Environmental Science & Engineering - www.esemag.com - May 2004
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Geophysics can reduce risks in trenchless technology
Associated Mining Consultants
Ltd. (AMCL) is an affiliate of
the Associated Engineering
group of companies. AMCL’s
geophysical staff conduct geophysical
investigations as part of horizontal
directional drill programs. Environmental
concerns have prompted the
use of directional drilling technology
as a trenchless method of crossing
watercourses along proposed utility
corridors. Several types of subsurface
features, if not identified prior to initiating
pipeline design, may affect the
success of directional drilling projects.
These features include boulders, cobbles
and sand seams, swelling clays,
extensive aquifers, infilled collapse
features, and abandoned mine workings.
Risks posed by unfavourable geologic
conditions may be minimized
with carefully planned geotechnical
investigations. Studies have recently
included geophysical surveying methods
to specify optimal test hole positioning,
to interpolate geologic conditions
between existing test holes, and
to assess engineering properties of
soils and underlying rock. Several geophysical techniques, including ground
penetrating radar, seismic refraction
(both land and marine) and electrical
resistivity/conductivity, have been
used to assess the different physical
properties of the subsurface material,
and to map stratigraphy between the
entry and exit points of the directionally
drilled installations.
Surveys conducted in the
Edmonton and Calgary areas illustrate the advantages of using complementary geophysical
methods in conjunction with optimally placed drill holes.
Figure 1 presents the combined results of a ground
penetrating radar and seismic refraction survey at a proposed
pipeline crossing of the Bow River in Calgary.
The ground penetrating radar delineated the depth to
weathered bedrock. The bedrock consisted of electrically
conductive shales and sandstones of the Paskapoo Formation,
which were overlain by electrically resistive gravels. The
large contrast in physical properties enabled detection using
the ground penetrating radar survey. The seismic refraction
survey delineated the depth to competent bedrock based on
contrasts in density between the competent bedrock and overlying
material. The combination of these boundaries proved
useful in the design of a horizontal directional drill program.
Figure 2 represents the results of a marine seismic survey
in the North Saskatchewan River in Edmonton. The survey
successfully mapped the depth to competent bedrock. When
combined with the results of a marine ground penetrating
radar survey and seismic refraction surveys on land, sufficient
information for design of a horizontal directional drill
program was provided.
The two examples show that geophysical surveys can
reduce risk in the planning and design of directional drill programs.
However, geophysical surveys rely on mappable contrasts
in physical properties to be successful. These contrasts
do not always exist. It is imperative that the local geology be
reviewed prior to conducting the surveys to ensure that the
optimum approach is undertaken. The experience of the geophysicists
in performing these types of surveys also plays a
role in reducing the risk.
Contact Jim Henderson at hendersonj@amcl.ca.
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