling of the recording levels is not required, that the first‐break times need not be measured with extreme accuracy, that noise rejection may be automatically included in the processing, and that eigenimages or sums of eigenimages which may be computed as part of the approach can provide important additional information.
Vertical seismic profile migration using the Kirchhoff integral
Authors: Dillon P. B.
Published: October 2002

Wave‐equation migration can form an accurate image of the subsurface from suitable VSP data. The image’s extent and resolution are determined by the receiver array dimensions and the source location(s). Experiments with synthetic and real data show that the region of reliable image extent is defined by the specular “zone of illumination.” Migration is achieved through wave‐field extrapolation, subject to an imaging procedure. Wave‐field extrapolation is based upon the scalar wave equation and, for VSP data, is conveniently handled by the Kirchhoff integral. The migration of VSP data calls for imaging very close to the borehole, as well as imaging in the far field. This dual requirement is met by retaining the near‐field term of the integral. The complete integral solution is readily controlled by various weighting devices and processing strategies, whose worth is demonstrated on real and synthetic data.
Ultrasonic laboratory tests of geophysical tomographic reconstruction
Authors: Lo Tien‐when , Toksöz M. Nafi , Xu Shao‐hui , Wu Ru‐Shan
Published: October 2002

In this study, we test geophysical ray tomography and geophysical diffraction tomography by scaled model ultrasonics experiments. First, we compare the performance of these two methods under limited view‐angle conditions. Second, we compare the adaptabilities of these two methods to objects of various sizes and acoustic properties. Finally, for diffraction tomography, we compare the Born and Rytov approximations based on the induced image distortion by using these two approximation methods. Our experimental results indicate the following: (1) When the scattered field can be obtained, geophysical diffraction tomography is in general superior to ray tomography because diffraction tomography is less sensitive to the limited view‐angle problem and can image small objects of size comparable to a wavelength. (2) The advantage of using ray tomography is that reconstruction can be done using the first arrivals only, the most easily measurable quantity; and there is no restriction on the properties of the object being imaged. (3) For geophysical diffraction tomography, the Rytov approximation is valid over a wider frequency range than the Born approximation in the cross‐borehole experiment. In the VSP and the surface reflection tomography experiments, no substantial difference between the Born and Rytov approximations is observed.
An approximation technique for determining optimal Combisweep parameters
Authors: Dougherty S. M. , Justice J. H.
Published: October 2002

The autocorrelation of a linear FM (frequency modulated) signal consists of a main peak, a primary side lobe, and far side lobes (Figure 1), the side lobes often being referred to as correlation noise (Werner and Krey, 1979; Edelmann and Werner, 1982). One of the aims of Vibroseis signal design is to find and input sweep signal whose autocorrelation has a narrow main peak (high temporal resolution) and low correlation noise as defined by the main peak to primary side lobe amplitude ratio [Formula: see text] (Figure 1) (Werner and Krey, 1979). A simple and easily implemented method of reducing correlation noise is amplitude tapering (Edelmann, 1966), but unfortunately tapering induces an increase in main peak width (Berkhout, 1984), an undesirable effect. A technique of linearly combining individual autocorrelations, called Combisweep, was shown to increase [Formula: see text] (Werner and Krey, 1979). However, Werner and Krey made no attempt to optimize parameter selection.
Synthetic seismograms for marine sediments and determination of porosity and permeability
Authors: Turgut Altan , Yamamoto Tokuo
Published: October 2002

We present numerical simulations of vertical seismic profiles (VSPs) of marine sediments. The theoretical seismograms, which are computed for vertically incident waves in flat layered poroelastic media, include the effects of dispersion and attenuation predicted by Biot theory. According to Biot theory, fast and slow compressional waves are excited and there is mode conversion at the interfaces. We include this effect in the calculation of reflection and transmission coefficients as an energy‐loss mechanism through the slow compressional waves. Finally, we examine the spectral ratio method for determining porosity and permeability from synthetic seismogram data. Analytical expressions for the velocity and specific attenuation are found based on the weak‐frame approximation. Once the frequency‐dependent velocity and specific attenuation are calculated, the porosity and permeability of marine sediments can be determined by using the proposed weak‐frame approximations. Spectral ratio calculations on synthetic examples show that Biot’s theory incorporated in the VSP method can be used to determine the porosity and permeability of marine sediments.
Determination of salt proximity by wave‐field imaging of transmitted energy
Authors: McMechan George A. , Hu Liang‐Zie , Stauber Douglas
Published: October 2002

