Sonar device for testing well drawdown9/10/2023 ![]() In groundwater studies, the theory behind this concept is that the aquifer is assumed to be homogeneous up to the wall of the borehole, while all the head losses are assumed to be concentrated in a thin, resistant skin against the wall of the borehole. ![]() If it is smaller, the well is usually poorly developed or its screen is clogged and this is referred to as a negative skin effect (De Marsily, 1986). If the effective radius of the well r is larger than the real radius of the borehole r w in groundwater studies, this concept is referred to as a positive skin effect. The concept of a skin effect is derived from petroleum engineering, which uses the concept to account for the head losses in the vicinity of the well (Ramey, 1982). For any well-flow equations, an adequate recovery equation can be mathematically represented (Kruseman and Ridder, 1994). The residual drawdown field data are more reliable than the pumping test data because recovery occurs at a constant rate, whereas in practise, a constant discharge during pumping is often difficult to achieve (Kruseman and Ridder, 1994). ![]() Recovery test measurements allow the transmissivity of the aquifer under investigation to be determined more accurately. In a hydraulic test, it is very important to measure the residual drawdown during the recovery period. In groundwater studies, this is expressed as the difference between the initial water level prior to the start of pumping and the water level measured at a time t' after the termination of pumping. This rise in water level is referred to as residual drawdown, s'. Keywords: recovery equations, residual drawdown, skin effect, variable dischargesĪs soon as the pump is closed up after a test, the water levels in the well and the piezometers will start to rise. On the basis of the suggested solution, a new analytical solution for the analysis of residual drawdown data after a pumping test with step-wise or intermittently changing discharge rates is provided. An analytical solution for the drawdown response in a confined aquifer that is pumped step-wise or intermittently at a different discharge rate is suggested. A new analytical solution for the determination of the skin factor without any restriction on the variables t and t' is derived. The proposed analytical solution is in perfect agreement with the experimental data for α = 0.01, in contrast to the Cooper-Jacob solution. A comparison between the proposed solution, existing solutions and experimental data from field observation are presented. ![]() Institute for Groundwater Studies, Faculty of Natural and Agricultural Sciences, University of the Free State, Bloemfontein, 9300, South AfricaĪ new analytical solution for residual drawdown during the recovery period after a constant rate pumping test is described. Analytical solutions for the recovery tests after constant-discharge tests in confined aquifers ![]()
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