REVERSED FLOW TEST - A BOREHOLE LOGGING METHOD FOR ESTIMATING PORE WATER-QUALITY AND INFLOW RATES ALONG AN UNCASED BOREHOLE PROFILE

There is often a need to estimate the variation in water quality and flow rate with depth in an aquifer given access only to an uncased borehole. In such situations, borehole logging techniques can be applied. This paper describes the Reversed Flow Test (RFT), a rarely used borehole logging method. The RFT is intended to provide information on pore water quality and inflow rates along the length of an uncased borehole profile. They are carried out by logging the conductivity of the borehole fluid under two pumping phases. During the first pumping phase the pump intake is located at the top of the borehole, and during the second the intake is located at the base. Provided the pumping rates are low and the system does not have marked lateral heterogeneity, stable conductivity profiles are often achieved within a relatively short time period. The data are interpreted to give estimates of electrical conductivity and inflow at each point in the profile. The test has been successfully carried out on a range of British aquifers, and four case histories are summarized here. In each case, the test was easily accomplished by two people in less than a day.