bluuebunny/crossref_metadata_2025 huggingface dataset:
OFFSHORE USE OF THE SELF‐POTENTIAL METHOD*
Authors: CORWIN R. F.
Published: April 2006

Abstract

An offshore self‐potential array, towed behind a small boat, has recorded anomalies of up to −300 mV. These anomalies were related to conductive onshore deposits, and appear to be caused by offshore extensions of the deposits. Along with locating onshore deposits and their offshore extensions, the system may be useful for locating offshore deposits with no onshore extension. The background noise level of the system typically is a few tenths of a millivolt, allowing reliable recording of one millivolt gradient anomalies under average sea conditions.
DEPTH ESTIMATION OF MAGNETIC SOURCES BY MEANS OF FOURIER AMPLITUDE SPECTRA*
Authors: HAHN A. , KIND E. G. , MISHRA D. C.
Published: April 2006

Abstract

The Fourier transform of a square‐shaped section of a magnetic survey, digitized in a square grid, forms a rectangular matrix of coefficients which can be condensed to a series of average amplitudes dependent only on their frequency and no longer on the direction of the respective partial waves. These average amplitudes together represent a spectrum which–when plotted in a semilogarithmic coordinate system (log amplitude versus frequency)–often shows straight segments which decrease with increasing frequency. By continuing the given field downwards these straight segments become horizontal at a certain depth, the so‐called “white depth”. This white depth may be used as a first estimate for the depth of magnetic sources producing the respective part of the field. It is shown that the sources which correspond to such use of the white depth can be expected to be “randomly distributed with some positive autocorrelation”.

As an example for such a depth estimation the interpretation of the aeromagnetic survey of NW‐Germany by a relief in 8–16 km depth is given. The relief divides the subsurface in an upper nonmagnetic layer and a lower layer with magnetization M= 2 Am−1.
EFFICIENT DESIGN OF AIR‐GUN ARRAYS*
Authors: SAFAR M. H.
Published: April 2006

Abstract

A new technique is developed for shaping the pressure bubble pulse radiated by an array of air‐guns. It involves the proper spacing of identical units. It is shown that considerable shortening of the pressure bubble pulse can be achieved provided there is sufficient mutual coupling between all the air‐guns.

The existing air‐gun array technique for reducing the bubble pulse involves the redistribution of the energy of the bubble pulses which are produced by an array of variable sized air‐guns such that no energy of the bubble pulses is radiated along the perpendicular to the array axis and only the sum of the initial pulses produced by the air‐guns forming the array is radiated along that direction. However, the new air‐guns array technique involves the damping of the bubble pulses which are produced by an array of identical air‐guns by means of mutual interaction and the effective pressure pulse radiated by the array is given by the sum of the damped bubble pulses produced by the mutually coupled identical air‐guns.

Preliminary field trials gave results consistent with the theoretical predictions. A comparison between the waveforms of the pressure bubble pulses radiated by a single air‐gun and by an array of four identical air‐guns shows that, due to the presence of mutual coupling between the four air‐guns, effective damping of the bubble pulse radiated by the array is about 50% greater than that of the bubble pulse radiated by the single air‐gun.
INTERPRETATION OF POTENTIAL FIELD DATA A GENERALIZED INVERSE APPROACH*
Authors: PEDERSEN L. BØRSTING
Published: April 2006

Abstract

The interpretation of potential field data from two‐dimensional structures with a single interface of density or susceptibility contrasts is solved in terms of generalized matrix inversion. The model equations are derived, and important features of generalized matrix inversion are treated. The method is subsequently used to solve two gravity problems, an artificial one where the solution is known, and a geophysical one related to the crust‐mantle interface. The solution is shown to compare well with the FFT results of Oldenburg. The method is also used to solve two magnetic problems, an artificial one with the solution known, and a geophysical one from the continental shelf of Greenland. The advantages and limitations of the method are finally discussed.
THE OLD AND THE NEW IN SEISMIC DECONVOLUTION AND WAVELET ESTIMATION*
Authors: LINES L. R. , ULRYCH T. J.
Published: April 2006

Abstract

Various seismic deconvolution operators can be determined by estimating a seismic wavelet and subsequently designing an appropriate inverse filter which converts the wavelet to a spike. Seismic wavelets and deconvolution operators must be estimated in a time adaptive sense due to the nonstationarity of the seismic trace. The wavelet estimation methods considered in this paper either use the assumption of a minimum phase wavelet and a random impulse response, or the assumption that the wavelet cepstrum is readily separable from the cepstrum of the seismic trace. The former assumption is required in using the Hilbert transform and Wiener‐Levinson wavelet estimations, while the latter assumption is used in homomorphic deconvolution. These wavelet estimates can be used in the design of multichannel Wiener and Kalman deconvolution operators. Multichannel usage of homomorphic deconvolution can also be implemented through various types of cepstral stacking.

The discussion of deconvolution filter design focuses on the problems of filter length, degree of prewhitening, and nonstationarity. In designing time adaptive deconvolution filters, the autocorrelation function can be used to monitor the nonstationarity of the seismic trace. The autocorrelation function, which is used in the computation of least squares inverse filters, can be estimated in an optimum fashion by using the maximum entropy method. Differences between minimum phase Wiener deconvolution and maximum entropy deconvolution become more pronounced for shorter data gates. As a result, the maximum entropy approach is preferred for time adaptive deconvolution.

