Geoscience Reference
In-Depth Information
collection mode, one electrode (
P
1
) remains stationary at a base station, while the second electrode
(
P
2
) is moved along a transect or a series of transects. The location referenced for each measure-
ment obtained is assumed to coincide with the position of the moving electrode (
P
2
in Figure 8.6b).
Potential differences (voltage) are the values normally reported for this second self-potential field
survey procedure.
Self-potential measurement transects, where possible, should be oriented perpendicular to the
known directional trends of subsurface features that are being investigated. Self-potential data col-
lected along a single transect can be plotted as a profile showing the potential difference or potential
gradient changes with respect to transect distance. If there are a series of self-potential measurement
transects, contour maps can be generated detailing horizontal spatial variations in electric potential
difference or electric potential gradient.
8.2.4 P
o s s i b l e
a
g R i c u l t u R a l
a
P P l i c at i o n s
f o R
s
e l f
-P
o t e n t i a l
M
e t h o d s
Self-potential anomalies associated with electrokinetic processes have been used to locate leaks in
dams, reservoirs, and landfills (Reynolds, 1997; Sharma, 1997). Self-potential methods may likewise
be useful for locating leaks in animal waste storage ponds and treatment lagoons. Again, profiled
or mapped self-potential anomalies caused by electrokinetic processes will be positive for loca-
tions where there is convergence of subsurface water flow and negative where there is divergence
of subsurface water flow. Consequently, self-potential surveys could prove valuable for determining
drainage pipe functioning condition with respect to water removal (positive anomalies should be
generated) or subirrigation (negative anomalies should be generated). In addition, based on the same
convergent flow electrokinetic potential considerations, self-potential methods might be employed
to provide insight on the magnitude and extent of the water table depression surrounding an irriga-
tion well. Finally, horizontal spatial patterns for soil salinity and soil clay content could be evaluated
with self-potential methods, if salinity and clay content spatial variations produce corresponding
spatial changes in electrochemically generated potential differences or gradients.
8.3
SeISMIC MethodS
8.3.1 s
e i s M i c
M
e t h o d
i
n t R o d u c t i o n
Seismic waves are essentially elastic vibrations that propagate through soil and rock materials.
Seismic waves can be introduced into the subsurface naturally, with earthquakes being a prime
example. Earthquakes are seismic waves, oftentimes extremely destructive, that typically result
from the energy released due to movement along large fractures (faults) in the Earth's crust. Data
obtained from earthquakes have allowed seismologists to resolve the overall structure of the Earth
(solid iron/nickel inner core, liquid iron/nickel outer core, mantle, and lithosphere).
Explosive, impact, vibratory, and acoustic artificial energy sources can also be used to intro-
duce seismic waves into the ground for the purpose of investigating subsurface conditions or fea-
tures. The use of active (artificial energy source) seismic geophysical methods is widespread in
the petroleum and mining industries. Active seismic methods have additionally been employed for
hydrological, environmental, geotechnical engineering, and archeological investigations. Although
presently used very little for agricultural purposes, seismic methods are likely to find significant
agricultural applications in the near future.
For seismic geophysical methods where artificial energy is supplied, the seismic waves gener-
ated are timed as they travel through the subsurface from the energy source to the sensors, which
are called geophones. Incoming seismic wave amplitudes, and hence energy, are also measured
at the geophones. The energy source is ordinarily positioned on the surface or at a shallow depth,
and the geophones are normally inserted at the ground surface. Data on the timed arrivals and
amplitudes of the seismic waves measured by the geophones are then used to gain insight on below-
ground conditions or to characterize and locate subsurface features. Numerous texts provide detailed
