bluuebunny/crossref_metadata_2025_split huggingface dataset:
AN INTEGRAL EQUATION FOR THE GEOELECTRIC RESPONSE OF THIN RESISTIVE BODIES1
Authors: FURNESS PETER 
Published: April 2006

AbstractThin sheet‐like forms are common target bodies in geoelectric prospecting. Depending on their mineralogy and other factors these bodies may be relatively conductive or relatively resistive with respect to their surroundings. For suitably remote field points (relative to the thickness) these features manifest themselves geoelectrically in terms of their conductivity‐thickness product for relatively conductive bodies or in terms of their resistivity‐thickness product for resistive forms.While the case of a conductive sheet has received some attention in the geophysical literature, resistive sheets have been largely ignored. Accordingly an efficient technique to model the geoelectric responses of a resistive lamina is presented here. The technique involves representing the lamina in terms of a distribution of normally directed current dipole moment whose density is shown to satisfy an inhomogeneous Fredholm integral equation of the second kind.The technique is rigorously tested in a 2D environment and is shown to produce reliable and suitably accurate results. An application of the method is presented in which the apparent resistivity and chargeability responses measured with a gradient array over a dipping resistive ribbon are computed. These are compared with the responses observed over a relatively conductive ribbon in the same orientation.
OPTIMAL ESTIMATION OF CRACK‐STRIKE1
Authors: MacBETH COLIN , YARDLEY GARETH S. 
Published: April 2006

AbstractThe accuracy of estimating crack‐strike from the algebraic equivalent of a popular technique, the dual source cumulative technique (DCT), for analysing shear‐wave splitting in seismic experiments is evaluated for earth models permeated by different alignments of micro‐cracks. A complementary analysis is performed using another analysis procedure, the dual‐independent source‐geophone technique (DIT), to investigate any benefits of the alternative formulation. The investigation considers synthetic vertical seismic profile (VSP) and reflection data for an earth model with two layers over a half‐space, and three different classes of crack‐strike variation with depth: uniform crack‐strike, an abrupt change of crack‐strike between the upper and lower layer, and a continuous increase over both layers. The synthetic data for zero‐offset and near‐offset VSPs and a reflection profile are computed using a full‐wave modelling package in which equivalent anisotropic media simulate distributions of aligned vertical, parallel, water‐filled microcracks. Estimates from the two techniques agree for the constant crack‐strike model, but differ for the VSP data with crack‐strike changes. The asymptotic behaviour of the two angular parameters θG and θS from DIT suggest that it may be used to determine crack‐strike under appropriate circumstances in these VSPs, when the time‐delay between the split shear‐waves for the layer of interest exceeds the peak period of the wavelet. In this limit, θG tends to follow the crack‐strike change with θS tending to a constant value, whereas DCT will give a misleading value between the upper and lower crack‐strike. Although the behaviour of DIT is not understood in all cases, θG and θS values from the VSP data always appear to diverge near the point where an abrupt crack‐strike change takes place. This could be used as a qualitative indicator for layer stripping. Both techniques agree for the reflection data as the recorded data matrix is necessarily symmetric, but still give misleading results for deeper layers in the presence of crack‐strike changes. This study suggests that more care should be taken when designing and analysing experimental configurations for detecting crack properties in reservoir rocks, to consider the response and resolution limits of the analysis techniques. A note of caution is offered to those who directly interpret polarization estimates as crack‐strike.
INDUCED GRAVITY ANOMALIES AND ROCK‐BURST RISK IN COAL MINES: A CASE HISTORY1
Authors: CASTEN U. , FAJKLEWICZ Z. 
Published: April 2006

