Geonics Limited EM31-SH, Operating Manual

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EM31-SH MANUAL Page
33
6.2
Example A: Heart Lake, Ontario (EM31)
Measurement Interval: 100 feet over till,
10 feet over sand/gravel
This survey line compares the results obtained with
conventional resistivity equipment (Wenner array with “a”
spacing of 1 foot and 20 feet) and the Geonics EM31. It is
seen that over the till, where the resistivity is slowly
varying, the agreement between the two techniques is
excellent. In the region shown as “sand and interbedded
gravel” there was a good deal of concretion which caused the
resistivity to vary greatly over short distances and which
accounts for the discrepancy between the two techniques.
Over the till the EM31 was read continuously although the
data was only recorded, with one exception, at every 100
feet. The exception occurred at station 7+50 where it was
noted that a local resistivity high occurred; this was of
course not observed on the Wenner array since measurements
were taken only at every 100 feet.
6.3
Example B: Sunnybrook Park (EM31, EM34)
Measurement Interval: 100 feet
This case history shows measurements made with both the EM31
and the EM34 and illustrates the effective depth penetration
of the two systems. The second sheet shows the results of
expanding Wenner spreads at station 4+00 and 12+00. At
station 4+00 we would expect the EM31 to read approximately
9.8 ohm-meters and the EM34 50 to 60 ohm-meters, which is the
case. At station 12+00 the resistivity increased with depth
and thus the EM31 should read a relatively low value and the
EM34 a higher value which increases with intercoil spacing.
This is indeed the case.