The Spectral Gamma measures individual radiation contributions from Potassium, Thorium and Uranium. The tool can also calculate the usual, average geological natural gamma radiation. This can now be done while drilling.
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)
Some of the uses for Spectral Gamma:
Determination of Lithology (sands, shales, other minerals, clay-bearing SS, arkose SS, micaceous SS, graywackes, greensands, heavy mineral SS, carbonates, evaporites, etc)
Recognition of Igneous Rocks
Cation Exchange Capacity
Our Spectral Gamma detection module and all electronics are being developed to standard 1.25" diameter/width. The modules can be used in Wireline and LWD applications