Electrode arrays for measuring groundwater flow direction and velocity

Abstract

The movement of 2000 liters of salt water after injection into groundwater within gravels a few meters below the ground surface at three injection sites was traced by six different resistivity monitoring arrays; the resistivity rectangle, Schlumberger sounding, Wenner sounding, Wenner fixed‐spacing, mise‐à‐la‐asse and downhole electrode array. Five of the arrays indicated groundwater flow direction and seepage velocity. As evidence indicates, similar geological and hydrogeological conditions exist at the injection sites. Therefore, comparisons between the sensitivity of the five arrays can be made and are as follows: resistivity rectangle—maximum decrease of 60 percent in derived potential differences; Schlumberger sounding— maximum decrease of 28 percent in measured apparent resistivity; Wenner sounding—maximum decrease of 20 percent in measured apparent resistivity, Wenner fixedspacing—maximum decrease of 22 percent in apparent resistivity; downhole electrode—maximum decrease of 38 percent in measured resistance. Measured potentials and derived values of potential gradient measured by the mise‐à‐la‐masse array indicated groundwater flow direction but not seepage velocity. Estimates of seepage velocity given by the resistivity arrays for the three salt water injection sites are between 260 ± 40 m/day and 700 ±100 m/day. These estimates are in broad agreement with values of seepage velocity derived from the point‐dilution technique, from previous salt water injection experiments, and from groundwater conductivity measurements using downhole probes.