MISE-À-LA-MASSE AND RESISITIVITY COMBINED TO MAP KARST TERRAIN, EASTON, PA

Mise-à-la-masse is a geophysical method traditionally used to delineate ore bodies. One current electrode is embedded in the target with the return electrode distant, effectively at infinity. The shape of the ore body is then roughly outlined by the distribution of voltage equipotentials measured at the surface. Our objective was to assess the usefulness of mise-à-la-masse for mapping fracture systems extending from karst cavities. To be effective, mise-à-la-masse requires a conductive target for current to propagate outward from the interior. We lowered an electrode down a borehole into the mud-covered floor of a cavity at Metzgar Field, an athletic complex belonging to Lafayette College near Easton, PA. The target cavity was an air-filled void roughly 2 m wide, 2 m high, 4 m long, and 7 m below the surface, located within the vadose zone of the Epler Formation, a karstified Ordovician limestone. The mise-à-la-masse lines were arrayed radiating outward in 15º increments from a point centered over the cavity. Each line consisted of 56 electrodes spaced 1.5 meters apart. Voltage equipotentials measured at the surface showed asymmetry. The current emerged along strike approximately 8 m SW of the current source, suggesting that a fracture system extends from the cavity along strike, and is in contact with overlying soil. This interpretation agrees with the results of a smoke test made using Hurco's Power Smoker II and LiquiSmoke®. After injecting smoke into the cavity for about 30 minutes, puffs arose from the surface of the field at points along strike. Smoke also emerged from a borehole along strike 21 m SW of the injection point, but not from any off-strike boreholes. Our initial results suggest mise-à-la-masse may improve karst characterization at sites where a borehole intersects a void, but work remains comparing this study and previous dipole-dipole and azimuthal resistivity surveys at Metzgar Field.