bluuebunny/crossref_metadata_2025 huggingface dataset:
SUBSURFACE GRAVITY MEASUREMENTS AND DENSITY MAPPING IN THE MAILARAM COPPER MINES, ANDHRA PRADESH, INDIA*
Authors: MURTY B. V. SATYANARAYANA , GHYASUDDIN M. , REDDY P. VENKAT
Published: April 2006

Abstract

Gravity measurements were made in the Mailaram copper mines, Andhra Pradesh. The observations were distributed between the two shafts situated about 220 m apart and in the three levels up to a maximum depth of 100 m. Assuming a normal free‐air gradient, average densities for the three layers were determined as 2.631, 2.604, and 2.823 g cm‐3, respectively. Upon incorporating the weighted mean density values from measurements on samples, the free‐air gradients were found to be 0.315 mGal m‐1 for the second layer (i.e. between the first and second levels) and 0.2978 mGal m‐1 for the third layer (i.e. between the second and third levels). The density variation map obtained from the gravity data, the deduced anomalies, the weighted mean density values from measurements on rock samples, and the varying free‐air coefficients all suggest correspondence with the concentration of ore lodes.
A METHOD FOR THE DETERMINATION OF THE THERMAL CONDUCTIVITY OF SANDSTONES USING A VARIABLE STATE APPROACH*
Authors: STRACK K. M. , IBRAHIM A. W. , KELLER G. V. , STOYER C. H.
Published: April 2006

Abstract

Common problems encountered during the determination of the thermal conductivities of tight sandstones are rock specimen‐to‐heat source contact and complicated sample preparation. An experimental technique using an electromagnetic heat source solves the direct contact problem between the heat source and the sample. Also, it simplifies the sample preparation and reduces the measurement time.

A CO2 laser operating in a pulsed mode is used as a heat source with about 500 W output power. Thus, heat losses due to radiation and air convection are negligible. Unpolished penny‐sized samples of tight sandstones are irradiated on one side and the temperature is measured on the opposite side. The temperature is recorded with 12‐bit accuracy by a digital data acquisition system. Carbon black is used to give the samples a uniform absorption.

The transient temperature data are acquired, processed, and interpreted with interactive computer programs. Transients for each sample can be stacked, to improve the signal‐to‐noise ratio, and normalized. The thermal conductivity is calculated from the rise portion of the transient using a ridge‐regression type generalized linear inversion scheme.

As many as 20 samples per hour can be measured with this simple but expensive set‐up. In the long run, this means a reduction of laboratory expenses. In addition, the resolution of this method is superior to other variable‐state measurement methods due to the real time controlled data acquisition and the numerical interpretation.
NON‐NORMAL INCIDENCE STATE‐SPACE MODEL AND LINE SOURCE REFLECTION SYNTHETIC SEISMOGRAM*
Authors: AMINZADEH F. , MENDEL J. M.
Published: April 2006

Abstract

This paper is directed at modeling layered media. We extend the plane‐wave normal‐incidence state‐space model developed by Mendel, Nahi and Chan in 1979, to the non‐normal incidence case. To do this we introduce a shifting principle, a zero‐offset wavefront, and zero‐offset travel times for different layers. We also develop an algorithm for obtaining a synthetic line source reflection seismogram. In this algorithm non‐normal incidence plane‐wave seismograms are summed over a range of incident angles. The algorithm is based on a modified version of Sommerfield's (1896) theorem. Simulations of acoustic and elastic media are included which illustrate the applicability of our plane‐wave and line source seismograms for both elastic and acoustic cases.
A NEW DERIVATION OF THE WAVEFRONT CURVATURE TRANSFORMATION AT AN INTERFACE BETWEEN TWO INHOMOGENEOUS MEDIA*
Authors: URSIN B.
Published: April 2006

Abstract

The principles for ray‐tracing and wavefront curvature calculations in a three‐dimensional medium are reviewed. A new derivation of the transformation of the wavefront curvature matrix at an interface between two inhomogeneous media is given. The derivation is based on a Taylor series expansion of the ray refraction equation at the interface between two inhomogeneous media, and only elementary geometric arguments are used. The wave‐front curvature transformation at the interface is obtained by neglecting all terms in the direction of the surface normal.

With proper definition of the variables, the derivation is also valid for a reflected wave‐front. A simplified transformation rule is derived for a reflected wave of the same type as the incident wave.
POROSITY PREDICTION FROM SEISMIC DATA*
Authors: ANGELERI G. P. , CARPI R.
Published: April 2006

Abstract

The inversion of seismic traces allows the estimation of reservoir porosity from an analysis of transit times derived from the pseudo‐velocity logs. A four‐step computational procedure is illustrated consisting of (i) inversion of seismic traces and calculation of interval velocities; (ii) accurate stratigraphic interpretation; (iii) determination of the petrophysical parameters for the porosity evaluation; (iv) analysis of the reliability of the results and final corrections.

Both the possibilities and the limits of the method are discussed. One of the causes of error is the fact that impedances—and not velocities—are readily obtainable from seismic data. Moreover, the porosity due to fracturation contributes only slightly to velocity, while it often contributes most importantly to the permeability.