In this paper, the performance of various wavelet estimators and inverse filters is discussed using real and synthetic seismic data. Discussions of homomorphic deconvolution and maximum entropy prediction error filtering are merged with descriptions of conventional approaches to deconvolution.
INVESTIGATION OF THE SEISMIC RESPONSE OF CYCLICALLY LAYERED CARBONIFEROUS ROCK BY MEANS OF SYNTHETIC SEISMOGRAMS *
Authors: RÜTER H. , SCHEPERS R.
Published: April 2006

Abstract

Seismic exploration for coal as well as basic scientific research indicate the existence of unsolved problems. These problems arise partly because the requirements are different from those in exploration for gas and oil and are partly due to the geological situation. The medium to be investigated is composed of cyclically changing layers with extremely high velocity and density contrast. Furthermore, the structure of the carboniferous rock is highly fractured and folded. This leads to difficulties in interpreting the seismic response of carboniferous rock. To overcome these difficulties synthetic seismograms are a useful tool.

Calculating synthetic seismograms the carboniferous rock has been modelled as a sequence of seams and rock. The following results have been obtained

A single seam gives rise to a distinct reflection signal even for a thickness of 1/50 of the wavelenght.

Individual reflections are not visible from a sequence of layers containing a great number of seams and interfaces. Due to constructive interference only a few high amplitudes appear. These high amplitudes are labelled “interference reflections”.

With increasing travel time the interference reflections are mainly composed of short lag multiples such that the primary reflections have no significant influence.

The sequence of seams acts on the reflected seismic signal as a high pass filter and on the transmitted signal as a low pass filter. The cut‐off frequencies are determined by the average seam thickness, and the steepness of the slopes increases with increasing number of seams.

The interference reflections can be used for determining the geological structure at least for the upper part of the sequence.
VELOCITY DETERMINATION AND ERROR ANALYSIS FOR THE SEISMIC REFRACTION METHOD*
Authors: DAMPNEY C.N.G. , WHITELEY R.J.
Published: April 2006

Abstract

A careful examination of the seismic refraction technique details the general assumptions and processes on which it is based. It is apparent that the normal interpretive process of determining velocity by hand fitting time breaks, while necessary to identify the refractors, is subject to imprecision.

We describe a digital technique that calculates velocity and its precision from the time break measurements. The technique also facilitates examining for the possibility of systematic errors. Having observed the kind of imprecision that was apparent with velocity we extended our study to layer thicknesses (and thus depth) and found that the imprecision was significantly magnified.

The technique and our claims for it are supported by an example of its application to a refraction survey in the Sturt Block, Western Australia.
LONG PERIOD SEA‐FLOOR MULTIPLES AND THEIR SUPPRESSION*
Authors: TANER M.T.
Published: April 2006

Abstract

Multiple sea‐floor reflections in deep water often are not effectively suppressed by either CDP stacking nor standard predictive deconvolution methods. These methods fail because the reflection coefficient varies markedly with angle of incidence and also because of the variation of arrival time with offset and because of dip. For a reasonablly flat sea‐floor, multiples of various orders and the primary sea‐floor reflection which have all been reflected at nearly the same angle lie along a straight line through the origin in time‐offset space. This line is called the “radial direction.” The multiples which lie along this line show a systematic relationship because they all experience the same water‐bottom reflection effect. In other words, multiples behave in a stationary manner along the radial directions on multi‐trace seismic records. A technique of multi‐channel predictive deconvolution, called “Radial Multiple Suppression,” utilizes this aspect to design Wiener operators for the prediciton and suppression of water bottom multiples.

The effectiveness of the technique is demonstrated by the study of field records, autocorrelations, velocity analyses, and stacked sections before and after Radial Multiple Suppression processing.
GEOPHYSICAL DATA BASES: SYSTEM CONSIDERATIONS*
Authors: DILLAHUNTY R.C. , FASH J.L. , PARSLEY L.R. , TOWNSEND D.W.
Published: April 2006

Abstract

The use of data file systems within the areas of geological well log and reservoir analysis is now accepted as a necessity in the storage and correlation of large volumes of diverse information.

Multi‐channel and three‐dimensional seismic recording techniques of various types result in a situation where the amount of processing parameters and derived seismic properties of a trace are rapidly approaching the amount of the recorded data themselves. The requirement to access these statistics efficiently has introduced the problems of data management to the seismic processing community.

Much software effort has been expended in the design of algorithms for cross reference and indexing of the statistics required in signal processing. These efforts, however, are generally in the direction of a localized solution to a specific requirement and tend to ignore the data base concepts well developed within the commercial data processing community.

