Geology Reference
In-Depth Information
also be inclined in order to examine lateral structures, when access and
installation conditions for the drill allow it or impose it.
There are various techniques, and the choice of which to use is dictated
by the geologic character of the formations in question, by the estimated
nature and depth of the hypothesized features to be examined, and by the
available budget. This last criterion can be of primary importance, given
the generally high costs of this method in the context of reconnaissance.
In unconsolidated terrain and at relatively shallow depths, a manual or
motorized auger can be used to create a cross-section and take samples of
the various units, as long as the short-term stability of the hole is secured.
When the terrain is unstable, underholing allows the creation of a vertical
excavation in which the walls are protected progressively by superposed
concrete rings, or by metallic tubing. The excavation can be done manually
if the diameter of the hole allows it, or with the help of a hammer-grab
sampler (Benoto method).
For greater depths or in indurated rock formations, autonomous drill
rigs on wheels or treads, or mounted on an all-terrain truck, are used.
Various drilling techniques enable perforation of the ground by percussion,
by rotation, or by rotary-percussion, and use cooling fl uid (air or water),
which also serves to carry rock fragments back up to the surface. A dense
recycled mud (often bentonitic mud) can also be used to bring up rock debris
and to maintain the walls of the hole. In other cases, such stabilization is
provided by tubing (steel or PVC).
The level of information provided by such drilling varies greatly with
the quality of the material brought back to the surface, either rock fragments
(destructive drilling) or actual continuous, linear samples, called cores
(core drilling).
The creation of the geologic cross-section of a borehole must be very
precise, taking into account not only the information obtained by a detailed
examination of the samples, but also observations made by the borer during
the drilling process (speed, sudden drops of the drill, water infl uxes, loss
of the cooling fl uid, lack of cuttings returning to the surface).
Cores allow the establishment of a perfectly reliable cross-section, which
it is wise to complete with and analysis of the recuperation rates in order
to take into account the washing out of fi ne particles by the cooling fl uid
in unconsolidated or poorly coherent terrain.
Destructive drilling requires regular samples to be taken throughout
the process, in order to not mix fragments from different depths. The
creation of a cross-section can be more diffi cult, especially when there is
little or no debris due to losses in the terrain (as is the case for karst cavities
for example). Additional information can be gleaned from instantaneous
drilling speed, which indicates the state and the compactness of the rock.
This parameter can be automatically registered, along with the drill rig's
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