MISE-À-LA-MASSE, RESISITIVITY TOMOGRAPHY AND SMOKE TESTS COMBINED TO MAP KARST, EASTON, PA

Mise-à-la-masse (MALM) 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 with respect to a base station. Our objective was to assess the usefulness of MALM for mapping fracture systems extending from karst cavities. MALM requires a conductive target for current to propagate outward from its 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, aligned along N60E strike. It is located within the vadose zone of the Epler Formation, a karstified Ordovician limestone. MALM 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. These results agree with previous dipole-dipole and azimuthal resistivity surveys at Metzgar Field; both indicate a highly conductive zone within the resistive limestone bedrock. This conductive zone suggests that water is gathering above the void, which acts as a capillary barrier. A resistivity inversion perpendicular to strike shows a small conductive anomaly, while parallel to strike it appears to be larger; this corresponds to the geometry of the void. Interpretations made from comparing the MALM and resistivity data agree 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 results suggest MALM may improve karst characterization at sites where a borehole intersects a void with sediments.