This paper examines the applicability of commercial data base concepts to the problems of data handling in the area of geophysical exploration, sets forth some basic definitions and organizational characteristics, and describes a system for the integration of several earth disciplines within a single data base from which parameters and mapped information may be derived and indexed for the purpose of processing and interpretation.
A FORWARD GENERATED SYNTHETIC SEISMOGRAM FOR EQUAL‐DELAY LAYERED MEDIA MODELS*
Authors: VETTER WILLIAM J.
Published: April 2006

Abstract

A forward solution for the reflection response of a parallel stratified lossless medium characterized by discrete reflection coefficients and unequal layer delays, for a normally incident pressure source signal, is presented. The notation, which details the reflection history of each wavelet in a response record, facilitates systematic enumeration of all terms in the reflection impulse response model, the determination of compact closed form expressions for amplitudes and delays of multiply reflected wavelets, and the aggregation of dynamic analog groups. An equal delay time constraint on layer thicknesses leads then to the reflection sequence or synthetic seismogram structure as an infinite sum of wavelets by their order of reflection.
ELECTROMAGNETIC PROSPECTING FOR GROUNDWATER IN PRECAMBRIAN TERRAINS IN THE REPUBLIC OF UPPER VOLTA*
Authors: PALACKY G.J. , RITSEMA I.L. , DE JONG S.J.
Published: April 2006

Abstract

The “Autorité des amenegements des valées des Voltas (AVV)” is establishing new rural settlements in the Volta valleys. First, a survey of available water supplies is performed. Economic aquifers in Precambrian terrains are deep (15–50 m) and usually occur in fractured zones accompanying faults. Such zones can be identified on aerial photographs, but their precise location on the ground is virtually impossible by visual means. Because of the small size of the aquifers, a location error of 5 m can make the difference between a productive well and a dry hole.

Traditionally, resistivity profiling has been used as the means of locating the fractured zones in the field. Our studies suggest that the task can be performed faster, cheaper and more accurately by VLF and EM methods. Because of the limited choice of transmitting stations reccivable in Upper Volta, the VLF method is not sufficiently sensitive to detect conductors with a strike between 45° and 105°. The results obtained with a multifrequency, horizontal‐loop EM (HLEM) system were satisfactory in all investigated areas. During the 1980 field season, 35 target areas were surveyed. Of the 24 holes drilled so far, 23 are productive.

The weathered layer is a source of distinctive HLEM anomalies, which are characteristic of the underlying rocks. Therefore, different interpretational procedures had to be developed for granitic and volcano‐sedimentary areas. Despite the high background level of in‐phase and quadrature components, which varied with thickness and conductivity of the weathered layer, aquifers could be detected at a depth greater than 30 m. Attempts were made to interpret the HLEM results quantitatively using two models: a three‐layer medium and a valley discontinuity. The latter model is more realistic, but more scale modelling will have to be performed to permit development of viable interpretational procedures. Meanwhile, phasor diagrams based on drilling and resistivity sounding data can be used to estimate the aquifer depth.
SOME ASPECTS OF HANDLING VELOCITY INVERSION AND HIDDEN LAYER PROBLEMS IN SEISMIC REFRACTION WORK*
Authors: SCHMÖLLER R.
Published: April 2006

Abstract

There are two types of masked layers in seismic refraction work: the velocity reversal (low‐velocity layer) and the hidden layer (insufficient velocity contrast or layer thickness). On the basis of an analytical formulation of the general case of a masked layer under an overburden of plane and parallel multiple refractors the two limiting cases are discussed: the solution resulting from an uncritical interpretation of the measured time‐distance curve and the blind zone solution. Between these two limiting cases there is a variety of possible masked layer solutions. These no‐blind zone solutions—as well as the blind zone solution itself—are formulated separately for the velocity inversion and the hidden layer case.

For the evaluation of some no‐blind zone solution a diagram is presented which can be used for any case of multiple refractors in the overburden of the masked layer. However, it is only for the three‐ and the four‐layer case that a blind zone interpretation by use of diagrams is advisable. Such diagrams are presented together with the basic sets of formulae which contain as parameters only ratios of velocities and layer thicknesses. As the velocity of the masked layer is usually unknown the diagrams are principally constructed to show the dependence on the masked layer velocity. This is useful for estimation of the largest possible error.
EXPLORING THE INTERIOR OF SALT DOMES FROM BOREHOLES*
Authors: NICKEL H. , SENDER F. , THIERBACH R. , WEICHART H.
Published: April 2006

Abstract

Presently, salt deposits are used for storage of energy from liquid or gaseous carrier materials. Another application being considered is the storage of radioactive material. In order to avoid possible future environmental risk, extremely high security has to be guaranteed in advance for a long period. Therefore, a very precise exploration is absolutely necessary.

Exploration methods already known—which are applicable during and after drilling of test holes—help in the recognition of structures in the salt only in the close vicinity of the holes. Radiowave electromagnetic methods developed for routine measurements in salt mines provide a penetration of some hundred meters. Appropriate well‐logging equipment has been designed here for measurements from surface boreholes.

Discontinuities around one borehole in the salt can be detected through the electromagnetic reflection method. When measuring between two boreholes, additional reflecting zones can be detected, and attenuation of directly pathed waves can point out material with greater absorption properties between the holes.

Additional attempts to use acoustic waves of nearly the same wavelength proved that it is possible to recognize the direction of signals arriving at the receiver and allow separation of P‐ and S‐waves.