Prestack reverse‐time migration for acoustic waves has recently been developed for vertical seismic profile (VSP) data (Chang and McMechan, 1986) and for cross‐hole (CH) data (Hu et al., 1988). Both sets of authors use the same migration software and produce images from the scattered (reflected and diffracted) energy in the recorded wave fields.
Reverse-time migration in parallel: A tutorial
Authors: Fricke J. Robert
Published: October 2002

A reverse-time migration is implemented on a fine-grain or massively parallel computer. With fine-grain architectures many processors are distributed throughout the memory space and can operate on data “in place.” In addition, via a general communication system, any processor can access data from anywhere in the entire memory-processor space. Thus, operations on both local and global data elements are possible. These capabilities are controlled by parallel language constructs which allow parallel variable declaration, parallel arithmetic operation, and parallel random memory access. Reverse-time migration was programmed on a fine-grain machine with these hardware and software features. The reverse-time migration process had a speed improvement of two orders of magnitude relative to a state-of-the-art serial machine. At least another order of magnitude performance can be achieved with currently available floating-point processors. Similar increases in performance are expected for other seismic processes such as velocity estimation, data interpolation, 2-D filtering, and others.
Antialiasing methods for two‐dimensional spatial wavelets in seismic modeling
Authors: Nautiyal Atul
Published: October 2002

Two methods of circumventing spatial aliasing in seismic reflection modeling of zero‐offset data were examined. The first approach used asymptotic approximations of the near‐ and far‐field variants of the space‐frequency domain wave‐field continuation operator. This technique controls spatial aliasing but puts a lower limit on the extrapolation step size and an upper limit on frequencies that can be used in the modeling. The second method used Fourier (wavenumber) analysis of the same operator to identify the aliased components and wavenumber windowing to remove the unwanted portions of the wavenumber spectrum. In contrast to the asymptotic analysis, the Fourier analysis approach was simple and flexible to use and did not restrict other variables.
Low‐frequency permittivities of skarns and associated rocks
Authors: Emerson D. W , Welsh H. K.
Published: October 2002

The lossy dielectric properties of dry heterogeneous skarns and associated rocks vary significantly when the real (ε′) and quadrature (ε″) permittivity responses are measured between 1 Hz and 160 kHz. Ores containing iron oxides and/or sulfides show very large permittivity values and high loss tangents with some dc conductivity. Unmineralized skarns manifest large ε′ and moderate ε″ values. Country rock, limestones, and granitoids show relatively low permittivities. The dielectric responses are ascribed generally to Maxwell‐Wagner mechanisms. No clear, convincing correlations are apparent between petrological characteristics and dielectric properties, but as iron and titanium content increases, so does the imaginary permittivity. Both real and imaginary permittivities are strongly influenced by density, and this effect must be removed before the effects of mineralogy can be observed.
A theoretical study of surface‐to‐borehole electromagnetic logging in cased holes
Authors: Augustin A. M. , Kennedy W. D. , Morrison H. F. , Lee K. H.
Published: October 2002

A new electromagnetic logging method, in which the source is a horizontal loop coaxial with a cased drill hole and the secondary axial fields are measured at depth within the casing, has been analyzed. The analysis, which is for an idealized model of an infinite pipe in a conductive whole space, has shown that the casing and formation are uncoupled at the low frequencies that would be used in field studies. The field inside the casing may be found by first finding the field in the formation and then using this field as an incident field for the pipe alone. This result permits the formation response to be recovered from the measured field in the borehole by applying a correction for the known properties of the casing. If the casing response cannot be accurately predicted, a separate logging tool employing a higher frequency transmitter could be used to determine the required casing parameters in the vicinity of the receiver. This logging technique shows excellent sensitivity to changes in formation conductivity, but it is not yet known how well horizontal stratification can be resolved. One of its most promising applications will be in monitoring, through repeated measurements, changes in formation conductivity during production or enhanced recovery operations.
Computation of refraction static corrections using first‐break traveltime differences
Authors: Lawton Don C.
Published: October 2002

