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.