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