Geophysical Reconnaissance in Bedrock Boreholes - Finding and Characterizing the Hydraulically Active Fractures

By Frederick L. Paillet

Abstract

Geophysical well logs are used to identify the depth where permeable fractures intersect boreholes in fractured bedrock aquifers. This information is used to sample fracture populations and to develop models for fracture flow networks at field sites. Conventional geophysical logs do not indicate the hydraulic properties of fractures and only characterize fractures in the immediate vicinity of the borehole. Direct hydraulic characterization is possible using high-resolution flow profile logging. A well-posed inversion problem can be formulated to solve for estimates of fracture-zone transmissivity and hydraulic head if two different steady or quasi-steady flow profiles are obtained, along with drawdown information. At the Mirror Lake, New Hampshire, study site, these profiles are obtained under ambient and steady pumping at about 4 liters per minute, but steady injection may be more convenient at other sites. When cross-borehole flow experiments are conducted, the technique can be expanded to generate estimates of fracture-zone storage coefficient and to infer patterns of fracture connections in the region between pumped and observation boreholes. The effectiveness of these techniques is demonstrated by the agreement between flow profile analysis and straddle-packer hydraulic tests conducted at the Mirror Lake site.