Differences in first‐arrival traveltimes between adjacent records in multifold reflection surveys can be used to compute the depth and velocity structure of near‐surface layers. The procedure uses the redundancy of first‐break data in multifold surveys to enable a statistically reliable refraction analysis to be undertaken for either end‐on or split‐spread recording geometries. The traveltime differences as a function of source‐receiver offset provide a direct indication of the number of refractors present, with each refractor being defined by an offset range with a constant time difference. For each refractor, the time‐difference value at a common receiver from two shotpoints is used to partition the intercept time into the delay time at each shotpoint. This procedure is repeated until the delay times at all shotpoints and for all refractors have been computed. Refractor depths and velocities are evaluated from this suite of delay times. A surface‐consistent static correction to a selected datum level is then calculated at each surface station, using a replacement velocity equal to that of the deepest refractor. In a case history from the Canadian Rocky Mountain foothills, short‐ and intermediate‐wavelength weathering static anomalies were resolved successfully. Elevation and weathering static corrections of up to 40 ms were computed, with an estimated error of less than ±3 ms.
Ray‐geometrical analysis of dip moveout amplitude distribution
Authors: Sorin Valery , Ronen Shuki
Published: October 2002

Amplitude analysis (such as for amplitude‐versus‐offset effects) requires a true‐amplitude correction at each process stage, including the dip moveout (DMO) process. The well known DMO operator is the “smile” smear, derived by Deregowski and Rocca (1981). In this note we continue their geometrical‐optics approach to obtain the amplitude on the operator they derived. The proposed solution is based on ray theory.
SV‐wave velocities from P‐P and P‐SV data for transversely isotropic solids
Authors: Levin Franklyn K.
Published: October 2002

Tessmer and Behle (1988) have shown that for P‐SV reflections from subsurfaces consisting of beds of isotropic solids separated by horizontal interfaces, the stacking velocity found for P‐SV reflection data is approximately the square root of the product of the P‐wave and SV‐wave stacking velocities from that reflection, i.e., [Formula: see text]. Even though P‐SV traveltime curves are not hyperbolic, the approximation is surprisingly good for the velocity fields considered by Tessmer and Behle. Given P‐wave and P‐SV stacking velocities, a reasonably accurate SV‐wave stacking velocity could be computed.
Triangle Ranch Headquarters field development using shallow core holes and high‐resolution seismic data
Authors: Renick Howard , Gunn R. D.
Published: October 2002

The Triangle Ranch Headquarters Canyon Reef field is long and narrow and in an area where near‐surface evaporites and associated collapse features degrade seismic data quality and interpretational reliability. Below this disturbed section, the structure of rocks is similar to the deeper Canyon Reef structure. The shallow structure exhibits very gentle relief and can be mapped by drilling shallow holes on a broad grid. The shallow structural interpretation provides a valuable reference datum for mapping, as well as providing a basis for planning a seismic program. By computing an isopach between the variable seismic datum and the Canyon Reef reflection and subtracting the isopach map from the datum map, we map Canyon Reef structure. The datum map is extrapolated from the shallow core holes. In the area, near‐surface complexities produce seismic noise and severe static variations. The crux of the exploration problem is to balance seismic signal‐to‐noise ratio and geologic resolution. Adequate geologic resolution is impossible without understanding the exploration target. As we understood the target better, we modified our seismic acquisition parameters. Studying examples of data with high signal‐to‐noise ratio and poor resolution and examples of better defined structure on apparently noisier data led us to design an acquisition program for resolution and to reduce noise with arithmetic processes that do not reduce structural resolution. Combining acquisition and processing parameters for optimum structural resolution with the isopach mapping method has improved wildcat success from about 1 in 20 to better than 1 in 2. It has also enabled an 80 percent development drilling success ratio as opposed to slightly over 50 percent in all previous drilling.
Influence of microgeometry on membrane potential of shaly sands
Authors: Sen Pabitra N.
Published: October 2002

