Geology Reference
In-Depth Information
areas. If the ground is soft and the main consideration is speed, large numbers
of metal tent pegs can be pushed in along a traverse line by an advance party.
Polarisation
voltages are generated wherever metals are in contact with
the ground water. Their magnitudes depend on the metals concerned and they
are relatively small when electrodes are made of materials such as stainless
steel. Polarisation voltages are unidirectional, and the routine reversal of
current flow generally achieves acceptable levels of cancellation of these
effects in conventional DC surveys.
6.2.2 Non-polarising electrodes
Polarisation voltages would be serious sources of noise in
SP
surveys, which
involve the measurement of small natural (and unidirectional) potentials,
and in induced polarisation (
IP
) surveys (see Chapter 7), and non-polarising
electrodes must therefore be used. Their design relies on the fact that no
contact potential exists at an interface between a metal and a saturated
solution of one of its own salts. The commonest type consists of a copper
rod in contact with a saturated solution of copper sulphate. The rod is
attached to the lid of a container (
pot
), which has a porous base of wood, or,
more commonly, unglazed earthenware. Contact with the ground is made
via the solution that leaks through the base. Some solid copper sulphate
should be kept in the pot to ensure saturation and the temptation to 'top
up' with fresh water must be resisted, because voltages will be generated if
any part of the solution is less than saturated. The high resistance of these
electrodes is not generally important because currents should not flow in
voltage-measuring circuits.
Despite some theoretical advantages, non-polarising electrodes are sel-
dom used in routine DC work. In induced polarisation surveys it may very
occasionally be desirable to use non-polarising electrodes for current in-
jection as well as voltage measurement, but not only does resistance then
become a problem but the electrodes deteriorate rapidly due to electrolytic
dissolution and deposition of copper.
Copper sulphate solution gets everywhere and rots everything and is there-
fore not liked. Non-polarising electrodes that use lead/lead chloride or tin/tin
chloride combinations are more pleasant to handle, but considerably more
expensive. Stainless steel polarises to only a small extent, and also makes ef-
ficient current electrodes, and is therefore favoured where the same electrode
is sometimes to be part of a current pair and sometimes of a voltage pair.
6.2.3 Cables
The cables used in DC and IP surveys have traditionally been single-core,
multi-strand copper wires insulated by plastic or rubber sleeving. Thickness
is usually dictated by the need for mechanical strength rather than low
