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Neutron Log Measurement of Moisture in Unsaturated Basalt

Progress and Problems

^a Idaho National Engineering and Environmental Laboratory, P.O. Box 1625, Mail Stop 2107, Idaho Falls, ID 83415-2107
^b 13786 Schoger Rd., Lathrop, CO, 81236
^c Idaho State University, Pocatello, ID 83209-8072
^d 2540 Grandville Ave., Henderson, NV 89052

View larger version (53K): [in a new window]   Fig. 1. Borehole geophysical logs for well C1A at the Idaho National Engineering and Environmental Laboratory (INEEL) constructed in the Quaternary basalts of the East Snake River Plain. Logs presented are, left to right, a caliper log in units of cm x 2.5 (black), a natural log in American Petroleum Institute ray (API-GR) units (green), a - density log in counts per second (cps) units (purple), and a neutron log in American Petroleum Institute neutron (API-N) units (red). API units are based on tool response in the API calibration test pits in Houston, TX. All log curves are plotted on a logarithmic scale, and all units increase to the right. The right hand panel shows the smoothed calculated neutron porosity curve in volumetric percentage (blue) based on the API limestone standard. The far right column shows a simplified stratigraphic column for the C1A well, where massive basalt is black, vesicular basalt is gray, thinly bedded or near-vent basalts are white, and sedimentary interbeds between basalt flows are yellow.

View larger version (65K): [in a new window]   Fig. 2. Measured and calculated physical properties for Well C1A. Property vs. depth curves are, from left to right, measured He porosity of 2.5 by 2.3 cm basalt sample cores (blue), neutron log response on a linear scale in API-N units (red), - (long-spacing) log response on a linear scale in cps (purple), measured bulk density of sample cores in grams per cubic centimeter (black), and the natural log of measured sample core permeability in millidarcies (red). All units increase to the right except for bulk density.

View larger version (39K): [in a new window]   Fig. 3. Plots of neutron log response vs. measured He porosity and - log response vs. measured bulk porosity. Regression fits were uniformly poor for these traditional log-to-physical property correlations. Samples were dried 2.5 by 2.3 cm cylinders taken from basalt cores, from locations in the saturated and vadose zones, respectively. Porosity was determined using a He porosimeter. Bulk density was determined from multiple measurements of physical dimensions and mass. Neutron and - log data used for the regression analyses were averaged over 30 cm. Gamma- data was from the "long-spacing" (LS) source-detector pair, recorded in counts per second. Neutron data was recorded in API-N units. Regression analyses were made for both neutron and - data, from the upper vesicular zones of basalt flows (red), lower vesicular zones of basalt flows (green), all vesicular basalt (purple), massive interiors of basalt flows (blue), the massive interiors of basalt flows with outliers points removed (brown, - only), and the near-vent facies of thinly bedded, fractured basalts and shelly pahoehoe (black). Neutron data regression coefficients for aquifer basalts were: upper vesicular, R = 0.0265; lower vesicular, R = 0.4121; all vesicular, R = 0.0200; near-vent, R = 0.9042; and massive, R = 0.4798. Neutron data regression coefficients for vadose basalts were: upper vesicular, R = 0.1233; lower vesicular, R = 0.1277; all vesicular, R = 0.0300; near-vent, R = 0.0970; and massive, R = 0.0424. Gamma- data regression coefficients for aquifer basalts were: upper vesicular, R = 0.2827; lower vesicular, R = 0.2470; all vesicular, R = 0.0332; near-vent, R = 0.1166; massive, R = 0.4592; and massive-no outliers, R = 0.7270. Gamma- data regression coefficients for vadose basalts were: upper vesicular, R = 0.0100; lower vesicular, R = 0.0906; all vesicular, R = 0.0624; near-vent, R = 0.0245; massive, R = 0.2903; and massive-no outliers, R = 0.3250.

View larger version (39K): [in a new window]   Fig. 4. Regression analysis of estimated H concentration in percentage (w/w) vs. neutron log response in API-N units for saturated massive basalt. Hydrogen concentration was estimated by assuming that 100% of total porosity was filled with unbound pore water. Neutron log data used for the regression analyses was averaged over 30 cm. The analyses were confined to massive saturated basalt to minimize the effects of infiltrated sediments, alteration minerals, and water in unquantified and/or unidentified fracture porosity, since these are rare in the massive interiors of basalt flows. Outliers associated with infiltrated sediments in the C1A cores were eliminated from the data set. Some of the correlations established were subsequently used to calculate the percentage (w/w) H curves shown in Fig. 5.

View larger version (32K): [in a new window]   Fig. 5. Four possible percentage (w/w) H plots are shown here, one for each correlation type. These plotted data were calculated using linear (purple), logarithmic (red), exponential (green), and inverse polynomial (black) regression fits shown on Fig. 4. The colors of the calculated weight percent plots on this figure correspond to the regressed correlations of the same color on Fig. 4. The natural log of permeability (orange) in millidarcies is also shown on this figure (1 darcy = 9.87 x 10–13 m2). For the sake of clarity, not all of the correlations from Fig. 4 were used.