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- Volume 30, Issue 4, 1982
Geophysical Prospecting - Volume 30, Issue 4, 1982
Volume 30, Issue 4, 1982
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IN‐SITU INVESTIGATION OF SEISMIC BODY WAVE ATTENUATION IN HETEROGENEOUS MEDIA*
Authors P. J. NEWMAN and M. H. WORTHINGTONAbstractField experiments have been carried out to study the nature and magnitude of seismic wave attenuation for a variety of lithologies. In each survey two three‐component sets of geophones with wall clamping mechanisms were lowered down boreholes and signals originating from surface compressional and shear‐wave sources were recorded. The data collected were corrected for spherical divergence and analyzed to determine intrinsic attenuation. For situations of anomalous wavefront expansion and in cases where multiple reflection losses may be significant, corrections were supplied by a synthetic seismogram programme to improve the estimates of intrinsic attenuation.
Values of attenuation were obtained for pure sandstone, sandstone‐marl sequences and fissured‐unfissured chalk sequences. These formations were all near surface and relatively porous. Significant differences in the relative values of compressional and shear‐wave attenuation in the various lithologies were noted. In particular compressional absorption in the fissured zones of chalk was twice the absorption associated with the seismic velocities and the absorption of shear waves fluctuated much less. A large contrast in P‐wave attenuation was also observed between pure Bunter Sandstone and the sandstone‐marl sequence (absorption was over three times as small in the former). A smaller contrast was noted for shear attenuation.
The results obtained suggest that, for the relatively homogeneous formations such as pure sandstone and “unfissured” chalk, “shear” absorption was dominant over “bulk” absorption. In contrast bulk absorption was larger than shear absorption for other formations, e.g. the “fissured” chalk sequences and a “partially saturated” chalk section. Attenuation was found to be approximately proportional to frequency in all experiments.
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APPLICATION OF THE GENERAL COMPLEX COMPLIANCE MODEL TO THE DIRECT‐DETECTION PROBLEM*
By E. STRICKAbstractA methodology is outlined for using general complex compliance functions—which contain the constant‐Q case and any kind of fluid saturant as special cases—to determine the nature of that saturant from seismic reflections observed on seismic records. The important attribute of such a formulation based upon complex compliance functions is that the presence of any kind of saturant is indicated by the values of the parameters contained within the complex compliance function, the latter having the same mathematical form for all rocks under any conditions as long as it remains a crystalline solid.
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THE POTENTIAL OF SHEAR‐WAVE OBSERVATIONS*
Authors K. HELBIG and C. S. MESDAGAbstractShear waves can today be generated and observed, though not with the flexibility and the technical standard of compressional waves, and they can be identified in seismograms by various means. Their potential lies not so much in their lower velocity (corresponding—for the same frequency—to shorter wavelength and higher resolution) but in the fact that they probe the earth with stresses and strains that differ from those of compressional waves. Full utilization of the information potential of shear waves, therefore, requires combined use of P‐and S‐waves.
Complications in the combined use of different wave types should be regarded as opportunities to obtain additional information. A typical example is the observation that the depth of one and the same interface estimated on the bases of P‐ and SH‐reflections, respectively, can differ significantly. This discrepancy may be due to the anisotropy of a finely layered medium. Under favorable circumstances some of the parameters describing this anisotropy can be deduced from the different depth estimates and the curvature of the squared‐offset/squared‐time representation of the different reflections. Since in anisotropic media vertically polarized shear waves are significantly different from horizontally polarized ones, the combined observation of all three waves opens up additional possibilities.
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UPSWEEP SIGNALS WITH HIGH‐FREQUENCY ATTENUATION AND THEIR USE IN THE CONSTRUCTION OF VIBROSEIS® SYNTHETIC SEISMOGRAMS*
By B. M. GURBUZAbstractAn analytical relationship for the autocorrelation function of an upsweep with high‐frequency attenuation is used in the construction of synthetic seismograms.
