Quantitative interpretation of pulsed neutron capture logs: Part 2 — Inversion of measurements in thinly bedded formations

Abstract

We have developed an inversion method to reduce shoulder-bed effects on pulsed neutron capture (PNC) logs for estimating layer-by-layer-capture cross sections Σ. The method is based on a previously developed rapid approximation of PNC logs. Tests performed on synthetic examples that include a variety of lithology, saturating-fluid, and bed-thickness configurations confirm the efficiency, reliability, and stability of the inversion procedure. Inversion consistently improves the vertical resolution and Σ definition of PNC logs across beds thinner than 45 cm. Our fast, iterative algorithm inverts Σ logs in seconds of CPU time and is therefore suitable for joint petrophysical interpretation with other open- and cased-hole logs.