Geology Reference
In-Depth Information
+20%
0
-20%
Imaginary
Real
S
N
BH
BH
BH
Moraine
BH
Ore
Fig. 9.10
Mobile transmitter-receiver
profile, employing horizontal coplanar
coils with a separation of 60 m and an
operating frequency of 3.6 kHz, in the
Kankberg area, north Sweden. Real and
imaginary components are expressed as a
percentage of the primary field. (After
Parasnis 1973.)
Phyllites
Precambrian
volcanics
0
100 m
voltage in the loop
e
(
t
) normalized with respect to the
current in the transmitter loop
I
.The response is shown
for a number of different times after primary cut-off.
The response persists into the latest channels, indicating
the presence of a good conductor which corresponds to
the graphitic shale. The asymmetry of the response
curves and their variation from channel to channel al-
lows the dip of the conductor to be estimated. The first
channel, which logs the response to relatively shallow
depths, peaks to the right. The maximum moves to the
left in later channels, which give the response to progres-
sively greater depths, indicating that the conductor dips
in that direction.
An example of a survey using a borehole TDEM
system is presented in Fig. 9.14, which shows results
from the Single Tree Hill area, NSW, Australia (Boyd &
Wiles 1984). Here semi-massive sulphides (pyrite and
pyrrhotite), which occur in intensely sericitized tuffs
with shale bands, have been penetrated by three drill-
holes. The TDEM responses at a suite of times after pri-
mary field cut-off, recorded as the receiver was lowered
down the three drillholes, are shown. In hole PDS1, the
response at early times indicates the presence of a con-
ductor at a depth of 145 m.The negative response at later
times at this depth is caused by the diffusion of eddy cur-
rents into the conductor past the receiver and indicates
that the hole is near the edge of the conductor. In holes
DS1 and DS2 the negative responses at 185 m and 225 m,
respectively, indicate that the receiver passed outside, but
near the edges of, the conductor at these depths. Also
shown in the section is an interpretation of the TDEM
data in terms of a model consisting of a rectangular
current-carrying loop.
V
123456
t
1
t
2
t
3
t
4
t
5
t
6
Time
Fig. 9.11
The quantification of a decaying TDEM response by
measurement of its amplitude in a number of channels (1-6) at
increasing times (
t
1-6
) after primary field cut-off.The amplitudes
of the responses in the different channels are recorded along a
profile.
TDEM (Frischnecht & Raab 1984). Only short offsets
of transmitter and receiver are necessary and the array
therefore crosses a minimum of geological boundaries
such as faults and lithological contacts. By contrast,VES
or continuous-wave EM methods are much more affect-
ed by near-surface conductivity inhomogeneities since
long arrays are required. It is claimed that penetration of
up to about 10 km can be achieved by TDEM sounding.
An example of a surface application of TDEM is pre-
sented in Fig. 9.12, which shows the results of a survey
undertaken near Mount Minza, Northern Territory,
Australia (Duckworth 1968, see also Spies 1976). The
target, which had been revealed by other geophysical
methods (Fig. 9.13), was a band of highly-conductive
graphitic black shale, which has a conductivity in excess
of 0.1 S m
-1
in its pristine condition. In Fig. 9.12 the
TDEM response is expressed in terms of the induced
