Geology Reference
In-Depth Information
structure rather than through it. This pattern of flow
causes distortion of the constant potential gradient at the
surface that would be associated with a homogeneous
subsurface and indicates the presence of the high-
resistivity salt. Figure 1.4 presents the results of a telluric
current survey of the Haynesville Salt Dome, Texas,
USA.The contour values represent quantities describing
the extent to which the telluric currents are distorted by
subsurface phenomena and their configuration reflects
the shape of the subsurface salt dome with some
accuracy.
1.3 The problem of ambiguity in
geophysical interpretation
If the internal structure and physical properties of the
Earth were precisely known, the magnitude of any par-
ticular geophysical measurement taken at the Earth's
surface could be predicted uniquely.Thus, for example,
it would be possible to predict the travel time of a seismic
wave reflected off any buried layer or to determine the
value of the gravity or magnetic field at any surface loca-
tion. In geophysical surveying the problem is the oppo-
site of the above, namely, to deduce some aspect of the
Earth's internal structure on the basis of geophysical
measurements taken at (or near to) the Earth's surface.
The former type of problem is known as a
direct
problem,
the latter as an
inverse
problem.Whereas direct problems
are theoretically capable of unambiguous solution,
inverse problems suffer from an inherent ambiguity, or
non-uniqueness, in the conclusions that can be drawn.
To exemplify this point a simple analogy to
geophysical surveying may be considered. In
echo-
sounding
, high-frequency acoustic pulses are transmitted
by a transducer mounted on the hull of a ship and echoes
returned from the sea bed are detected by the same
transducer. The travel time of the echo is measured and
converted into a water depth, multiplying the travel time
by the velocity with which sound waves travel through
water; that is, 1500 m s
-1
. Thus an echo time of 0.10 s
indicates a path length of 0.10 ¥ 1500 = 150 m, or a
water depth of 150/2 = 75 m, since the pulse travels
down to the sea bed and back up to the ship.
Using the same principle, a simple seismic survey may
be used to determine the depth of a buried geological
interface (e.g. the top of a limestone layer). This would
involve generating a seismic pulse at the Earth's surface
and measuring the travel time of a pulse reflected back to
the surface from the top of the limestone. However, the
0 m2
50
Fig. 1.4
Perturbation of telluric currents over the Haynesville
Salt Dome,Texas, USA (for explanation of units see Chapter 9).
The stippled area represents the subcrop of the dome. (Redrawn
from Boissonas & Leonardon 1948.)
configuration of closely-spaced shots and detectors is
moved systematically along a profile line and the travel
times of rays reflected back from any subsurface geologi-
cal interfaces are measured. If a salt dome is encountered,
rays reflected off its top surface will delineate the shape of
the concealed body.
4.
Earth materials with anomalous electrical resistivity
may be located using either electrical or electromagnetic
geophysical techniques. Shallow features are normally
investigated using artificial field methods in which an
electrical current is introduced into the ground and
potential differences between points on the surface are
measured to reveal anomalous material in the subsurface
(Chapter 8). However, this method is restricted in its
depth of penetration by the limited power that can be
introduced into the ground. Much greater penetration
can be achieved by making use of the natural Earth cur-
rents (telluric currents) generated by the motions of
charged particles in the ionosphere. These currents ex-
tend to great depths within the Earth and, in the absence
of any electrically anomalous material, flow parallel to
the surface. A salt dome, however, possesses an anom-
alously high electrical resistivity and electric currents
preferentially flow around and over the top of such a
