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aspccts of thc operation are complcy, the basic principle is not, and can be
dcscribed without rclying on high levcl physical and mathcmatical explanations.
Th٥ EM-31 consists of a ccntrally located instrunent box with two arms extending
approximatcly l.6 metcrs from thc centcr (Figure 2). One arm contains the
transittcr, thc othcr thc reccivcr, The tra١smitter coil creates a magnetic field
that penctratcs the earth setting up elcctrical currents in undcrground conducters.
٨ secondary magnctic field is generatcd by the electrical currents, The secondary
magnetic field, which dcpcnds significantly on the ground's accumulative conduc­
tivity, is sensed at the receiver end, The results are read from an analog meter
placed on top of the instrunent box (McNeill, 1979, Frohlich and Orter, 1982
Frohlich and Lancaster 1986). The employed measuring unit for thc accumulative
conductivity is millisiemens per meter (mSln), which is a newer unit name for the
earlier used millimho per meter (mmholm) (Bevan 1983; and Frohlich and
Lancaster 1986).

The depth penetration is dependent on four factors: (1) intercoil spacing or
distance between the transmitter and the receiver coils (fixed at 3.6 meters); (2)
frequency (fixed at 9.8 kH%); (3) carrying height (relatively constant, but
dependent on the person carrying the equipment); and (4) the dipole configuration
(two configurations are available by turning the transmitter and the receiver
modules 90 degrees, resulting in either a vertical or a horizontal mode). The two
different dipole configurations allow a change in the depth penetration; the vertical
mode allows an accumulative depth penetration of about 6 meters, while the
horizontal mode results in depth penetration of about 2 meters, both measured
from the carrying height.

It should be noted, however, that the instrument is most sensitive to features in
the upper part of the soil, and that the sensitivity gradually decreases with depth
(Bevan, 1983, and Frohlich and Lancaster 1986). For practical reasons, the carrying
height is assumed to be fixed due to the added strain that is placed upon the person
when carrying the equipment at different vertical heights for long periods of time.
If data from the same survey area is obtained using both modes, features at
different levels can be resolved more effectively. Furthermore, the difference in the
patterns of conductivity between the two modes provides additional interpretive
data.

We applied different recording techniques, wto of which appear most effective:
(1) measuring of traverses (one, two or three parallel lines, 1 to 2 meters apart, and
with 1 or 2 meters between each measurement), and (2) measuring of grids (up to
100 by 100 meters, with measurements taken every meter).

The recording of traverses is quick and efficient, as well as being helpufl in
making a preliminary decision on how to proceed with more detailed prospecting.
Obtaining detailed conductivity data from a grid is appropriate as a basis for

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