bluuebunny/crossref_metadata_2025 huggingface dataset:
SIMULTANEOUS INTERACTIVE MAGNETIC AND GRAVITY INVERSION*
Authors: MENICHETTI V. , GUILLEN A.
Published: April 2006

Abstract

Gravity and magnetic anomalies may have the same source but it is always difficult to achieve correlated solutions if interpretations are carried out separately. Therefore it is useful to invert both anomalous gravity and magnetic profiles at the same time, so that the solution mav be more constrained.

Existing inversion techniques do not lend themselves equally to this kind of extension, since the parameters—such as density and susceptibility contrasts—should not be related. The algorithms more easily adaptable to simultaneous inversion are those which look for the shape and the position of anomalous bodies with constant unknown density or susceptibility contrasts.

In this study, we use the generalized inverse method in the 2 1/2 D case. Parameters are the coordinates of the vertices of the polygonal cross‐sections of the anomalous bodies and both density and susceptibility contrasts of each body. The two types of profile to be processed must be superposable but may have different lengths, spacings, and coordinate origins. Both synthetic examples and field data from geologically known areas have been processed, and it appears that the simultaneous inversion technique may provide an important additional degree of control in the interactive interpretation process.
ELECTROMAGNETIC FIELDS DUE TO A THIN RESISTIVE LAYER*
Authors: PASSALACQUA H.
Published: April 2006

Abstract

Using approximate boundary conditions, expressions for electromagnetic fields have been derived for a thin, highly resistive layer lying between two homogeneous layers excited by an electric dipole grounded on the surface of the earth. The variations of the fields with the parameter T/T1 (ratio of the transverse resistance of the thin layer to the transverse resistance of the first layer) were studied in relation to frequency, time, the normalized separation source—receiver, and the angle between the source and the radius to the observation point. For a value of h2/h1 (ratio of thickness of second layer to the thickness of the first layer) approximately equal to 0.2, the general three‐layer medium case gives the same results as this approach. It was found that the electric fields have a very strong dependence on the parameter T (transverse resistance) which characterizes the thin, highly resistive layer. However, the magnetic fields depend only very weakly on this parameter.
A METHOD FOR CALCULATING IP ANOMALIES FOR MODELS WITH SURFACE POLARIZATION*
Authors: ESKOLA L. , ELORANTA E. , PURANEN R.
Published: April 2006

Abstract

A numerical method is given for calculating resistivity and induced polarization anomalies produced by a surface polarization model. Surface polarization is generated when a purely electronic conductor is located in an electrolyte environment. The system that develops on the boundary between the conductor and the electrolyte is described macroscopically by a net surface charge distribution and an electric double layer. An integral equation is derived for the potential by assuming that the electronic conductor forms an equipotential system and that the polarization impedance across the boundary is linear. The integral equation is solved by means of the method of subsections. As an application some numerical modeling results are presented. The surface impedance values used in calculations are based on laboratory measurements that are briefly described. Implications of the results for scale modeling are discussed.
THE USE OF TWO‐ELECTRODE AND SCHLUMBERGER FILTERS FOR COMPUTING RESISTIVITY AND EM SOUNDING CURVES*
Authors: BAŞOKUR A. T.
Published: April 2006

Abstract

Different sets of filter coefficients for the linear filter technique for the computations of resistivity and EM sounding curves are evaluated for several electrode and coil configurations. Instead of this procedure, the two‐electrode filter can be used for computations of Wenner, Schlumberger, and dipole—dipole apparent resistivity model curves by defining convolutional expressions which contain the new input functions in terms of the resistivity transform function. Similarly, the Schlumberger filter performs the computations of dipole—dipole apparent resistivity model curves. The Wenner, Schlumberger, and dipole—dipole filter functions are defined in terms of the two‐electrode filter using the new convolutional expressions. A relationship between the Schlumberger and dipole—dipole filter functions is given.

The above arguments are adopted for the computations of EM sounding curves. It is shown that the EM filter for the horizontal coplanar loop system (which is identical to the two‐electrode filter) performs the computations of the mutual coupling ratios for perpendicular, vertical coplanar, and vertical coaxial loop systems. In the same way, the Schlumberger filter can be used to compute vertical coaxial sounding curves. The corresponding input functions are defined in terms of the EM kernel for all convolutional expressions presented.

After these considerations, integral expressions of the mutual coupling ratios involving zero‐order Bessel function are derived. The mutual coupling ratio for the vertical coaxial loop system is given in the same form as the mutual coupling ratio for the vertical coplanar loop system.
INTERPRETATION OF LANDSAT MSS DATA IN AN IRON‐ORE‐BEARING ZONE IN GOA, INDIA*
Authors: MURTHY Y. V. S. , MALLICK K.
Published: April 2006

Abstract

In South and Central Goa iron ore occurs in two parallel belts with the general NW‐SE Dharawar trend. The ore occurrence, however, is not continuous. There are barren zones as well as zones of very high concentration in some of which there are mining activities.

Landsat MSS data have been interpreted over a zone covering both mineral belts in order to delineate the ore occurrences. As a guide line a known ore‐bearing area has been considered along with the unknown zones.

