Geoscience Reference
In-Depth Information
secondary EM field propagates into the surrounding media. Some of the secondary EM energy
(flux) is detected by an EM receiver on the surface. The receiver records both the primary field from
the source and the secondary field from the object in the subsurface. These two signals must be
separated in the electronics of an EM system.
6.3 eQUIpMent
There are many implementations of EM methods that have been developed for geophysical applica-
tions over the years. In addition to the type and orientation of transmitter and receiver, EM methods
may be classified in a fundamental way as those that use an artificial source of EM energy, and
those that use the earth's natural EM field (telluric, magnetotelluric [MT], audio magnetotelluric
[AMT] methods). Artificial source methods are often classified by the type of source and detector
(ground electric field line, or magnetic field coils or magnetometers), and the nature of the transmit-
ter and receiver (e.g., long line or dipole electric field, small loop or large loop). Artificial source
methods may be further subdivided by the type of wave that is transmitted and received: a system
that transmits and receives a wave of a single frequency is called a frequency domain system, and
a system that transmits and receives a multiple-frequency pulse of energy is called a time-domain
system. Artificial and natural systems are further classified by the range of frequencies (e.g., low
frequency, very low frequency [VLF], extremely low frequency [ELF], audio frequency, etc.), and
the characteristics of the EM field vector that is measured (e.g., ellipticity, tilt, amplitude, or phase).
To further complicate things, methods have come to be known by their commercial names, and EM
techniques have been developed for use on the ground, in the air, and in boreholes.
The EM field at any point in space is a vector, with a magnitude and a direction. The magnitude
of the field at a given point in space is a function of the orientation of the transmitted field, the modi-
fication of the direction of the field by materials between the transmitter and receiver (the object
of the measurements), and the direction of the field measured by the receiver (receiver orientation).
EM measurement instruments and field surveys are designed to exploit the vector nature of the EM
field, and to measure attributes of the EM field that indicate the size, depth, and orientation of the
objects in the subsurface. The attributes measured relate to either the spatial or time relationships
of the measured secondary EM field. The field attributes include the amplitude, time delay (phase),
and orientation of the received field with respect to the primary transmitted field. Specifically, the
following parameters can be measured: (1) the phase of the spatial components with respect to
the source, (2) the orientation of the field (tilt, and the axis of the field ellipsoid), (3) the relative
amplitudes of measurements at different frequencies, (4) the amplitude ratio and phase differences
between different spatial components, or (5) the phase and amplitude of individual spatial compo-
nents with respect to the source. Methods are designed to “normalize” the effect of the primary
field. The separation of most transmitter-receiver pairs is fixed to eliminate geometric effects from
the primary field caused by varying the relative positions of the transmitter and receiver.
If the transmitter and receiver are located above the surface of the ground, then the field mea-
sured at the receiver is a combination of the field that propagates directly through the air (the
primary field), the EM field that is influenced by the background material in the subsurface (some-
times referred to as the terrain), and the induced secondary field from the object of a contrasting
conductivity located in the subsurface. The field that propagates directly from the transmitter is
called the primary field, and the field caused by the eddy currents induced on the subsurface terrain
and any buried object in the subsurface is called the secondary field, as shown in Figure 6.4a. The
direction (sign) of the secondary field induced by the conductive object in the subsurface is opposite
to the primary field, in accordance with Lenz's law.
The secondary field is also “shifted” in time, and this shift is called a phase
shift. The terminol-
ogy describing the phase of a wave is not discussed very clearly in most of the literature. However,
popular usage of the term “phase” in describing induction EM methods usually refers to the fact
that the secondary field has a certain phase relationship to the primary field. The phase shift is