The microgeometry of the pore space influences the membrane potential [Formula: see text] and the dc electrical conductivity σ of a shaly sand in a similar manner, independent of the details of the geometry. [Formula: see text] and σ are related via the conductivities of cations and anions [Formula: see text] and [Formula: see text], and [Formula: see text]. This explicit relationship is used to investigate the role of the geometrical factors that influence both [Formula: see text] and σ in a related manner. The dependence of σ on water conductivity [Formula: see text] can be well approximated with four geometrical parameters, which can be obtained from the slopes and the intercepts of curves of σ versus [Formula: see text] at high and low salinities. I show how these geometrical factors appear in the expression for [Formula: see text] as well. The geometrical parameters, one of them being the formation factor, vary from rock to rock; and any trend in the parameters depends on the local geology. For the data on a group of 140 different cores, the geometrical factors could be well approximated by functions of porosity, cementation exponent, and charge density, to give a simple conductivity formula analogous to the empirical formulas that are most widely used in formation evaluation. These empirical factors are used to obtain an approximate formula for [Formula: see text].
On modeling airborne very low‐frequency measurements
Authors: Poikonen Ari , Suppala Ilkka
Published: October 2002

Numerical models employed in ground VLF modeling use a normally incident (homogeneous) plane wave as a primary field. We show that these models are not directly applicable to modeling the impedance and wavetilt in the air, quantities needed in the interpretation of airborne VLF resistivity measurements. Instead, the primary field must be replaced by an inhomogeneous plane wave incident on the ground at an angle close to 90 degrees in order to provide the correct behavior of the apparent resistivities in the air. VLF magnetic polarization parameters, however, can be modeled in the air using the normally incident plane wave as a primary field. We also show that the plane‐wave analysis provides the same attenuation characteristics for the wavetilt in the air that is predicted by the Norton’s surface wave obtained by using the vertical electric dipole as a source. Use of the inhomogeneous plane wave introduces the vertical component of the electric field in the model. A 2‐D modeling technique based on the network solution is used to demonstrate the effects of the vertical electric field in the H‐polarization case. The vertical electric field generates charge distributions on the horizontal boundaries of conductors. In the case of a vertical sheet‐like conductor, these charges cause a slight asymmetry in apparent‐resistivity anomalies. Attenuation characteristics of various VLF anomalies with altitude are also presented. The H‐polarization anomalies attenuate much more rapidly in the air than those for E‐polarization due to the difference in the dominating source of EM fields in each polarization.
Full‐waveform inversion of plane‐wave seismograms in stratified acoustic media: Applicability and limitations
Authors: Pan G. S. , Phinney R. A.
Published: October 2002

A nonlinear waveform inversion procedure derived from Tarantola and Valette’s inversion formalism has been implemented in the horizontal slowness and intercept time (p-τ) domain. It infers simultaneously the velocity (bulk modulus) and density profiles of layered acoustic media. An arbitrary partition of the full wavefield information, i.e., an arbitrary selection of traces (and time ranges) of the whole p-τ seismic section is used as input to the inversion procedure. To explore its applicability and limitations, the algorithm has been tested against a series of “noise‐free” and noisy minimum‐phase band‐limited p-τ data sets synthesized from models of many microlayers which are meant to emulate the behavior of well logs. For a given data covariance, we confirmed that the specification of the starting model covariance determines the “generalized” damping factor which predicts the resolution of the final inverted model and the convergence rate of the inversion procedure. We found three major obstacles in this type of inversion: under‐parameterization of the model space with fixed thickness, large variation in material properties of the model, and lack of low‐wavenumber information from observed seismic signals. Underparameterization of the model space introduces substantial noise and artifacts into an inversion procedure, and large variations in material properties cause severe multiples which cause strong nonlinearity in an inversion. For multichannel seismic reflection data, the best low‐wavenumber information for the model is provided by the moveout of the signals. A well‐estimated starting model from any available source helps in stabilizing the inversion in two aspects; it provides necessary low‐wavenumber information to reconstruct the correct model profiles, and it reduces the magnitude of model parameter perturbations (with respect to the starting model) to suppress the strong nonlinearity. With proper inversion parameters, using limited data traces (less than 15 p values), the procedure is capable of reconstructing the velocity and density profiles simultaneously and satisfactorily.
One‐way versions of the Kirchhoff Integral
Authors: Berkhout A. J. , Wapenaar C. P. A.
Published: October 2002

