Measurement of hydraulic conductivity, porosity and lithology by neutron activation borehole logging at high spatial resolution increments

Abstract

A new method of measuring the continuously variable hydraulic conductivity at 20 cm increments surrounding a borehole is described. The method requires injection of a tracer solution and measurement of the variable lateral distance the tracer has moved by prompt gamma neutron activation analysis (PGNAA) geophysical logging. Gamma spectra collected by PGNAA logging from 0.16 to 10 MeV are analysed to provide a relative abundance of elements H, Si, Al, Fe, Cl and possibly others if sufficiently abundant. The distance a NaCl or KCl tracer solution has migrated into the rock surrounding the borehole is calculated from the greater energy attenuation of a 1.95 MeV low energy Cl gamma emission compared to a 6.1 or 7.4 MeV high energy Cl emission. The differential gamma attenuation is verified by experiment. A simple but sensitive method for measuring relative porosity surrounding a borehole is also presented by measuring the elemental abundances of common rock forming minerals and water, allocating elements to minerals and presenting a water/rock ratio. Relative porosity may be further simplified to H/(H+Si) particularly for sandstones typical from the Sydney Basin. Many boreholes of hydrological interest are drilled into sedimentary rocks and alluvium dominated by abundant quartz and clay, which can be quantified by relative Si and Al. Similarly, many sedimentary lithologies may be defined by variations in their mineralogy reflected in proportional changes in elemental abundance. Subtle variations in lithology not apparent by visual inspection such as degree of cementation or clay pore filling in sandstone may also be detected. Porosity and lithology estimation by PGNAA geophysical logging does not require a tracer solution to be injected and may be measured through borehole casing with screened or unscreened intervals.