Geology Reference
In-Depth Information
current flow.The telluric method can also be adapted to
mineral exploration.
(a)
9.11.3 Magnetotelluric surveying
Prospecting using magnetotelluric fields is more
complex than the telluric method as both the electric
and magnetic fields must be measured. The technique
does, however, provide more information on subsurface
structure.The method is, for example, used in investiga-
tions of the crust and upper mantle (e.g. Hutton
et al.
1980).
Telluric currents are monitored as before, although no
base station is required. The magnetotelluric field is
measured by its inductive effect on a coil about a metre in
diameter or by use of a sensitive fluxgate magnetometer.
Two orthogonal components are measured at each
station.
The depth
z
to which a magnetotelluric field pene-
trates is dependent on its frequency
f
and the resistivity
r
of the substrate, according to equations of the form of
(9.2) and (9.3), that is,
Base
Mobile
(b)
Fig. 9.22
(a) Base and mobile potential electrode sets used
in telluric surveys. (b) The figure traced by the horizontal
component of the telluric field over an undisturbed area (circle)
and in the presence of a subsurface conductor (ellipse) after
correction for temporal variations in telluric current intensity.
r
zk
f
=
(9.7)
when applied to the base electrode results, constrains the
resultant electric vector to describe a circle of unit radius.
The same function is then applied to the mobile elec-
trode data. Over an anomalous structure the conductiv-
ity of the ground is not the same in all directions and the
magnitude of the corrected resultant electric field varies
with direction.The resultant field vector traces an ellipse
whose major axis lies in the direction of maximum con-
ductivity.The relative disturbance at this point is conve-
niently measured by the ratio of the area of the ellipse to
the area of the corresponding base circle.The results of a
survey of this type over the Haynesville Salt Dome,
Texas, USA are presented in Fig. 1.4. The solid circles
represent locations where ellipse areas relative to a unit
base circle have been computed. Contours of these
values outline the known location of the dome with
reasonable accuracy.
The telluric method is applicable to oil exploration as
it is capable of detecting salt domes and anticlinal struc-
tures, both of which constitute potential hydrocarbon
traps. As such, the method has been used in Europe,
North Africa and the former Soviet Union. It is not
widely used in the USA where oil traps tend to be too
small in area to cause a significant distortion of telluric
where
k
is a constant. Consequently, depth penetration
increases as frequency decreases. It can be shown that the
amplitudes of the electric and magnetic fields,
E
and
B
,
are related
2
02
.
f
E
B
Ë
¯
=
r
a
(9.8)
where
f
is in Hz,
E
in mV km
-1
and
B
in nT.The appar-
ent resistivity
r
a
thus varies inversely with frequency.The
calculation of
r
a
for a number of decreasing frequencies
thus provides resistivity information at progressively
increasing depths and is essentially a form of vertical
electrical sounding (see Section 8.2.3).
Interpretation of magnetotelluric data is most reliable
in the case of horizontal layering. Master curves of ap-
parent resistivity against period are available for two and
three horizontal layers, vertical contacts and dykes, and
interpretation may proceed in a similar manner to curve-
matching techniques in the resistivity method (see Sec-
tion 8.2.7). Routines are now available, however, which
allow the computer modelling of two-dimensional
structures.