Results are shown for two of the most significant reservoir types, i.e. carbonatic and clastic. Two cases belonging to the latter type will be examined. In the first case the primary porosity is dominant. The second case is very complex and both primary and secondary porosity are present.
RECIPROCAL AVERAGING TECHNIQUES IN THE GEOELECTRICAL BOUNDARY ELEMENT APPROACH*
Authors: OKABE M.
Published: April 2006

Abstract

A Fredholm integral equation of the first kind with respect to the surface charge density is presented via a weighted residual formulation in the standard isotropic problem. The surface charge densities are numerically obtained by solving the well‐known Fredholm integral equation of the second kind, where the new equation can be regarded as a constraint. The accuracy of boundary element solutions is examined in connection with the violation of such a constraint and the “modified reciprocal averaging techniques” are proposed.
THE INFLUENCE OF SHALE EFFECTS UPON THE ELECTRICAL RESISTIVITY OF RESERVOIR ROCKS*
Authors: WORTHINGTON P. F.
Published: April 2006

Abstract

The roots of the so‐called shaly‐sand problem in hydrocarbon evaluation lie in the effect of relatively fine‐grained minerals upon measured electrical parameters of granular reservoirs. This influence manifests itself as an excess conductivity, over and above that due to the purely geometric effects of electrolyte distribution within the pore space. For formations with low shaliness, this excess conductivity is usually insignificant in typical oilfield situations.

The influence of shaliness upon observed values of formation resistivity has been appraised by collating core‐sample data from four different reservoirs. It has been demonstrated that during the course of electrical measurement under conditions of full electrolyte saturation, any given lithology can exhibit both negligible and highly significant shale effects depending upon the resistivity of the interstitial aqueous electrolyte. The effects of shaliness are also governed by the degree of water saturation.

Because the manifestation of shaliness in electrical data is not a function of lithology alone, recourse is made to a more realistic concept of shale effects whereby a formation, or section of a formation, is classified as “effectively clean” or “effectively shaly” according to whether it obeys or defies, respectively, the fundamental empirical laws of Archie (1942). In particular, since an intrinsic formation factor can be obtained directly in fully‐saturated effectively clean reservoirs, whereas only an apparent quantity may be recorded directly in fully‐saturated effectively shaly reservoirs, the ratio of apparent to intrinsic formation factor serves as a useful conceptual indicator of shale effects, attaining the limiting value of unity only under effectively clean conditions.

In the context of electrical measurement the terms “shaliness” and “shale effects” are evidently not synonymous and it is the latter which should be considered when selecting equations for the computation of water saturation. The implications for well‐log analysis follow through formulated guidelines that describe the relative levels of shale effects in different zones of lithologically uniform reservoirs.
APPLICATION OF BOREHOLE GEOPHYSICS AT AN EXPERIMENTAL WASTE STORAGE SITE*
Authors: NELSON P. H. , MAGNUSSON K. A. , RACHIELE R.
Published: April 2006

Abstract

A suite of electrical, radiation, and mechanical borehole probes were run in a 76‐mm‐diameter borehole drilled to a slant depth of 380 m in leptite and granite. The hole is located in Precambrian bedrock in central Sweden where a site is dedicated to in‐situ experiments pertaining to the disposal of radioactive wastes. The challenge to borehole logging methods for such site investigations is to resolve geological features and fluid flow parameters in geological sites which are initially chosen for their homogeneity, low porosity, and minimal fracturing. The Stripa borehole is characterized by high electrical resistivity values in the 20–100 kΩm range, by acoustic velocities around 5800 m s‐1 (which is close to laboratory values on intact specimens), and by total porosity of around one volume percent. In this context, probe resolution was adequate to produce interpretable information on almost all of the logs.

Two principal rock types were encountered in the hole: granite, of quartz monzonitic composition, and leptite. The granite and leptite intercepts are subdivided into units characterized by mafic mineral content, sulfide mineral content, and electrical and radiation properties. Iron‐rich zones in the leptite are highly anomalous on the gamma‐gamma and neutron logs; thin mafic zones in the granite can also be distinguished. Occurrences of a few percent pyrite are detected by the electrical, gamma‐gamma, and neutron logs. Although overall porosity is quite low throughout the hole, analysis of the resistivity and neutron logs indicates the porosity increases by a few volume percent at fracture zones. The differential resistance and caliper probes detect borehole diameter roughness of less than 1 mm, helping to confirm acoustic waveform anomalies which are indicative of fracture zones. Compres‐sional wave transit time and shear‐wave interference patterns usually occur coincident with open fractures observed in core, the correlation being especially good at major fracture zones.
DOWNHOLE PERIODIC SEISMIC SOURCES*
Authors: HARDEE H. C.
Published: April 2006

Abstract

Downhole periodic seismic sources have been proposed as a means for studying fluid zones in hydrothermal/magma systems encountered in deep scientific holes drilled as part of the Continental Scientific Drilling Program. Scientific measurements in these holes will require the use of downhole, nondestructive, high resolution, seismic tools. Downhole periodic seismic sources tested in a shallow geothermal zone showed that a pneumatic oscillator could effectively generate downhole low‐frequency (10–100 Hz) seismic waves and that these waves could effectively penetrate a highly fractured formation. An improved version of these downhole sources, capable of swept‐frequency operation, is currently under development. This type of downhole seismic source has applications in crustal studies and in the evaluation of petroleum, groundwater and geothermal reservoirs.
HIGH‐RESOLUTION PREDICTION OF ACOUSTIC IMPEDANCES BELOW BOTTOM‐OF‐HOLE*
Authors: TAL‐VIRSKY B. B. , TABAKOV A. A.
Published: April 2006

