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2D Resistivity Data. Image CC

Why Do A Resistivity Survey?
The resistivity of the subsurface varies depending on rock type, porosity, permeability, the amount and
type of salts present in the ground water and the type of clay present. When resistivity is low,
conductivity is correspondingly high. Therefore, the method is useful for locating massive sulphides
deposits and graphite-rich zones, both of which conduct electricity. When exploring for epithermal gold
deposits the presence of quartz-rich zones and veins is important. Quartz, the most common mineral
in epithermal veins and associated breccia, has a very high resistivity relative to other minerals. For this
reason, quartz-rich zones that may host gold mineralization, show up in surveys as zones of high
resistivity or low conductivity.

How Is a Resistivity Survey Done in the Field?

An electrical resistivity survey involves laying out a series of electrodes, each driven into the ground
about 150 millimeters. Their spacing is dependent on the depth of penetration required. The further apart
the electrodes, the deeper the resistivity measurements of the subsurface that can be taken. Typically
resistivity surveys range in depth from a few metres to more than 100 metres. The disadvantage of trying
to obtain measurements from greater depths is a resultant loss in resolution.
There are number of possible layouts for electrodes; the Wenner and Schlumberger arrays are the most
common. The Wenner array is the simplest, having an equal spacing between the electrodes. In the
Schlumberger array, the spacing between the current electrodes is greater than the spacing between
the voltage electrodes. Other configurations include the dipole-dipole and pole-dipole arrays.
The simplest electrode
arrangement used in resistivity
surveys is the Wenner array.

A resistivity survey may consist
of a single line profile across a
point of interest. Alternatively,
readings are taken in a grid
pattern to obtain a three-
dimensional image of the
subsurface resistivity. A grid
survey requires either
advancing the electrodes along a
given line or by having a number
of arrays connected along a line
with data simultaneously collected and recorded by the instrumentation.
The normal result of a resistivity survey is a number of profiles that show the variation in resistivity with
depth. These profiles are examined and interpreted by a geologist who locates targets for follow-up
exploration.
Induced Polarization (IP) Method

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