Geonics Limited EM31-SH, Operating Manual

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EM31-SH MANUAL Page
22
4.0 SURVEY TECHNIQUE
Surveying with the EM31 is straightforward. As pointed out
in 2.1.3. measurements may be made either continuously or on
a station-by-station basis. In either case it is always
recommended, as for any other geophysical survey, that survey
lines and measurement stations be carefully laid out, and the
survey performed in a systematic fashion with the resulting
data accurately plotted for each measurement station. The
most common survey error is to have the survey lines too
short, in which case they do not extend sufficiently far off
the expected anomalous region to permit the operator to
establish the background values of terrain conductivity.
The decision as to the correct spacing will be based on a
knowledge of the lateral dimension of the anticipated
resistivity anomaly. To ensure the correct spacing, it is
useful at the start of the survey to continuously observe the
conductivity values that are encountered as the operator
moves alone the survey line.
The resolution in conductivity of the EM31 is also high, with
changes of 5% being quickly perceived. This instrument is
capable of giving an extremely precise survey with
information on small variations in the terrain.
It was seen in Section 1 that current flow within the earth
consists of a series of concentric circles, assuming that the
conductivity is laterally uniform. Therefore, in the case of
a uniform half-space, rotation of the instrument in a
horizontal plane about the transmitter coil as a pivot will
produce no change in the meter reading. Conversely, any
change in the reading as this procedure is carried out is an
indication of lateral inhomogeneities in conductivity. It is
simpler and usually sufficiently accurate for the operator to
rotate the instrument through 90° using himself as pivot