Abstract

Prediction of acoustic impedances below bottom‐of‐hole developed in the paper is based on VSP data processing. The pulse form and reflections are deduced from records on the vertical array by the method of subtraction. A deterministic prediction error and a wave‐shaping Wiener filter are used to transform the pulse train and reflections into a short symmetrical pulse. Additional broadening of the pulse spectrum is achieved by stacking of seismograms from shots with various dominant frequencies after zero‐phase transformation of downgoing waves.

The inversion of seismograms is made after ideal spiking by means of a subtraction procedure. Acoustic impedances deduced are closely related to lithology of rocks below bottom‐of‐hole.
CALCULATIONS ON THE THERMAL CONDITIONS AROUND A SALT DIAPIR*
Authors: JENSEN P. K.
Published: April 2006

Abstract

Temperature modeling around the Vejrum salt structure in Denmark shows that the heat flow near the top is twice the regional heat flow. The temperature in the vicinity of the top of the diapir is thus anomalously high. Overlying permeable formations could therefore be attractive for geothermal energy exploitation. A negative temperature anomaly of −20°C is calculated around the root of the salt body. Comparisons between calculated and measured temperatures are used to test the model. It is shown that measurements of heat flow (or temperature gradients) in wells penetrating the region of groundwater circulation could be used in identification of salt diapirs.
ELECTROMAGNETIC SOUNDING NEAR A LARGE SQUARE LOOP SOURCE OF CURRENT*
Authors: PODDAR M.
Published: April 2006

Abstract

An electromagnetic sounding experiment with a large square loop as source was carried out on the dried‐up bed of a water reservoir near the town of Dharmavaram in Andhra Pradesh. The sounding was performed in both geometric and parametric modes, and involved measurements of phase as well as amplitude of the vertical magnetic field inside and outside the loop. The six‐frequency EM system used for the experiment was found to be a workable system for electromagnetic sounding. The results of the experiment more or less confirm the conclusions from theoretical modeling. They also show that even though the earth is not always electrically horizontally layered over a sufficiently large lateral extent for the one‐dimensional model to be strictly valid, it is still possible to apply such modeling to sounding curves taken one part at a time and obtain layer parameters which check qualitatively with the layer parameters obtained from direct current resistivity sounding.
MEASURED ANISOTROPY IN PIERRE SHALE*
Authors: WHITE J. E. , MARTINEAU‐NICOLETIS L. , MONASH C.
Published: April 2006

Abstract

The five elastic coefficients which characterize a transversely isotropic medium have been measured for the Pierre Shale. The first‐arrival times deduced from two closely spaced vertical seismic profiles provide values of magnitude and direction of the phase velocity at a given depth for different locations of the sources. From these velocities, we can estimate the five elastic coefficients by assuming that the P‐ and SV‐wave velocities have only a small dependence on the angle of propagation. On the other hand, it is shown that the accuracy in the difference between the directions of propagation and particle motion is not sufficient to determine the anisotropy.

This work was industrially sponsored through the Integrated Geophysics Project.
LINEAR FILTERING OF VLF DIP‐ANGLE MEASUREMENTS*
Authors: KAROUS M. , HJELT S. E.
Published: April 2006

Abstract

The suggested interpretation technique is based on discrete linear filtering of VLF data. The output of the described filtering results is expressed in terms of an equivalent current density at a specified depth that would cause the measured magnetic field. The most practical six‐point filter gives an accuracy of 8%. The filter is an extension of the commonly used Fraser filter to process VLF dip‐angle data.

Filtering the same data set for various depths gives an idea about the change of current density with depth. Areas with high current‐density correspond to good conductors. The conductor dip can also be determined.

The use of the technique is illustrated on theoretical and field examples. In all cases a good correlation with original models and other types of geophysical measurements was obtained. As shown in the last example, the filtering technique is also applicable in interpretation of other electromagnetic methods.
ON THE ORIGIN AND INTERPRETATION OF SELF‐POTENTIAL ANOMALIES*
Authors: KILTY K. T.
Published: April 2006

Abstract

The renewed interest in the self‐potential method of exploration for mineral deposits gives an understanding of the self‐potential mechanism new importance. The cause of SP anomalies in general lies in the interference between simultaneously occurring nonequilibrium phenomena. However, theories of the mechanism of mineral SP anomalies generally relate the SP anomaly to the equilibrium potential of the chemical reaction supposed to occur on the ore body surface. In this paper, I reformulate these equilibrium mechanisms in terms of nonequilibrium thermodynamics. The result is that the SP anomaly depends not on the equilibrium potential alone, but also on the potential resulting from current transferred across the ore body—electrolyte interface. It is not possible to calculate the overpotential theoretically because of the number of complicating factors, and experimental data are not available. This does not imply that SP data are uninterpretable quantitatively. SP data may be interpreted similarly to other potential field data.
THE CLASSIFICATION OF PHYSICO‐GEOLOGICAL MODELS OF MINERAL DEPOSITS*
Authors: VAKHROMEYEV G. S. , BARYSHEV A. S.
Published: April 2006

Abstract

We describe the concept of physico‐geological models (PGMs) in geophysical exploration. They represent a “general model”, a spatial combination of a set of particular models (disturbing bodies). The modeling is called complete, incomplete or approximate, depending on the degree of characterization of the PGM by parameters such as dimension, shape and petrophysical property. Each of the three modeling types can be realized as a conceptual, and analytical, or a material PGM. Both deterministic and stochastic PGMs exist; deterministic models are mainly used to investigate the possibilities of a geophysical method, while stochastic models serve to substantiate complex geophysical interpretations.

