A comparison of the moisture gauge and the neutron log in air-filled holes at NTS

Two methods are commonly used to measure water content of geologic materials by neutron diffusion, the moisture gauge and the neutron log. Both are used at NTS, the moisture gauge in tunnels, the neutron log in vertical drilled holes. In this work, the moisture gauge and the neutron log are compared for use in air-filled holes NTS. The measurement instruments have evolved with very different operational characteristics and one important physics difference, the source to detector spacing. The moisture gauge has a very short, 0--6 cm spacing, with little internal shielding, and count increases with water. The neutron log has a long spacing, 30--50 cm, substantial internal shielding, and exhibits decreasing count with increasing water. The moisture gauge gives better bed resolution than the neutron log. Because its count increases with water, the moisture gauge is more strongly affected by water in the borehole, especially in dry formations. In these conditions the neutron log is the method of choice. In air-filled holes, if source size or logging time is not a constraint, the relative sensitivity of the two tools to water is determined by the relative strengths of borehole effects as fluid, holesize, or tool-wall gap. If source size ismore » a constraint for safety reasons, the short spacing provides higher countrates for a given detector efficiency and thus better relative precision in determining the true count. If source size is limited because of detector or electronics saturation, the short spacing will be better at high water content, while the long spacing will be better at low water content. The short spacing may have an advantage because it can make better contact with the hole wall and can be more easily corrected for gap. The long spacing tool is currently used in vertical holes at NTS because that is the only tool available from logging contractors. Since they are most concerned with high water contents, the short spacing tool could prove to be better.« less