MISE-á-LA-MASSE; AN OLD GEOPHYSICAL METHOD NEWLY RE-PURPOSED FOR CHARACTERIZING KARST AQUIFERS

The mise-á-la-masse method has been a standard tool for delineating buried conductors for nearly 100 years. Originally developed for ore bodies, it has seen recent use in hydrogeology for mapping of ionic contaminant plumes, flooded mines, and geothermal waters. But, there are few examples in the literature on delineation of karst conduits, and it has been largely abandoned by practicing geophysicists in favor of 2D and 3D electrical imaging with conventional electrode arrays. This leaves many hydrogeologists unaware of its applicability to karst. Based on years of testing at numerous sites with differing degrees of karstification, this paper proposes that mise-á-la-masse remains a simple and rapid method for delineating karst conduits under phreatic, epiphreatic, and some vadose conditions, and that the data can be interpreted with minimal processing, modelling, or inversion. Furthermore, a modification of the method is presented for delineation of laterally-extensive resistive targets at the water table such as LNAPL or caverns. In all cases, the method relies upon direct access to water in the conduit system – at a swallow hole or spring, or in a well that penetrates the target. The water is energized by one current electrode, with a second current electrode at electrical infinity. The surface equipotentials are delineated with a pair of voltage electrodes to provide mapping of the target. For hydrological purposes, the method essentially uses electrons as an artificial tracer, with the benefit that electrons can be made to flow hydraulically upgradient. In addition, since the target is directly energized, some of the non-uniqueness inherent in other geophysical methods is mitigated. This paper presents a mathematical model and several case histories (successful and unsuccessful) to illustrate the suitability of various targets/applications, and the need for careful matching of survey parameters (e.g. current electrode placement, voltage electrode spacing, absolute versus gradient measurements, etc.) to target parameters (e.g. depth, diameter, resistivity contrast, etc.). The conclusion is that at suitable sites, "antiquated" mise-á-la-masse can resolve discrete hydrogeologic features that are typically undetectable and/or unidentifiable with modern (often more expensive) geophysical methods.