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Abstract

Gamma rays from a subsurface earth formation are detected, and measurement of their respective energy levels transmitted to the surface where the resultant spectral data is stored in a plurality of narrow energy windows. Functional relationships relating photoelectric cross-section, density, and soft-to-hard ratio are developed from a formation model. These relationships are utilized with the stored spectral data in a perturbative process whereby effect of photoelectric cross-section on density is reduced and improved accuracy in density determination effected. Additionally, density is factored into photoelectric cross-section determination, compensating for effects of density on the soft-to-hard ratio. Availability of the entire spectrum permits improved resolution of the windows, monitor peaks, and detectors, and permits compensation for detector resolution in the observed spectrum. In one embodiment, the energy range over which the long space count rate is derived for determination of density is varied as a function of logging conditions.

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First Claim