On the basis of two‐dimensional plotting of gray level values it has been found that the MSS bands 4 and 7 are most suitable for the studies over iron‐rich areas in Goa. Two techniques are described here for the processing of the MSS data; the separation of residual from the regional and MSS band‐ratioing. It is observed that (i) the gray level residual maps of MSS bands 4 and 7 are of use in demarcating the iron‐ore‐bearing zones, and (ii) an existing mine, an abandoned mine, and a proved iron ore zone could be delineated by MSS band‐ratioing. On the basis of the latter technique, a few areas with ore occurrence potential have been indicated.
ON THE INTERPRETATION OF AIRBORNE ELECTROMAGNETIC DATA FOR THE TWO‐LAYER CASE*
Authors: MUNDRY E.
Published: April 2006

Abstract

For helicopter‐borne electromagnetic systems, the distance between the transmitting and the receiving coils is small compared with the altitude above ground. For this case, a major simplification can be made for the calculation of model curves. Some two‐layer curves for the interpretation of frequency measurements are presented.

A very simple procedure is demonstrated for the conversion of the relative secondary field into apparent resistivity and apparent distance for the mapping of airborne electromagnetic data. Furthermore, an approximation is described for the determination of the thickness and the resistivity of a layer lying on a perfectly conducting half‐space.
OFFSET CONTINUATION IN THEORY AND PRACTICE*
Authors: BOLONDI G. , LOINGER E. , ROCCA F.
Published: April 2006

Abstract

Offset continuation is a technique that was recently proposed for the dip moveout correction. This correction can be carried out in the time‐wavenumber domain using a proper partial differential equation that links sections with different offset.

It has been shown that a single spike in a common‐offset section—corresponds to a semi‐elliptically shaped reflector with foci located at the source and receiver in the section migrated after dip moveout correction.

The sections that result after offset continuation, stack, and migration are thus a superposition not only of semicircles, but also of semi‐ellipses with different lengths of axes. This effect smears the migration alias‐noise which, without offset continuation, would appear as migration circles not close enough together to interfere destructively.

Offset continuation can improve the quality of seismic sections in several ways:

—the velocity analyses are more readable, less dispersed and dip independent; diffraction tails arrive with the same normal moveout velocity as the apex and thus diffraction‐noise can be “stacked out”;

—noise produced by aliasing in the migrated section is reduced.

In this paper we give a practical and conceptual interpretation of the offset continuation method, with a generalization to three‐dimensional volumes of data. A critical examination of several synthetic and field data examples shows the actual possibilities and advantages of offset continuation.
STATIC CORRECTIONS FOR SHEAR WAVE SECTIONS*
Authors: WIEST B. , EDELMANN H. A. K.
Published: April 2006

Abstract

The computation of static corrections requires information about subsurface velocities. This information can be obtained by different methods: surface wave analysis, short refraction lines, downhole times, uphole times and first arrivals from seismograms.

For pure shear waves generated by SH sources the analysis of first arrivals from seismograms combined, if necessary, with short refraction lines has proved to be most accurate and economic.

A comparison of first‐arrival plots from P‐ and S‐wave surveys of the same line measured in areas of unconsolidated sediments in northern Germany illustrates the characteristic differences between the two velocity models. P‐waves show a marked velocity increase at the water table from about 600 to 1800 m/s. S‐wave velocities of the same strata increase gradually from about 100 to 400 m/s. As a consequence, S‐wave models are vertically and laterally more complex and, in general, show no significant velocity increase at a defined boundary as P‐wave models do. Therefore, other suitable correction levels with specific velocities must be chosen.

A comparison of “tgd‐corrections” (correction time between geophone position and datum level) for P‐ and S‐waves in areas of unconsolidated sediments shows that their ratio is different from the P‐/S‐velocity ratio for the respective correction level because of the greater depth of the S‐wave refractor. Therefore, P‐ and S‐waves are influenced by different near‐surface anomalies, and time corrections calculated for both wave types are largely independent.
ESTIMATES OF TERRESTRIAL THERMAL GRADIENTS AND HEAT FLOW VARIATIONS WITH DEPTH IN THE HINTON‐EDSON AREA OF THE ALBERTA BASIN DERIVED FROM PETROLEUM BOTTOM‐HOLE TEMPERATURE DATA*
Authors: KUSHIGBOR C. , LAM H. L. , MAJOROWICZ J. A. , RAHMAN M.
Published: April 2006

Abstract

There is a significant increase in terrestrial heat flow with depth in the Hinton‐Edson area of the deep part of the western Canadian sedimentary basin in Alberta. This is especially true near the Rocky Mountain foothills which is an area of high relief, high hydraulic head and regional water recharge. Gravity‐imposed downward movement of meteoric water through the thick sedimentary strata with velocities as low as 10–10 m/s to 0.5 × 10–9 m/s may cause an increase of heat flow with depth. Such disturbance of heat flow with depth on a regional scale in the sedimentary strata means that it is not possible to determine the background conductive steady‐state heat flow associated with crustal or upper mantle heat sources in such an area from measurement of conductive heat flow in the part of the sedimentary column where water movement occurs. This is because the convective portion cannot be determined, particularly when measurements are made in only part of the regional hydrodynamic system of the basin.
ZERO—PHASE FORWARD FILTERS FOR RESISTIVITY SOUNDING*
Authors: MANSINHA L.
Published: April 2006

