An auger tool to estimate hydraulic conductivity using a resistivity analogy

Abstract

A field instrument and analysis method was developed to estimate the vertical distribution of hydraulic conductivity, K, in shallow unconsolidated aquifers. The field method uses fluid injection ports and four pressure transducers in a hollow auger that measure the hydraulic head outside the auger at several distances from the injection point. A constant injection rate is maintained for a duration time sufficient for the system to become steady state. The novelty of this method lies in the fact that K is determined while the drill string is in the ground and the change in hydraulic head is monitored in the same drill stem. Dense vertical sampling and the application of four transducer offsets provide a detailed resolution of the vertical variability in hydraulic conductivity. Exploiting the analogy between electrical resistivity in geophysics and hydraulic flow, data are processed with a 1-D inversion algorithm for the pole-pole resistivity method resulting in models consistent with the known geology. The injection methodology, conducted in three separate drilling operations, was investigated for repeatability, reproducibility, linearity, and for different injection sources. Repeatability tests, conducted at ten levels, demonstrated spreads of generally less than 10%. Reproducibility tests conducted in three closely spaced drilling operations showed a spread of less than 20%, which may be due to lateral variations in hydraulic conductivity. Linearity tests, made to determine dependency on flow rates, showed no indication that the applied flow rate biased the measurements given the uncertainty of repeated measurements. In order to obtain estimates of the hydraulic conductivity by an independent means, a series of measurements were made by injecting water through screens installed at two separate depths in a monitoring pipe near the measurement site.