Geology Reference
In-Depth Information
The preliminary research generally leaves more or less important
uncertainties, particularly in areas where the geologic formations are not
directly accessible (vegetative cover, superfi cial deposits, urbanisation and
privatisation of land). The classic survey can also reveal its limitations in
very diffi cult natural contexts and enables various interpretations (fl uvial
or glacial erosion, slopes entirely masked by scree, gravitational reworking
of slopes, very complex structure).
The remaining unknowns are then clarifi ed using other techniques,
the choice of which is dictated by the goals of the study, but also by the
accessibility of the study area and by the project budget. The most reliable
methods provide
in situ
observation and representative samples (coring,
wells, pits, galleries). They allow a paleontological identifi cation of the
series in question (when it is justifi ed) and a calibration reference for indirect
methods (geophysical surveys, destructive boring, well logging).
2 GEOPHYSICAL PROSPECTING
2.1 General characteristics
The principle of geophysical prospection is to determine, based on
measurements taken on the ground, the physical characteristics of the
bedrock and to deduce information on its makeup. A number of methods
exist based on the physical parameter in question.
2.1.1 Electrical methods
a) Spontaneous polarization or self-potential (SP)
The natural electric currents tied to the movement of water in the soil
(infi ltration, groundwater fl ow) or to the differences in potential between
different aquifers are measured. They can then highlight fractures draining
aquifers.
b) Electrical resistivity method
This method is based on the study of the electrical conductivity of rocks,
which is a direct function of its water content. What is actually being
measured is the electrical resistivity (the inverse of conductivity), which is
expressed in ohm⋅meters (Ω⋅m). The resistivity of formations varies by a few
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