Integrating Multidisciplinary Investigations in the
Characterization of Fractured Rock
By Allen M. Shapiro, Paul A. Hsieh, F. Peter Haeni
ABSTRACT
Because site conditions and project objectives vary from site to
site, there is no single approach to the characterization of
fractured rock for problems of environmental assessment. There
is, however, a unifying theme that is consistent in problems of
site characterization; that is, most problems depend to a great
extent on advective ground-water movement. Once the spatially
heterogeneous hydraulic properties of the bedrock are
conceptualized to define advective ground-water flow in the
detail consistent with project objectives, other physical,
chemical, and biological processes of interest can then be the
focal point of characterization. Without first characterizing
advective ground-water movement, it is likely that ambiguous and
perhaps erroneous interpretations of complex physical, chemical,
and biological processes will result. Because of the extreme
heterogeneity that is anticipated in fractured rock, no single
method of characterization can explicitly and unambiguously map
the spatial distribution of hydraulic properties that control
advective fluid movement. The integration of geologic,
geophysical, hydrologic and geochemical information is a
necessity in developing a defensible model of heterogeneity in
fractured rock. A multiple-method hierarchial approach to
characterization in fractured rock is presented, where in
regional geologic and hydrologic information is first
synthesized to act as a background to more detailed
site-specific reconnaissance using fracture network mapping and
surface geophysics. These noninvasive reconnaissance techniques
can be used to observe or infer the location and orientation of
fractures and fracture zones and fracture properties that are
critical in siting bedrock boreholes in which to conduct in
situ investigations using single- and multiple-borehole
techniques. Single-hole techniques that can be used to
characterize heterogeneity in the bedrock include conventional
geophysical logging, borehole scanning, borehole flow, borehole
radar, hydraulic testing, and chemical
sampling. Multiple-borehole methods, such as seismic and radar
tomography, borehole flow, hydraulic and tracer testing, are
conducted to infer the connectivity and spatial extent of
transmissive fractures. Although the characterization methods
summarized in this article can be conducted independently, the
synthesis of information from multiple characterization methods
is the only means of developing an accurate understanding of
heterogeneity in bedrock.