The conventional Kirchhoff integral, based on the two‐way wave equation, states how the acoustic pressure at a point A inside a closed surface S can be calculated when the acoustic wave field is known on S. In its general form, the integrand consists of two terms: one term contains the gradient of a Green’s function and the acoustic pressure; the other term contains a Green’s function and the gradient of the acoustic pressure. The integrand can be simplified by choosing reflecting boundary conditions for the two‐way Green’s functions in such a way that either the first term or the second term vanishes on S. This conventional approach to deriving Rayleigh‐type integrals has practical value only for media with small contrasts, so that the two‐way Green’s functions do not contain significant multiple reflections. We present a modified approach for simplifying the integrand of the Kirchhoff integral by choosing absorbing boundary conditions for the one‐way Green’s functions. The resulting Rayleigh‐type integrals are the theoretical basis for true amplitude one‐way wave‐field extrapolation techniques in inhomogeneous media with significant contrasts.
Induced‐polarization effects in time‐domain electromagnetic measurements
Authors: Flis Marcus F. , Newman Gregory A. , Hohmann Gerald W.
Published: October 2002

Sign reversals in the coincident‐loop transient response can be produced by employing a Cole‐Cole model in numerical TEM modeling of polarizable conductors. These reversals may be thought of in terms of a polarization current which changes sign during the transient, passing from a charging current at early times to a discharging current at late times. In a layered earth, the relative strength of this current compared to the normally induced vortex current dictates whether or not a reversal is seen. If the earth is conductive, the effects of the polarization current may never be seen. If, however, the earth is only moderately conductive, the polarization current may dominate. In the case of a 3-D polarizable conductor in a conductive host, the addition of a host response serves to delay the time of any sign reversal in the transient. Reducing the host rock response by increasing its resistivity enables the polarization current to dominate earlier. By bringing the conductor closer to the surface, the amplitude of the negative response can be made greater and hence the sign reversal brought earlier in time. In such cases, moderate polarization parameters may cause substantial negative responses. It is possible to interpret TEM anomalies exhibiting sign reversals. The location and geometry of a discrete polarizable conductor can be correctly assessed, and a valid but approximate TEM time constant can be measured, from the positive part of the transient before the sign reversal.
Generalized form of the Dix equation for the calculation of interval velocities and layer thicknesses
Authors: Nowroozi Ali A.
Published: October 2002

Over three decades ago, Dix (1955) derived an approximate equation for the determination of interval velocity from observed reflection seismic data. Assuming a stack of m horizontal layers, with interval velocities [Formula: see text], layer thicknesses [Formula: see text], j = 1, m, and near‐vertical raypaths, Dix (1955) showed that [Formula: see text]where [Formula: see text] and [Formula: see text] are the two‐way vertical times and [Formula: see text] and [Formula: see text] are the root‐mean‐square (rms) velocities to interfaces j + 1 and j, respectively.
Velocity analysis by iterative profile migration
Authors: Al‐Yahya Kamal
Published: October 2002

