Cross-borehole flow profiling—delineating subsurface flow paths within a geophysically defined aquifer framework

Abstract

An extensive geophysical tool box is available with which to construct models of subsurface flow paths in complex aquifers using a combination of surface and borehole methods. These methods include electromagnetic and seismic sounding from the surface and downhole-imaging methods based on acoustic, optical, and electrical properties of rocks surrounding the borehole. However, such methods are inherently limited in hydrogeophysics because of the need to measure properties such as rock alteration or local fracture density that are indirectly related to subsurface flow rather than directly sampling the hydraulic parameters (permeability, storage coefficient, and hydraulic head) of subsurface flow paths. High-resolution borehole flow profiles provide an opportunity to relate geophysical characterization to hydraulic properties as an integral part of the standard geophysical logging program. Single-borehole flow analysis provides information on the hydraulic properties of flow paths in the limited region immediately adjacent to the borehole. Cross-borehole methods, where flow is measured in one borehole while an adjacent borehole is stressed, characterize the large-scale flow paths that connect the individual flow zones identified in individual boreholes.