Use of borehole neutron logs to estimate moisture content in the unsaturated zone of an alluvial aquifer

Vicente Quinones-Aponte; Ramon A. Carrasquillo

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Use of borehole neutron logs to estimate moisture content in the unsaturated zone of an alluvial aquifer

Vicente Quinones-Aponte·Ramon A. Carrasquillo

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Abstract

he neutron borehole logging tool was calibrated for the determination of moisture content in the unsaturated zone of an alluvial aquifer . A continuous core sample was collected to a depth of 15 ft from the unsaturated zone . Moisture content and other soil properties were determined by standard soil-laboratory techniques. The neutron logs profile was correlated to moisture content . The first correlation included all the data points within the unsaturated zone . A correlation coefficient of 0.72 was obtained. An inspection of the plotted data showed that points at the upper extreme of the fitted line departed significantly. A second correlation was tried eliminating data points 1, 3, and 5 which fell too low in the first trial. An almost perfect correlation coefficient of 0.99 was obtained for the data points that were included in the second trial. The criteria used to select the data points for the second correlation trial was that all these points also fell on the left most limit of the neutron profile. The neutron logs profile, which is like a type of harmonic curve seems to be affected by an undefined factor. Analyzing the shape of the neutron profile it is inferred that hysteresis is the most probable phenomena that is affecting this moisture content profile. Although correlating neutron logs to moisture content is not a straight forward procedure, with an appropriate analysis, calibrated neutron logging tools can be used to determine the moisture content within the unsaturated zone of aquifers. Moisture content should be determined for calibration purposes at least at every foot of depth. Measurements of other elements or factors which may produce the harmonic curve effect might be included in further studies.

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Vicente Quinones-Aponte, Ramon A. Carrasquillo. 1986. Use of borehole neutron logs to estimate moisture content in the unsaturated zone of an alluvial aquifer. https://pubs.usgs.gov/publication/70171485

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