AbstractThe use of the gravity method to predict rock bursts in mines is based on the relationship between the development of a dilatancy process in the exploited rock mass and the time‐dependent gravity anomalies induced by this phenomenon. The differences between successive observations of anomalies and the time behaviour of their trend amplitudes as precursors of preceding changes of rock stability are interpreted. The centres of zones of induced rock density variation are determined by computing the position of singular points of the differences between anomalies.Two gravity surveys have been carried out in the Radbod coal mine (Germany). The first survey took place at the level of the Dickebank seam (depth 1030 m), the second in the Sonnenschein seam (depth 1090 m). The observations were made with Worden and LaCoste‐Romberg (D‐type) gravimeters. The differences between successive anomalies were less than 100 μGal.In the case of the Dickebank seam, the position of singular points demonstrates the effect of two approaching longwalls on a previously mined‐out seam and on the gallery in which the gravity observations were made. In the case of the Sonnenschein seam, the trend amplitudes show distinct variations in the formation of the approaching longwall below the edges of all previously mined‐out seams. In particular, the effect of a remnant pillar has caused the largest gravity gradients. This result corresponds to the existence of a zone of rock‐burst hazard known from test drilling. The computed singular points are grouped together under the remnant pillar indicating two local hazard zones.Both results, the observed development of rock instability with time and the information about the position of the disturbed rock mass relative to the mine workings, are of importance, subsurface gravity surveying can therefore be a valuable tool for predicting rock‐bursts.
DEGHOSTING USING A TWIN STREAMER CONFIGURATION1
Authors: POSTHUMUS B. J. 
Published: April 2006

AbstractA processing method is presented to attenuate the surface ghost using marine twin streamer data. It is an extension to the dephase and sum method which corrects for the phase of the ghost in both streamer outputs and then adds them in an attempt to fill the notches in the amplitude spectrum. The method presented corrects both the phase and the amplitude effect of the surface ghost by combining both signals as a weighted sum.This method is applicable to all types of twin streamer data, ranging from deep exploration data to very shallow high‐resolution surveys. Both the synthetic and real data examples shown are of the high‐resolution type, using frequencies above 2 kHz and short streamers (active sections of the order of one metre).Both the dephase and sum method and the weighted sum method are applied to synthetic high‐resolution data and the results are compared. This has been done for noise‐free data, data with a high noise level and data with strong geometrical spreading on the ghost reflections. From these test results it can be concluded that in general the weighted sum method gives better results. The improvement in the signal‐to‐noise ratio appears to be the same, due to the additive character of both methods. In the case of high‐resolution twin streamer data recorded in shallow water, the delay time of the ghost reflection can be of the same order of magnitude as the traveltime of the primary. For this situation, geometrical spreading can have a considerable effect on the amplitude of the ghost reflection. If no correction can be made for the spreading function, it might be better to use the dephase and sum method.Both methods are also applied to a real data set recorded in a high‐resolution survey. Because the ghost delay is of the same order of magnitude as the arrival time of the primary, the ghost reflection is strongly affected by geometrical spreading. Since the spreading function of the source is unknown, it cannot be corrected for. This causes the result of the weighted sum method to be less reliable compared to the case where no spreading is involved.
SECONDARY SHEAR WAVES FROM SOURCE BOREHOLES1
Authors: MEREDITH J. A. , TOKSÖZ M. N. , CHENG C. H. 
Published: April 2006

