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in the range of spacinf source to detector. When the ne~ns become thermal, they are <br />cap[ured by formation which in turn emits gamma rays. <br />Figure 2 depicts a Cuard tool which is a focused current device. It is used primarily in <br />either salt water or salt mud-filled well bores. The guard electrodes are mechanically and elec- <br />trically connected and extend 64 inches (more than ten times the measuring electrode spacing) <br />above and below the measuring electrode. Their function is to maintain an equal potential in the <br />well bore. The measuring electrode is usually five and one half inches long. It makes an excel- <br />lent tool for thin bed definition. It is the only resistivity device with the capability of measuring <br />the resistivity of the wall while operating in the immediate environment of a very low resistivity <br />liquid-filled bore hole. Salt water is such a low resistivity environment. <br />GAMMA RAY - DETECTOR <br />°. CASING <br />COIIA <br />R ' <br />- <br />_ <br />_ COIL <br />- <br />=~LOCATOR_ __ -_ <br />---= <br />--~ MAGNEi ~_ <br />_ <br />_ <br />- -- <br />_'-- ~ I COLLAR -~ _=- <br />- - - <br />_~ SPACER_ Ii ~ _ __ r <br />~ ~ ~ = GAMMA RAY - <br />NeuiRON- - <br /> <br />- - - -- = NEUTRON STACK <br />- - <br /> DETECTOR ; <br />_- -_ , .~ DETECTOR _~_ <br />_= _ = S OU RC E ~ =--- ---. <br />Figure 1 <br />G <br />J <br />4 <br />V <br />REGULATOR CO\TROL x <br />SECTION SECTION ~ <br />CONTROL <br />SECTION <br /> <br />H" GU.\FD ELECTF.`DE m <br />GA,NMA <br />DETECTOR <br />H" GUARD ELECTRODE <br />Figure <br />58 <br />