Depending on the geological problem, PGMs are subdivided into multi‐alternative models (geological mapping, prediction, general prospecting) and double alternative models (specialized prospecting).

An exploration‐oriented classification of the PGMs of mineral deposits is discussed. According to this classification the variety of known genetic deposit types is reduced to a limited number of generalized PGM types. The development of typical PGMs is illustrated with examples of magnetitic deposits of Siberia.
A MODIFIED MODE OF OPERATION FOR THE TURAM ELECTROMAGNETIC EXPLORATION SYSTEM WITH BENEFITS FOR DEEP EXPLORATION*
Authors: DUCKWORTH K. , BAYS A. R.
Published: April 2006

Abstract

A new mode of operation for the Turam electromagnetic exploration system is proposed in which the transmitter loop is placed across the expected trend of a conductor and the receiver is operated along lines parallel to one side of the transmitter. The concept appears to offer several benefits which include greatly extended traverse length, the use of large coil spacing, rejection of the effects of conductive environments, and consistency in the indication of target dip.
OFFSET SOURCE VSP SURVEYS AND THEIR IMAGE RECONSTRUCTION*
Authors: DILLON P. B. , THOMSON R. C.
Published: April 2006

Abstract

Zero‐offset‐source VSP surveys provide information about the subsurface only within the Fresnel zone centered at the well. Offsetting the source location moves the reflection zones away from the well thus providing lateral cover.

Conventional processing of this type of data gives rise to a distorted image of the subsurface. Using a simple ray‐tracing scheme, this image may be reconstructed into the more familiar coordinate system of the surface seismic section. This simple data‐independent mapping is based on the assumption of horizontal layering and requires a vertical velocity profile.

The technique of placing the source away from the borehole was first applied to the single‐offset‐source VSP survey. However, data from any survey geometry (such as deviated well with rig source, walkaway VSP, etc.) can be mapped to the coordinate system defined by the appropriate seismic section.

To obtain the best results from this type of survey the target area must be defined and simple modeling techniques used to optimize the source location(s). These pre‐survey modeling methods may also be used to anticipate—and hence avoid a number of problem areas which experience has highlighted.

The data from any VSP survey is the result of a realizable experiment and as such obeys the wave equation. This implies that the wave equation may be used to migrate the data to its true subsurface location. Theoretically, such a process is more secure than ray‐tracing techniques, although its practice presents many difficulties.
ODD‐DEPTH STRUCTURES FOR DETERMINISTIC DECONVOLUTION AND SEISMOGRAM TESTING*
Authors: SZARANIEC E.
Published: April 2006

Abstract

In odd‐depth structure the two‐way traveltime to each boundary is constrained to be an odd integer. The odd‐depth property of a model is exposed to possible refutation under a seismogram test. Test function is a simple transformation of a synthetic seismogram. For an odd‐depth model the test function has identically the value 1.

The testability of a synthetic seismogram over an odd‐depth structure provides a method of deterministic deconvolution. There is no need of specialized assumptions, like the minimum‐phase property, about the source wavelet. The deconvolution may be performed in the absence of the early segment of a seismogram.
AN OPERATOR APPROACH TO FORWARD MODELING, DATA PROCESSING AND MIGRATION*
Authors: KENNETT B. L. N.
Published: April 2006

Abstract

The response of a seismic model to excitation by a source can be represented in terms of the action of reflection and transmission operators for portions of the structure. This approach provides a flexible framework for both modeling and processing problems.

The operator development provides a physical description of the wave propagation process and, via the expansion of reverberation operators, gives a mechanism for assessing the accuracy of approximate developments. The representation suggests new ways of developing modeling algorithms by balancing the computational effort expended on minor and major features of the model.

For processing problems, the operator representation shows the relation of processing stages to the seismic wave field and thereby indicates effective sequences of operations. For migration it is possible to specify an ideal pre‐stack migration procedure in terms of the inverse of the propagation operators and to examine the problems which need to be overcome by practical algorithms.
PHASE DISTORTION DUE TO ABSORPTION IN SEISMOGRAMS AND VSP*
Authors: ANGELERI G. P. , LOINGER E.
Published: April 2006

Abstract

Absorption of seismic energy in the earth reduces amplitudes and changes phases of the propagating seismic waves. Amplitudes are usually recovered according to an estimated exponential decay curve, while phase distortions are generally disregarded. Therefore, accurate processing of seismic data requires a careful investigation of the relationship between absorption and phases.

In this paper a procedure is suggested to achieve this goal, and some related topics are worked out. A method is outlined for computing synthetic seismograms and vertical seismic profiles with phase distortion due to absorption.

