Natural gamma ray spectrum analysis and filtering technique

摘要

Disclosed are a method and a system for well logging in which gamma radiation detected in five energy windows is converted into a log of thorium, uranium and potassium (Th, U, K) in a process which makes use of an adaptive filter determined both by changes in the detected gamma radiation with borehole depht and by the nature of the logging process. In a first, simpler embodiment, the filter uses the changes with borehole depth in the total amount of detected gamma radiation in the vicinity of the depth level of interest. In a second, more exacting embodiment, the filter uses estimates of the geological noise reflected in the individual energy windows, an estimate of statistical variations in the Th, U, K, concentrations, and an estimate of the statistical variations in the measurements, and also changes depending on whether the depth level of interest is in a homogenous or transient zone in the borehole. The invention relates to induced as well as natural gamma radiation, and to other types of well logging in which energy windows can be measured, e.g., in sonic logging.