BOREHOLE GEOPHYSICAL LOGGING BY USE OF THE ELECTROMAGNETIC FLOWMETER AND NUCLEAR MAGNETIC RESONANCE AT THE MASSACHUSETTS MILITARY RESERVATION, CAPE COD, MASSACHUSETTS

Over the last 30 years, advancements have been made in developing borehole geophysical tools and methods for determining aquifer hydraulic properties that control solute transport. Two methods, the electromagnetic flowmeter (EMFM) and nuclear magnetic resonance (NMR), have been tested and applied at the Massachusetts Military Reservation (MMR), and the results were compared to historical geophysical logs and to hydraulic tests conducted at the site. In general, the EMFM has improved the resolution of flow measurements, and NMR has facilitated obtaining water content and permeability estimates. Both methods have provided physical and hydraulic information consistent with previously established results at the site.

The EMFM has increased the resolution of vertical borehole flow measurements compared with previously used impeller flowmeters, allowing for measurements at lower pumping rates and slower trolling speeds. EMFM logs showed flow distributions that are used to identify variations in hydraulic conductivity with depth in the borehole.

In recent years, NMR technology has advanced to allow for NMR tools that can be used in small-diameter boreholes typical of environmental studies. At MMR, the boreholes are 5- to 10.2-cm in diameter. NMR measurements were made at fixed depths within eight boreholes. Each measurement provided estimates of total water content, bound- and mobile- fraction of water, and an estimate of permeability over depth intervals as small as 0.5 m and radially 5 to 14.6 cm from the borehole, depending on the NMR probe that was used. The NMR total water content estimates compared well to historic active-source logs and to the effective porosity determined from hydraulic and transport tests at the MMR site.

The NMR technique provides two major advancements in geophysical logging: (1) because NMR does not use a nuclear source, it greatly simplifies the logistics and legal requirements for active-source logging that has historically been used to determine water content; and (2) NMR logging provides estimates of pore-size distribution that allow for the distinction between bound- and mobile- water fractions, which is not possible with active-source logs.