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- Volume 4, Issue 3, 1986
First Break - Volume 4, Issue 3, 1986
Volume 4, Issue 3, 1986
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Neutron porosity devices - what do they measure?
By D.V. EllisIn exploration, seismie methods are used to locate interesting formations. Well logging measurements are often relied upon to make the quantitative determination of formation petrophysical properties in these formations. One important formation property is porosity, which can be estimated using acoustic or nuclear measurements. The nuclear radiation scattering techniques used for its estimation employ gamma ravs or neutrons. Thc motivation for the use of gamma rays for the determination of formation porosity is straightforward. The electron density of the formation affects the scattering and transmission of gamma rays in a well known manner: the logarithm of the gamma ray flux at the detector depends linearly on the reciprocal of the electron density. A measurement of the gamma ray flux yields the density. To convert the density measurement of the formation into porosity, the appropriate electron density of the rock matrix must be known as well as that of the formation fluid, although this second factor is of somewhat less importance than the former. Therefore it is important to know the rock lithology in order to use the appropriate matrix density for extracting the correct porosity value from the density measurement. The application of neutron scattering to determine porosity is somewhat more complicated due to the nature of neutron interactions with matter. In porosity logging applications, the most important phenomenon is elastic scattering of the neutrons with the formation nuclei. In this process the kinetic energy of the neutron is reduced at each scattering depending on the scattering angle and, more importantly, the mass of struck particle: the closer its mass is to the neutron mass, the more efficient the collision can be in reducing the neutron energy. For this reason, hydrogen is the most efficient moderator of neutrons in earth formations. A determination of the moderating efficiency of a formation should be related to its hydrogen content. A measurement of the spatial distribution of multiply scattered low energy neutrons is sufficient to characterise the moderating efficiency of the formation. As in the case of density logging, the accurate interpretation of the neutron measurement depends on a knowledge of the lithology of the formation. However, unlike the density measurement, the nature of the fluid filling the pores is quite important. In this artiele the measurement principles of the neutron porosity device will be reviewed and related to some useful formation parameters. The interpretation of the measured parameters in terms of porosity will be discussed and shown to be dependent on the rock lithology, the pore fluid composition and the presence of shales or c1ay minerals.
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Resistivity method applied to the investigation of earth embankments
More LessElectrical resistivity is one of the most important and interesting physical parameters of an earth embankment in that it can be related to the lithology and geomechanical properties. By applying the electrical profiling method, resistivity measurements were made along two typical sections of river embankment: one along the Danube embankment and the other along the Tisa embankment. The resistivity was measured during the two hydrologically extreme periods, at low and high water level. Statistical analysis of the data made it possible to give an interpretation of the geomechanical conditions along the two sections studied. The attempt to use geophysical methods in investigating the condition of earth embankments is based on the assumption that the geophysical parameters of the embankment vary with the hydrological state which controls its geochemical properties. Two factors are favourable for the geophysical investigation of an embankment: the body has a regular, constant shape and much is known about its lithology and the hydrogeological situation. Naturally, the embankment includes heterogeneous lithological and geomechanical zones of various dimensions. These zones may reasonably be expected to give rise to geophysical anomalies, whose amplitudes will vary with the hydrological state of the embankment. The moisture content of the embankment is controlled by variations in the river water level, the water table on the landward side of the embankment and the rainfall. The effects of moisture will be amplified in less compact or geomechanically weaker zones. Compared to other geophysical parameters, resistivity is the most sensitive to moisture content. Figure 1 shows the relationship between the resistivity of the embankment material, which consists of clay and sand, and the moisture content measured in the laboratory. Extremely high resistivity values were obtained for dried cores, and the lowest resistivity values were obtained for wet and water-saturated cores. It may be concluded that systematic measurement of resistivity throughout an embankment would have a high probability of locating zones with the highest moisture variation. Naturally, these changes will be associated with changes in porosity or geomechanical properties (relative compactness) of the embankment.
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Volumes & issues
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Volume 42 (2024)
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Volume 41 (2023)
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Volume 40 (2022)
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Volume 39 (2021)
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Volume 38 (2020)
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Volume 37 (2019)
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Volume 36 (2018)
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Volume 35 (2017)
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Volume 34 (2016)
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Volume 33 (2015)
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Volume 32 (2014)
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Volume 31 (2013)
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Volume 30 (2012)
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Volume 29 (2011)
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Volume 28 (2010)
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Volume 27 (2009)
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Volume 26 (2008)
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Volume 25 (2007)
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Volume 24 (2006)
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Volume 23 (2005)
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Volume 22 (2004)
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Volume 21 (2003)
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Volume 20 (2002)
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Volume 19 (2001)
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Volume 18 (2000)
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Volume 17 (1999)
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Volume 16 (1998)
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Volume 15 (1997)
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Volume 14 (1996)
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Volume 13 (1995)
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Volume 12 (1994)
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Volume 11 (1993)
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Volume 10 (1992)
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Volume 9 (1991)
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Volume 8 (1990)
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Volume 7 (1989)
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Volume 6 (1988)
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Volume 5 (1987)
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Volume 4 (1986)
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Volume 3 (1985)
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Volume 2 (1984)
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Volume 1 (1983)