Method and apparatus for determining borehole and formation constituents

Abstract

A method and apparatus for determining the contents of a borehole traversing a subterranean formation is provided. In the method, a well-logging sonde is used to bombard the contents of a borehole and the formation with fast neutrons. An early capture spectrum signal is generated to reflect the composition of the formation and the contents of the borehole. A late capture spectrum signal is generated to reflect the composition of the formation. The early capture spectrum signal is compared with the late capture spectrum signal to derive an output signal representative of the spectral composition of the contents of the borehole. Additionally provided are improved methods and apparatus for determining the contents of a subterranean formation. An inelastic spectrum signal is generated to reflect the detailed combined composition of the formation and contents of the borehole. The output signal representative of the spectral composition of the contents of the borehole previously derived is used to eliminate the borehole contribution to the detailed combined composition of the formation and borehole and to correct the resulting detailed formation composition signal. Alternatively, an improved formation signal may be derived without the inelastic spectra information. The output signal representative of the spectral composition of the contents of the borehole may be used to eliminate any borehole contribution to the late capture spectrum signal.