SUMMARYThis report describes the 1991 yearly calibrationof a gross gamma-ray geophysicalpulse logging system owned by the U.S. Departmentof Energy (DOE) and operated by the Pacific Northwest Laboratory. The calibrationwas conducted to permit the continued use of this system for geologic and hydro-" logic studies associatedwith remedial investigationsat the Hanford Site.Primary calibrationsto equivalenturanium units were conducted in DOE B borehole model standards that reside on the Hanford Site. The calibrations were performed in boreholemodels SBL/SBH and SBA/SBB, which contain lowequivalenturanium concentrations.Correlationswere establishedbased on two similar approaches for relating observed count rate in before-and after-loggingfield calibrationsto equivalenturanium concentrations.A new field source (Ra-20S-82)was fabricatedto replace the old source (Ra-20S-204),whose activity led to variable fitld calibration results previously caused by a nonfixed geometry. A cross-calibrationstudy was performed to compare the operationof the new source relative to the old source.A digitally based collection/recordingsystem was recently acquired, so that many of the procedureswere performed with the old analog system and the new digital system to compare the performance of the digital system.iii CONTENTS