Abstract

Forward filters to transform the apparent resistivity function over a layered half‐space into the resistivity transform have been derived for a number of sample intervals. The filters have no apparent Gibbs' oscillations and hence require no phase shift. In addition, the end points of the filter were modified to compensate for truncation. The filters were tested on simulated ascending and descending two‐layer cases. As expected, “dense” filters with sample spacing of In (10)/6 or smaller performed very well. However, even “sparse” filters with spacing of In (10)/2 and a total of nine coefficients have peak errors of less than 5% for p1:p2 ratios of 10–6 to 106. If a peak error of 5.5% is acceptable, then an even sparser filter with only seven coefficients at a spacing of 3 In (10)/5 may be used.
THE USE OF SEISMIC SHEAR WAVES AND COMPRESSIONAL WAVES FOR LITHOLOGICAL PROBLEMS OF SHALLOW SEDIMENTS*
Authors: STÜMPEL H. , KÄHLER S. , MEISSNER R. , MILKEREIT B.
Published: April 2006

Abstract

From a great variety of in situ shear wave experiments, i.e., reflection, refraction and borehole surveys in the shallow sediments of the north German plains, several specific properties have been derived. Shear waves (S) differ from compressional waves (P) in that:

they are not affected by the degree of water saturation. Thus, they provide a better correlation between the velocity Vs and (solid) lithology;

they generally have lower frequencies, but shorter wavelength and, hence, a better resolution of thin layers;

they have lower absorption Qs−1 and hence a better penetration in partially saturated and gas‐containing sediments than P‐waves.

Correlations have been established between Vs and the confining pressure and between reduced Vs values and several lithological parameters like the grain size of sandy material. More lithological and hydrological information is obtained by using S‐ and P‐wave surveys along the same profile. The best information on a sedimentological structure is obtained by the simultaneous observation of Vs, Vp, Qs and Qp.
INTERACTION BETWEEN AIRGUNS*
Authors: VAAGE S. , URSIN B. , HAUGLAND K.
Published: April 2006

Abstract

In the design of linear airgun arrays the interaction between the airguns is usually neglected. We review the different formulae which have been proposed for the minimum separation between airguns at which the interaction is negligible. These formulae can all be approximated by a linear function of a single variable.

We have analyzed a large number of measurements in order to establish the amount of interaction between two airguns of various volumes at different pressures and depths. The resulting far‐field signature has been measured and compared with the sum of the signatures from the two airguns measured in the same experimental situation. The changes in primary pulse amplitude, bubble period and primary/bubble peak‐to‐peak amplitude ratio were computed from the measurement data as a function of airgun separation, chamber volume, chamber pressure and airgun depth. The influence of a waveshape kit was investigated, and the effects of interaction and the effects of using a waveshape kit were compared.
EM COUPLING IN MULTIFREQUENCY IP AND A GENERALIZATION OF THE COLE‐COLE IMPEDANCE MODEL*
Authors: BROWN R. J.
Published: April 2006

The Cole‐Cole relaxation model has been found to provide good fits to multifrequency IP data and is derivable mathematically from a reasonable, albeit greatly simplified, physical model of conduction in porous rocks. However, the Cole‐Cole model is used to represent the mutual impedance due to inductive or electromagnetic coupling on an empirical basis: this use has not been similarly justified by derivation from any simple physical representation of, say, a half‐space, layered or uniform.

A uniform conductive half‐space can be represented as a simple subsurface loop with particular resistive and inductive properties. Based upon this, a mathematical expression for the mutual impedance between the two pairs of electrodes of a dipole‐dipole array is derived and designated “model I”. It is seen that a degenerate case of model I is the Cole‐Cole model with frequency exponent c= 1. Model I is thus more general than the Cole‐Cole expression and must provide at least as good a fit to a set of field data. Provision for variation of c from unity could be made in model I equally well as for the Cole‐Cole model although, at present, this would be a purely empirical alteration.

Model I contains four parameters, one of which is, in effect, the resistivity of the half‐space. Therefore only three parameters are involved in the model I expressions for normalized amplitude and for phase of the EM‐coupling mutual impedance. Model I is compared with previously published “standard” values for two different dipole separations. Under particular constraints, model I is shown to provide better fits than the Cole‐Cole model (with c= 1) over particular frequency ranges, specifically at very low frequencies and at moderately high frequencies where the model I phase curve follows the standard phase curve across the axis to positive values (negative coupling).
COMBINED SOUNDING‐PROFILING RESISTIVITY MEASUREMENTS WITH THE THREE‐ELECTRODE ARRAYS*
Authors: KAROUS M. , PERNU T. K.
Published: April 2006

A combination of DC‐resistivity sounding and profiling measurements can be used to obtain the maximum information about distribution of resistivities in the earth. Combined sounding‐profiling measurements with the AMN, MNB arrays are considered. The resistivity data from such measurements can be presented as: (a) normal sounding curves, (b) combined sounding‐profiling curves, (c) profiling curves, (d) pseudo‐sections, or as transformations obtained by the so‐called gradient processing, to emphasize the influence of the target objects.

