Scintillation Gamma Ray Tool (SGT*)


During DSDP and the early part of ODP, the Scintillation Gamma Ray Tool (SGT) was more widely known as the Natural Gamma Ray Tool (GR). All gamma ray tools record naturally occurring gamma rays in the formation adjacent to the borehole. Therefore, they provide a measurement of the radioactive content of the formation. Unlike the HNGS, run on the triple combo and FMS/Sonic tool strings, the SGT did not acquire spectral gamma data; it measured only the total number of gamma counts in API units. It was last run during IODP Expedition 321.


  • Depth correlation between runs
  • Lithology identification
  • Evaluation of clay content

Tool Specifications

Temperature rating: 350° F (177°C)
Pressure rating: 20 kpsi (138 MPa)
Diameter: 3.375 in (8.57 cm)
Length: 5.5 ft (1.68 m)
Weight: 83 lbs (37.7 kg)
Sampling interval: 6 in (15.24 cm) and 2 in (5.08 cm)
Maximum logging speed: 3600 ft/hr (1,097 m/hr)

Measurement Specifications

Measurement range: 0-20,00 GAPI
Vertical resolution: 12 in (30.48 cm)
Depth of investigation: 24 in (60.96 cm)
Accuracy: 5.5 ft (1.68 m)

Major Outputs

ECGR: Environmentally corrected gamma ray (GAPI)

Deployment Notes

The SGT has been run in combination with a variety of tools.

* ®trademark of Schlumberger