Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 315

subsurface conductors present (see Section 5.7.1.3 ) varies
along the survey traverse and new eddy current systems are
induced at each station (see Section 5.7.1.4 and Fig. 5.70 ) .
This is very useful for detecting conductors whose dip and
orientations are unknown. A portable generator is used to
supply current to the transmitter loop, typically up to 30 A.
For a 200

the orientations of the various conductivity planes (current
systems) in thick bodies and for resolving close-spaced
conductors.
Anomalies measured with a fixed loop have a fairly
simple form because the coupling between the loop and
the conductor is constant along the survey traverse. Fixed-
loop surveying has simpler operational logistics than
moving-loop mode as only the receiver is moved along
the survey traverse, a distinct advantage when working in
rugged and densely vegetated terrains. However, consider-
able effort may be required in these terrains to lay out the
large loop, and access restrictions may limit the survey to
just one loop location. Careful planning is required to
optimise loop location relative to the target zone.

200 m single-turn loop the dipole moment
( Eq. (5.21) ) would be as much as 1,200,000 A m 2 . Moving-
loop mode provides high resolution of conductors, but
anomalies have complex shapes.
Moving-loop mode requires across-line access for laying
the loops, which may be logistically difficult in rugged and
densely vegetated terrains. The in-loop configuration also
requires access to the centre of the transmitter loop.

Fixed-loop mode
For investigating the geometry of a known conductor, or
where the dip of the target conductor zone is known, and
for deep penetration, the large fixed-loop mode can be
used. The transmitter loop is fixed, or stationary, for the
duration of the survey. The loop may have dimensions as
large as 2000 m by 1000 m, with its longest side oriented
parallel to the expected strike of sheet-like conductors and
located so as to couple well with the target zone (see
Section 5.7.1.3 ) . Prior knowledge about the dip and strike
of the conductor is required, possibly obtained from a
moving-loop survey, to optimise the location of the loop.
The receiver is systematically moved relative to the
transmitter loop, in surface surveys along traverses perpen-
dicular to the loop

Transmitter loop size
As described in Section 5.7.1.2 , larger transmitter loops
produce a stronger EM field penetrating a larger volume of
the ground. The increased signal-to-noise ratio at the later
delay times increases the depth of investigation. Maximum
signal-to-noise ratio and vertical resolution are obtained
when the size of the loop is of the same order as the depth
being investigated. However, large loops couple well with
conductive background rocks and conductive near-surface
layers, causing these to produce strong responses, the effects
of which can obliterate the target response (see Late-time
measurements in Section 5.7.2.3 ). In conductive environ-
ments a compromise has to be made between depth of
penetration and overburden and background responses. In
moving-loop mode, larger loops reduce spatial resolution so,
overall, there is a trade-off between depth of investigation
and spatial resolution when specifying loop size. Note that a
larger loop produces a longer turn-off ramp time (see
Section 5.7.1.2 ) which, and depending on the shape of
the primary pulse, determines the delay time of the earliest
off-time measurement ( Fig. 5.73 ) .
In general, a loop-size to investigation-depth ratio of
about 1:3 is suitable in resistive environments, with larger
loops required in conductive areas and where thick con-
ductive overburden is present, a ratio of say 1:2. However,
the response of conductive overburden and the background
increase, but this may not present a problem for a deep
probing DHEM receiver distant from the overburden.
A surface survey may bene t from using a smaller loop.
A sounding may require data to be collected with more than
one loop size. The optimum size of the loop for detecting
the expected target at a particular depth in the host environ-
ment can be determined with computer modelling.

s longest side and passing through and
away from the loop ( Fig. 5.82c ) . In DHEM surveying the
receiver is placed at different depths in one or more drill-
holes (see Section 5.8.1 ). The data are located to the
receiver location. Survey distance from the side of the loop
is limited by the decreasing amplitude of the primary

'

eld
away from the loop. Since the loop is stationary, it could, if
access permits, be energised by a high-power truck-
mounted generator producing currents up to about
100 A. For an 800

200 m single-turn loop the dipole
moment ( Eq. (5.21) ) would as much as 16,000,000 A m 2 .
Fixed-loop con guration provides lower resolution than
moving-loop con gurations, so three-component meas-
urements (see Section 5.7.1.5 ) are important for obtaining
diagnostic information about a conductor. The survey may
be repeated with the fixed loop at different locations rela-
tive to the conductor zone to vary the coupling of the
primary field with the target. This is useful for detecting
conductors with opposite dip direction, for investigating