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- Volume 20, Issue 1, 1972
Geophysical Prospecting - Volume 20, Issue 1, 1972
Volume 20, Issue 1, 1972
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INTERPRETATION OF GRAVITY ANOMALIES BY NON‐LINEAR OPTIMISATION*
More LessAbstractThe interpretation of a gravity anomaly in terms of the shape of the anomalous body is a non‐linear problem and may, therefore, be carried out using non‐linear optimisation techniques. The formulation is extended to include cases where the density contrast and the regional background are also unknown. For a given model the objective function is provided by the discrepancy between the observed anomaly and the calculated anomaly due to the model. Given an initial model, the optimisation procedure searches for a minimum of the objective function by an iterative adjustment of the parameters. A number of suitable objective functions is given. The behaviour of these functions in the parameter hyperspace is quite complex. Accordingly, direct search methods should be employed at the early stages of the search, changing to gradient methods at later stages. The use of constraints is also necessary to ensure the geological feasibility of the model. The required computer time may be largely reduced by careful programming. Two examples of interpretation by optimisation methods are given.
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CORRECTION OF ACCURATE GRAVITY SURVEYS BY CAREFULLY OBSERVED VERTICAL GRADIENTS OF GRAVITY*
Authors S THYSSEN‐BORNEMISZA, E. GROTEN and W. O. BAZHAWAbstractGravity surveys in western Texas with station spacing of about 400 m were complemented by carefully observed vertical gradients of gravity making use of specially designed and automated instrumentation. The presented areal surveys of 51 and 52 stations taken in relatively flat terrain are parts of a large survey of close to 1000 stations complemented by an equal number of vertical gradient measurements. Quite irregular anomalous vertical gravity gradients surpassing 10 microgal/m were often encountered. Assuming the causative density contrasts to be located not more than 10 m below ground surface then free air correction errors of ± 0.1 mgal and more must be accepted. From a practical point of view there seems to be no other way to control such unpredictable errors than to carefully observe the local variation of vertical gravity gradients with adequate equipment. Making use of very closely spaced gravity measurements to derive these anomalous features seems more costly and cumbersome.
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THREE‐DIMENSIONAL OPTIMUM MULTICHANNEL VELOCITY FILTERS*
By PETER HUBRALAbstractDesign procedures and characteristics of three stacking filters are discussed which may find application in various three‐dimensional velocity filtering problems. These filters are derived in the time‐domain as optimum multichannel Wiener filters. Random stationary functions are assumed as stochastic models for the seismic traces. All power and crosspower spectra which are the basic elements of the multichannel normal equations are statistically averaged according to specific three‐dimensional considerations. Various properties of the input traces may be incorporated in the design of the optimum filters. With fairly general assumptions about the input these filters are deterministic in the sense that they are applicable to a broad class of input traces with similar statistics in amplitudes and arrival times of signals.
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A FOURIER METHOD FOR THE REGIONAL‐RESIDUAL PROBLEM OF POTENTIAL FIELDS*
More LessAbstractAn equivalent stratum model is used to provide an explanation for the power spectrum characteristics of potential fields.
The power spectrum of observed magnetic fields is found to consist of effects due to sources which can be represented by apparent monopoles and dipoles. Similarly, the power spectrum of observed gravity fields is found to consist of two groups of effects. A matched filter is proposed whereby the effects due to the two components in the potential field can be separated. As a consequence of the established theoretical expressions a scheme is suggested whereby the aliasing power of sampled data can be estimated. Also, the concepts of downward continuation, reduction to the pole, and reduction to pseudo‐gravity of magnetic fields are re‐examined in light of the theoretical expressions due to the equivalent stratum model.
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SEISMIC NOISE MEASUREMENTS IN A GEOTHERMAL AREA*
Authors T. T. GOFORTH, E. J. DOUZE and G. G. SORRELLSAbstractGeneration of electrical power through the use of natural steam obtained from the Earth has the potential to be a significant source of pollution‐free power. Successful future exploitation of geothermal power depends upon the development of cost‐effective exploration techniques. A field experiment in the Imperial Valley of California indicates that there is a close empirical relationship between hot water deposits at depth and an anomalously high seismic background level at the surface. If this relationship proves to be a general one, it will serve as the basis of a simple and inexpensive method of geothermal exploration.