The algorithm works in the frequency domain, and it provides for absorption according to the usual model of exponential decay of amplitude with distance. The absorption coefficient is a linear function of frequency and is related to the quality factor Q of the rocks. Complex seismic velocities are introduced and minimum‐phase delay due to absorption is assumed for all cases considered.

Methods for estimating Q profiles from seismic well surveys and seismic data are described. Comparison between field and synthetic data shows the effectiveness and benefits of the procedure. Some applications of the method to phase distortion recovery and wavelet processing are presented.
RESOLUTION ENHANCEMENT OF WELL LOG AND SEISMIC DATA*
Authors: MICHETTE A. G. , DARLING A. M. , FIDDY M. A. , WARD W. A. , FRANKLIN E.
Published: April 2006

Abstract

A new method with general applications for seismic data is presented for spectral extrapolation. The method gives a restored image that is optimum in the sense of minimum norm and can be adapted to incorporate any constraints on the reconstruction. As an illustration of the technique it is first applied to a synthesized (noise‐free) image. Then it is shown that enhancement of a density log (run in a hole drilled entirely in Coal Measures rocks) is possible using constraints taken from the geologist's log. A synthetic trace with significantly improved event arrival times and general character is obtained.
NOMOGRAMS TO INTERPRET CRONE PEM DATA USING A DIPPING SHEET MODEL*
Authors: RAI S. S. , VERMA S. K.
Published: April 2006

Abstract

The response of a rectangular plate to the Crone PEM system has been investigated for different values of depth, dip and conductance parameters. The conductance aperture diagram is presented to estimate the conductance and depth to the top of a vertical sheet. Information regarding dip and depth may be obtained using the appropriate nomogram.
HOW DO SHEAR WAVE EVENTS AFFECT NORMAL P‐WAVE RECORDS?*
Authors: AMEELY L. , EDELMANN H. A. K. , FERTIG J.
Published: April 2006

It has been known since the beginning of reflection seismics that several disturbing events seen in seismic records are caused by waves with S‐wave velocities instead of P‐wave velocity. When using dynamite and recording with vertical geophones these events are primarily caused by converted waves.

On the basis of known P‐ and S‐wave velocities in a certain area a theoretical seismogram is calculated, displaying traveltime as well as energy relation for different wave configurations. By comparison with seismograms recorded in the same area it can be shown that converted wave events can be clearly recognized.

These events can be described theoretically. Thus, either more effective computer programs can be applied to eliminate these disturbing events, or these events can be evaluated to get additional information about specific strata.
CONTINUOUS CALIBRATION OF MARINE SEISMIC SOURCES*
Authors: NEWMAN P.
Published: April 2006

The success of signature deconvolution in optimizing both signal‐to‐noise ratio and time resolution in the seismic section depends critically upon obtaining an accurate estimate of the far‐field source signature. Various deterministic estimation schemes have been proposed in recent years, most of which involve direct monitoring of source output within the water layer.

As an alternative to elaborate and error‐prone source monitoring schemes during data acquisition, a simple modification to any source array permits subsequent estimation of far‐field signatures directly from reflected signal. The new method requires the inclusion within any chosen source array of a simple point source, the “reference” source. Initial experiments employed a water gun as the reference source, characterized by a concise implosive signature with peak‐to‐peak amplitude of approximately 2 bar·m within the seismic sprectrum.

In operation the reference source is fired shortly before the main array (typically 2 s during initial trials) and the usual record length is extended by a similar amount. Each recorded trace then comprises two results: the subsurface response to the reference source signal followed by the response of the same subsurface to the main array. The disparities in source amplitudes and NMO differentials ensure that interference effects are negligible in the main recording.

Time‐ or frequency‐domain methods can be employed to extract the main array signature from the dual dataset or to invert this to some preferred wavelet simultaneously. As an additional benefit the reference source yields excellent high‐resolution profiles of the shallow geology.
MAXIMUM‐LIKELIHOOD ESTIMATION OF SEISMIC IMPULSE RESPONSES*
Authors: URSIN B. , HOLBERG O.
Published: April 2006

A seismic trace is assumed to consist of a known signal pulse convolved with a reflection coefficient series plus a moving average noise process (colored noise). Multiple reflections and reverberations are assumed to be removed from the trace by conventional means. The method of maximum likelihood (ML) is used to estimate the reflection coefficients and the unknown noise parameters. If the reflection coefficients are known from well logs, the seismic pulse and the noise parameters can be estimated.

The maximum likelihood estimation problem is reduced to a nonlinear least‐squares problem. When the further assumption is made that the noise is white, the method of maximum likelihood is equivalent to the method of least squares (LS). In that case the sampling rate should be chosen approximately equal to the Nyquist rate of the trace. Statistical and numerical properties of the ML‐ and the LS‐estimates are discussed briefly. Synthetic data examples demonstrate that the ML‐method gives better resolution and improved numerical stability compared to the LS‐method.

