| Popis: |
As part of a larger programme to understand the behaviour of dense non-aqueous phase liquids (DNAPLs) in the subsurface (Redman, 1992), a controlled release of 770 litres of perchloroethylene (PCE) into a 9 by 9 metre cell within a sandy aquifer was monitored geophysically during the summer of 1991. Surface and in situ resistivity, terrain conductivity and neutron, induction, and density logging data were collected over a period of several weeks to document the movement of the PCE away from the point of injection towards its final resting place in pools on confining layers within the cell. The very high resistivity of the PCE in its liquid state makes it a target for resistivity, induction logging and terrain conductivity methods. Its high density, low hydrogen and high chlorine content make it a good target, in principle, for density and neutron logs. For resistivity measurements, two specifically designed in situ probes were installed one metre on either side of the injection point. Two surface arrays were used to monitor apparent resistivity across the diagonals of the cell. A resistivity scanning system was developed to ensure rapid acquisition of surface and probe resistivity data. The density, neutron and induction logs were performed on a regular basis in nine PVC-cased boreholes arranged on a 3 metre radius circle around the injection point. In all cases, extensive background measurements were made. |