Instrumentation as well as recent results are reported.
WAVEFRONT DIVERGENCE, MULTIPLES, AND CONVERTED WAVES IN SYNTHETIC SEISMOGRAMS*
Authors: HRON F. , COVEY J. D.
Published: April 2006

Abstract

Synthetic seismograms can be very useful in aiding understanding of wave propagation through models of real media, verification of geologic models derived from interpretation of field seismic data, and understanding the nature and complexity of wave phenomena. If meaningful results are to be obtained from synthetic seismograms, the method of their computation must, in general, include three‐dimensional geometrical spreading of wavefronts associated with highly concentrated (i.e., point) sources. The method should also adequately represent the seismic response of solid‐layered media by including enough primaries, multiples, and converted phases to accurately approximate the total wavefield. In addition to these features, it is also very helpful, although not always essential, if the method of seismogram computation provides for explicit identification of wave type and ray path for each arrival. Various seismograms, computed via asymptotic ray theory and an automatic ray generation scheme, are presented for a highly simplified North Sea velocity structure. This is done to illustrate the importance of the above features and to demonstrate the inadequacy of the plane‐wave synthesis method of seismogram computation for point sources and the limitations of acoustic models of solid‐layered media.
DETECTABILITY OF AN INTERMEDIATE LAYER BY PERPENDICULAR AND VERTICAL COPLANAR ELECTROMAGNETIC SOUNDING SYSTEMS EMPLOYING DIFFERENT PRIMARY EXCITATIONS*
Authors: VERMA R. K. , MALLICK K.
Published: April 2006

Abstract

The detectability of an intermediate layer in a three‐layer earth model in the time domain has been investigated. The calculations were made for the perpendicular loop (designated system II) and vertical‐coplanar (designated system III) electromagnetic (EM) sounding systems. The primary excitation employed is a train of half‐sinusoidal and square waveforms of alternating polarity. The time‐domain response has been determined by Fourier transformation of the matched complex mutual coupling ratios into the time domain and by linear digital filtering. Top and bottom layers have equal resistivity. EM responses have been computed for conductive and resistive intermediate layer with a wide range of thickness and for two values (500 m and 1000 m) of loop‐separation. For the detectability analyses, the root mean square (rms) difference between three‐layer and homogeneous‐earth responses is adapted. The threshold value for detectability is defined as an rms difference of 10% and the measurement error is arbitrarily assumed to be of the order of 3%. It is observed that the perpendicular‐loop system is better than the vertical‐coplanar system in detecting thin intermediate layers (either conductive or resistive). For a loop separation of 1000 m and half‐sinusoidal pulse excitation, the detectable thickness ratio (h2/h1) is 0.10 by system II for the conducting middle layers; for square pulse excitation the corresponding thickness ratios are 0.06 for system II and 0.12 for system III. For a loop separation of 1000 m and half‐sinusoidal pulse excitation in detecting the resistive intermediate layers, the corresponding thickness ratios are 0.9 for system II and 2.25 for system III; while for square pulse excitation the thickness ratios are 0.55 for system II and 1.55 for system III.

Results in the frequency domain and time domain (for half‐sinusoidal and square pulsed field) have also been presented for systems II and III for detecting conducting layers by considering an earth model where p1≠ p3 and p3 > p1 (p is the resistivity). The loop separa‐ tion used is 1000 m. Direct comparisons between the frequency domain and time‐domain results clearly demonstrate the superiority of frequency‐domain systems for detecting con‐ ducting intermediate layers.
ON THE EQUALIZATION OF THE TRANSFORMERLESS SEISMIC STREAMER RESPONSE*
Authors: SAFAR M. H.
Published: April 2006

Abstract

The use of very long streamers in marine seismic surveys makes the response offset dependent. Line equalization networks are used to balance this effect.

In this paper the response of the transformerless streamer system driving a charge amplifier is analyzed. Examples are given for a seismic cable 2500 m long, showing that the delay and the reduction in amplitude of the streamer impulse response in the absence of leakage are of the order of 1 ms and 0.6 dB, respectively. This means that the line equalization network currently used with the transformerless streamer system is not needed. We have also demonstrated that the presence of leakage resistance across the hydrophone group render the sensitivity of the transformerless streamer system offset dependent.
FRACTURES AS CONDUCTING DYKES AND CORRESPONDING TWO‐DIMENSIONAL MODELS*
Authors: ÁDÁM A.
Published: April 2006

Abstract

A statistical treatment of MT data from the Pannonian Basin, Hungary, based on the distortion theory of the S‐effect suggests that the local increase in crustal conductivity is connected with deep fractures.

Field model measurements have recently been carried out in a shallow, quasi two‐dimensional basin of well known tectonics using MT station distances of 1–3 km. The results of the investigation and numerical modelling of near surface distortions, support the idea that deep fractures (faults) contain the conducting formations. According to 2‐D model computations, several conducting dykes at considerable depths can cause anomalies which are indiscernible from an anomaly due to a conducting layer. The significance of these results is discussed from the point of view of geothermal energy exploitation.
LIMITS OF STRUCTURAL INVERSION OF SEISMIC HORIZONS*
Authors: BILGERI D. , CARLINI A.
Published: April 2006

Abstract

Time horizons can be depth‐migrated when interval velocities are known; on the other hand, the velocity distribution can be found when traveltimes and NMO velocities at zero offset are known (wavefront curvatures; Shah 1973).