Field experiments were conducted in two areas to investigate the attenuation of upsweep where the near‐surface materials were different. The results showed that the attenuation of high frequencies occurs at the source point depending on the near‐surface lithology.
The attenuation effect is usually neglected in the construction of the input wavelet of synthetic seismograms for Vibroseis data. In this study, the high‐frequency attenuation of upsweep was considered in the construction of the input wavelet for the synthetic seismogram in an area where the Vibroseis technique was used. The synthetic seismogram generated in this manner had a better correlation with the Vibroseis section than that of corresponding synthetics using minimum‐phase and the unattenuated autocorrelation wavelet of the upsweep.
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SUBSURFACE GRAVITY MEASUREMENTS AND DENSITY MAPPING IN THE MAILARAM COPPER MINES, ANDHRA PRADESH, INDIA*
Authors B. V. SATYANARAYANA MURTY, M. GHYASUDDIN and P. VENKAT REDDYAbstractGravity measurements were made in the Mailaram copper mines, Andhra Pradesh. The observations were distributed between the two shafts situated about 220 m apart and in the three levels up to a maximum depth of 100 m. Assuming a normal free‐air gradient, average densities for the three layers were determined as 2.631, 2.604, and 2.823 g cm‐3, respectively. Upon incorporating the weighted mean density values from measurements on samples, the free‐air gradients were found to be 0.315 mGal m‐1 for the second layer (i.e. between the first and second levels) and 0.2978 mGal m‐1 for the third layer (i.e. between the second and third levels). The density variation map obtained from the gravity data, the deduced anomalies, the weighted mean density values from measurements on rock samples, and the varying free‐air coefficients all suggest correspondence with the concentration of ore lodes.
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A METHOD FOR THE DETERMINATION OF THE THERMAL CONDUCTIVITY OF SANDSTONES USING A VARIABLE STATE APPROACH*
Authors K. M. STRACK, A. W. IBRAHIM, G. V. KELLER and C. H. STOYERAbstractCommon problems encountered during the determination of the thermal conductivities of tight sandstones are rock specimen‐to‐heat source contact and complicated sample preparation. An experimental technique using an electromagnetic heat source solves the direct contact problem between the heat source and the sample. Also, it simplifies the sample preparation and reduces the measurement time.
A CO2 laser operating in a pulsed mode is used as a heat source with about 500 W output power. Thus, heat losses due to radiation and air convection are negligible. Unpolished penny‐sized samples of tight sandstones are irradiated on one side and the temperature is measured on the opposite side. The temperature is recorded with 12‐bit accuracy by a digital data acquisition system. Carbon black is used to give the samples a uniform absorption.
The transient temperature data are acquired, processed, and interpreted with interactive computer programs. Transients for each sample can be stacked, to improve the signal‐to‐noise ratio, and normalized. The thermal conductivity is calculated from the rise portion of the transient using a ridge‐regression type generalized linear inversion scheme.
As many as 20 samples per hour can be measured with this simple but expensive set‐up. In the long run, this means a reduction of laboratory expenses. In addition, the resolution of this method is superior to other variable‐state measurement methods due to the real time controlled data acquisition and the numerical interpretation.
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TRANSIENT COUPLING BETWEEN FINITE CIRCUITS ON AN ANISOTROPIC CONDUCTING HALF‐SPACE*
Authors K. A. NABULSI and J. R. WAITAbstractThe frequency domain response of a grounded linear circuit is deduced. The model of the ground is a homogeneous conducting half‐space whose horizontal and vertical conductivities are specified. The source itself is formulated as an insulated current‐carrying wire of finite length lying on the surface that is grounded at its end points. The tangential electric field expressions are used to calculate the induced voltage between two additional electrodes that are connected by an insulated wire. Particular attention is paid to the transient coupling response in the potential circuit when the source current is a step‐function of time. Among other things, it is shown that the voltage response is a step‐function only if the circuits are at right‐angles and if the ground is effectively isotropic and nonpolarizable.
