Geology Reference
In-Depth Information
9.1.1 Audio-frequency natural signals
Of all natural electrical phenomena, sferics are the most useful in geophys-
ical exploration, because their wide frequency range allows conductivity to
be investigated over depths ranging from a few metres to several kilome-
tres. Modern AMT (audio-magnetotelluric) methods differ from the earlier
AFMAG (audio-frequency magnetics, now generally confined to airborne
surveys) in that measurements are made of electrical as well as magnetic
field components.
Unfortunately, like so many things that come for free, natural signals are
not always reliable. Thunderstorm activity varies both seasonally and on
a daily basis, with a minimum in most temperate zones during the late
morning. Signals are stronger in the evening and at night, times that are not
very convenient for survey work. They are also strongest during the summer,
which unfortunately is the blackfly season in high northern latitudes. Low
signal strengths and high noise levels were major obstacles to acquiring
good data in the early days of AMT, and are still factors today, despite the
enormous improvements in equipment sensitivity.
9.1.2 AF wave propagation
Regardless of whether they are produced naturally or artificially, electro-
magnetic waves from distant sources propagate in the spherical wave-guide
formed by the surface of the Earth and the base of the ionosphere. Attenu-
ation is determined by the physical properties of the two conducting media
and their separation. The properties of the ionosphere, and the height of its
base, vary in a 24-hour cycle, since the effect of solar radiation is to lower
the base to about 60 km and render it more diffuse. At night, the base rises
to about 90 km and is more sharply defined, and attenuation is reduced.
The best signals are obtained when the entire path, from thunderstorm to
receiver, is in darkness.
If the Earth were a perfect conductor, the electric field vector in the sferic
wave would be at right angles to the ground surface, and the magnetic
vector would be horizontal and at right angles to the propagation direction.
Such a wave would be magnetically
horizontally polarised
and electrically
vertically polarised
. However, the ground has a finite resistance, and sferic
waves can penetrate some distance into it. In homogeneous conducting
ground, this downgoing wavefront is planar and horizontal, i.e. there is a
horizontal electrical field, E
x
, as well as a horizontal magnetic field, H
y
.The
ratio between these two fields is a measure of the subsurface conductivity,
governed by the Cagniard equation:
ρ
=
(E
x
/
H
y
)
2
/
5
f
