AN ELECTROMAGNETIC (EM) INDUCTION SURVEY FOR GROUNDWATER EXPLORATION AT THE QUEBRADA DEL ORO AQUIFER: AN OUTCROP-BASED VALIDATION EXPERIMENT

An EM Induction Survey imaged the deeper and most productive part of the Quebrada del Oro Aquifer at the UPRM Hydrologic Study Site as an intermediate electrical conductivity zone (0-to-14 mS/m) displaced by reverse fault zones. The UPRM Hydrogeologic Study Site is part of a major hydrogeologic province that consists of weathered volcaniclastic rocks where water production is structurally controlled by NNW and NNE trending faults. Measured hydraulic conductivities range in the order of 0.0015 to 0.0005 cm/s, with individual coarse grained beds reaching hydraulic conductivities of about 0.07 cm/s. Which suggest that there could be preferential fluid flow along individual coarse grained-beds, in addition to preferential fluid flow along fracture zones. To a first order constraint, field geophysical measurements and laboratory results suggest that saprolite (Zone I-C, according to the Deere and Patton(1991) weathering Classification Scheme) composed of silty fine sand and with a 20% of water content can be characterized with electrical conductivities ranging ( 23- to 41 mS/m), and a fully saturated sample can be characterized with electrical conductivities ranging (14 to 55 mS/m).