Geonics Limited EM31-SH, Руководство по эксплуатации

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EM31-SH MANUAL Page
36
6.8
Example G: Pre-glacial River Valley (EM31, EM34)
Measurement Interval: 100 feet
This survey was performed to outline a pre-glacial river
valley whose existence had been suggested from water-well
data. The survey was performed with the EM34 at a station
interval of 100 feet and with intercoil spacing of both 50
and 100 feet. (An earlier model of the EM34 made use of 50
and 100 feet intercoil spacings. These spacings are not
available with the current EM34 model). At either spacing
the time required to complete the 8400 foot survey line was
1.5 hours. The same line was subsequently surveyed with the
EM31.
Typical bedrock conductivity in the area is approximately 32
millimhos per meter, whereas an average value for the
conductivity of the infilling glacial till is of the order of
8 to 12 millimhos per meter. Thus the EM34 at either
intercoil spacing yields approximately 30 to 34 millimhos per
meter at the valley edges where the overburden is thin and 12
to 24 millimhos per meter at the valley centre. The EM31
yields values of 14 to 18 millimhos per meter at the valley
edges (slightly affected by the presence of bedrock) and
approximately 10 millimhos per meter at the valley centre.
The interpreted depth of the valley, based on the model shown
in the figure, is approximately 120 feet which is in
reasonable agreement with the water-well data value of 150
feet, bearing in mind that the three sets of data show that a
two layer model is an over simplification.
The conductivity high which occurs between station 32 and 38
results from a very large pile of waste furnace ash lying on
the surface.