The lithodensity tool combines a borehole-corrected bulk density measurement with a photoelectric lithology log (Pe). The radioactive source and detectors are mounted in an articulated skid that is maintained in contact with the borehole wall by a powered backup arm to minimise borehole rugosity effects. The arm also doubles as a caliper measurement. The tool may be combined with compensated neutron and focused induction measurements in the classic ‘triple-combo’ configuration.
PRINCIPLE OF MEASUREMENT:
Gamma radiation from a 137Cs source in the tool is Compton scattered by the formation and detected by two scintillation detectors. The relative intensities of the radiation at each detector gives a measurement of formation bulk density.
The photoelectric measurement is derived from the ratio of the gamma intensities in high and low energy windows at a detector. The Pe measurement depends in the atomic number of the formation and is a good lithology indicator.
Both density and Pe measurements are influenced by the borehole environment. These effects are minimised by corrections calculated by extensive Monte Carlo modelling and benchmarked to standards at the Callisto facility in Leicestershire, UK.
FEATURES
Well characterised tool response based on Monte Carlo models
Fully digital telemetry
Combines with density, induction and other logging probes
Drift eliminated by active calibration loop using internal source
Hardened wear-pad can be replaced in the field
High resolution measurement. Maximum data sampling interval 1cm (0.4")
