Geology Reference
In-Depth Information
effectively increases the distance between the capacitor 'plates'. Similar
effects would be observed over other highly resistive surface layers.
Only signal amplitudes are used by the OhmMapper
TM
, but there will
generally also be phase differences between the currents circulating in the
receiving and transmitting aerials, and these could provide additional useful
information.
The depth of investigation in a galvanic survey is determined mainly by
the total length,
L
, of the array, and this is true of CCR surveys also. Rough
rules of thumb are that if
n
≥
3, then the investigation depth is equal to
L
/
5,
but if
n
is 2, the penetration drops to
L
1 the penetration
is only equal to
L
/
7.2. At the frequencies and separations characteristic of
the OhmMapper
TM
, there will often also be some element of skin-depth
limitation.
Although the intervals between readings are very small in manually towed
surveys, this does not provide equivalently high resolution, since this is
determined by aerial length and separation.
/
5.7, and for
n
=
6.6.4 Advantages and disadvantages of capacitive coupling
Capacitively coupled resistivity surveys allow resistivity data to be obtained
very rapidly, even in areas where ground contact via electrodes would be
difficult or impossible. Traverses can be repeated with different separations
between the aerials, and commercially available inversion programs allow
resistivity cross-sections to be constructed from multi-spaced data. However,
as with all geophysical methods, there are problems, both practical and
theoretical.
Results of CCR will be reliable only if the coupling between the ground
and the aerials remains reasonably constant, so changes due to surface
irregularities are sources of noise. These are minimised by weighting the
aerials but with obvious disadvantages for one-man operations. Considerable
force is needed to drag the system over anything but the smoothest terrain,
and especially uphill.
External EM noise sources can affect data quality. However, signal fre-
quencies lie within a narrow band, and noise sources such as power lines
or telluric currents actually affect CCR less than galvanic systems. Both are
adversely affected by long linear conductors that run parallel to survey lines.
