Environmental Engineering Reference
In-Depth Information
At dam sites in valleys the results of resistivity surveys are affected by the irregular ter-
rain and by changes in the electrical properties between dry materials on the abutments
and the wet material beneath the valley floor. In situations where the valley floor width is
less than five times the thickness of alluvial material there are significant side effects that
produce misleading results.
These limitations and the generally poor depth resolution (about 10% of depth) indi-
cate that the resistivity method has limited application in dam site investigations. It has
however been successfully used in the review of some existing embankment structures
(Corwin, 1999).
5.5.1.4
Electromagnetic conductivity
EM surveys have been widely used in North America (Corwin, 1999) in the assessment of
existing embankments. The development of commercial software packages for the inter-
pretation of EM data has considerably assisted the analysis of the data. Care has to be
taken to select the correct frequency and to ensure that the geophysical interpretation
matches the likely geotechnical model. EM is affected by above ground metal objects and
by AC noise sources, e.g. power lines.
The transient electromagnetic (SIROTEM) method produces resistivity-depth profiles
(Whiteley, 1983). The method is quicker and requires less space than the resistivity method,
but may not be suited to the shallow depths normally associated with dam foundations.
5.5.1.5
Magnetic
Surveys using a sensitive magnetometer do not provide significant detail on the subsurface
profile but can indicate the presence of buried ferrous objects, e.g. pipelines that can have
a major effect on EM and resistivity traverses.
5.5.1.6
Microgravity
Accurate measurement of the earth's gravitational field, together with tomographic pro-
cessing of the data, can indicate the presence of anomalous subsurface structures e.g.
dykes, cavities.
The survey procedure is simple but requires precise location and level of each measur-
ing point, which can make the method tedious, particularly in rough terrain.
5.5.1.7
Ground penetrating radar
Ground penetrating radar (GPR) uses the ability of VHF electromagnetic (radio) signals
to penetrate through soils and rocks. Usually the waves are radiated from an antenna
mounted on a wheeled sled. Reflections are received from features such as fractured zones
and cavities in rock, and from distinct layers in soils. The method has also been used
downhole.
The method has been claimed achieve to some success in the location of sinkholes and
cavities in karst areas and this appears to be its main potential in dam engineering. The
results of surface surveys in karst areas are described by Wilson and Beck (1988). Cooper
and Ballard (1988) also describe the location of cavities at the El Cajon dam site in
Honduras, using a downhole technique.
Limitations of the method include the need for a near regular surface to permit passage
of the sled and very limited depth penetration, particularly through clayey soils.
Siggins (1990) and Morey (1974) describe details of the method.
5.5.2
Downhole logging of boreholes
Table 5.2
summarizes the available methods for downhole logging of boreholes.
