Geonics Limited EM31-SH, Operating Manual

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EM31-SH MANUAL Page
30
5.2 Multi-Layered Earth
(cont’d)
The table accompanying the figure shows the calculations
which yield the value of apparent conductivity at various
thicknesses of the upper layer. Thus as we traverse such a
structure we would find that the apparent conductivity fell
from 30.7 millsiemens per meter at a larger distance from the
buried river valley to 12.5 milsiemens per meter at the
centre of the valley.
Figure 6 illustrates a second situation where the objective
is to locate bedrock highs within muskeg. This is again a two
layer case and the table illustrates the calculations to
determine the apparent conductivity. For this configuration,
i.e. a conductor on top of an insulator and k
2
a small
quantity, the measured apparent conductivity becomes
relatively independent of the actual value of the
conductivity of the lower layer. For k
2
<< 1, the apparent
conductivity simply becomes a function of the thickness of
the upper conductive layer.
Finally, a third example is given in Figure 7. In this case
we wish to traverse a thickening gravel deposit; the maximum
thickness is such that the instrument still responds to
material below the deposit. This is an example of the
situation of an insulator sandwiched between two conductors
and is inevitably the most difficult geometry for
electromagnetic systems to detect, as evidenced by the
tabulated values of apparent conductivity. Even at a
thickness of five meters (station D) there is still
significant response from the till beneath the deposit and
this response tends to keep the apparent conductivity high.
The three examples show how to calculate the response of the
EM31 over a variety of geological environments.