The Application Of Automated High Resolution DC Resistivity In Monitoring Hydrogeological Field Experiments

Over the past several years, an automated high resolution DC resistivity system<br>has been developed and used at the University of Waterloo as a spatial and temporal<br>monitoring technique in hydrogeological field experiments. This paper focuses on the<br>technology and presents three case histories of its application.<br>Measuring DC resistivity in situ can be an effective technique with which to<br>monitor hydrogeological field experiments, and it can pose some interesting challenges<br>to those wishing to employ the method.<br>During the summer of 1991, in situ probes and surface arrays were used to<br>monitor the migration of perchloroethylene in a saturated sandy aquifer. PC based<br>automated DC resistivity scanning equipment was developed in order to determine the<br>utility of resistivity to detect PCE in situ. The probes used were a casing with embedded<br>electrodes and a series of removable probes which contained all cabling, making them<br>transparent to ground penetrating radar when the probes are temporarily removed.<br>Conventional, removable, surface arrays were also employed.<br>In September of 1992, a series of kerosene injections were completed in a 3.6<br>metre diameter by 1.7 metre deep tank packed with a medium sand. A current booster<br>was used in conjunction with the same apparatus developed for the PCE experiment to<br>monitor the migration of kerosene in the unsaturated zone using both an in situ probe<br>and surface arrays.<br>During a soil flushing field experiment the system was used to monitor a fresh<br>water infiltration event, salt plume emplacement event and a drainage event. In this<br>case, more economical in situ probes were constructed (since GPR was not an issue),<br>and in greater quantity, to monitor at a higher spatial density. The kerosene and the soil<br>flushing experiments were conducted concurrently using a serial network of distributed<br>multiplexers linked to a common data acquisition system.<br>The technique has shown to be able to acquire a data set sampled very frequently<br>in time and space, during parts of experiments when time constraints may not allow such<br>data to have been gathered by any other means.