The examples chosen from numerical modeling and field tests show the efficiency of measurements with three‐electrode arrays to accurately locate thin conductors and contacts of lithological units of different resistivities. An interpreted cross‐section is compared with the results of other geophysical measurements (VLF‐R, dipole EM, Δ, IP) showing good correlation.
A REGIONAL GRAVITY STUDY OF THE NORTHERN NORTH SEA (56–62° N)*
Authors: HOSPERS J. , FINNSTRØM E. G. , RATHORE J. S.
Published: April 2006

Abstract

A Bouguer gravity anomaly map is presented of the North Sea and adjacent land areas in Norway and Denmark, covering an area situated between 56° and 62°N, 1°W and 10°E. The gravity data from the UK sector of the North Sea, the land and offshore areas of Denmark, and the land areas of Norway have been published before. However, the gravity data from the Norwegian sector of the North Sea are new. A large number (about 60) of individual gravity features can be defined in the mapped area. Most of those situated in the UK sector of the North Sea and on land in Norway have been discussed earlier; however, most of the anomalies found elsewhere which are qualitatively interpreted here have not been discussed before. An interpreted Bouguer anomaly map is presented which identifies all these features.

The majority of the gravity anomalies encountered in the mapped area can be shown to be associated with one of the following geological features: (i) basement highs, (ii) large bodies of heavy basic or ultrabasic rock in the crystalline basement, (iii) large igneous intrusions within the sedimentary column and thick accumulations of volcanic rocks or their associated eruption centers, (iv) major basement faults.

Large‐scale geological structures such as the Central, Viking and Sogn Grabens and the East Shetland, Stord, Forth Approaches and Norwegian‐Danish Basins are essentially in isostatic equilibrium and are only locally marked by relatively weak gravity minima.

A residual gravity anomaly map has been produced by subtracting from the observed Bouguer anomalies the estimated gravity effect of an assumed thinned crust. This residual gravity anomaly map shows a number of features of the Bouguer anomaly field with greater clarity.
THE GRAVIMETRIC DETECTION OF MINING SUBSIDENCE*
Authors: LYNESS D.
Published: April 2006

Abstract

A series of gravity measurements were taken over a period of time (c. 150 days each) above two adjacent working coal faces. Precise levelling and gravity measurements were taken along the same profile before, during, and after seam extraction. The observed change in gravity agrees well with the levelling data. The combined data sets illustrate the validity of a simple Bouguer relationship for the gravity gradient.

The results of two‐dimensional modelling are used to estimate the gravitational effect of the extracted material. A local feature detected only on the gravity signal may be due to an outcropping limestone layer.

This controlled experiment demonstrates the possibility of using high‐precision gravity measurements as a substitute for levelling, particularly in surveys of large areal extent where the cost of obtaining comprehensive coverage by levelling may be prohibitive.
CALCULATION OF DISPERSION CURVES AND AMPLITUDE‐DEPTH DISTRIBUTIONS OF LOVE CHANNEL WAVES IN HORIZONTALLY‐LAYERED MEDIA*
Authors: RÄDER D. , SCHOTT W. , DRESEN L. , RÜTER H.
Published: April 2006

Abstract

We present dispersion curves, and amplitude‐depth distributions of the fundamental and first higher mode of Love seam waves for two characteristic seam models. The first model consists of four layers, representing a coal seam underlain by a root clay of variable thickness. The second model consists of five layers, representing coal seams containing a dirt band with variable position and thickness. The simple three‐layer model is used for reference.

It is shown that at higher frequencies, depending on the thickness of the root clay and the dirt band, the coal layers alone act as a wave guide, whereas at low frequencies all layers act together as a channel. Depending on the thickness, and position of the dirt band and the root clay, in the dispersion curves of the group velocity, secondary minima grow in addition to the absolute minima. Furthermore, the dispersion curves of the group velocity of the two modes can overlap. In all these cases, wave groups in addition to the Airy phase of the fundamental mode (propagating with minimum group velocity) occur on the seismograms recorded in in‐seam seismic surveys, thus impeding their interpretation. Hence, we suggest the estimation of the dispersion characteristics of Love seam waves in coal seams under investigation preceding actual field surveys.

All numerical calculations were performed using a fast and stable phase recursion algorithm.
SIMPLE DATA PROCESSING OF TRIPOTENTIAL APPARENT RESISTIVITY MEASUREMENTS AS AN AID TO THE INTERPRETATION OF SUBSURFACE STRUCTURE*
Authors: ACWORTH R. I. , GRIFFITHS D. H.
Published: April 2006

Abstract

The advantages of the Wenner tripotential method (Carpenter 1955) for apparent resistivity profiling are described and two new data processing techniques introduced as an aid to the interpretation of apparent resistivity sections (pseudo‐sections). These techniques were developed from model data computed using a two‐dimensional finite difference method.

Oscillatory components present in anomalies on tripotential profiles and related to electrode spacing are shown to be effectively removed by linear filtering that also simplifies their form and aids recognition. Furthermore, it is shown that the ratio of the beta‐ and gamma‐apparent resistivities is a good indicator of resistivity variation, and is particularly sensitive to lateral change.

Model data indicates that, over a wide range of conditions, enough subsurface information can be obtained by inspection of tripotential resistivity and ratio profiles, and from space sections to make possible a useful—and sometimes semi‐quantitative—interpretation.

