Geology Reference
In-Depth Information
be taken for specifi c projects, in the case of large study projects. The use of
satellite images (Landsat, Spot, Quickbird, Ikonos,…) is now common.
Geologic surveys must extend past the limits of the study area, and aim
to cartographically pinpoint the extent of aquifers and, particularly, their
contacts with bordering impermeable terrain and/or with other reservoirs.
They consist of identifying and positioning a maximum number of direct
observations on the map, both at natural outcrops (steep slopes, rises,
shelves, little valleys) and at new or old construction sites (excavations,
rubble heaps, quarries, passageways). Field mapping is directly aimed at the
identifi cation and mapping of different formations, the survey of eventual
rock-type variations and zones of weathering or superfi cial covering, and
of structural measures (strike and dip, joints, faults), which are severely
lacking in commercial geologic maps.
Cartographic boundaries do not always coincide with stratigraphic
limits, tending instead to identify lithologic units in terms of their
hydrogeology (impermeable formations, porous, fractured, or karst
reservoirs).
It is equally important to take into account the paleogeographies that
succeeded each other in the study area, in order to well understand possible
variations in thickness and facies characterizing certain formations, to
be able to reconstitute the geometry of glacial or fl uvial sediments, and
to identify spatio-temporal relationships between the sea and coastal
aquifers.
Certain normal or abnormal contacts can be revealed by morphologic
(rises and shelves in the landscape) or hydrogeologic (dogleg turns in
valleys) clues, but also by contrasts in vegetation or weathered soils.
In areas with very dense vegetation, paths are sometimes cleared in
order to be able to see the nature of the terrain. It can also be diffi cult to
precisely locate certain important observations in such areas, as well as
in topographically rugged areas, which may require the intervention of a
geometer or the use of a GPS.
The mapping can be completed by locating boreholes and/or
geophysical surveys found during the preliminary research phase, as well
as by hydrogeologic features identifi ed through archives and on the ground
(springs, wells, water boreholes, karst cavities, losses). When inventorying
these features, it is always very helpful to ask the local people and the well
users.
The exploration of natural cavities (caves, chasms) developed in karst
terrain can also be helpful in determining the cross-section of geologic
series and the position of major tectonic accidents. When they are very
large or highly-developed, these underground networks can indeed provide
fi rst-class lithologic cross-sections, and can even serve as calibration by
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