Time-lapse streaming-potential measurement in a sand-box and calculation of streaming potential coupling coefficient

Abstract

Groundwater flow due to pumping in a phreatic aquifer generates a self-potential (SP) signal at the ground surface which can be used to determine aquifer parameters and direction of groundwater flow. An SP measurement system was assembled using 80 non-polarizing CuSO4 electrodes and an 80-channel high precision logging multi-meter (Keithley 2701) capable of scanning all channels at 8-second interval. The system can automatically log data in spatio-temporal domain. A group of electrode cables and patch cables were made allowing different electrode layouts for field measurement of SP on an earthen dam, over a groundwater pumping or injection site and over a subsurface contamination site. In order to understand the functionality of the SP system, a test box of dimensions 120cm × 120cm × 60cm was built using Plexiglas with a drain at the bottom. The bottom of the box was filled with 2 inches of pea pebbles to facilitate injection and draining from bottom, and the remainder of the box was filled with very coarse to fine grained sand. Electrode placement in the test sandbox was on a 12.33cm × 15.33cm grid. Five observation wells instrumented with high precision pressure transducers and one central pumping well were installed in the tank for hydrogeological investigation. The SP system was checked and verified against a conventional 2 electrode SP system. Time-lapse SP data were recorded using 80 electrodes during a pumping test in the sandbox. In-situ streaming potential coupling coefficient, C (mV/m) were calculated from the collocated observation well and SP electrode data. The intrinsic streaming potential coupling coefficient, C, was measured in the lab using core samples. The in-situ and intrinsic streaming potential coupling coefficient were compared and coupled forward modeling and measured SP data will be discussed.