A rationale for the general interpretation of tripotential data is developed. Field data are described from an area of weathered granite basement in Nigeria. A model of the subsurface is developed using parameters derived from the processed observations. The observed and calculated apparent resistivity space sections are very similar.
FULL WAVE EQUATION DOWNWARD CONTINUATION OF SEISMIC REFLECTION DATA*
Authors: FOSTER D. J. , CARRION Ph. M.
Published: April 2006

Abstract

A new method for suppressing multiple reflections in seismograms is developed. It is based on a downward continuation procedure which uses the full acoustic wave equation (hyperbolic form) as a downward continuation operator. We demonstrate that the downward continuation of the recorded wave field maps a reflectivity function without multiply reflected events. The method is applied successfully to individual traces of plane‐wave decomposed (slant‐stacked) synthetic and field data.
FIRST ARRIVAL PICKING ON COMMON‐OFFSET TRACE COLLECTIONS FOR AUTOMATIC ESTIMATION OF STATIC CORRECTIONS*
Authors: COPPENS F.
Published: April 2006

Abstract

The increase in the number of geophone groups in production records during recent years and the requirement for accurate basic static corrections for high resolution records have made it necessary to develop sufficiently accurate automatic techniques for the determination of static corrections.

A fully automatic method is presented which makes use of the delay‐time method in order to compute static corrections at each shot position. Delay times, weathering and subweathering velocities are determined from automatic picks of the first arrivals on common‐offset trace collections.

It is assumed that the weathering is a single layer and that the dip of the subweathering layer under the geophone groups is small.

The picking routine is fully automatic and successful in most cases, provided the signal‐to‐noise ratio is sufficiently high.

The subsequent filtering of erroneous values for picked times is performed by means of statistical techniques, using curves of picked times on common‐offset trace collections. If the distance between receivers and shot‐points on the profile is sufficiently short, one can expect only little change in the picked times of two contiguous traces.

The method is well adapted to end‐on spreads with a great number of traces, where distances between geophone groups are short.

Examples are presented showing the possibilities of the method for the determination of long wavelength as well as short wavelength components of static corrections.
ACOUSTIC MODELING OF THE SEAFLOOR*
Authors: BERGE A. M. , DRIVENES G. , KANESTRØM R. , BESKOW B.
Published: April 2006

Data from routine seismic surveys contain considerable information about the geo‐acoustic properties of the seafloor. Waves are reflected at a wide range of angles of incidence from near‐vertical reflections (higher multiples) to supercritical reflections (primary and lower multiples). The reflection coefficient is approximately constant for small angles of incidence (< 10°) but varies greatly for larger angles of incidence. Near‐vertical reflections are used to determine the seafloor density. The P‐velocity in the seafloor is determined in advance from the critical distance using the amplitude variation of the primary as well as the multiples. The Vp/VS ratio is determined by modeling the amplitude variation with the angle of incidence. The primary reflection from the seafloor and the first three multiples are included in the modeling.

Seismic data obtained with both conventional and superlong airgun arrays have been modeled. Data collected from the Barents Sea show that even if the P‐velocity is the same at different sites, the Vp/Vs ratio, density and Poisson's ratio vary significantly. The most extreme example shows that for a P‐velocity of 2.80 km/s the Vp/Vs ratio varies between 1.9 and 6.0. The corresponding densities vary from 2.36 g/cm3 to 1.80 g/cm3 and the Poisson's ratio varies from 0.31 to 0.49.

The acoustic modeling offers a method of assessing the mean geotechnical or mechanical properties of larger volumes of marine sediments in terms of incompressibility, shear modulus and Poisson's ratio.
A NEW APPROACH TO THE INVERSION OF SEISMIC DATA*
Authors: THORNTON B. S. , SUTTON G. R.
Published: April 2006

A new inversion method for seismic reflection data is based on impedance concepts and uses transformations to reduce the essentially non‐linear problem to a linear eigenvalue formulation without approximation. A set of reflection data has been inverted using this method. The characteristic impedances of the layers so determined are compared with log data from a reference borehole.
WHY WATERGUNS?*
Authors: TREE E.L. , LUGG R.D. , BRUMMITT J.G.
Published: April 2006

A single watergun generates a ‘stand‐alone’ narrow pulse of acceptable energy content and broad‐band spectrum.

When multiple units of equal volume are fired simultaneously and are accurately synchronized, carefully depth‐controlled and spaced beyond their near‐field interactive distance, the energy measured in the far‐field increases proportional to the square of the number of individual units employed.

When these multiple units are configured in areal array designs based on distance weighting or varying density distribution of the equal elements, further gains in downward directivity and attenuation of horizontally traveling interference accrue. Such pattern designs take account of the spectral content of the particular watergun used, while the wavenumber response is concentrated to attenuate the dominant interference at water velocity.

Finally, the high‐frequency content, and high repeatability and synchronizing accuracy of the new generation of waterguns improves the high resolution potential to help combat natural earth absorption losses. We should, therefore, keep the ‘in‐line’ dimensions at both source and receiver as short as possible. Unacceptable interference that persists during subsequent processing can be controlled by the long‐array simulation technique.
A COMPARISON OF STATISTICAL AND DETERMINISTIC WIENER DECONVOLUTION OF DEEP‐TOW SEISMIC DATA*
Authors: BOWEN A. N.
Published: April 2006

Wiener ‘spiking’ deconvolution of seismic traces in the absence of a known source wavelet relies upon the use of digital filters, which are optimum in a least‐squares error sense only if the wavelet to be deconvolved is minimum phase. In the marine environment in particular this condition is frequently violated, since bubble pulse oscillations result in source signatures which deviate significantly from minimum phase. The degree to which the deconvolution is impaired by such violation is generally difficult to assess, since without a measured source signature there is no optimally deconvolved trace with which the spiked trace may be compared.

