Detection of Radioactive Cement in Cased Wells

Abstract

In a previous article we have described a technique for measuring therelative intensities of gamma rays from the radioactive elements occurringnaturally in geological formations along the walls of a borehole. The apparatusconsisted of two Geiger-Muller tubes mounted inside a pressure tight chamber.Each counter tube was coupled separately to an amplifier and a frequency meterat the surface by means of a multi-conductor cable upon which the chamber wassuspended. The output currents of the two frequency meters were recorded on aphotographic paper as the chamber was moved in the hole. These traces showedremarkable correlation with the geologic formations, as evidenced by comparisonwith commercial electrical logs. Owing to the penetrating power of the gammarays, logs may be taken in both open and cased boreholes. The apparatus has recently been changed somewhat. One of the counter tubes hasbeen removed and a stage of amplification has been added inside the chamber.This arrangement seems to give a more favorable signal-to-noise ratio andsatisfactory logs can be now made in cased holes and also in open holes aspreviously. Briefly described, the chamber moving in the hole is made of 372-in. drill pipe and is about 10 ft. long. There is an additional extensionabout 2 ft. long at the top, inside which cable connections are made. TheGeiger-Muller tube is mounted inside the bottom end of the chamber. Above thisis the vacuum-tube amplifier with its batteries and transformer. In the toppart of the chamber, small 45-volt batteries are mounted, which in part supplythe high voltage for the counter tube. Additional voltage IS supplied from thesurface through the multi-conductor cable upon which the chamber is suspended.Also, the output of the transformer in the single-stage amplifier is coupled toanother single-stage amplifier at the surface through this cable. T.P. 1113