The conventional Gamma Ray tool takes a bulk reading of gamma rays emitted mainly by U238, Th232 and K39. The tool doesnt distinugish between the three isotopes so it will for example give a high GR reading in a shale but also in a sand high in potassium feldspar.
SLB's natural Gamma Ray Tool (NGRT) provides GR readings for each of the three main isotopes by sorting the counts from each isotope by looking at the energy associated with each count.
While the NGRT provides more useful GR data than a conventional GR tool the NGRT uses a BGO crystal in the detector which is very sensitive to temperature......increasing temperature results in a decrease in resolution. For this reason the detector is usually encased in a Dewar flask and the tool is cooled with dry ice prior to running to keep it below 40degC. The need to cool the tool down adds to the time taken for pre-run preparations and the dewar flask makes the tool bulky.
