J. G. Conaway and Q. Bristow
ABSTRACT
This paper deals with two problems of concern in quantitative gamma ray log interpretation. These are (1) problems involved in using 241Am for temperature stabilization of spectral logs and (2) pitfalls in the use of calibration sleeves for spectral and gross count system calibration.
Borehole scintillation detectors are available implanted with 241Am, an alpha emitter, as a gain stabilization source. In recent experiments we have found that the relative position of the 241Am gamma equivalent peak with respect to a given natural gamma peak depends not only on temperature, but on rate and direction of change of temperature as well. In other words, a size-able hysteresis effect has been observed in the relative gamma and alpha peak positions which eliminates 241Am as a viable stabilization technique under conditions where the temperature is changing more than 1 or 2°C per hour, as encountered in most logging situations.
Calibration sleeves have a legitimate use in natural gamma logging to monitor the long term stability and performance of a logging system. However, considerable error can be expected in attempting to use such sleeves to calibrate KUT or total count logging systems rather than simply monitor their long term performance. These errors are due to a difference between the spectrum resulting from a calibration sleeve and the spectrum observed in a radioactive zone in a borehole, due to a relative lack of scattered gamma rays in the former case. In this paper we present results from a calibration sleeve and a model borehole illustrating this problem.