The Study of the Geometry of Aquifers - Hydrogeology: Objectives, Methods, Applications - page 109

This method is particularly well-adapted to mapping the subsurface
topography of a bedrock layer covered by a variable thickness of material
(scree over limestone for example), and to investigating the extent of the
alteration zone of a compact rock unit, sandy regolith over granite for
example. (Figure 58).
b) High resolution seismic refl ection
The source and the geophones are moved along a line. Computer software
can then create cross-sections describing the geologic structure.
2.2 The limitations of geophysical methods
In the domain of hydrogeology, the use of geophysics is generally limited
to electric resistivity and seismic refraction.
The experimental determination of the values of the physical parameter
under study and its variations can generally indicate several different
possibilities, and only a strong initial understanding of the natural context
can enable the best geologic interpretation. A geophysical survey therefore
cannot provide detailed information, but can give a good overall conception
of the study area as a whole. It can resolve some uncertainties, particularly
with respect to whether the bedrock is homogeneous or not, and can help
determine the optimal positioning for verifi cation and calibration drilling
projects. Geophysical interpretation obviously requires a suffi ciently precise
topographic map.
Each geophysical method has its applications and its limitations. They
should therefore not be used for systematic surveys, but instead be used to
answer specifi c questions in a manner appropriate to the given context, as a
function of the topography and the nature and characteristics of the known
or supposed terrain. In certain cases, several methods can be combined
in order to facilitate the fi nal interpretations. The appeal of geophysical
surveying lies in its ease of application in the fi eld, without requiring the
creation of specifi c access points and without causing any destruction, as
well as in its relatively low cost.
3 RECONNAISSANCE DRILLING
The goal of reconnaissance drilling is to provide point data on the nature
and the vertical succession of the subsurface terrain. The boreholes can