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GOSSES BLUFF AIRBORNE MAGNETIC SURVEY*
By G. A. YOUNGAbstractAn airborne magnetic survey of an area of 580 km2 centred on Gosses Bluff was flown in 1968. This survey formed part of a joint project by the Australian Bureau of Mineral Resources and the United States Geological Survey to evaluate the hypothesis that Gosses Bluff is of impact origin.
Analyses of the magnetic data show that shallow sources of magnetic disturbance flank the Bluff on its southern side. Interpretation of the magnetic anomalies indicates their source to be shock‐melted breccias of possible Jurassic age.
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INVESTIGATION OF SOME NON‐LINEAR PHENOMENA OF INDUCED POLARIZATION USING A PHYSICAL‐AND‐MATHEMATICAL MODEL*
Authors G. FINZI‐CONTINI and G. BENVENUTIAbstractThe electrical behaviour of a polarizable sample formerly studied (Time‐Domain) through both the classical and the condenser methods is simulated using the theoretical responses given by a three dimensional physical‐and‐mathematical model, which has been proposed in a previous paper by one of us on the basis of certain hypotheses.
The apparent capacitance responses of the sample are qualitatively simulated in order to tackle the problem under this point of view; moreover, an attempt to obtain a quantitative simulation of experimental data within the limits of the present reliability of the model, using theoretically obtained Model Master Curves characterized by dimensionless parameters, is done.
This research is carried out in the frame of a broader study referring to a possible scheme of simulation of the so‐called induced ionic polarization, concerning a number of idealized pseudo‐reversible, potentially irreversible, non‐linear phenomena.
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TWO‐DIMENSIONAL MASS DISTRIBUTIONS FROM GRAVITY ANOMALIES*
Authors I. R. QURESHI and F. M. IDRIESAbstractThe oscillatory character of the solution obtained by Qureshi and Mula's method for the direct determination of two‐dimensional mass distributions from gravity anomalies is examined.
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THE STUDY OF STREAMING POTENTIALS ON FISSURED MEDIA MODELS*
Authors V. A. BOGOSLOVSKY and A. A. OGILVYAbstractIn massive rocks ground waters mainly flow in fracture zones whose permeability greatly changes depending on their filling. When studying ground‐water flow in fissures, the results of observations of the electric fields of filtrational origin—which, in this case, considerably differ from those in porous media—can be used. Therefore the authors have made experiments on fissured media models. The measurements have been made in a special filtration tube with the fissured media simulated by a system of quartz glass plates. The spacings between plates were regarded as fissure widths. The observations have been made in fully open “fissures” and in those partially filled with sand or sandy‐clay material.
These experiments have resulted in establishing a dependence between the values of streaming potentials and pressure drops. The SP values have particularly been found to decrease with the opening of “fissures”. The most intensive electro‐filtrational fields were observed at 40 per cent filling with medium grained sand. Additions of argillaceous material to the sand filler brought about sharp reductions in the intensity of electro‐filtrational potentials.
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INTERPRETATION OF THE GRAVITY ANOMALY OVER A CAUSATIVE BODY WITH CIRCULAR SYMMETRY*
By M. K. PAULAbstractThe interpretation of the gravity anomaly on a horizontal plane over a causative body having circular symmetry about a vertical axis is considered from a rather unconventional approach. As the analytical expression for the gravity effect of a circular body assumes a closed form only on the axis of symmetry, the interpretation in this approach is carried out with the anomaly profile along the axis—which leads to simpler and faster computation.
A numerical method is developed for computation of the anomaly profile along the vertical axis from the horizontal radial profile of the symmetric anomaly by upward continuation. Provision is also made for an end correction when the radial profile has only a limited extension.
Some simple geometrical shapes are assumed for the causative body. Its parameters are then determined from least squares fitting of its gravity effects to the observed (upward continued) vertical profile (i) by the steepest descent method and (ii) by the Newton‐Raphson method.
Some applications of these methods are demonstrated.
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RESISTIVITY PROFILES OVER SOME DYKES OF DHANBAD*
More LessAbstractA number of resistivity profiles over dolerite and metadolerite dykes have been taken using Wenner and Schlumberger configurations around Dhanbad in order to find the potential of these for water accumulation. These dykes give rise to both resistivity highs and lows as compared to the background. The results are discussed in the light of known theories. Some suitable sites for water accumulation have been delineated.