A real data example shows the ML‐ and LS‐method applied to stacked seismic data. The results are compared with reflection coefficients obtained from well log data.
SELF POTENTIAL AND CURRENT CHANNELLING*
Authors: CULL J. P.
Published: April 2006

Anomalies in self potential (SP) are usually explained in terms of geochemical activity. However, there are other sources of current which cause complications in the interpretation. In particular, DC telluric currents can be generated outside the survey region. Contrasts in resistivity cause current channelling and secondary voltages can be induced by leakage. The effect can be demonstrated using network theory and internal sources can be emphasized by subtraction. Joint surveys are required to demonstrate correlations with apparent resistivity.
ON DIRECT RECOVERY OF THE IMPULSE RESPONSE*
Authors: SZARANIEC E.
Published: April 2006

Abstract

The deconvolution equation is solved in the z‐transform domain directly for an impluse response. The principal assumption is the odd‐depth model: two‐way traveltimes to the boundaries are constrained to be odd integers only. It is further assumed that the length of the wavelet sequence is known to be less than half the length of the data sequence.

An inverse of the impulse response is constrained by the zero samples of the source function. The resulting underdetermined set of equations is supplemented with the equations provided by the odd‐depth model. The impulse response is found from the inverse by polynomial division.
POSSIBILITIES OF USING VERTICAL GEOPHONE ARRAYS (CVA) IN REFLECTION SEISMOLOGY*
Authors: RÜTER H. , KÖHLER K. , ARNETZL H.
Published: April 2006

Abstract

Vertical geophone arrays in boreholes have been used for many years to study seismic velocities by investigating the first arrivals of records. The development of the vertical seismic profiling (VSP) technique shows possibilities of using the reflected events to close the gap between interpretation of conventional seismic data and physical observations made in the well. Reflected events recorded by vertical arrays (as in VSP) generally have higher signal‐to‐noise ratio, larger bandwidth and can easily be separated from multiples. The new Continuous Vertical Array (CVA) technique combines vertical arrays in several boreholes with a line of source points near the surface. The result is a multi‐covered seismic line similar to that of a conventional seismic survey, but it retains the benefits of observations with vertical arrays. The possibilities of the new technique are discussed with the aid of theoretical considerations, model studies, and a first field case using nine boreholes 500 m apart with depths of 400 m.

New data acquisition and processing techniques (mainly migration before stack) have been developed. The CVA‐seismic method is still in the development stage but promises new possibilities for detailed surveys in difficult areas.
THE INFLUENCE OF AN ASYMMETRY IN THE SEQUENCE ROCK/COAL/ROCK ON THE PROPAGATION OF RAYLEIGH SEAM WAVES*
Authors: DRESEN L. , KERNER C. , KÜHBACH B.
Published: April 2006

Abstract

Small offsets in hard coal seams can be detected with the aid of seam (channel) waves. Transmission and reflection of seam waves depend, among other parameters, upon the symmetry properties of the sequence rock/coal/rock. Two typical unsymmetrical sequences are found in European coal deposits: (i) coal seams with roof and floor of differing acoustic impedance and (ii) coal seams interlayered with rock and soil.

Two‐dimensional analog models with appropriate impedance contrasts are used to study the effect of the unsymmetrical layers upon the propagation of Rayleigh seam waves. Data analysis is based upon amplitude measurements both parallel and perpendicular to the layers and dispersion curves.

The effect of unsymmetrical roof (rock 1) and floor (rock 2) was studied with models containing homogeneous coal seams. Leaky mode wave groups with phase velocities (cR) in the range between the SV‐wave velocities (βr1, 2) of the two rock materials, i.e. βr1≥cR > βr2, form a characteristic part of the Rayleigh seam wave signal. Using Knott's energy coefficient calculations it is shown that in that range energy leakage into the surrounding rock by refracted SV‐waves is restricted to only one of the two interfaces, namely coal/rock 2. At the other interface, coal/rock 1, all waves are totally reflected. Thus, the high amplitudes of these leaky mode wave groups are explained by “quasi‐normal mode” features.

The influence of a dirt bed on wave propagation was studied in models where the roof and the floor have the same elastic properties. The maximum thickness of the dirt bed did not exceed 20% of the total seam thickness. The effect of the bed's location within the seam was also investigated.

For all recorded normal‐mode wave groups either the total seam or the coal layers could be regarded as wave guides. This was shown by the fact that the phases could be associated with the phase velocity dispersion curves calculated for the symmetrical sequence rock/coal/rock. These curves are relevant under the condition that the thickness of the coal layer assumed under the calculation coincides with the thickness of the effective wave guide of the respective wave groups.

Wave groups guided in the total seam are not influenced by either the thickness or the position of the dirt bed. On the other hand, for wave groups guided in the coal layers, the quotient of signal amplitudes in the coal layers is influenced by the position of the dirt bed.
LEAST‐SQUARES FILTERS WITHOUT TRANSIENT ERRORS: AN EXAMINATION OF THE ERRORS IN LEAST‐SQUARES FILTER DESIGN*
Authors: BUNCH A. W. H. , WHITE R. E.
Published: April 2006

Abstract

A new approach has been developed for the design of cross‐equalization filters by the least‐squares method. The filters estimated by this new exact method are subject to only two types of error: bias and random error. Cross‐equalization filters estimated by a more conventional least‐squares method are further subject to “transient error”. This type of error becomes important when designing filters from a data gate of a length comparable with the length of the filter, i.e., less than four times the length of the filter.

