A logging-while-drilling system which measures one gamma ray spectrum from a formation element, and another gamma ray spectrum from a calibration source

摘要

A gamma-ray logging-while-drilling system (15, fig. 1) comprises a calibration source such as Americium 241 (not shown) disposed near or within a gamma-ray detector 32, e.g. within one or more scintillation crystals 30 of the detector 32. The calibration source emits a calibration radiation which has a calibration spectrum. The gamma ray detector measures one gamma ray spectrum from at least one radiating element within an earth formation (22, fig. 1) and a second spectrum from the calibration source. A dual gain circuitry (34, fig. 2) or amplification circuit provides a standard amplification for the received gamma-ray radiation signals and a high gain spectrum for the radiation signals of the calibration source. The system then produces a first gain's correction F1 (92, fig. 10) from the standard amplification spectrum S (90, fig. 10). A second gains correction F2 is then produced from the high gain spectrum and the two gains' corrections are compared to correct for the gain shifts of the gamma-ray detector (102, fig. 10). The system may comprise a collar 10 in which the detector 32 is disposed in a detector channel 40 at the periphery of the collar.