Geology Reference
In-Depth Information
SELF POTENTIAL (SP) METHOD
Certain natural or spontaneous potentials occurring in the underground caused
by electro-chemical or mechanical activity are called self-potentials. These
potentials, when associated with groundwater at geologic contacts, bioelectric
activity zones etc., are known as electro-kinetic or streaming potentials.
Streaming Potentials generated by subsurface water flow are the source of
the great majority of SP anomalies of groundwater interest. In a porous or
fractured media, the relative movement between solid matrix and electrolyte
(groundwater) causes an electrical potential at the interface, called zeta
potential. If the water movement were brought by a hydraulic gradient (
P
),
a difference of electric potential
E
, called streaming potential, would result
between any two points in the direction of motion. The following relation
can be observed.
E
=
C
×
P
where
C
, the streaming potential coefficient is dependent on a number of
parametres like resistivity, dielectric constant and viscosity of fluid in the
rock, the zeta potential, the grain size, the water path and others (Ahmed,
1964; Parkhomenko, 1971; Bogoslovsky and Ogilvy, 1973). The presence of
a pressure gradient in the sub-surface, however, is not a sufficient condition
to ensure the existence of an electric potential on the surface. As defined by
Fitterman (1979), it is necessary to have a pressure gradient parallel to a
boundary that separates regions of different streaming potential coefficients.
An electric field equivalent to that by a surface distribution of current dipoles
along the boundary is developed. The equipment required is extremely simple,
consisting merely of a pair of non-polarisable electrodes (to eliminate electrode
polarization effect) connected by a wire to a millivoltmeter of high impedance
(greater than 108 ohms, so that negligible current will be drawn from the
ground during the measurement). The procedure for carrying out the field
studies is described by Krishnamurthy et al. (2001).
MISE-À-LA-MASSE METHOD
Schlumberger first attempted the mise-à-la-masse method in 1920. Only
very limited case histories are available for this method (Parasnis, 1967,
1979; Ketola, 1972). The idea is to use a subsurface conductive mass (in this
case water bearing fracture) as energisation point. The conductor is energized
by a point source by lowering one current electrode in borehole below the
water table. The other current electrode is kept on the surface at far off place
(infinity). Potential on the surface is mapped in a grid pattern by keeping one
electrode fixed as the reference electrode on the surface and moving the
other potential electrode along the various profiles. The mise-à-la-masse
equipotential maps are prepared by normalizing the potential values for 1A
current (i.e, units are volts/amp).