The effect of altering various design parameters has been investigated for the new method. It has been found that the proportion of bias in the filter decreases as the effective filter length increases, whereas the random error in the filter decreases with increase in either the signal‐to‐noise ratio of the data or the ratio of the data duration to the filter length. The level of whitening applied to the auto‐correlation matrix before inversion was not found to be a critical design parameter. Also, two techniques have been tested for reducing any anomalous d.c. component in the calculated filter.
EM AND IP RESPONSE OF A STEEL WELL CASING FOR A FOUR‐ELECTRODE SURFACE ARRAY. PART II: NUMERICAL RESULTS*
Authors: WILLIAMS J. T. , WAIT J. R.
Published: April 2006

Abstract

Some numerical results for the apparent resistivity of a homogeneous half space containing a vertical steel well casing as measured by a dipole‐dipole array are presented. The basic model includes the electromagnetic and induced polarization effects by allowing for frequency dependent eddy currents and an electrochemical boundary of the well casing. It is shown that the well casing produces an apparent complex resistivity response that could be mistaken for an induced polarization anomaly existing above a hydrocarbon deposit. This response of the well casing is strongly dependent on frequency.
MAXIMUM LIKELIHOOD ESTIMATION OF SEISMIC REFLECTION COEFFICIENTS*
Authors: ÖZDEMİR H.
Published: April 2006

Abstract

A seismic trace is modeled as a moving average (MA) process both in signal and noise: a signal wavelet convolved with a reflection coefficient series plus colored random noise. Seismic reflection coefficients can be estimated from seismic traces using suitable estimation algorithms if the input wavelet is known and vice versa. The maximum likelihood (ML) algorithm is used to estimate the system order and the reflection coefficients. The system order is related to the arrival time of the latest signal in a complex seismic reflection event. The least‐squares (LS) method does not provide such information. The ML algorithm makes assumptions only about the Gaussian nature of the noise. It is better suited for seismic applications since the LS method inherits the white noise assumption. The Gauss‐Newton (G‐N) and Newton‐Raphson (N‐R) optimization algorithms are used to obtain the ML and the LS estimates.

Reflection coefficient estimations are affected by the choice of sampling rate of seismic data. Theoretically, the optimum choice in system identification is the Nyquist rate. Experience with synthetic data confirms the theory.

In practice, good estimates of reflection coefficients are possible only up to certain pulse separations (or, equivalently, orders). This is mostly due to numerical problems with the optimization algorithms used and partly due to the limited bandwidth of seismic signals. Good estimates from data simulated using three airgun array pulses recorded with 6–128 Hz filter setting are possible up to about 40.0 ms pulse separations. Successful estimations from pinchout and thin layer simulations and well controlled offshore “bright‐spots” are given.
SEPARATION OF P‐ AND S‐WAVES*
Authors: DANKBAAR J. W. M.
Published: April 2006

Abstract

In combined P‐ and SV‐observations with vertical and in‐line horizontal geophones, recognition and separation of the individual P‐ and S‐arrivals may form a considerable problem. Due to the geophone receiving characteristics, P‐ and S‐waves are observed on both horizontal and vertical geophones. A new method of separating P‐ and S‐waves is presented that inverts the geophone records for the receiving characteristics, thereby separating the two wavetypes.

The filter coefficients are determined from the near‐surface P‐ and S‐wave velocity and from the geometry of the geophone groups. The P/S‐separation filter was tested on synthetic data with good results. Even PS‐ and SP‐converted reflections, which are completely coinciding for horizontal reflectors, could be well separated. The filter appears to be stable and to have low sensitivity to noise in the data or to errors in the near‐surface P‐ and S‐wave velocity. The filter is superior to other separation methods, which were applied to the same dataset for comparison. This method can also be applied to VSP data.
TUTORIAL LINEAR INVERSE FILTERS IN ORTHOGONAL COORDINATES*
Authors: DOUZE E. J.
Published: April 2006

Abstract

The linear filter is used extensively in exploration geophysics, and is usually computed using the least squares normal equations. In the general field of time series, the inverse problem is often solved through eigenvalue expansion solutions to integral equations.

The normal equations can be solved in terms of the eigenvalues and eigenvectors of the autocorrelation matrix. It has been suggested that a spectral expansion technique should be used which computes the inverse directly without explicit use of the normal equations. If all possible spiking positions are calculated using the normal equations, the spiking operator matrix is obtained. The matrix operator obtained from the spectral expansion is closely related to the spiking operator matrix. Thus, it is possible to compute the spectral expansion filter using the normal equations. Therefore, it is possible to use the best features of both methods, i.e. obtaining the optimum filter with the normal equations, and discarding the poorly determined parts of the solution based on spectral expansion.
ON TIMING THE VSP FIRST ARRIVAL*
Authors: DILLON P. B. , COLLYER V. A.
Published: April 2006

Abstract

The advent of signal energy on a VSP or check‐shot trace may be defined as the first break. An accurate pick of this first break would be possible in the absence of noise. However, real data traces are inevitably corrupted by noise and this leads to difficulty in identifying a break because the signal‐to‐noise ratio is low in its neighbourhood.

