The use of the natural gamma log in engineering geological investigations

The natural gamma log is widely used in both cased and uncased boreholes to identify changes in lithology down the length of a borehole. This is particularly important in the ground investigation process where 100% core recovery is often not achieved in the borehole programme. In this paper the use of the gamma log to provide additional information from the ground investigation boreholes is examined and illustrated by a number of case histories. It is shown that the gamma log can be used to study both the lithological variations within an individual borehole and changes in geological structure across a construction site. It is also demonstrated that a geological formation or sequence can be classified in terms of its natural gamma signature, which enables the geologist to observe its presence over large distances on a regional basis. The importance of the calibration of the gamma sonde and the use of the correct logging speed is emphasised in the paper. The successful calibration of the log against a quarry wall using a known observed geological structure as a reference is also described together with the use of the resulting information to obtain a geological log for boreholes drilled behind the quarry face. Its use in extrapolating between boreholes to detect and trace the presence of a particular geological formation to estimate its vertical and lateral extent for possible extraction is also discussed.