A recently developed near‐bottom seismic profiler used in conjunction with a surface air gun source produces traces which contain the far‐field source signature as the first arrival. Knowledge of this characteristic wavelet permits the design of two‐sided Wiener spiking and shaping filters which can be used to accurately deconvolve the remainder of the trace. In this paper the performance of such optimum‐lag filters is compared with that of the zero‐lag (one‐sided) operators which can be evaluated from the reflected arrival sequence alone by assuming a minimum phase source wavelet. Results indicate that the use of zero‐lag operators on traces containing non‐minimum phase wavelets introduces significant quantities of noise energy into the seismic record. Signal to noise ratios may however be preserved or even increased during deconvolution by the use of optimum‐lag spiking or shaping filters.

A debubbling technique involving matched filtering of the trace with the source wavelet followed by optimum‐lag Wiener deconvolution did not give a higher quality result than can be obtained simply by the application of a suitably chosen Wiener shaping filter. However, cross correlation of an optimum‐lag spike filtered trace with the known ‘actual output’ of the filter when presented with the source signature is found to enhance signal‐to‐noise ratio whilst maintaining improved resolution.
THE SCALING OF AIRGUN ARRAYS, INCLUDING DEPTH DEPENDENCE AND INTERACTIONS*
Authors: ZIOLKOWSKI A.M.
Published: April 2006

The article provides a theoretical basis for the extension of the method of scaling law deconvolution to three dimensions using airgun arrays as a sound source.

Earlier papers by the author required the dimensions of the scaled sources to be different while the depths and firing pressures were maintained the same in order to preserve the same dynamics of the scaled sources at scaled time. However, this forces the source ghost to be considered as part of the impulse response of the earth rather than as part of the downgoing source wave. And, in fact, the dynamics of the scaled sources are not the same at the same depth because the ghost reflection modulates the behaviour of the oscillating bubbles generated by the airguns, and this modulation does not scale.

To force the sources to scale properly, including the ghost interaction, the larger source must be put at greater depth, where hydrostatic pressure is greater, and the initial firing pressure must be adjusted accordingly. Thus, the depth, initial firing pressure and gun volume are all variables. The interaction among guns in scaled airgun arrays also scales exactly if the geometry of an array and the depth of its deployment are scaled by the same factor.
REFLECTION‐WINDOW MAPPING OF SHALLOW BEDROCK*
Authors: SINGH S.
Published: April 2006

A field study was undertaken to evaluate the effectiveness of the high frequency seismic reflection technique for mapping of shallow and irregular bedrock. Bedrock reflections were obtained using a hammer source with both in‐line and common offset field layouts. The recording equipment included 12‐channel enhancement seismographs, 28 Hz vertical geo‐phones and a microcomputer. The latter increased the overall versatility of the seismic system.

Field sites for this study are typical of the geological settings of the tin mining areas of Malaysia. The topographical ‘lows’ of the irregular bedrock control the localization of tin ore. The subsurface geology consists of a thin low velocity layer (± 300 m/s) overlying the compact overburden (± 1700 m/s) which in turn lies on bedrock.

This paper discusses various criteria for designing an optimum window for obtaining usable reflections between the first arrival and the leading edge of the ground roll cone.

Detailed mapping of the overburden and the bedrock interface by the reflection method can be useful in delineating areas for exploratory drilling and for optimum planning of mining operations.
IMPROVED PHASE‐ELLIPSE METHOD FOR IN SITU GEOPHONE CALIBRATION*
Authors: LIU H.‐P. , PESELNICK L.
Published: April 2006

For amplitude and phase response calibration of moving‐coil electromagnetic geophones two parameters are needed, namely, the geophone natural frequency, f0, and the geophone upper resonance frequency fu. The phase‐ellipse method is commonly used for the in situ determination of these parameters. For a given signal‐to‐noise ratio, the precision of the measurement off0 andfu depends on the phase sensitivity, f(δφ/δf) For some commercial geophones f(δφ/δf) atfu can be an order of magnitude less than the sensitivity atf0. In this paper we present an improved phase‐ellipse method with increased precision. Compared to measurements made with the existing phase‐ellipse methods, our method shows a 6‐ and 3‐fold improvement in the precision, respectively, on measurements of f0 andfu on a commercial geophone.
MIGRATION OF VSP DATA BY RAY EQUATION EXTRAPOLATION IN 2‐D VARIABLE VELOCITY MEDIA*
Authors: HU LIANG‐ZIE , McMECHAN G.A.
Published: April 2006

The standard Kirchhoff algorithm can be generalized for migration of pre‐stack finite‐offset data from variable‐velocity media. The concentric ellipses over which the data are spread in constant velocity media become significantly distorted (even multi‐valued) in the variable velocity case. The specific shapes can be explicitly defined by kinematic extrapolation of the source and recorded wave fields with the ray equation.

