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Transmitter current and primary magnetic field
Transmitter current and primary magnetic field
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+
0
0
Time
Time
-
-
Transmitter
off
Transmitter
on
Transmitter
off
Transmitter
on
Transmitter
off
Transmitter
on
emf in receiver coil due to primary magnetic field
emf in receiver coil due to primary magnetic field
+
+
0
0
Time
Time
-
-
Secondary field
measured
Secondary field
measured
Secondary field
measured
Secondary field
measured
Secondary field
measured
Impulse response
Step response
Figure 5.74
Transmitter waveforms and the responses obtained with a coil sensor. A B-
eld sensor reproduces the transmitted waveform. The
(approximate) impulse and step response waveforms are shown in the absence of a ground response.
eld sensors capable of
making low-noise measurements of the weak time-varying
secondary
field were unavailable. Instead, coils were used
to obtain the impulse response from a square-like pulse of
the primary
field, and the step response was obtained by
the transmission of a triangular pulse. Most of the modern
TEM systems produce both B-
eld and dB/dt data. The
nature of the responses obtained critically depends on the
actual waveform transmitted, but the respective responses
contain many of the attributes of the ideal step and impulse
responses. Without further quali
cation, we refer to these
as the step and impulse responses throughout our descrip-
tion of TDEM.
There are significant differences between the step and
impulse responses and the information they contain about
a conductor, which are described in
Section 5.7.2
.An
example of dB/dt and B-field data from the same location
is presented in
Section 5.7.6
.
Until comparatively recently, B-
practice the situation is further complicated by several other
subsurface conduction processes that can occur and that
interfere with the responses (see
Section 5.7.6
).
5.7.2.1
Homogeneous subsurface
It is convenient to begin the description of TDEM
responses using the simplest case of an electrically homo-
geneous ground where the only contrast in electrical prop-
erties occurs at the ground surface, i.e. the ground
-
air
interface. This is known as a half-space (see Half-space
model in
Section 2.11.1.3
)
. It is a useful model to introduce
the key concepts of diffusion, smoke ring, diffusion depth
and decay rate which describe the electromagnetic diffu-
sion process and are used for the analysis of the measured
secondary decay.
Recall from
Section 5.2.2.2
that initially eddy currents
flow so as to try to oppose any change in the primary field,
i.e. they attempt to maintain the primary
field everywhere
as it was prior to turn-off. For the case of the surface
transmitter loop, the eddy current is an image of the loop
(
Fig. 5.75a
). When the loop is elevated above the ground,
as in AEM surveying, the eddy current at the instant of
turn-off will be laterally more expansive than the loop.
Figures 5.76a
to c show a cross-section through a hori-
zontal transmitter loop on the surface of a half-space and
the induced migrating eddy current at progressively greater
delay times. As described in
Section 5.7.1.4
,
immediately
upon its creation the eddy current begins to expand and
migrate outward and downward, losing energy rapidly and
causing the region to experience a changing magnetic field.
5.7.2
Subsurface conductivity and EM responses
Time domain EM responses of the subsurface can be com-
plex, but they can be understood in terms of the responses
of an electrically homogeneous background, a conductive
overburden (if present) and localised
regions of
contrasting conductivity. Key to understanding TDEM
responses is that at the instant of current turn-off, eddy
currents are created at all interfaces across which there are
contrasts in electrical conductivity, and the observed decay
is due to a combination of their individual decays. In
'
target
'
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