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 The earliest detectors on logging tools were Geiger counters, but these have been replaced by scintillation detectors in most modern tools.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
Figures & Tables
This manual was created in 1994 to assist the geologist to interpret logs. In the not too distant past, the reading of geology from wireline logs was highly interpretive. The ability of a rock to conduct electrical current or sound waves is several steps removed from traditional outcrop descriptions based on the eye and hammer. However, the range of logging measurements has expanded markedly over the years. In particular, the addition of nuclear tools has introduced log traces that reflect both rock composition and geochemistry in a more direct manner. Taken together, both new and old logs contain a host of keys to patterns of rock formation and diagenesis. The majority of books on log analysis focus on the reservoir engineering properties of formations penetrated in the borehole. The promise of potential porous and hydrocarbon-saturated rocks generally pays for both the hole and the logging run. There are many examples of common log types from a variety of sequences.