In conventional seismic processing, velocity analysis is performed by using the normal moveout (NMO) equation which is based on the assumption of flat, horizontal reflectors. Imaging by migration (either before or after stack) is done normally in a subsequent step using these velocities. In this paper, velocity analysis and imaging are combined in one step, and migration itself is used as a velocity indicator. Because, unlike NMO, migration can be formulated for any velocity function, migration‐based velocity analysis methods are capable of handling arbitrary structures, i.e., those with lateral velocity variations. In the proposed scheme, each shot gather (profile) is migrated with an initial depth‐velocity model. Profile migration is implemented in the (x, ω) domain, but the actual implementation of profile migration is not critical, as long as it is not done in a spatial‐wavenumber domain, which would preclude handling of lateral velocity variations. After migration with an initial velocity model, the velocity error is estimated, and the initial velocity model is updated; the process is repeated until convergence is achieved. The velocity analysis is based on the principle that after prestack migration with the correct velocity model, an image in a common‐receiver gather (CRG) is aligned horizontally regardless of structure. The deviation from horizontal alignment is therefore a measure of the error in velocity. If the migration velocity is lower than the velocity of the medium, events curve upward, whereas if the migration velocity is higher than the velocity of the medium, events curve downward.
Postdrilling hydrocarbon indicators study on South Marsh Island, Blocks 128 and 141: A 1975 case history
Authors: Pan P. H.
Published: October 2002

The South Marsh Island Block 128 Field (Figure 1) is located offshore Louisiana, 150 miles southwest of New Orleans. Two abnormal phenomena, seismic wave tuning and high‐amplitude reflections caused by small amounts of gas in the water sands, were observed after several wells were drilled in Blocks 128 and 141 by Mobil in 1975.
Shot dip moveout with logarithmic transformations
Authors: Cabrera Julian , Levy Shlomo
Published: October 2002

In a recent paper, Notfors and Godfrey (1987) presented an approximate analytical expression for a common‐offset dip moveout (DMO) operator in the space, logarithmic‐time domain. This operator yields an efficient frequency‐wavenumber DMO algorithm. In this note we extend Notfors and Godfrey’s work to obtain an approximate DMO operator in the logarithmic space and time domains and propose an efficient frequency‐wavenumber DMO algorithm that is applied to shot gathers. A synthetic example is presented.
Construction of the generalized one‐dimensional synthetic seismograms by a three‐step extrapolation procedure
Authors: Shtivelman V. , Loewenthal D.
Published: October 2002

In a recent publication, Gutowski and Treitel (1987) gave a clear exposition of one‐dimensional (1-D) modeling. They considered the most general case of arbitrary locations of source and receiver within the layered medium, where the source may have different downgoing and upgoing strengths and may be above or below the receiver. The solution, in the form of two rational polynomial expressions in the Z-transform domain, was analyzed in detail for some interesting special cases.
On: “Direct time‐domain calculation of the transient response for a rectangular loop over a two‐layer medium” by M. M. Goldman and D. V. Fitterman (GEOPHYSICS, 52, 997–1006, July 1987).
Authors: Lee Terry , Ignetik Rainer
Published: October 2002

Goldman and Fitterman (1987) condsider the transient response of a conducting slab above a perfect conductor or insulator. Crucial to this problem is the location of a series of roots of two transcendental equations, to which there are useful approximations.
Modeling magnetostratigraphy in a borehole
Authors: Gallet Yves , Courtillot Vincent
Published: October 2002

The magnetic field along a hole bored through a sequence of dipping layers with varying magnetization and planar interfaces is calculated. The emphasis is on recovering remanent magnetization polarity transitions for magnetostratigraphic applications (dating, correlations), although intensity of remanent magnetization can in itself be a useful rock property. Results are presented for various cases of geologic interest in the form of axial vertical profiles and section maps of the holes at varying distances from polarity reversal interfaces. The vertical profiles demonstrate a resolving power of about six times the hole radius; meaningful magnetostratigraphies can be expected for rocks with a magnetization larger than [Formula: see text], for instruments with a sensitivity of 0.1 nT. In a number of natural occurrences, it may not be necessary to resort to progressive demagnetization to recover the polarity sequence. Depending on which magnetic field and magnetization component one looks at, the section maps display characteristic patterns, in which, for instance, the direction of magnetization and the dip of the layers interfere. These maps are discussed in some detail. They can be used as guidelines to build a multisensor vector magnetometer (downhole magnetostratigraphic tool), whose output should be coupled with measurement of magnetic susceptibility for reduction of induced magnetization and with output from a surface instrument in a differential operation mode to reduce transient magnetic variations.
Seismic resolution and field design: Success and failure at Taber, Alberta, Canada
Authors: Taylor Gary
Published: October 2002

