Field Demonstration of Slim-hole Borehole Nuclear Magnetic Resonance (NMR) Logging Tool for Groundwater Investigations

Nuclear magnetic resonance (NMR) methods provide estimates of free and bound water content and hydraulic conductivity, which are critically important for groundwater investigations. Borehole NMR tools have been available and widely used in the oil industry for decades, but only recently have been designed for small diameter boreholes typical of groundwater investigations. Field tests of an 89-mm-diameter borehole NMR logging tool are presented. This borehole NMR logging tool was developed for economical NMR logging of 100- to 200-mm-diameter boreholes, and specifically for characterizing hydraulic properties in the top 200 m of the subsurface. The tool has a vertical resolution of 0.5 m, a minimum echo spacing of 2.0 ms, and a radial depth of investigation of 178 to 203 mm, which typically is beyond the annulus of observation wells. It takes about 15 minutes to collect a data sample for each 0.5-m interval. The borehole NMR logging tool was field tested during spring 2010, in PVC-cased wells at sites in East Haddam and Storrs, Connecticut; Cape Cod, Massachusetts; Lexington, Nebraska; Lawrence, Kansas; and Rifle, Colorado. NMR logging yielded estimates of bound water, free water, and total-water content, as well as continuous distributions of water content versus transverse relaxation time (T2) at all depth levels. The derived water-content data were compared to the available ground-truth hydrogeologic data from each well, including drilling logs, neutron and other geophysical logs, and direct measurements of hydraulic conductivity. The results indicate that the borehole NMR logging tool provides information on porosity, pore-size distribution, and estimated hydraulic conductivity that cannot be duplicated by any other single geophysical logging tool.