Using these concepts, exact recursive inversion formulae for the calculation of interval velocities are given. The assumption of rectilinear raypath propagation within each layer is made; interval velocities and curvatures of the interfaces between layers can be found if traveltimes together with their gradients and curvatures and very precise VNMO velocities at zero offset are known.

However, the available stacking velocity is a numerical quantity which has no direct physical significance; its deviation from zero offset NMO velocity is examined in terms of horizon curvatures, cable length and lateral velocity inhomogeneities.

A method has been derived to estimate the geological depth model by searching, iteratively, for the best solution that minimizes the difference between stacking velocities from the real data and from the structural model.

Results show the limits and capabilities of the approach; perhaps, owing to the low resolution of conventional velocity analyses, a simplified version of the given formulae would be more robust.
INFLUENCE OF VELOCITY ERRORS ON THE FOCUSING ASPECTS OF MIGRATION*
Authors: DE VRIES D. , BERKHOUT A. J.
Published: April 2006

Abstract

In migration procedures, the velocity profile of the subsurface is the most important input information. Since, in general, this information is only approximately known, errors in the migration output due to errors in the velocity input occur in all practical applications. In migration, velocity errors and depth errors can be interchanged. This interchange property is perfect in the paraxial approximation. From this result it follows that migration with incorrect velocities may still yield correctly migrated data if the imaging principle is modified. This attractive property can be used in the stripping version of migration (recursive migration).
GUIDED LOW‐FREQUENCY NOISE FROM AIRGUN SOURCES*
Authors: KENNETT B. L. N. , HARDING A. J.
Published: April 2006

Abstract

The presence of the water layer in marine seismic prospecting provides an effective waveguide for acoustic energy trapped between the sea‐bed and the sea‐surface. This energy persists to large ranges and can be the dominant early feature on far‐offset traces. On airgun records, there is commonly a lower frequency set of arrivals following the water‐trapped waves. These arrivals are not as obvious with higher frequency watergun sources. By using a combination of intercept‐time/slowness (τ—p) mapping on observational data and theoretical modelling, we are able to identify the origin of the events. If a very rapid increase in a seismic wavespeed occurs beneath the sea‐bed sediments, a new waveguide is formed bounded by the sea surface and this transition zone. The low frequency waves are principally guided within this thicker waveguide. Numerical filtering in the τ—p domain followed by trace reconstruction is very effective in removing the low frequency noise.
MIGRATION STRATEGY*
Authors: HOSKEN J. W. J. , DEREGOWSKI S. M.
Published: April 2006

The geophysical data processor today has on offer a great variety of tools for the inversion of seismic reflection data to estimate geological structure. The major subset of these comprises migration procedures, which span a wide range of sophistication and cost in terms both of computation time and manual effort on the part of interpreters and processing staff. The choice of an over‐powerful process can be very wasteful, but on the other hand too naive a migration procedure can lead to wrong interpretations which are much more costly still.

Complete inversion procedures which aim to delineate all changes in rock densities and elastic properties in the subsurface are still in the imaginative stages of research. Not even the most sophisticated migration procedure in current use with real data, however, provides a complete inversion, but all depend in some measure on prior knowledge of the velocity structure of the section of the earth traversed by the seismic energy. Such knowledge may be very approximate at first, but each inversion should, through the skill of the interpreter, allow him to revise his velocity model and, up to some limit imposed by the quality and ambiguity of the original data, to improve the next inversion. Paradoxically, he can often be helped by using forward modeling procedures to check the implications of his ideas in the data domain, both in deciding how to update the velocity model and in selecting the most appropriate migration process to use next.

We review here the currently available toolkit of migration and modeling processes and make suggestions as to how each process can fit into a learning strategy which can improve the interpretation as economically as possible and in as many iterative steps as the complexity of the earth's velocity structure makes necessary. An example is shown of the strategy being used in a complex overthrust region.

The authors wish to thank the Chairman and Board of Directors of BP Exploration Co. for permission to publish this paper, and also make acknowledgment to our colleagues whose labours in research and development have made available to our use many of the essential tools required to implement the strategies we describe.
COMPUTING THE KERNEL FUNCTION IN RESISTIVITY SOUNDING WITH AN ARBITRARY ELECTRODE CONFIGURATION*
Authors: KOHLBECK F.
Published: April 2006

A power series expansion can be used to obtain the kernel from apparent resistivities for an arbitrary electrode configuration. Three types of function are most appropriate for this purpose. The expansion coefficients can be by a least‐squares method. In this case, ortho‐normalization of the functions is of great advantage. An example with the Wenner configuration is given.
THE USE OF VERTICAL LINE SOURCES IN ELECTRICAL PROSPECTING FOR HYDROCARBON*
Authors: ROCROI J. P. , KOULIKOV A. V.
Published: April 2006

