ABEM Terrameter SAS 1000 / SAS 4000
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10
APPENDIX
A.
BASIC
PRINCIPLES
OF
RESISTIVITY SURVEYING
10.1
GENERAL
The SAS 1000 / 4000 measures different parameters that characterizes the ground:
•
Resistivity
•
Induced Polarization
•
Self Potential
The electrical resistivity varies between different geological materials, dependent mainly on
variations in water contents and dissolved ions in the water. Resistivity investigations can thus
be used to identify zones with different electrical properties, which can then be referred to
different geological strata. Resistivity is also called specific resistance, which is the inverse of
conductivity or specific conductance. A very brief introduction to resistivity surveying is
given below, for further information the reader is referred to textbooks on the subject (e.g.
Parasnis 1997; Reynolds 1997; Sharma 1997; Telford et al. 1990; Ward 1989).
A brief section introducing multiple gradient array surveying is also included since it is not
described in the textbooks mentioned above.
10.1.1
Resistivitity of Natural Materials
The most common minerals forming soils and rocks have very high resistivity in a dry
condition, and the resistivity of soils and rocks is therefore normally a function of the amount
and quality of water in pore spaces and fractures. The degree of connection between the
cavities is also important. Consequently, the resistivity of a type of soil or rock may vary
widely, as shown in Figure 23. However, the variation may be more limited within a confined
geological area, and variations in resistivity within a certain soil or rock type will reflect
variations in physical properties. For example: the lowest resistivities encountered for
sandstones and limestones mean that the pore spaces in the rock are saturated with water,
whereas the highest values represent strongly consolidated sedimentary rock or dry rock
above the groundwater surface. Sand, gravel and sedimentary rock may also have very low
resistivities, provided the pore spaces are saturated with saline water, which is not indicated in
Figure 23. Fresh crystalline rock is highly resistive, apart from certain ore minerals, but
weathering commonly produces highly conductive clay-rich saprolite. The variations in
characteristics within one type of geological material makes it necessary to calibrate
resistivity data against geological documentation, from for example surface mapping, test pits
or drilling. However, this applies to all geophysical methods.