Geophysical mapping is presented that suggests a genetic relationship may exist between a band of Glauconitic sandstones and the edge of a large Mississippian structure. A seismic line of late 1970s vintage, covering a thick Glauconitic sandstone reservoir, was reshot with a broader band vibrator sweep, reduced geophone offsets, and shorter arrays. These changes significantly improved the seismic visibility of the sandstone reservoir. However, they also significantly enhanced the vulnerability of the signal to high‐noise environments. Such an environment is identified beneath a 163 000 V transmission line with wooden poles, where ground roll from resonating poles and not induced 60 Hz current is the dominant noise system. Examples are shown of the seismic signatures for both structural and stratigraphically controlled reservoirs in the area.
Amplitude‐offset relationships over shallow velocity inversions
Authors: Poley D. F. , Lawton D. C. , Blasco S. M.
Published: October 2002

The amplitude and phase of reflected seismic data can vary significantly with the offset distance between source and receiver. Analysis of amplitude‐offset relationships in high‐resolution seismic data from areas of shallow permafrost in the Beaufort Sea is undertaken in this paper. Reflection attributes produced by energy partitioning at interfaces under nonnormal angles of incidence were studied by means of ray tracing and the Zoeppritz equations. Observed amplitude and phase variations were found to cause CDP stacking techniques to degrade reflection data quality, particularly where the zone of interest features velocity inversions or is shallow in relation to receiver lengths. In these cases velocity analysis based on coherency measures may result in the selection of incorrect stacking velocities.
A new approach to modeling the electromagnetic response of conductive media
Authors: Lee K. H. , Liu G. , Morrison H. F.
Published: October 2002

We introduce a new and potentially useful method for computing electromagnetic (EM) responses of arbitrary conductivity distributions in the earth. The diffusive EM field is known to have a unique integral representation in terms of a fictitious wave field that satisfies a wave equation. We show that this integral transform can be extended to include vector fields. Our algorithm takes advantage of this relationship between the wave field and the actual EM field. Specifically, numerical computation is carried out for the wave field, and the result is transformed back to the EM field in the time domain. The proposed approach has been successfully demonstrated using two‐dimensional (2‐D) models. The appropriate TE‐mode diffusion equation in the time domain for the electric field is initially transformed into a scalar wave equation in an imaginary q domain, where q is a time‐like variable. The corresponding scalar wave field is computed numerically using an explicit q‐stepping technique. Standard finite‐difference methods are used to approximate the fields, and absorbing boundary conditions are implemented. The computed wave field is then transformed back to the time domain. The result agrees fairly well with the solution computed directly in the time domain. We also present an approach for general three‐dimensional (3‐D) EM problems for future studies. In this approach, Maxwell’s equations in the time domain are first transformed into a system of coupled first‐order wave equations in the q domain. These coupled equations are slightly modified and then cast into a “symmetric” and “divergence‐free” form. We show that it is to this particular form of equations that numerical schemes developed for solving wave equations can be applied efficiently.
On the construction and efficiency of staggered numerical differentiators for the wave equation
Authors: Kindelan M. , Kamel A. , Sguazzero P.
Published: October 2002

Finite‐difference (FD) techniques have established themselves as viable tools for the numerical modeling of wave propagation. The accuracy and the computational efficiency of numerical modeling can be enhanced by using high‐order spatial differential operators (Dablain,1986).
Clay mineralogy and pressure analysis from seismic information in Krishna‐Godavari basin, India
Authors: Basu Saibal
Published: October 2002

Interval transit time data derived from seismic velocities have been analyzed to predict clay mineralogy and overpressures from compaction trends in the offshore areas of Krishna‐Godavari basin located on the east coast of India. Compaction trends from rms velocity fairly well match the trends obtained from sonic logs. This study has been extended to the undrilled part of the basin with the help of seismic information. Areal distribution of clay mineral types has been determined from the compaction trends. Prospective areas are identified on the basis of clay mineral distribution and overpressure zones related to the areas of different clay mineral associations.