WRRCTR No. 75 Neutron Well Logging in Hawaii

Abstract

In 1970, the Water Resources Research Center initiated a study of neutron well logging techniques in Hawaii. The primary objective of this study was to apply neutron logging techniques to the problem of obtaining reliable porosity data and relating these to water yields from Hawaiian aquifers. Neutron well logging in Hawaiian basaltic formations has produced much important qualitative and quantitative information. The most important application has been the determination of porosity in the saturated zones. Interpretation of neutron log responses in Hawaiian basaltic formations is similar to conventional neutron log interpretation in sedimentary formations. Neutron count varies as an inverse function of porosity. High neutron counts are indicative of low porosities and low neutron counts are indicative of high porosities. In general, the neutron logs are extremely responsive to detailed changes in formation porosity, and of particular importance is that the reproducibility of neutron logs from the same well is excellent. Neutron logs from Hawaiian wells also are extremely responsive to the saturated-unsaturated boundary and to the casing termination. Surprisingly, the neutron logs show little consistent response to nominal hole diameter or borehole fluid salinity. Probably the best use of the neutron logs is in conjunction with other borehole geophysical logs. Correlation between neutron Logs and electric resistivity logs is particularly good.