The spectral gamma ray tool uses the same sensor as the total gamma ray tool. The output from the sensor is fed into a multi-channel analyzer that calculates the amount of radiation coming from the energies associated with each of the major peaks. This is done by measuring the gamma ray count rate for 3 energy windows centred around the energies:

1.46 MeV for potassium-40 

1.76 MeV for the uranium-radium series 

2.62 MeV for the thorium series. 

These readings represent the gamma ray radioactivity from each of these sources. Their sum should be the same as the total gamma ray value measured by the total gamma ray tool, and is coded SGR if measured with a spectral gamma ray tool. Any combination of the three components can be summed and analyzed. However, the most important is the sum of the potassium-40 and thorium radiation, which is called the computed gamma ray response (CGR). 

The depth of investigation is identical to the usual gamma ray tool. It should be noted that potassium is reported as a percentage, while U and Th are reported in parts per million (ppm)