Bringing Together Co-located Geological, Geophysical and Hydrogeologic Observations of Cemented Fault-Zone Permeability

Abstract:

We examine co-located outcrop and subsurface geological, geophysical, and hydrogeological observations of a cemented fault in the Rio Grande rift. The partially cemented Loma Blanca fault offsets poorly consolidated piedmont slope and alluvial sediments; along parts of its exposure the fault consists of a 2- to 5-m-wide calcium carbonate cemented zone. The degree of cementation of the fault, which varies along its >4 km long surface exposure, is closely related to host-sediment grain size; coarser grained sections of the fault are more strongly cemented than finer grained portions. We hypothesize that the spatially variable degree and thickness of cementation is a key control on the ability of the Loma Blanca fault to act as a barrier to fault-normal fluid flow. We use geologically-validated geophysical observations to map variations in fault-zone parameters in the subsurface, collecting co-located GPR, DC-resistivity, and audio-magnetotellurics data along a transect across the fault. Differences in the electrical properties between strata offset by the fault allow us to constrain fault orientation and throw. Future investigations of the relationships between the degree of fault-zone cementation and normalized chargeability may allow us to map the estimated degree of cementation in the sub-surface. Using mathematical modeling we test alternative designs for future pumping and injection well tests that will be used to update estimates of the location, orientation, and properties of the fault, especially fault-normal permeability. The goal is a novel fault-zone permeability model based on the integration of geological, geophysical, and hydrogeological observations and models.