Conventional electrical prospecting can be extended to the search for deep‐seated hydrocarbon deposits, by using the steel casings of drill‐holes as vertical line sources. These sources produce at depth a density of current higher than the density created by point sources located at the ground surface. Several tests have shown that the contrast of conductivity between resistive hydrocarbon deposits and the surrounding salt water produces relevant anomalies on a resistivity map obtained with vertical line sources, especially where there exists a superficial masking effect caused by a highly resistive layer. In a survey carried out in the USSR, combined measurements were performed, both with line source and with surface point sources. The detected residual resistivity anomaly roughly delineates the contours of the known hydrocarbon deposit.
AN INVESTIGATION OF THE SPECTRAL PROPERTIES OF PRIMARY REFLECTION COEFFICIENTS*
Authors: WALDEN A. T. , HOSKEN J. W. J.
Published: April 2006

This paper investigates the form of the nonwhiteness found in the reflection coefficients from a wide variety of rock sequences around the world. In all but one case densities are taken as constant due to the paucity of suitable density data. The reflection sequences are pseudo‐white only above a corner frequency, below which their power spectrum falls away according to a power law fβ, where β is between 0.5 and 1.5. This spectrum can be adequately modelled in practice very simply with an ARMA (1, 1) process which acts on a white innovation sequence. The corollary of this is that before wavelet estimation methods are applied (all of which‐except those based on synthetic seismograms—presuppose white reflection sequences) or deconvolution filters estimated, seismic traces should be filtered with the inverse of this process.

Interestingly, the estimated ARMA processes group themselves into two clearly differentiated categories, having very different indices of predictability (or, strictly, indices of linear determinism). The two categories apparently correlate precisely with two kinds of sedimentation: one which consists largely of sequences of rocks with repeating properties, called “repetitive” in this paper but perhaps loosely describable as “cyclic”, and the other which is randomly bedded with no apparent pattern of components. The former has indices of predictability which are two to four times as great as those of the latter. Another, probably related, property is that β for the repetitive sequences tends to be greater than that for non‐repetitive rock columns.

The observed power spectra are shown to be consistent with a simple model for the logarithm of acoustic impedance consisting of a mixture of processes where the distribution of (time) scale parameters is reciprocal.

Detailed effects of block‐averaging and sampling the logs are shown to depend on the type of sequence under examination.
INVERSION METHODS FOR τ‐p MAPS OF NEAR OFFSET DATA—LINEAR INVERSION*
Authors: HARDING A. J.
Published: April 2006

Abstract

Conventional velocity analysis, based on the ideas of rms velocity and hyperbolic reflection events in the x‐t domain, is restricted in validity to near vertical incidence. Thus analysis of near‐offset datasets usually requires the muting of wide‐angle reflections from shallow interfaces before the rms velocities are determined. The ray‐theoretical integral for the delay time τ, which depends on the slowness p and the velocity function, is valid for all angles. The wide‐angle reflections can be used to improve the accuracy of the derived velocity function in the near surface region, if the recorded x‐t data are mapped into the τ‐p domain. By representing the velocity function between reflectors as a series of gradient zones, i.e. regions with a uniform increase in velocity with depth, the recovery of the velocities may be posed as a matrix linear inverse problem for the slopes of the gradient zones. In order to convert the problem to a linear one, the velocity discontinuities at the reflecting interfaces must be fixed in advance. Their positions are based on the behaviour of the τ‐p map of the data. Finding a stable velocity model may require several iterations with the reflecting interfaces at different positions. An understanding of the workings of the inversion algorithm allied with an analysis of the causes of instability aids the search for a stable model.
DETERMINISTIC ESTIMATION OF A WAVELET USING IMPEDANCE TYPE TECHNIQUE*
Authors: LOEWENTHAL D. , LEE S. S. , GARDNER G. H. F.
Published: April 2006

Abstract

A new deterministic technique for estimating a wavelet suggested by Loewenthal and Jakubowicz requires measurement of both pressure and vertical particle velocity. Through construction of the impedance function a deterministic estimation of the wavelet and the reflectivity can be obtained.

This idea is tested for a one‐dimensional model. The test is carried out by forming a synthetic seismogram of both pressure and particle velocity and checking the formulas for obtaining the estimated wavelet under noisy conditions.
IS IT POSSIBLE TO INCREASE THE RESOLUTION IN SEISMIC EXPLORATION FOR COAL BY USING HIGH FREQUENCY SIGNALS?*
Authors: RÜTER H. , SCHEPERS R.
Published: April 2006

Abstract

In seismic exploration for coal data resolution is a fundamental problem. Modeling helps to understand those details of the geology that can be interpreted from the seismic image. For single seam exploration, the vertical resolution of a seismic section is defined by the bandwidth of the signals. If there are several seams, each seam acts as a high‐pass filter for reflections and as a low‐pass filter for transmitted waves. Synthetic seismograms show that reflections from deep seams have a low frequency content.

Within a layered sequence of coal seams, many multiples are generated which disturb later primary reflections. The ratio of primaries to multiples depends on the frequency content of the seismic data and on the number of overlying seams. The multiple problem is more severe with high frequencies.