The use of Kirchhoff migration with a surface source and a subsurface recorder requires that two sets of Kirchhoff loci be superimposed. For each trace, the first set of loci is computed with the source and the actual recorder position as foci; the second set is computed with the source and the virtual recorder position as foci. This dual procedure explicitly incorporates the primary diffracted energy and the free‐surface reflections, respectively.

Implementation involves the construction of a virtual medium, lying above the free surface, with a velocity distribution that is the mirror image of the actual distribution below the free surface. Ray‐equation extrapolation is performed through the real/virtual boundary. The resulting image is produced in a split form, with all the contributions of the primary reflected and diffracted energy lying in the lower ‘real’ half and all the contributions of the energy that was reflected at the free surface lying in the upper ‘virtual’ half. The final image is produced by folding the split image about the free surface and adding the two halves. A practical advantage is that the origin of various contributions (and artifacts) can be more readily identified (for interpretation or removal) in the split images.

The ray‐equation pre‐stack migration algorithm is very general. It is applicable to all source‐recorder geometries and variable velocity media and reduces exactly to the standard Kirchhoff algorithm when applied to zero or finite‐offset surface survey data. The algorithm is illustrated by application to VSP data. For the VSP geometry, the algorithm does not require any specific trace spacing (in depth) and can be used for data from deviated as well as vertical holes.
ON THE UPWARD CONTINUATION OF POTENTIAL FIELD DATA BETWEEN IRREGULAR SURFACES*
Authors: IVAN M.
Published: April 2006

Abstract

The potential field and its derivatives at points above an irregular surface can be approximately obtained from the sampled potential field data acquired on that surface.

A method of minimizing the truncation effect, which appears when gravity and magnetic maps are processed with the aid of surface integrals, is derived.

The results are compared with those of the most relevant similar methods by using a theoretical, but realistic, model.
COMPLEX RESISTIVITY OF SYNTHETIC SULPHIDE BEARING ROCKS*
Authors: MAHAN M.K. , REDMAN J.D. , STRANGWAY D.W.
Published: April 2006

The complex resistivity of synthetic samples has been measured over the frequency range 0.01 Hz to 1.0 MHz. The synthetic samples were made of a quartz matrix and sulphide particles. Grain sizes, volume fractions, and sulphide particles of different nature were used. The samples were saturated to different degrees, with different types of electrolyte. The results obtained were used to check the validity of the theoretical electrochemical model developed by Wong (1979). This model fits the experimental data reasonably well in the frequency range where the dispersion and the phase maxima occur. Against the model expectations, the experiment did not duplicate the Debye‐form of spectra because most of the common metals are oxidized to the ionic form and an electrochemical charge transfer reaction occurs at the interface. The model prediction regarding the effect of the distribution of the grain radii on the complex resistivity could not be duplicated experimentally.
AN ALGORITHM FOR FAST TIME‐DOMAIN COMPUTATION OF ONE‐DIMENSIONAL SYNTHETIC VERTICAL SEISMIC PROFILES*
Authors: THYBO H.
Published: April 2006

The normal incidence seismic wavefields in a horizontally layered model of the earth have a well‐known matrix description if the earth model has been transformed into a layer stack built up by thin layers, all of equal traveltime. To compute a Synthetic Vertical Seismic Profile (SVSP) the wavefields are iterated through the whole layer stack by means of the matrix description, either expressed in the time domain or in the frequency domain. In conventional methods a single layer is added to a resulting stack of layers at each recursion step. However in the present algorithm, a whole layer stack is added to another layer stack at each recursion step. Improvement in computing time obtained by doing so is approximately linearly dependent on the typical number of layers between receivers. This makes the method very suitable for detailed modeling wherever high resolution of the model is desired.
A PORTABLE LOCAL LOOP VLF TRANSMITTER FOR GEOLOGICAL FRACTURE MAPPING*
Authors: HAYLES J.G. , SINHA A.K.
Published: April 2006

A portable low‐power Very Low Frequency (VLF) transmitter using a large square loop antenna has been designed, assembled and tested by the Geological Survey of Canada (GSC) for geological studies of fracture patterns in igneous rock masses. Standard laboratory equipment, consisting of a signal generator, a 1100‐W power amplifier and several high‐power tuning capacitors, was used for the purpose.

Field tests at the Chalk River facilities of Atomic Energy of Canada Limited have demonstrated a remarkable similarity between survey results obtained using the VLF signals from the local loop transmitter and from distant US Navy VLF transmitters. The local loop was used to simulate the fields from navy stations NAA in Cutler, Maine and NSS in Annapolis, Maryland. Conductor axes, mapped by using these navy stations, and by using the loop antenna yielded almost identical results. A survey was also done in the same area with the local loop placed in such a manner that the direction of the VLF field was at 45° to the field directions from NAA and NSS. In this case, the same conductor axes were located with only minor shifts in position, indicating that conductors whose axes lie within 45° of the direction of the primary horizontal magnetic field are mapped. Thus, it is probably sufficient to have two sources with orthogonal VLF fields to map all VLF conductors in an area. Since in most areas at least one navy VLF station can be used, the local loop transmitter can be used to generate a signal at right angles to the direction from the navy transmitter to allow a more complete VLF survey coverage.
AN OPTIMIZED DIGITAL FILTER FOR THE FOURIER TRANSFORM*
Authors: NISSEN J. , ENMARK T.
Published: April 2006

Abstract

Short filters for calculating Hankel‐transformations, with special attention to the d.c.‐sounding problem, have been published in recent years. These filters, with a typical length of less than 25 coefficients, have made it possible to implement, e.g., VES‐interpretation programs on microcomputers and 3‐D electric and electromagnetic modeling programs on minicomputers. Initially the performance of the short filters was rather poor, but with the introduction of short optimized filters there has been a considerable improvement in the accuracy.