AbstractPrevious studies of radiation from point sources in fluid‐filled boreholes have most often been based on far‐field, stationary phase analysis. In these papers, the explicit contribution of the borehole itself acting as a waveguide has not been properly considered, with a few exceptions. In general, these studies accurately describe S‐wave radiation in high‐velocity rocks such as granites and limestones and P‐wave radiation in most rocks, and experiments have confirmed this. However, tube waves directly influence the external wavefield and in fact create a shear‐wave ‘wake’ outside the borehole due to constructive interference of tube‐wave emission if a velocity condition is met. This constructive interference or wake is generated when the tube‐wave velocity is greater than the shear‐wave velocity. When this happens, a tube‐wave complex pole invalidates the mathematical assumptions for stationary phase analysis and the stationary phase predictions do not agree with experimentally derived radiation patterns. Shales at shallow depths and other soft sediments characteristically have tube‐wave velocities greater than shear‐wave velocities. Because the tube‐wave is of relatively high amplitude compared to body waves generated directly by the source, these secondary shear waves can be the highest amplitude arrivals on receiver arrays.The shape and properties of these secondary shear waves are calculated and shown to have identical properties to Mach waves of aerodynamics and seismology. For instance, these waves are geometrically conical and the aperture of the cone and the moveout velocity can be calculated. This paper also demonstrates the important effect that casing has on the Mach waves and provides predictions about when these waves are likely to be observed. Finally, evidence of Mach waves in data sets is examined and it is shown how these waves have been confused with receiver borehole tube waves.It is possible, though rare, that the tube‐wave velocity of the borehole is greater than the compressional‐wave velocity of the surrounding medium. In this case secondary compressional or compressional Mach waves would be generated although this problem is not addressed here.
BOREHOLE STONELEY WAVE PROPAGATION ACROSS PERMEABLE STRUCTURES1
Authors: TANG X. M. , CHENG C. H. 
Published: April 2006

AbstractThis study investigates the propagation of borehole Stoneley waves across permeable structures. By modelling the structure as a zone intersecting the borehole, a simple 1D theory is formulated to treat the interaction of the Stoneley wave with the structure. This is possible because the Stoneley wave is a guided wave, with no geometric spreading as it propagates along the borehole. The interaction occurs because the zone and the surrounding formation possess different Stoneley wavenumbers. Given appropriate representations of the wavenum‐ber, the theory can be applied to treat a variety of structures, including a fluid‐filled fracture. Of special interest are the cases of permeable porous zones and fracture zones. The results show that, while Stoneley wave reflections are generated, strong Stoneley wave attenuation is produced across a very permeable zone. This result is particularly important in explaining the observed strong Stoneley wave attenuation at major fractures where it has been difficult to explain the attenuation in terms of the single planar fracture theory. In addition, by using a simple and sufficiently accurate theory to model the effects of the permeable zone, a fast and efficient method is developed to characterize the fluid transport properties of a permeable fracture zone.
WAVE‐EQUATION MULTIPLE SUPPRESSION USING CONSTRAINED GROSS‐EQUALIZATION1
Authors: MONK DAVID J. 
Published: April 2006

AbstractA method for improving the attenuation of water‐layer multiple energy is suggested. The improvement is achieved using wave‐equation extrapolation to generate an initial model of the multiple energy, and then constraining the way in which this model is modified to fit the observed multiple energy. Reconciling the initial multiple model with the input data is a critical part of this process and several techniques have been suggested previously by other authors. The approach used here is to fit the time, amplitude and phase of the wavelets by adapting the initial model trace using a weighted sum of four traces which can each be derived from the initial multiple model trace.Results on real data suggest that attenuation of primary energy is minimized using this technique, without diminishing the level of multiple attenuation.
FREQUENCY‐DEPENDENT COMPLEX RESISTIVITY OF ROCK‐SALT SAMPLES AND RELATED PETROPHYSICAL PARAMETERS1
Authors: KULENKAMPFF J. M. , YARAMANCI U. 
Published: April 2006