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QUANTITATIVE INTERPRETATION OF HORIZONTAL‐LOOP EM MEASUREMENTS USING A PERMEABLE SPHERE MODEL*
Authors S. S. RAI and S. K. VERMAAbstractAn interpretation scheme for horizontal‐loop EM measurements is presented for a permeable sphere model. The induced multipole moments are found to contribute significantly even at very low frequencies for a permeable conductor. The anomaly profiles are computed considering multipole excitation (up to 20) to study the effect of depth of burial and permeability of conductor. The anomaly half‐width along with the inphase and quadrature anomaly amplitudes allow direct interpretation of the parameters of the sphere. The above scheme is suitable for results of Dighem II (coplanar configuration), Slingram and Max Min II measurements.
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OPTIMIZATION OF SHORT DIGITAL LINEAR FILTERS FOR INCREASED ACCURACY*
More LessAbstractThe accuracy of short length digital linear filter operators can be substantially increased if the sampling interval as well as the abscissa shift are properly adjusted. This may be done by a trial and error process of adjustment of these parameters until the error made by the filter operator, applied to a suitably chosen test function, is smallest.
As an illustration of the application of this method, 7‐, 11‐ and 19‐point filters for the calculation of Schlumberger apparent resistivity from a known resistivity transform are designed. Errors with the new 7‐point filter are seen to be less than those with a 19‐point filter of conventional design. The errors with the new 19‐point filter are two to three orders of magnitude smaller than those made by the conventional 19‐point filter.
The new method should provide digital linear operators that allow significant improvements in accuracy for comparable computation efforts, or substantial reduction in computation for comparable accuracy of results, or something of both.
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Volumes & issues
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Volume 72 (2023 - 2024)
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Volume 71 (2022 - 2023)
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Volume 70 (2021 - 2022)
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Volume 69 (2021)
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Volume 68 (2020)
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Volume 67 (2019)
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Volume 66 (2018)
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Volume 65 (2017)
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Volume 64 (2015 - 2016)
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Volume 63 (2015)
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Volume 62 (2014)
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Volume 61 (2013)
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Volume 60 (2012)
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Volume 59 (2011)
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Volume 58 (2010)
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Volume 57 (2009)
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Volume 56 (2008)
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Volume 55 (2007)
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Volume 54 (2006)
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Volume 53 (2005)
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Volume 52 (2004)
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Volume 51 (2003)
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Volume 50 (2002)
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Volume 49 (2001)
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Volume 48 (2000)
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Volume 47 (1999)
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Volume 46 (1998)
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Volume 45 (1997)
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Volume 44 (1996)
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Volume 43 (1995)
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Volume 42 (1994)
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Volume 41 (1993)
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Volume 40 (1992)
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Volume 39 (1991)
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Volume 38 (1990)
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Volume 37 (1989)
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Volume 36 (1988)
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Volume 35 (1987)
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Volume 34 (1986)
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Volume 33 (1985)
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Volume 32 (1984)
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Volume 31 (1983)
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Volume 30 (1982)
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Volume 29 (1981)
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Volume 28 (1980)
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Volume 27 (1979)
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Volume 26 (1978)
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Volume 25 (1977)
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Volume 24 (1976)
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Volume 23 (1975)
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Volume 22 (1974)
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Volume 21 (1973)
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Volume 20 (1972)
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Volume 19 (1971)
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Volume 18 (1970)
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Volume 17 (1969)
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Volume 16 (1968)
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Volume 15 (1967)
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Volume 14 (1966)
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Volume 13 (1965)
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Volume 12 (1964)
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Volume 11 (1963)
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Volume 10 (1962)
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Volume 9 (1961)
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Volume 8 (1960)
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Volume 7 (1959)
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Volume 6 (1958)
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Volume 5 (1957)
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Volume 4 (1956)
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Volume 3 (1955)
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Volume 2 (1954)
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Volume 1 (1953)