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- Volume 23, Issue 3, 1975
Geophysical Prospecting - Volume 23, Issue 3, 1975
Volume 23, Issue 3, 1975
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MICROGRAVIMETRY FOR ENGINEERING APPLICATIONS*
By A. A. ARZIAbstractMicrogravimetric surveys for verifying bedrock soundness at the foundation of a nuclear power plant and for delineating a zone of small cavities and tracing grout emplacement at the foundation of a large cooling tower are described and used to illustrate an analysis of the method.
It is pointed out that the difference between the usual large scale gravity surveys and microgravimetric surveys for engineering applications involves significantly more than a mere scaling down of grid spacings and precision tolerances. In addition to the smaller scale, the special physical environments for such microgravimetric surveys as well as strict timing and economy considerations and the demand for conclusive answers to specific questions require careful revisions of field procedures as well as data reduction and interpretation.
Where properly applied, microgravimetric surveys—usually in conjunction with selective drilling—yield reliable subsurface information with a high resolution that is still limited by the gravimeter noise. Costly extensive drilling can thus be avoided. The method offers special advantages over other subsurface exploration methods in a wide variety of applications.
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NEW RESULTS IN RESISTIVITY WELL LOGGING*
By A. ROYAbstractTwo independent theoretical analyses show that, spacing for spacing, the two‐electrode normal device investigates deeper than the seven‐electrode focused Laterolog 7. The first analysis, published earlier, compares the radii of investigation of the different sondes from the portion of the ground that contributes maximum to, or fifty percent of, the measured signal. The second relates to the nature of the departure curves for the two sondes for an infinitely thick resistive formation pierced by a bore hole. With increasing spacing, the apparent resistivity for the normal device rises much faster and asymptotically approaches the true resistivity much earlier than that for Laterolog 7. These two analyses also prove that, in a Laterolog 7, the radius of investigation increases as O1O2 decreases relatively to A1A2. Since any complex point electrode system is equivalent to a superposition of elementary dipoles or two‐electrode devices, it follows that no resistivity sonde of discrete point electrodes can have a radius of investigation larger than that for the normal device of the same spacing. Laboratory measurements in a model tank confirm these theoretical results.
For thin highly resistive formations we find from theoretical and model studies that the normal device is markedly superior to Laterolog 7 as it gives apparent resistivity values much closer to the true. It has other advantages as well. Thus, our results on the comparative performances of the normal and Laterolog 7 devices are at variance with those published so far since the introduction of Laterolog 7 in 1951.
For reasons given in the text, the chief characteristic of Laterolog 7—namely, focusing the current from the central electrode in a thin horizontal sheet into the target formation—does not endow it with any special advantage. The main reason is that the measured potential in a Laterolog 7 is caused not only by the current focused into the target formation but also by the currents from the outer two electrodes which flow, due to the same focusing process, entirely through the adjacent formations and the mud column. Indeed, the contribution to the measured signal by the outer two power electrodes is considerably larger than that from the central.
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AN INTERACTIVE COMPUTER/GRAPHIC‐DISPLAY‐TERMINAL SYSTEM FOR INTERPRETATION OF RESISTIVITY SOUNDINGS*
More LessAbstractA fast computer‐procedure giving the apparent resistivity curve as well as the partial derivatives with respect to the layer‐parameters is presented. It is based on the linear filter method developed by D. P. Ghosh in 1971.
The sampling frequency is 10 points per decade, and 3 decades are covered. The maximum relative error is less than 10−3, and in most cases orders of magnitude smaller. The computation time on a CDC 6400 for one curve given in 30 points ranges linearly from .17s for a two‐layer case to .36s for a ten‐layer case.
The procedure is used to plot master curves interactively on a graphic display terminal (Tektronix 4010) connected to the CDC 6400. By trial‐and‐error adjustments a set of layer‐parameters is found, giving essentially the measured curve.
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A STUDY ON THE POLES AND ZEROES OF STEFANESCU'S RAISED KERNEL FUNCTION
More LessAbstractThe theory of geoelectric sounding exploration is based on Stefanescu's kernel. In order to study the analytic properties of the kernel, necessary and sufficient conditions on it's poles and zeroes are developed. A distinction is made between “physical” and “perfect” models.
It is shown that a physical model may be determined from either the set of poles or the set of zeroes of the raised kernel function at the top of the model. These roots may be of desired multiplicities.
For determining a perfect model, both sets of poles and zeroes are required and they all have to be simple ones.
