Geology Reference
In-Depth Information
Transmitter
Receiver
a
d
t
Thin conductor
15
-25
10
7.5
Dip 60
30
5.0
-20
50
2.5
-15
d
a
- = 0.10
-10
a
- = 0.25
Fig. 9.20
Example of a vector diagram
used in estimating the parameters of a thin
dipping conductor from the peak real and
imaginary component values. (Redrawn
from Nair
et al.
1968.)
-5
0
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
-54
Negative peak value in percentage, in-phase component
mineral surveys. The potential electrodes are connected
to an amplifier which drives a strip chart recorder or tape
recorder.
If the electrical conductivity of the subsurface were
uniform the potential gradient at the surface would
be constant (Fig. 9.21(a)). Zones of differing conduc-
tivity deflect the current flow from the horizontal and
cause distortion of the potential gradients measured
at the surface. Figure 9.21(b) shows the distortion of
current flow lines caused by a salt dome which, since it
is a poor conductor, deflects the current lines into the
overlying layers. Similar effects may be produced by anti-
clinal structures. Interpretation of anomalous potential
gradients measured at the surface permits the location of
subsurface zones of distinctive conductivity.
Telluric potential gradients are measured using
orthogonal electrode pairs (Fig. 9.22(a)). In practice, the
survey technique is complicated by temporal variation in
direction and intensity of the telluric currents. To over-
come this problem, one orthogonal electrode pair is read
at a fixed base located on nearby barren ground and
another moved over the survey area.At each observation
point the potential differences between the pairs of elec-
trodes at the base and at the mobile station are recorded
simultaneously over a period of about 10 min. From the
magnitude of the two horizontal components of the
electrical field it is simple to find the variation in direc-
tion and magnitude of the resultant field at the two
locations over the recording interval. The assumption is
made that the ground is uniform beneath the base
(b) disturbed
(a) normal
Distance
(a)
Undisturbed
telluric
currents
(b)
Deflected
telluric
currents
Salt
Fig. 9.21
The instantaneous potential gradient associated with
telluric currents. (a) Normal, undisturbed gradient. (b) Disturbed
gradient resulting from deflection of current flow by a salt dome.
electrodes so that the conductivity is the same in all di-
rections.The resultant electrical field should also be con-
stant in all directions and would describe a circle with
time (Fig. 9.22(b)).To correct for variations in intensity
of the telluric currents, a function is determined which,
