DETERMINATION OF CASING THICKNESS USING A GAIN STABILIZED GAMMA RAY SPECTROSCOPY TECHNIQUE

摘要

A method of well logging utilizing gamma radiation serves to measure the thickness of the casing in a well borehole. The photoelectric absorption by the iron in the casing of gamma rays scattered from the formation varies as a function of the energy of the gamma radiation. The detected gamma spectrum below about 350 KeV from the surrounding earth formation is preferably divided into two energy ranges or windows, and a ratio between the two indicates the thickness of any intervening steel casing material. With suitable scale factors in calibration of the system, casing thickness can be determined from the ratio of the summed gamma count rate in the range of about 35 to about 120 KeV divided by the count rate in a higher energy range from about 180-325 KeV. A radiation source (22) in the sonde (10) emits gamma photons into the formation adjacent the borehole and these are scattered back toward a counting device.