An optimization procedure is applied to design a 20‐point filter for the Fourier sine‐transformation. This filter may be useful in electromagnetic prospecting theory, e.g., in the calculation of the electric and magnetic field from a line source.
ON THE TIME‐VARYING WIENER FILTER*
Authors: DIMRI V.P.
Published: April 2006

For a new approach to designing the time‐varying Wiener filter, the input is first divided into sections and then the time‐varying filter is determined from the entire input and the desired output. The technique differs from the existing one in which the time‐invariant filter is determined from each section. Hence, the main difference, between the proposed and the existing technique lies in the arrangement of input data. The proposed technique requires fewer computational operations and performs better than the time‐invariant Wiener filter, as illustrated by numerical examples.
SOME IMPROVEMENTS IN THE INTERPRETATION OF VERTICAL ELECTRIC SOUNDING CURVES*
Authors: CHYBA J.
Published: April 2006

The process of VES interpretation is discussed, including the following points.

(a) Preliminary interpretation by means of master curves. It is shown that the positions of the auxiliary points K and Q depend on the resistivity of the substratum. The interpretation is improved if the auxiliary points are determined separately for each master curve.

(b) The individual parts of the measured curves are shifted in overlapping MN electrode positions so that the total sum of squares of the shifts is minimal.

(c) The ambiguity may be reduced by means of supplementary information or assumptions on the resistivities. Fixing the resistivities is not always possible because discrepancies may arise between the ground measurement and the well‐logging data. The simultaneous interpretation of several VES curves is recommended assuming constant resistivities. This assumption may be subsequently verified by means of the F‐test.

(d) A nonlinear algorithm is proposed for the determination of confidence intervals. As the multi‐dimensional confidence intervals are often very complicated, it is recommended to construct only one‐dimensional confidence intervals for the estimable parametric functions.

(e) A ‘double‐least‐squares’ optimization technique is presented. The optimization is performed on the estimable parametric functions, and the individual parameters are determined so that the solution remains near the initial guess. This technique is faster than the singular value decomposition.
TUTORIAL THE PROGRESSIVE ATTENUATION OF HIGH‐FREQUENCY ENERGY IN SEISMIC REFLECTION DATA*
Authors: ZIOLKOWSKI A. , FOKKEMA J. T.
Published: April 2006

Seismic reflection data always exhibit a progressive loss of high‐frequency energy with time. This effect is partly attributable to irreversible processes such as the conversion of elastic energy into heat (commonly known as absorption), and partly to reversible processes associated with interference between reflected waves arriving at different times. This paper looks only at reversible linear elastic effects at normal incidence and asks the following question: if there were no such absorption, would there still be a progressive loss of high‐frequency energy?

Using normal incidence and a layered elastic earth model we prove the following results.

1. The normal incidence response of a sequence of plane parallel elastic layers is non‐white.

2. The pressure wave reflected by a layer that is thin compared with a wavelength is differentiated with respect to the incident wave.

3. The transmission response of a thin layer is consequently low‐pass and the transmission response of a sequence containing many thin layers is very low‐pass.

4. The well‐known effect of the transport of acoustic energy by peg‐leg multiples within thin layers is identical with this low‐pass transmission response.

5. It follows that the high frequency energy is reflected back early in the seismogram.

6. By comparison, very low‐frequencies are transmitted through the layered sequence easily and are reflected with difficulty. There is probably a lack of low‐frequency energy in the reflection seismogram, by comparison with the spectrum of the incident plane wave.

It follows that any meaningful evaluation of frequency‐dependent absorption in seismic data cannot take place unless the frequency‐dependent linear elastic effects are taken into account first.
SEISMIC MEASUREMENTS FOR SAFETY IN MINES*
Authors: KÖRMENDI A. , BODOKY T. , HERMANN L. , DIANISKA L. , KÀLMÀN T.
Published: April 2006

The study of rock stresses and their changes is of great importance for safety in mines. To detect dangerous stress accumulations in coal mines an empirical method, Jahn's drilling test, is generally used. An experimental survey to solve the same problem by geophysical measurements was undertaken in a Hungarian coal mine. The basic idea was to determine the easily measurable seismic velocities instead of the more difficult to measure stresses in the rocks, since there is a monotonic relation between them.

During the survey seismic transmission‐type measurements were carried out in the fore‐field of longwall faces between the top and tail roads. The seismic velocity data obtained were processed using an iterative algebraic reconstruction technique to determine the ‘velocity field’, i.e., the seismic velocity distribution, of the area covered by the ray paths. By periodically repeating the measurements in the same area, it was possible to follow the changes in the stress conditions caused by mining operations.