Engineering Geology

Geological mapping frequently forms the initial phase of exploration and should identify
potential aquifers such as sandstones and limestones, and distinguish them from aquicludes.
Superficial deposits may perform a confining function in relation to any major aquifers they
overlie, or because of their lithology, they may play an important role in controlling recharge
to major aquifers. Furthermore, geological mapping should locate igneous intrusions
and major faults. Obviously, it is important during the mapping programme to establish the
geological structure.
Direct subsurface exploration techniques are dealt with in Chapter 7.
Geophysical Methods and Groundwater Investigation
As far as seismic methods are concerned, velocities in crystalline rocks are generally high to
very high (Table 7.4). Velocities in sedimentary rocks increase concomitantly with consolidation
and with increase in the degree of cementation and diagenesis. Unconsolidated sedimentary
accumulations have maximum velocities varying as a function of the mineralogy, the
volume of voids (either air-filled or water-filled) and grain size.
Porosity tends to lower the velocity of shock waves through a material. Indeed, the compressional
wave velocity, V p , is related to the porosity, n, of a normally consolidated sediment
as follows:
1 n 1 n
V V V
= + - Chapter 4
p pf pl
(4.32)
where V pf is the velocity in the pore fluid and V pl is the compressional wave velocity for the
intact material as determined in the laboratory. The compressional wave velocities may be
raised appreciably by the presence of water.
The resistivity method does not provide satisfactory quantitative results if the potential
aquifers being surveyed are thin, that is, 6 m or less in thickness, especially if they are
separated by thick argillaceous horizons. In such situations, either cumulative effects are
obtained or anomalous resistivities are measured, the interpretation of which is extremely
difficult, if not impossible. In addition, the resistivity method is more successful when used
to investigate a formation that is thicker than the one above it.
Most rocks and soils conduct electric current only because they contain water. But the widely
differing resistivity of the various types of pore water can cause variations in the resistivity of soil
and rock formations, ranging from a few tenths of an ohm-metre to hundreds of ohm-metres.
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