Field tests of geophysical techniques for predicting and monitoring leach solution flow during in situ mining

Abstract

The Bureau of Mines is conducting research to develop improved methods for predicting and monitoring the flow of leach solution during in situ mining. Potential benefits include higher metal recovery through better solution distribution and more cost-effective environmental monitoring. The ability of seismic tomography to detect fractured zones and saturated areas was field tested for applications in predicting flow patterns and in monitoring leach solution above the water table. Seismic refraction tomography located fractured zones in a shallow refracting rock layer. A crosshole field test located water injected between source and receiver boreholes. In field tests at the University of Arizona's San Xavier experimental mine, tomograms of the seismic velocity distribution indicated dipping rock layers of contrasting seismic velocities consistent with borehole logs. Comparing data collected before and during water injection located wetted regions. Six electromagnetic methods for determining where high-conductivity leach solution has replaced groundwater were tested at the San Xavier mine in cooperative research with the University of Arizona and Sandia National Laboratory. The methods were variations of surface and borehole electromagnetic induction and controlled source audio-frequency magnetotellurics. They were tested by conducting surveys both before and during injection of a brine solution. The salt-water brine was injected into boreholes and allowed to permeate the surrounding rock, creating a conductive plume. The surface methods located the brine at the water table, and the borehole methods located brine-filled fractures. The analysis is not complete, but preliminary results appear promising for applying these methods to monitoring leach solution.