AbstractThe presence of water is one of the main concerns of nuclear waste disposal in rock‐salt. It can be investigated using electrical properties of the rock. Laboratory measurements of frequency‐dependent resistivity and other petrophysical parameters, such as porosity, water content, and specific internal surface area, have been carried out on rock‐salt from the Asse mine in Germany, in order to obtain characteristic resistivity responses for the evaluation of geoelectric field methods and to develop new methods for the estimation of the water content and saturation. The laboratory method, on a.c. half‐bridge for very high impedances, allows measurements of the resistivity spectrum of rock‐salt in the frequency range from 15 Hz to 10 kHz. The saturation of the samples was varied artificially and was approximately 5%, 10%, 20% and 100%.The porosity varies between 0.1% and 0.5%, the water content is approximately 0.05% or less, and the initial saturation is less than 50%. The resistivity ranges from 10 MΩm at the initial saturation down to 1 kΩm for fully saturated samples. In the low‐frequency range up to 100 Hz, an Archie‐type relationship may be used to estimate the water content of the rock‐salt from resistivity measurements. The Archie exponent m is found to be approximately 2.The resistivity is observed to be strongly dependent on frequency. The resistivity decreases with increasing frequency, with a greater decrease for small saturations and vanishing frequency dependence at complete saturation. The relative dielectric constant was found to be 6 ± 1. Saturation dependence was not observed within this error range.The measurements imply that, by measuring resistivity in rock‐salt, estimations of water content and saturation, and thus the porosity, can be made in situ. This is particularly important for the safety of nuclear waste disposal in rock‐salt.
2D finite‐difference elastic wave modelling including surface topography1
Authors: Hestholm Stig , Ruud Bent 
Published: April 2006

AbstractA 2D numerical finite‐difference algorithm accounting for surface topography is presented. Higher‐order, dispersion‐bounded, cost‐optimized finite‐difference operators are used in the interior of the numerical grid, while non‐reflecting absorbing boundary conditions are used along the edges. Transformation from a curved to a rectangular grid achieves the modelling of the surface topography. We use free‐surface boundary conditions along the surface. In order to obtain complete modelling of the effects of wave propagation, it is important to account for the surface topography, otherwise near‐surface effects, such as scattering, are not modelled adequately. Even if other properties of the medium, for instance randomization, can improve numerical simulations, inclusion of the surface topography makes them more realistic.
Normal moveout in focus1
Authors: de Bazelaire Eric , Viallix Jean Rene 
Published: April 2006

AbstractImproving the accuracy of NMO corrections and of the corresponding interval velocities entails implementing a better approximation than the formula used since the beginning of seismic processing. The exact equations are not practical as they include many unknowns. The approximate expression has only two unknowns, the reflection time and the rms velocity, but becomes inaccurate for large apertures of the recording system and heterogeneous vertical velocities. Several methods of improving the accuracy have been considered, but the gains do not compensate for the dramatic increase in computing time. Two alternative equations are proposed: the first containing two parameters, the reflection time and the focusing time, is not valid for apertures much greater than is the standard formula, but has a much faster computing time and does not stretch the far traces; the other, containing three parameters, the reflection time, like focusing time and the tuning velocity, retains high frequencies for apertures about twice those allowed by the standard equation. Its computing time can be kept within the same limits. NMO equations, old and new, are designed strictly for horizontal layering, but remain reliable as long as the rays travel through the same layers in both the down and up directions.An equation, similar to Dix's formula, is given to compute the interval velocities. The entire scheme can be automated to produce interval‐velocity sections without manual picking.
On small‐scale near‐surface distortion in controlled source tensor electromagnetics1
Authors: Qian W. 
Published: April 2006

AbstractBased on a Born approximation of a thin sheet integral equation, it is shown that small‐scale surficial heterogeneity significantly distorts the electromagnetic field excited by electric dipoles only when either the source or the receiver are located on the heterogeneities. When a surface heterogeneity is beneath the source, the associated distortion of the electromagnetic field is manifest as a change in the effective electric dipole moment. Hence the magnetotelluric transfer functions and impedance relations remain undistorted in this case. When a surface heterogeneity is beneath the receiver, the electric field is severely distorted, but the magnetic field is only slightly distorted. The impedance tensor is therefore strongly distorted, but the tipper vector is almost unaltered. Since the controlled source tipper is a function of 1D earth conductivity, it is proposed that tipper data should be used in the first stage of 1D interpretation. For a 1D earth, the tipper vector must always point towards the source and, in the near‐field limit, should have unit length. These two necessary conditions must be met by the measured tipper before it is interpreted one dimensionally.
Sign‐change correction for prestack migration of P‐S converted wave reflections1
Authors: Balch A.H. , Erdemir Cemal 
Published: April 2006

