The MGT uses an array of four independent, self-contained gamma spectrometry modules with 2-ft spacing between detectors (Table T1). Figure F1 illustrates the geometry of the MGT, which enables the received gamma ray counts from the four detectors to be stacked at sequential depths. Common-depth stacking is based on the known logging speed and known detector geometry of the MGT. This maintains both signal strength and the high vertical resolution of the MGT. Each detector module comprises a cylindrical, 2-in-diameter NaI crystal fixed along the centerline of the tool. The NaI crystals are each 4 in long and packaged with a programmable 256-channel amplitude analyzer in the detector module. The spectrum stabilization system is based on an 241Am calibration source in each detector, enabling the spectral resolution of stacked data to be maintained at the same level (~8%) as individual detectors (Fig. F2).
Goldberg et al. (2001) report preliminary tests that indicate the vertical resolution of the MGT to be increased by a factor of three or four over conventional logging tools (e.g., Schlumberger Natural Gamma Ray Tool) while maintaining high gamma ray count statistics comparable to commercial tools at typical logging speeds. The tool also includes an accelerometer for improving data stacking based on logging speed. The MGT may be deployed in series with the Schlumberger tool string, as configured during ODP Leg 191, or as a stand-alone tool.