Use of Borehole and Surface Nuclear Magnetic Resonance Methods at Haddam Meadows State Park, Connecticut

Abstract

During summer 2011, borehole and surface nuclear magnetic resonance (NMR) data were collected at Haddam Meadows State Park, Connecticut, a site where alluvium, stratified drift, and ice‐contact deposits overlie bedrock that is about 43 m below land surface. The NMR methods were tested at this site because there is a rich data set that includes drilling and borehole geophysical logs and surface geophysical surveys. Borehole NMR data were collected in two PVC‐cased boreholes. The slim‐hole borehole tool used for this study measures water content with a 1‐m vertical resolution at a focused radial distance of 100 mm from the center of the tool. Total water content ranged from 0.05 to 0.57, with about 75 percent of the values between 0.2 and 0.4. In the saturated zone, about two thirds of the total water content was interpreted as “mobile” water and the other third was bound water. These values were comparable with porosity estimates from neutron logs and were consistent with these surficial materials, which were interpreted from gamma, electromagnetic induction, and lithology logs. The surface NMR array data were acquired near the boreholes using a circular figure‐eight loop with a 40‐m diameter. The surface NMR imaged to a depth of 40 m below land surface. The NMR‐interpreted depth to water was about 5 m below land surface, with a range of total water content for the saturated materials of about 0.1 to 0.22. The interpreted thickness of the stratified drift and the presence of a low‐permeability, bound‐water layer are consistent with observations in the boreholes and with results from 2D resistivity and time‐domain electromagnetic data collected over the same area. Results from the borehole and surface NMR data were generally comparable, although the surface NMR estimates of water content were consistently lower than the borehole NMR results. This may be the result of differences in the measurement locations, different system sensitivities and/or spatial averaging effects, or borehole NMR measurement of increased water content in the zone disturbed by drilling.