The probe measures the quantity and energy of natural gamma emissions from the formation. The main emitters of these are the isotopes of Potassium, Thorium and Uranium. These radioactive isotopes each emit gamma rays that have a characteristic energy level measured in MeV. The quantity and energy of these gamma rays is measured by the probe. A log of the spectroscopic response to natural gamma ray radiation is presented as a total gamma ray log that plots the weight fraction of potassium (%), thorium (ppm) and uranium (ppm). Different types of rock emit different amounts and different spectra of natural gamma radiation.
The fully stackable spectral gamma version can operate in both spectral and in 5 window mode. The standard channel numbers are 256 but due to Geovista’s constantly improved high tech electronics, we can also supply the probe in 512, 1016 or 2032 channels.
The standard Geovista sonde uses a large 150 x 38 mm Sodium Iodide scintillation crystal coupled to a photomultiplier to detect natural gamma radiation and display it as a 256-channel energy spectrum. This helps to distinguish the main radioisotopes at the source of the radiation (typically 232Th, 238U and 40K). A large crystal is used because of low number of gammas detected in the higher energy range.
A continuous log of Total Gamma counts and of counts within five energy band windows can also be recorded. The range of energies detected by the sonde is from 100 KeV up to 3.0 MeV.
The sonde is pre-calibrated so that data from an internal temperature sensor in intimate contact with the crystal can be used to automatically adjust the high voltage supply to the detector to compensate for any temperature induced changes in its gain. The sonde also provides for automatic dead time measurement and correction.
For much improved resolution and sensitivty, Geovista also supply a choice of a LaBr, BGO or a CeBr3 scintillation crystals.
- NaI is a good all-round crystal. It is also the least expensive. It is the most commonly used crystal.
- BGO is somewhat similar to NaI in terms of resolution and decay time. However, it is a much more efficient GR absorber due to its high Z.
- LaBr has excellent resolution, a capacity to deal with high count rates, and a good output linearity with temperature. However, it can exhibit higher intrinsic background counts.
- CeBr is an alternative to LaBr and as an added advantage, it has the same form factor as NaI. It also exhibits far fewer intrinsic background counts.
Geovista provides a calibration service that physcially calibrates the probe in a known, standard reference borehole to obtain the API and stripping ratios for K, U and Th.
Optionally, we can also supply a Medusa calibration if required.