New casing and backfill design for neutron logging access boreholes

In an effort to enhance the usefulness of neutron logging for environmental applications, a new combination of backfill and casing materials for access boreholes has been developed. The combination of acrylic casing and polyurethane foam backfill has been tested under laboratory and field conditions. Acrylic casing does not significantly attenuate fluxes of either high energy or thermal neutrons, in contrast with polyvinyl chloride casing which reduces the thermal neutron flux by more than 40 percent due to neutron absorption by chlorine, Polyurethane foam, which is inert, hydrophobic, and insoluble in water, adheres well to both dry and wetted soils, sediments, and rocks. It can be formed in situ at a low but controllable, bulk density. At a bulk density of 0.08 g cm(-3), and in combination with acrylic casing, polyurethane foam increases the thermal neutron count by less than 5 percent in a saturated sand, relative to background. In addition to its small effect on the neutron flux, polyurethane foam, unlike bentonite or cement, does not affect the moisture content of the surrounding formation during installation. Furthermore, because it is a closed-cell foam, its moisture content does not change under varying formation moisture conditions. As was shown in related field tests, polyurethane foam is especially well suited for backfilling boreholes in fractured rocks because of its fast set time which minimizes penetration into fractures. The design proved to be convenient and durable under rugged field conditions.