AbstractThe polarization direction or 'sign’ of reflected converted P–S waves depends upon the angle of incidence of the incident P‐wave. Sign reversal due to reversal of the angle of incidence is often encountered and is an impediment to P–S wave processing and imaging, because when P–S events or P‐S migrated images with mixed signs are stacked, destructive interference occurs. We have created and demonstrated a means of correcting for this reversal. To do this, a P‐wave angle of incidence is calculated for every point in the image space. This is done by calculating a P–S reflected waveform for every point, by extrapolating the reflected S‐wavefield backwards from the receiver line, and then cross‐correlating this waveform with the S‐wave reflections observed at the receiver line. A multiplier, (sgn α) is assigned to each point in the image space, where α is the angle of incidence of the P‐wave.The multiplier was applied to a set of prestack reverse time migration images derived from a cross‐borehole physical elastic model data set. The improvement in the stacked image when the sign correction is applied is spectacular. The P‐S image quality is comparable to, or better than, stacked migrated P‐P images.The method appears to be applicable to all reflection modes and to all recording geometries.
An analytic signal approach to the interpretation of total field magnetic anomalies1
Authors: Qin Shuang 
Published: April 2006

AbstractThe analytic signal (AS) is defined as the square root of the sum of the squares of the vertical and the two horizontal derivatives of the total magnetic field ΔT. This paper verifies theoretically that peaks of the AS correlate directly with their magnetic causative bodies and are positioned symmetrically over them, i.e. the main feature of the AS is that it is independent of the inclination of the magnetic field. This avoids the difficulties that are often faced in the conventional process of reduction to pole for ΔT, when the direction of magnetization of the causative bodies is not known. In addition, the AS has characteristics similar to the derivative features of the magnetic field, so that it is very sensitive to edge effects of the causative magnetic bodies. The theoretical derivations are tested by comparison with calculations on models, and, in a field example from Hunan Province, China, the AS is applied successfully to the interpretation of ΔT, whereas the conventional process of reduction to pole fails, due to the reverse magnetization of the causative body.
Cost‐effective 3D one‐pass depth migration1
Authors: Sollid A. , Arntsen B. 
Published: April 2006

AbstractA crucial point in the processing of 3D seismic data is the migration step, both because of its 3D nature and the computational cost involved. The efficiency and accuracy of 3D migration are determined by the wavefield extrapolation technique employed. Wavefield extrapolation based on second‐order differential operators of variable‐length is very efficient and accurate at the same time. Compared to migration based on the McClellan transform and operator splitting, the use of variable‐length second‐order differential operators offers significant advantages. The 3D migration operator has an almost perfect circular symmetry. No positioning errors in the 45° azimuth between the in‐line and cross‐line directions are evident. The method is, in practice, only limited by spatial aliasing and does not require expensive interpolation of data to reduce numerical artifacts. This reduces the computational cost of 3D one‐pass depth migration by a large factor.
The potential of a noise‐reducing antenna for surface NMR groundwater surveys in the earth's magnetic field1
Authors: Trushkin D.V. , Shushakov O.A. , Legchenko A.V. 
Published: April 2006

AbstractA method of non‐invasive NMR in the earth's field has been developed and is now used for groundwater surveys to depths of investigation of 100 m or more.A circular wire loop of diameter 100 m, laid out on the ground, is employed to excite and receive the NMR signal in the earth's field. However, in areas with high electromagnetic noise, the NMR measurements may be inaccurate.To overcome this problem, a noise‐reducing figure‐of‐eight‐shaped antenna, consisting of two touching coils each of diameter 50 m, has been utilized.Using this antenna, the NMR signal has been calculated for different depths of water‐saturated layers with various inclinations of the geomagnetic field. The model calculations and experimental data have been compared and found to be mutually consistent. The two‐coil antenna is shown to be suitable for studies at depths of up to 30–40 m, which is of practical importance for engineering geology.