Geology Reference
In-Depth Information
multiple 'n' of the dipole lengths, so the system geometrically resembles a
dipole-dipole galvanic array.
The dipole-dipole system is the industry standard for CCR surveys for
practical reasons. Since every bit of unshielded wire contributes to the cou-
pling, current flow must be confined to the aerials. The dipole-dipole con-
figuration allows this, and minimises transmitter-receiver direct coupling,
because the transmitter and receiver electronics and power sources are en-
closed in nacelles situated at the midpoints of their respective aerials. The
signal from the receiver electronics to the data logger is, in the target area
close to the aerials, carried by optical pulses in a fibre-optic cable ('optical
wand') and not by current flow.
Measurements are obtained at fixed time intervals and the data logger
is, when the system is being dragged manually, strapped to the opera-
tor's belt, which also takes the strain on the cable. The logger display can
show the resistivity profile as it develops, and also several of the preceding
profiles. The precautions discussed in Section 1.3.3 regarding all 'continu-
ous' measurements need to be observed to ensure data validity. Even when
towed behind a vehicle, speeds should be kept below about 5 km/h.
6.6.3 CCR parameters
The factor used to convert resistance measurements to apparent resistivity
for a DC galvanic dipole-dipole array was shown in Figure 6.2e. The equiv-
alent for CCR line-source measurements is very different, being quoted by
Geometrics as:
l
π
ln
(
b
−
1)
2
(
b
−
2)
K
=
2
b
b
2
(
b
+
1)
2
b
+
2
b
2
b
2
+
2
b
−
2
b
b
2
−
1
where
l
is the dipole length and
b
=
2(
n
+
1) and ln indicates that the loga-
rithm is being taken to the base 'e'.
The OhmMapper
TM
only measures electric fields. The magnetic field
is ignored, which is equivalent to assuming that the skin depth (Section
5.2.5) is greater than the transmitter-receiver separation. At the 16.5-kHz
frequency, this implies that the separation should be, numerically, less than
four times the square root of the resistivity. EM effects will then be negligible
and apparent resistivities should be comparable to those obtained galvani-
cally. However, dielectric effects can also influence results. Comparisons of
CCR and galvanic apparent resistivities over permafrost have shown that the
former may be only 25% of the latter, even though the two are compara-
ble over unfrozen soils. This occurs because the permafrost, an insulator,
