Geology Reference
In-Depth Information
Primary EM field
Transmitter
Receiver
H
p
Surface
Modified
primary
field
Secondary
field
Eddy
currents
Fig. 9.2
The rotation of a search coil about an axis corresponding
to the direction of arriving electromagnetic radiation
H
p
producing an infinite number of null positions.
magnetic field is proportional to the component of the
field perpendicular to the plane of the coil. Conse-
quently, the strength of the signal in the earphones is at a
maximum when the plane of the coil is at right angles to
the direction of the arriving field. Since the ear is more
sensitive to sound minima than maxima, the coil is usu-
ally turned until a null position is reached. The plane of
the coil then lies in the direction of the arriving field.
Conductor
Fig. 9.1
General principle of electromagnetic surveying.
frequency used in an EM survey can be tuned to a
desired depth range in any particular medium. For ex-
ample, in relatively dry glacial clays with a conductivity
of 5 ¥ 10
-4
Sm
-1
,
d
is about 225 m at a frequency of
10 kHz.
Equation (9.2) represents a theoretical relationship.
Empirically, an effective depth of penetration
z
e
can be
defined which represents the maximum depth at which
a conductor may lie and still produce a recognizable elec-
tromagnetic anomaly
9.4 Tilt-angle methods
When only a primary electromagnetic field
H
p
is present
at a receiver coil, a null reading is obtained when the
plane of the coil lies parallel to the field direction.There
are an infinite number of such null positions as the coil
is rotated about a horizontal axis in the direction of the
field (Fig. 9.2).
In many EM systems the induced secondary field
H
s
lies in a vertical plane. Since the primary and secondary
fields are both alternating, the total field vector describes
an ellipse in the vertical plane with time (Fig. 9.3). The
resultant field is then said to be
elliptically polarized
in the
vertical plane. In this case there is only one null position
of the search coil, namely where the plane of the coil
coincides with the plane of polarization.
For good conductors it can be shown that the direc-
tion of the major axis of the ellipse of polarization corre-
sponds reasonably accurately to that of the resultant of
the primary and secondary electromagnetic field direc-
tions. The angular deviation of this axis from the
horizontal is known as the
tilt-angle
q
of the resultant
field (Fig. 9.3). There are a number of EM techniques
(known as
tilt-angle
or
dip-angle
methods) which simply
measure spatial variations in this angle.The primary field
may be generated by a fixed transmitter, which usually
consists of a large horizontal or vertical coil, or by a small
mobile transmitter. Traverses are made across the survey
area normal to the geological strike. At each station the
100
s
z
e
ª
(9.3)
f
The relationship is approximate as the penetration de-
pends upon such factors as the nature and magnitude
of the effects of near-surface variations in conductivity,
the geometry of the subsurface conductor and instru-
mental noise. The frequency dependence of the depth
of penetration places constraints on the EM method.
Normally, very low frequencies are difficult to generate
and measure and the maximum achievable penetration is
usually of the order of 500 m.
9.3 Detection of electromagnetic fields
Electromagnetic fields may be mapped in a number of
ways, the simplest of which employs a small search coil
consisting of several hundred turns of copper wire
wound on a circular or rectangular frame typically be-
tween 0.5 m and 1 m across.The ends of the coil are con-
nected via an amplifier to earphones. The amplitude of
the alternating voltage induced in the coil by an electro-
