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A639 Induction Conductivity / Gamma Tool APPLICATIONS Conductivity is the inverse of resistivity. Induction logging systems were originally designed to facilitate resistivity measurements in oil-based drilling mud, where there is no conductive medium occurred between the tool and the formation. It is also often used in dry holes or when the borehole fluid is very fresh. The purpose is, as usual, to measure resistivity or conductivity in order to calculate porosity and to interpret lithology. Induction is useful in several types of ground investigation. Mineral Investigation The resistivity of rocks depends on several factors. Rocks are usually poor conductors. Rock resistivity is determined, primarily, by porosity and the salinity of the pore fluid. Clay minerals reduce the resistivity of the formation. Some alteration processes result in the formation of clay minerals, reducing resistivity, while others reduce rock porosity and increase formation resistivity. Decreases in resistivity may be a result of either significant concentration of conductive metal sulphides or oxides. Fracturing increases effective porosity and can, therefore, decrease resistivity. The complexity of resistivity interpretation means that complementary information from other geophysical measurements or other geological logs is usually required. However, massive sulphide deposits generally consist of conductive ore zones and resistivity logs are often used to delineate these ore zones. Coal Investigation Coal usually has high resistivity. This may be reduced according to its porosity, clay, and water content. The porosity of coal varies with rank and so its resistivity increases from lignite to semi-anthracite and then decreases slightly with anthracite. Clay inclusions reduce the resistivity of coal. Clay-rich lithologies, such as shales, have low resistivity. Clean, relatively dry sandstone has high resistivity. This may be reduced according to porosity, salinity, and clay content. Resistivity (or conductivity) logs may also indicate oxidation of a coal seam or alteration by an intrusion. Ground-water Investigation The Archie equation defines the relationship between porosity and resistivity in reservoir rocks: for more information on this please contact us. The A634(E)Induction Conductivity tool is designed for measurement of electrical conductivity of rocks in boreholes. High stability and exceptionally wide dynamic range permit precise measurements of conductivity of sand-clay layers and also mineralised water-soaked sands. The tool may be used in water filled, dry and plastic cased boreholes. The response of the probe is practically instantaneous (<0.5 s). Gamma Gamma rays are very high frequency electromagnetic radiation and are derived from the radioactive decay of various elements. In borehole applications the most common radioactive elements are radioisotopes of potassium, thorium and uranium. Gamma Ray logs provide a clear indication of variations in lithology and also accurately define bed thickness. Shaly bands are obvious and can be readily identified because clay content results in a high gamma count. The Gamma Ray log is used for identification of coal, as coal typically exhibits a low counting rate. In this application a high sensitivity Gamma tool generally provides best results. In petroleum applications the higher gamma activity of the clay size fraction is due to the potassium contained in clay minerals, fine mica and feldspars. This is not only useful information in its own right, but is a crucial parameter in the accurate interpretation of Density and Neutron logs. Gamma ray tools are an obvious choice in uranium exploration and development. The relationship between uranium content and count rate may be employed to determine ore grades. Ground water and engineering applications utilize the gamma ray log for lithology indicators and strata thickness as well as clay content, to determine permeability and rock strength. 1 to 300mS/m tool is intended to suit the requirements of those concerned with relatively low conductivity. 300mS/m to 1000S/m is suited to the examination of highly conductive metallic minerals and ores. Suitable for mineral exploration, mine development and planning. Specifications
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