Geoscience Reference
In-Depth Information
unidirectional. Self-potential noise is often associated with alternating electric current (AC) gener-
ated from thunderstorm activity, variations in the Earth's magnetic field, heavy rainfall effects, and
so forth (Reynolds, 1997).
8.2.2 s
o u R c e s
o f
c
o n s i s t e n t
a n d
u
n i d i R e c t i o n a l
n
a t u R a l
e
l e c t R i c
P
of t e n t i a l
g
R a d i e n t s
Consistent and unidirectional natural potential differences generally fall into three main catego-
ries—electrokinetic, electrochemical, and mineral:
1.
Electrokinetic
—This type of potential is also called electrofiltration potential or stream-
ing potential and is associated with electric current generated by the movement of a dis-
solved ion containing water through a porous media such as soil or rock. The underlying
mechanism creating current and potential differences is believed to be electrokinetic cou-
pling between pore side walls and dissolved ions (Sharma, 1997). Electrokinetic potentials
increase in the direction of water flow, and electric charge moves in the opposite direction
(Reynolds, 1997). Therefore, profiled or mapped self-potential anomalies caused by elec-
trokinetic processes will be positive for locations where there is convergence of subsurface
water flow and negative where there is divergence of subsurface water flow.
2.
Electrochemical
—Differences in pore water dissolved ion concentrations between two
subsurface locations results in spontaneous transport of electric charge carriers (dissolved
ions) and development of spatial gradients of electric potential. Diffusion potential gradi-
ents exist with this spatial charge imbalance scenario when pore water cation and anion
mobilities differ. Nernst potential gradients exist under these circumstances when there are
no differences in dissolved cation and anion mobilities. For diffusion and Nernst electric
potential gradients to persist, subsurface spatial differences in pore water dissolved ion
concentrations must be maintained. The mechanism for maintaining spatial differences in
pore water dissolved ion concentrations has not yet been identified, but the most likely can-
didate is some form of below-ground chemical interaction involving atmospheric oxygen
(Parasnis, 1986). Electrochemical potential gradients are also formed due to the subsurface
presence of quartz veins, pegmatite intrusives, or clay deposits, and all three cases are
probably a response to cation adsorption processes.
3.
Mineral
—Spatial self-potential anomalies are often measured at the ground surface over
massive metal ore bodies. The mechanism generating these anomalies is not completely
understood. However, the explanation having the greatest acceptance involves natural elec-
tric currents originating from chemical oxidation and reduction reactions between a metal
ore body straddling the water table and the pore water dissolved ions present in the sur-
rounding rock or soil.
It should be noted that electric potential differences measured at the surface in agricultural settings
are in all likelihood due to either electrokinetic or electrochemical sources.
8.2.3 s
e l f
-P
of t e n t i a l
e
q u i P M e n t
, f
i e l d
P
R o c e d u R e s
,
a n d
d
a t a
a
n a l y s i s
P
R o d u c t s
The basic equipment setup needed for self-potential surveying is very simple and includes only
three components: electrodes, a voltmeter, and electric cable. The voltmeter measures the electric
potential difference between two electrodes placed at the ground surface that are connected to the
voltmeter via electric cable. The metal stakes commonly used as electrodes for resistivity geo-
physical surveys are not acceptable for obtaining self-potential measurements. The problem with
using metal stakes for electrodes is the formation of unwanted electric potentials due to electrode
polarization caused by contact between the bare metal in the stake and the electrolytic aqueous
solution present within the soil. For resistivity surveys, the unwanted potentials produced by metal