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“REMOTE SENSING”— AN EVALUATION OF ITS IMPACT ON EARTH SCIENCE—*
By R. CASSINISSummary“Remote sensing” techniques have been used for many years as an exploration tool; broadly speaking, any method using sensors not directly connected to the earth's surface falls into this group.
It is preferable, however, to use the term solely with reference to a number of new sensors which have recently become available to earth scientists for the detection of various radiations of the electromagnetic spectrum in the wave length range from 0.1 micron (UV) to some centimeters including the visible and infrared regions.
Radiations artificially induced in the earth (active sensing) or spontaneous radiations (passive sensing) can be detected. Some of the bands investigated seem promising for application to geological surveys, especially using sensors on aircraft and satellites, which allow synoptic and large scale investigations.
A brief account is given of the results already achieved and of future developments such as the ERTS and manned satellite (post Apollo) “earth observation” programmes.
The potential role of Europe in the field is discussed.
Details are given of some applications of thermal I.R. imagery to geological and vulcanological problems, with special emphasis on the research carried out in Italy.
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PHYSICAL ASPECTS OF THE “AIRPULSER” AS A SEISMIC ENERGY SOURCE*
More LessAbstractDuring the last few years the airpulser, or air gun, has become very common as an energy source for marine seismic surveys. This paper describes the physical processes which take place during the operation of the pulser and develops theoretical results concerning the energy and frequency of the radiated signal and the amplitude decay of the secondary bubble pulses. The theory takes into account the presence of the airpulser itself which is assumed to be a rigid sphere within the bubble of released air. The theoretical results are combined and compared with measurements made of the pressure within the airpulser, the acceleration of the body of the pulser, and the amplitude and frequency of the signal radiated into the surrounding water. A formula for calculating the bubble frequency is given and a diagram made of the energy partition between mechanical losses, radiated energy, etc. Finally, a comparison is made of the energy release from the airpulser with that from TNT.
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INDUCTION ANOMALY DUE TO AN ELONGATED COVERED ORE‐ZONE EXCITED BY A LONG CURRENT CARRYING CABLE*
Authors J. G. NEGI, C. P. GUPTA and U. RAVALAbstractA detailed analysis of the electromagnetic anomaly due to an elongated mineralized zone of low resistivity in presence of a fixed‐transmitter (a long cable carrying an, alternating current IeIωt) is presented where (i) the host rock is not highly resistive and might contain some disseminated mineralization, (ii) the ore‐body has an inhomogeneous conductivity, (iii) there is a contrast in magnetic properties of the ore‐body and the host rocks, and (iv) source is close to the conducting system i.e., anomalous zone is lying shallow.
The numerical computation of the generalized analytical expressions have been made. The selective screening behaviour of the cover is delineated and suitable frequency ranges for maximum detectability of a covered conductor have been obtained. A paradoxical decrease of the in‐phase component of the response function with the increase of the core conductivity has been found for large conductivities of the cover. For moderate values of response parameter the inhomogeneity in the conductivity is found to significantly affect the induced field. The present analysis of the various features of the secondary field will aid to the existing interpretation sensitivity of the induction prospecting data for porphyry conducting ore deposits with zonal wall‐rock alteration and sulphide distributions.
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LES COURBES DAR ZARROUK ET LES SONDAGES DIFFÉRENTIELS DE RÉSISTIVÉ*
By E. SZARANIECAbstractDifferential and difference soundings of resistivity for real, imaginary and complex values of the newly introduced parameters ν and μ are considered. The transverse and longitudinal differential soundings, known up to the present, correspond with the values ν= 1 and ν=— 1.
The point of departure is the generalization of the concept of Dar Zarrouk parameters and the dimensional analogy between D.Z. resistivity and transverse and longitudinal differential resistivities. Properties of generalized D.Z. curves as well as the method of their construction are given. The examples of differential and difference curves are presented and the properties of these curves are discussed; particulary the effective spacings are determined. The problems of the realization of differential and difference soundings are considered; the bisymmetrical and asymmetrical arrays are proposed as a practical ones.
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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)