Under such conditions, an obvious alternative is to pick “troughs” where the local signal‐to‐noise ratio is likely to be much higher. Although trough picking is an effective way to minimize the noise problem, it is sensitive to signal properties (such as absorption and multiple reflections) which have no effect upon the accuracy of break picks. Thus, trough picking is signal‐sensitive and break picking is noise‐sensitive.

Clearly, an ideal first‐arrival picking scheme would combine the noise‐tolerant features of trough picking with the signal‐tolerant features of break picking. This ideal may be approached by exploiting known properties of the VSP trace using conventional signal processing techniques. The result of such processing is to reduce the problem to that of picking a trough correctly centered about the true break time.
PHASE AND GROUP TIME SECTIONS AND POSSIBILITIES FOR THEIR USE IN SEISMIC INTERPRETATION OF COMPLEX MEDIA*
Authors: SHTIVELMAN V. , LANDA E. , GELCHINSKY B.
Published: April 2006

Interpretation problems are discussed for a new class of models for complex seismic media, called heterogeneous models formed by inclusions (HMI). Examples of such models in geology are destructive deformation zones, tectonic raptures, complex folds, magmatogene formations, fronts of metamorphism and of phase transition, etc., which are of importance in interpretation of seismic data. The wavefields in such media have a complicated interferential character and should be considered as complex wave groups characterized by their phase and group properties.

To study the phase and group characteristics of such wavefields, a method of construction and comparative analysis of so‐called phase and group sections is introduced. This method is based on a transformation of the wavefield (seismogram) into a normalized seismogram (cos of the phase) and a perigram (a low cut version of the trace envelope). The group sections obtained on the perigrams represent zones of energy concentration and give stable estimates of the average characteristics of model structure. The phase sections are obtained on the normalized seismograms and represent primarily, the inner structure of the model.

The method was applied to both synthetic and field data. The results of the combined analysis of the phase and group sections show that in many cases there are significant differences between them. On the basis of this analysis, several types of seismic objects may be distinguished which can serve as a basis for seismic interpretation.
TWO‐AND‐ONE‐HALF DIMENSIONAL IN‐PLANE WAVE PROPAGATION*
Authors: BLEISTEIN N.
Published: April 2006

This paper collects certain results concerning wave propagation in two‐and‐one‐half dimensions, i.e., three‐dimensional (3‐D) wave propagation in a medium that has variations in two dimensions only. The results of interest are for sources and receivers in the plane determined by the two directions of parameter variation. The objective of this work is to reduce the analysis of the in‐plane propagation to 2‐D analysis while retaining–at least asymptotically–the proper 3‐D geometrical spreading. We do this for the free space Green's function and for the Kirchhoff approximate upward scattered field from a single reflector. In both cases the derivation is carried out under the assumption of a background velocity c(x, z) with the special cases c = c0 and c = c(z).
POTENTIAL FIELD OF A STATIONARY ELECTRIC CURRENT USING FREDHOLM'S INTEGRAL EQUATIONS OF THE SECOND KIND*
Authors: ELORANTA E.H.
Published: April 2006

An integral equation method is described for solving the potential problem of a stationary electric current in a medium that is linear, isotropic and piecewise homogeneous in terms of electrical conductivity. The integral equations are Fredholm's equations of the ‘second kind’ developed for the potential of the electric field. In this method the discontinuity‐surfaces of electrical conductivity are divided into ‘sub‐areas’ that are so small that the value of their potential can be regarded as constant.

The equations are applied to 3‐D galvanic modeling. In the numerical examples the convergence is examined. The results are also compared with solutions derived with other integral equations. Examples are given of anomalies of apparent resistivity and mise‐a‐la‐masse methods, assuming finite conductivity contrast. We show that the numerical solutions converge more rapidly than compared to solutions published earlier for the electric field. This results from the fact that the potential (as a function of the location coordinate) behaves more regularly than the electric field. The equations are applicable to all cases where conductivity contrast is finite.
GEOPHYSICAL INVESTIGATION OF A FAULT ZONE–CASE HISTORY FROM ILE‐IFE, SOUTHWEST NIGERIA*
Authors: OLORUNFEMI M.O. , OLAREWAJU V.O. , AVCI M.
Published: April 2006

A fault zone, inferred as a major linear structure from aerial photographs of the University of Ife Campus, has been investigated.

Results of a multi‐method geophysical survey indicate that the zone is characterized by relatively low resistivity and high magnetic effect. They are characteristic of a sheared and perhaps saturated fault zone with magnetic mineral infillings along its plane.

The near symmetry of the magnetic anomalies over the fault zone may not be indicative of a step faulting. The geoelectric sections show no indication of any significant displacement. The above supports results of a previous study that the fault is a strike‐slip fault.
THE EXACT SEISMIC RESPONSE OF AN OCEAN AND A N‐LAYER CONFIGURATION*
Authors: DRIJKONINGEN G.G. , FOKKEMA J.T.
Published: April 2006

The space‐time acoustic wave motion generated by an impulsive monopole source is calculated with the aid of the Cagniard‐de Hoop technique. Two configurations with plane interfaces are discussed: an air/fluid/solid configuration with the source and the receiver located in the fluid layer; and a stack of n fluid layers between two acoustic half‐spaces where the source and the receiver are located in the upper half‐space. Synthetic seismograms are generated for the pressure of the reflected wavefield, using the source signature of an airgun.