Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 314

loop and receiver coil lie in the same plane the con gur-
ation has co-planar geometry, and it has co-axial geometry
when their axes are aligned.
Two survey modes are in common use: moving loop and
fixed loop.

The asymmetric separated-loop con guration ( Fig. 5.82b )
is used in towed-bird AEM systems (see Section 5.9.2.1 )
and for ground surveying. It produces the best resolution
of steeply dipping thin conductors. Separation between the
centre of the loop and the receiver for ground surveying
is typically twice the loop size. The data are located to
the midpoint between the loop centre and the receiver.
The laterally separated geometry produces responses
dependent on the relative positions of the transmitter and
receiver with respect to the conductor. This means that the
responses of a dipping conductor obtained from surveying
adjacent parallel survey lines in opposite directions (as
often occurs with airborne data) will be different and
laterally offset (see Profile analysis in Section 5.7.5.3 ).
The loop size in moving-loop mode is typically 50 m to
500 m and the whole array usually moved at an interval of
50% of the loop size. The transmitter is at a different
location for each measurement,

Moving-loop mode
For reconnaissance surveying and when the dip direction
of a conductor is unknown, the moving-loop mode is very
effective and provides good resolution. Both the transmit-
ter and receiver are moved along the survey line with their
relative positions fixed. The most common configuration
has the receiver located at the centre of the transmitter
loop and is known as the in-loop configuration ( Fig. 5.82a ).
The data are located to the loop centre. The array is
insensitive to the horizontal (X and Y) components of the
secondary fields of horizontal conductive layers and

conductive half-space (see Fig. 5.84 ) .

so coupling to any

a)

Z

Tx Y

Traverse line

X

Rx

b)

Z

Tx Y

Traverse line

X

Rx

Z

c)

Figure 5.82 The transmitter - receiver configurations for
various survey modes. (a) The in-loop and (b) separated-
loop configurations of the moving-loop mode, and (c)
the

Y

Traverse lines

Tx

X

fixed-loop mode. Orientations of the X, Y and Z
components of the receiver are shown. The Z component
and the transmitter loop are co-planar in all cases and
also co-axial in (a). Tx and Rx are the transmitter and
receiver, respectively.

Rx