The Gamma-Ray Log
Radioactive isotopes of elements continuously decay to more stable forms and emit radiation of several types. Although many radioactive isotopes are known, only three types occur in any appreciable abundance in nature: the uranium series, the thorium series, and the potassium-40 isotope. Gamma rays have longer penetrations than either alpha or beta rays, and can be measured by simple counter devices. The earliest detectors on logging tools were Geiger counters, but these have been replaced by scintillation detectors in most modern tools. Older tools were recorded in "counts" or as equivalent weights of radium per ton. All modern logs are scaled in terms of the API (American Petroleum Institute) gamma ray unit. The API test pit at the University of Houston uses a concrete calibration standard with a value of 200 API units thought to be about double that of an "average" mid-continent shale.
Under typical borehole conditions, about 90% of radiation measured comes from the first six inches of adjacent formation, which sets the approximate radius of investigation. Because radioactive decay is a stochastic process, some degree of smoothing is made by an averaging (time constant) circuit to cut down the statistical noise. The count rate is also smoothed by the logging speed of the tool as it is raised through the borehole. The time constant and logging speed therefore influence both the bed resolution and the amount of noise recorded on the gamma ray log.
In most stratigraphic and petroleum geological applications, the gamma ray log is used