Introduction to the resistivity surveying method. The resistivity of ...

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2.7.8 Marine bottom resistivity survey - U.S.A.
Contrary to popular belief, it is actually possible to carry out resistivity surveys
underwater, even in marine environments. Figure 23a shows the apparent resistivity
pseudosection from a survey along the seabed between Fisher Island and the mainland in
Miami, Florida (Lagmanson 1998). The seabed consists of mud with a thickness of up to
about 3 metres followed by sand (up to 2 metres thick) overlying a sandstone and limestone
bedrock which also contains cavities. Due to the low resistivity of the seawater, the electric
potentials measured were extremely small, even with the Wenner-Schlumberger array. To get
accurate readings under such adverse conditions, a very sensitive resistivity meter system was
used (Lagmanson 1998). The subsurface model after inversion is shown in Figure 23c. Note
the seabed topography, and the very low resistivity mud and sand upper layers overlying the
higher resistivity bedrock.
Figure 23. (a) The measured apparent resistivity pseudosection, (b) the calculated apparent
resistivity pseudosection for the (c) model section from an underwater marine survey.
2.7.9 Time-lapse water infiltration survey - U.K.
Resistivity imaging surveys have not only been carried out in space, but also in time!
In some studies, the change of the subsurface resistivity with time has important applications.
Such studies include the flow of water through the vadose (unsaturated) zone, changes in the
water table due to water extraction (Barker and Moore 1998), flow of chemical pollutants and
leakage from dams (Johansson and Dahlin 1996).
Copyright (1999-2001) M.H.Loke