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QUANTITATIVE INTERPRETATION OF A COMBINED ELECTROMAGNETIC AND MAGNETIC HELIPORTED SURVEY FOR THE SEARCH OF ECONOMIC MAGNETIC TACONITE*
By M. K. SEGUINAbstractThe present paper shows the practical use of a combined electromagnetic and magnetic helicopter transported system in the search for steeply dipping high‐grade magnetic taconite bodies (magnetic ribbons). The principles of operation of the heliported system and the essential parts of the instrumentation are described. The quantitative interpretation of the data allows the geophysicist to calculate the tonnage of magnetite within the iron ores and to obtain an approximate idea of their grade. According to the tonnage and grade calculations obtained from the surveys, three regions in the west‐central sector of the Labrador trough (Wishart Creek, Knob Lake Ridge, and Swampy Bay areas) appear to be the most interesting locations for the site of a magnetic taconite deposit (the Howells River area, which has been evaluated previously, is excluded from the discussion). The tonnages calculated by this geophysical method are not to be compared with probable or possible mineable ore reserves, but are only rough estimates in terms of mining engineering standard.
The physical parameters determined with such a system are also used as a tool for indirect geological mapping of the areas flown when interpreted carefully. Practical examples related to results obtained on the magnetic iron formations of the Schefferville mining district are presented.
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TRANSIENT ELECTROMAGNETIC RESPONSE OF A SPHERE IN A LAYERED MEDIUM*
By T. LEEAbstractThe Galerkin method of solving integral equations is well suited to the solution of the integral equations describing the transient response of a sphere embedded in a layered medium, which is excited by a large co‐axial loop.
The transient response is calculated by transforming the steady state solutions obtained in the frequency domain.
The analysis shows that the scattering matrix is extremely diagonally dominant and the maximum number of modes required to obtain convergence does not rapidly increase with frequency. The number of modes required is about eight. This type of scattering matrix can be taken to be an expression of the principle of elementary superposition. This principle is reflected in the decay curves. These show that the early part of the decay curves asymptotically approach the decay curves to be expected for a layered structure without the sphere. The slope of the latter stages of the decay curve gives a decay constant that is the same as was obtained for spheres in free space excited by planar or dipolar sources.
The point of departure in time of these curves from the layered ground curves is delayed either by placing the sphere at a greater depth or by placing a more conductive overburden above the sphere.
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THERMAL INFRA‐RED TECHNIQUE APPLIED TO MODERN INVESTIGATION*
Authors J. THOMAS, J. P. PEDEUX and C. ARNAUDAbstractDuring the last few years a substantial improvement has been made in the development of the thermal infra‐red scanner so that 1.5 milli‐radian resolution is now commonly obtained. Direct transfer of information on 70 mm film through fiber glass transcoder allows immediate and refined interpretation.
Although the collected data represent the image of thermal anomalies within a pellicular section of the earth's crust, it is proved that because of “per ascensum” phenomena, deep buried sources induce surface thermal changes. The difficulty lies in showing an absolute relationship between these deep phenomena and thermal surface occurrences.
A careful examination of several cases related to hydrology, karstic geology, civil engineering, pollution, hot springs should help the geophysicist by providing him a new tool for shallow investigations. Thermographics with interpretation are shown to support this statement.
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THE GRAVITATIONAL ATTRACTION OF VERTICAL TRIANGULAR PRISMS*
More LessAbstractFormulae for the gravitational attraction, at a point, of vertical triangular prisms with oblique ends are presented. They are suited to computation on an electronic computer and are useful for determining the thickness of sediments in a sedimentary basin by iterative means.
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FLAT SPOT EXPLORATION*
Authors M. M. Backus and R. L. ChenAbstractThe hydrocarbon‐brine contact produces a flat reflection, unconformable with the lithologic reflections from the trap boundaries, and over a limited area bounded by structural contours. When it can be reliably detected and mapped, the flat spot can provide (i) a reasonably unambiguous indication and areal extent of a reservoir and (ii) an estimate of reservoir thickness.
The gas‐brine contact in thick reservoirs in offshore elastic sections is the easiest target. An example of gas mapping is presented in the paper. Other reservoirs represent a continuum of increasingly elusive targets.
Increasing the range of applicability of flat spot exploration will require (i) increases in signal/noise and multiple ratio, increases in the three dimensional depth point density (or grid density), increased resolution and static and dynamic correction accuracy, and (ii) processing and interpretation aimed directly at flat spot mapping.
Low relief structural and stratigraphic traps should provide the most attractive targets. The approach should be at least theoretically feasible, though not necessarily cost effective, for most major reservoirs with a well‐behaved hydrocarbon brine contact.
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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)