Analyzing outcrop-scale fracture features to supplement investigations of bedrock aquifers

A case study was conducted of 79 outcrops within 150 meters of the nine, 7590 m deep boreholes at the Lawrence Berkeley Laboratory (LBL) Fracture Hydrology Field Site in Raymond, California, USA, in order to make preliminary comparisons between surface fracture data and geophysical and hydrologic testing conducted in the boreholes. The orientation, trace length, spacing, roughness, planarity, associated mineralization, and domains (the geographic distribution of specific fracture sets) of 471 fractures were measured. Five families of steeply-dipping fractures and one family of shallow dipping fractures comprise 75 percent of the data and trend 52, 62, 130, 147, 173, and 35, respectively. The geographic distributions (domains) of the families, however, show the well field to be within the domains of the 62-, 173- and 35-trending families. The steeply-dipping fractures detected in the boreholes by LBL via acoustic televiewer logging trend about 65, 173, and 30 corroborating the findings of the fracture-domain analysis. Results indicate that the boreholes are located within a laumontite-mineralized area, including a steeply-dipping, 160-trending zone, 520 cm wide, of laumontite-rich pods that transects the boreholes. Independent hydrologic tests by LBL revealed a 160-trending barrier to groundwater flow between some of the boreholes, precisely where the 160-trending zone of laumontite-mineralized pods was mapped.