Determination of casing thickness using a natural gamma ray spectroscopy technique

摘要

A method of well logging utilizing naturally occurring gamma radiation is disclosed. In the preferred and illustrated embodiment, the thickness of the casing in a well borehole is measured. The photoelectric absorption of gamma rays from the formation by the iron in the casing 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 to indicate 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 naturally occurring 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.