Primary reflections from deep coal seams within a sequence can be detected only if low‐frequency signals are used. However, the use of low‐frequency signals reduces the resolution of the deeper data.
LOW‐PASS FILTERING OF NOISY SCHLUMBERGER SOUNDING CURVES PART 1: THEORY*
Authors: PATELLA D.
Published: April 2006

A contribution is given to the solution of the problem of filtering noise‐degraded Schlumberger sounding curves. It is shown that the transformation to the pole‐pole system is actually a smoothing operation that filters high‐frequency noise. In the case of residual noise contamination in the transformed pole‐pole curve, it is demonstrated that a subsequent application of a conventional rectangular low‐pass filter, with cut‐off frequency not less than the right‐hand frequency limit of the main message pass‐band, may satisfactorily solve the problem by leaving a pole‐pole curve available for interpretation. An attempt is also made to understand the essential peculiarities of the pole‐pole system as far as penetration depth, resolving power and selectivity power are concerned.
LOW‐PASS FILTERING OF NOISY FIELD SCHLUMBERGER SOUNDING CURVES PART II: APPLICATION*
Authors: GHOSH N. , WADHWA R.S. , SHROTRI B.S. , PATELLA D.
Published: April 2006

The basic principles of the application of the linear system theory for smoothing noise‐degraded d.c. geoelectrical sounding curves were recently established by Patella. A field Schlumberger sounding is presented to demonstrate first their application and validity. To achieve this purpose, firstly it is pointed out that the required smoothing or low‐pass filtering can be considered as an intrinsic property of the transformation of original Schlumberger sounding curves into pole‐pole (two‐electrode) curves. Then we sketch a numerical algorithm to perform the transformation, opportunely modified from a known procedure for transforming dipole diagrams into Schlumberger ones. Finally we show a field example with the double aim of demonstrating (i) the high quality of the low‐pass filtering, and (ii) the reliability of the transformed pole‐pole curve as far as quantitative interpretation is concerned.
A LAYER‐STRIPPING TECHNIQUE FOR THE SUPPRESSION OF WATER‐BOTTOM MULTIPLE REFLECTIONS*
Authors: CARRION Ph.M.
Published: April 2006

A new method to suppress water‐bottom multiples (water‐bottom reverberations) uses the fact that in the domain of intercept time and ray parameter (τ–p domain) the water‐bottom reverberations are strictly periodical for a horizontal flat sea bottom.

Using this property a comb filter can be designed. The window of the filter should be approximately equal to the duration of a source pulse. The algorithm finds the maximum of the periodical energy throughout the τ–p domain and then designs the comb filter which eliminates the water bottom reverberations from each trace in the τ– p domain. This process can be repeated for higher order reverberations. Finally the τ–p domain with attenuated multiples is transformed back to the conventional x ‐‐ t space.

The method is illustrated on a variety of synthetic data and on a set of real marine CMP data acquired in the North Sea near the Norwegian shore.
OMNI‐DIRECTIONAL DOWNHOLE EM PROBES*
Authors: CULL J. P. , COBCROFT R.
Published: April 2006

Borehole surveys are usually commissioned to provide structural details as a guide to further drilling. Conductors remote from the hole are readily detected using EM probes. Vectors can be established by noting variations in wavelength, particularly for reversals of polarity. However, several holes in the same area are required to remove rotational ambiguities, and single axis systems may generate spurious anomalies related to errors in alignment. Consequently, a three‐component sensor has been designed to compare the orthogonal response at each depth. Data are expressed in terms of the invariant parameters of the polarization ellipse, and secondary fields are readily identified.
ELECTROMAGNETIC SOUNDING OF THE KAPURDI LIGNITE DEPOSIT IN WESTERN RAJASTHAN, INDIA*
Authors: PODDAR M. , DHANASEKARAN P.C.
Published: April 2006

In the Kapurdi block of the Barmer lignite field in western Rajasthan, India, lignite occurs as a number of discontinuous seams of varying thickness up to a few m. The country rock is a succession of clay, sand and various members of the clay‐sand family. The overburden, comprising sand, Fuller's earth, bentonitic clay, clayey sand/sandy clay and clay, is thick and, electrically, highly conductive. Both its thickness (40–90 m) and conductivity (0.5–1.0 S/m) are highly variable. The lignite seams may occur anywhere in the clay‐sand sequence below this overburden.

A ten‐layered electrical model of the Kapurdi area was parameterized on the basis of galvanic resistivity sounding and core drilling results. This model was then validated by electromagnetic sounding measurements made with a 200 × 200 m square loop transmitter and energizing current of variable frequency (80 Hz to 11 kHz), using central induction sounding techniques. With a loop of this size, only formations in the overburden could be mapped. Subsequent computer modeling has shown that a larger loop of size, say, 800 m and a frequency band of 30–500 Hz are required to map formations below–as well as those within–the overburden. Even then only various clay‐sand interfaces can be mapped, i.e., only over‐burden thickness and the likely depth range of lignite seams (if they occur) can be estimated. Direct mapping of lignite seams is not feasible in practice. This is true of loop‐dipole, as well as central induction sounding. It is interesting that skin depth need not always be a factor limiting depth of exploration in electromagnetic prospecting.
THE P400 WATERGUN SOURCE IN THE QUEST FOR IMPROVED RESOLUTION AT ALL DEPTHS*
Authors: LUGG R.D. , BRUMMITT J.G.
Published: April 2006

The low‐frequency response of the P400 watergun is an improvement over that of the S80 version. It has been further enhanced by deployment in a vertically staggered array. The notch in the amplitude spectrum at about 35 Hz due to the interference between the precursor and main implosion pulse has been virtually eliminated, and depth notches due to the free surface interface at the upper end of the spectrum have been greatly reduced. The spectral band is thus very broad and well‐shaped and corresponds to an even shorter signature.

