Chapter%2011 - Petrophysics MSc Course Notes 11. Total...

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Petrophysics MSc Course Notes Total Gamma Ray Log Dr. Paul Glover Page 98 11. THE TOTAL GAMMA RAY LOG 11.1 Introduction The gamma ray log measures the total natural gamma radiation emanating from a formation. This gamma radiation originates from potassium-40 and the isotopes of the Uranium-Radium and Thorium series. The gamma ray log is commonly given the symbol GR . Once the gamma rays are emitted from an isotope in the formation, they progressively reduce in energy as the result of collisions with other atoms in the rock ( compton scattering ). Compton scattering occurs until the gamma ray is of such a low energy that it is completely absorbed by the formation. Hence, the gamma ray intensity that the log measures is a function of: The initial intensity of gamma ray emission, which is a property of the elemental composition of the rock. The amount of compton scattering that the gamma rays encounter, which is related to the distance between the gamma emission and the detector and the density of the intervening material. The tool therefore has a limited depth of investigation. Note that the gamma ray measurement device accepts gamma rays from almost a hemisphere that includes the formation and the drilling mud between the formation and the sensor. Gamma rays may therefore come from the formation at any angle from horizontal to almost vertically, and indeed may come from the drilling mud itself (beware: some drilling muds are very radioactive!). The gamma ray log is combinable with all tools, and is almost always used as part of every logging combination run because of its ability to match the depths of data from each run. 11.2 Principles The tool consists simply of a highly sensitive gamma ray detector in the form of a scintillation counter. The scintillation counter is composed of a thalium activated single sodium iodide crystal backed by a photomultiplier. When a gamma ray strikes the crystal a small flash of light is produced. This flash is too small to be measured using conventional electronics. Instead, it is amplified by a photomultiplier, which consists of a photocathode and a series of anodes held at progressively higher electrical potentials, all of which are arranged serially in a high vacuum. The flash of light hits the photocathode and causes a number of primary electrons to be produced. These few electrons still represent too small a signal to be measured. The primary electrons are accelerated towards the first anode. For every electron that hits the anode, a number of secondary electrons are emitted (between 4 and 8 usually). These electrons are accelerated towards the next anode, where each of the secondary electrons produce even more secondary electrons. This process is repeated for each of say 10 anodes. If 6 electrons are emitted at each anode for each incident electron, we can see that a single incident gamma ray ultimately produces 6 10 = 60,466,176 electrons, which represents a current that can be amplified further by conventional amplifiers.
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This note was uploaded on 10/30/2011 for the course PETROLEUM Short cour taught by Professor Dr.paulglover during the Winter '11 term at University of Aberdeen.

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Chapter%2011 - Petrophysics MSc Course Notes 11. Total...

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