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 acces s boreholes has been developed, The combination of acrylic casing and polyu rethane foam backfill has been tested under laboratory and field conditions . Acrylic casing does not significantly attenuate fluxes of either high ene rgy or thermal neutrons, in contrast with polyvinyl chloride casing which r educes the thermal neutron nux by more than 40% due to neutron absorption b y chlorine, Polyurethane foam, which is inert, hydrophobic, and insoluble i n water, adheres well to both dry and wetted soils, sediments, and rocks. I t 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, polyureth ane foam increases the thermal neutron count by less than 5% in a saturated sand, relative to background. In addition to its small effect on the neutr on flux, polyurethane foam, unlike bentonite or cement, does not affect the moisture content of the surrounding formation during installation. Further more, because it is a closed-cell foam, its moisture content does not chang e under varying formation moisture conditions. As was shown in related fiel d tests, polyurethane foam is especially well suited for backfilling boreho les in fractured rocks because of its fast set time which minimizes penetra tion into fractures. The design proved to be convenient and durable under r ugged field conditions.