After convolution with the streamer ghost, the amplitude of the implosion pulse from the composite far‐field signature is more than 30 times the composite precursor amplitude. Thus the signal approaches the minimum‐phase condition.

Shot‐generated noise scattered back by diffractors located at or near seabed is the single greatest impediment to increased penetration, especially in high resolution surveys. Such noise decays much less rapidly than signal, particularly so with broad band data. Furthermore, back‐scattered interference coming from a cone of bearings between 15–45° with respect to the line is particularly damaging since in these directions the stack enhances the interference and confuses the primary velocity analysis. The watergun source patterns are therefore element‐weighted to maximize directivity within these critical directions in the frequency band 50–155 Hz.

When spatial aliasing is reduced by increased streamer spatial sampling and by discrete anti‐alias filters during the record cycle,f‐k filtering can be applied to the field records as an effective supplement to the source and receiver patterns without the aliased ‘wrapped around’ noise alignments destroying the upper end of the spectrum of genuine reflectors. Furthermore, increased spatial sampling is a pre‐requisite in thef‐k‐migration process if the valuable higher frequencies of broad band data are to be migrated in steep‐dip situations without aliasing.

Thus, spatial resolution is a necessary complement to temporal resolution if maximum advantage of the broad‐band P400 watergun source is to be realized.
GEOELECTRIC DETERMINATION OF QUALITY CHANGES AND TECTONIC DISTURBANCES IN COAL DEPOSITS*
Authors: CSÓKÁS J. , DOBRÓKA M. , GYULAI Á.
Published: April 2006

A new underground geoelectric method is presented for the determination of small tectonic disturbances and barrenings in coal seams. The distribution of the apparent resistivity can be mapped from the measured apparent resistance data by using a recently developed geoelectric imaging method. The applicability of the methods are proved by in situ measurements and by a model experiment.
EXTENDED MARINE ARRAYS VERSUS SIMULATED EXTENDED ARRAYS*
Authors: ROKSANDIý M.M.
Published: April 2006

Two factors are responsible for the fact that an extended marine source array performs better than a point source:

1. a higher degree of transmission of the radiated seismic energy through the water‐sediment interface providing a better penetration;

2. filtering effects.

The higher degree of transmission is due to: (a) the directivity of extended sources, (b) the lower reflection coefficient at the water‐sediment interface for seismic waves radiated from an extended source array than for spherical seismic waves radiated from a point source, (c) the lower amplitude decay of the pulses from an extended source than from a point source. In addition, signature characteristic of an extended source array and Fresnel zone of waves generated by such a source differ from those corresponding to a point source.

The propagating wavelet radiated from a point source array may not be, in a sedimentary sequence below the sea‐floor, the linear combination of wavelets emitted from point sources. In such cases, there is a noticeable difference between the performance of a field‐implemented source array and that of the corresponding simulated source array. The performances of the field‐implemented and simulated extended receiver arrays can be identical if the recording system is adequate and the processing technique appropriate.
LOVE‐TYPE SEAM‐WAVES IN WASHOUT MODELS OF COAL SEAMS*
Authors: BREITZKE M. , DRESEN L.
Published: April 2006

The propagation of Love seam‐waves across washouts of coal seams was studied by calculating synthetic seismograms with a finite‐difference method. Seam interruption, seam end and seam thinning models were investigated. The horizontal offset, the dip of the discontinuities and the degree of erosion served as variable parameters. Maximum displacement amplitudes, relative spectral amplitudes and phase and group slowness curves were extracted from the synthetic seismograms.

Both seam interruption and seam thinning reduce the maximum displacement amplitudes of the transmitted Love seam‐waves. The degree of amplitude reduction depends on the horizontal offset and the degree of erosion. It is four times greater for a total seam interruption than for an equivalent seam thinning with a horizontal offset of four times the seam thickness. In a seam cut vertically, the impedance contrast between the coal and the washout filling determines the maximum displacement amplitudes of the reflected Love seam‐waves. They diminish by a maximum factor of four in oblique interruption zone discontinuities with a dip of maximum 27°, and by a maximum factor of ten in a seam thinning with a degree of erosion of at least 22%.

The analysis of the relative spectral amplitudes indicates a preferential transmission of those phases with frequencies below, and a preferential reflection of those phases with frequencies above the first mode Airy‐phase. The relative spectral amplitudes of the reflected Love seam‐waves show a distinct interference pattern of the waves reflected at both interruption zone discontinuities.

The dispersion analysis reveals a flattening of the phase and group slowness curves with increasing frequencies, horizontal offset and degrees of erosion.

These results imply that a detection of washouts in‐mine will be possible in a frequency range including at least the first mode Airy‐phase. An interference pattern and a flattening of the dispersion curve indicate a washout rather than other seam obstructions and leads to an estimate of the washout dimension.