Geology Reference
In-Depth Information
between the voltage electrodes should not be very much greater than the
width of the smallest target that would be of interest.
7.5.1 Gradient-array data
Current paths are roughly horizontal in the central areas investigated using
gradient-arrays, and chargeable bodies will be horizontally polarised. Pro-
files can be interpreted by methods analogous to those used for magnetic
data, with approximate depths estimated using the techniques of Section
3.5.2.
7.5.2 Dipole-dipole data
Dipole-dipole traverses at a constant
n
value can be used to construct profiles,
but multi-spaced results are almost always displayed as pseudo-sections
(Figure 7.5). The relationships between the positions of highs on pseudo-
sections and source body locations are even less simple with dipole-dipole
than with Wenner arrays (see Section 6.5.1). In particular, the very common
pant's leg
anomaly (Figure 7.5) is usually produced by a near-surface body
with little extent in depth, since every measurement made with either the
current or the voltage dipole near the body will record high chargeability.
Anomaly shapes are thus very dependent on electrode positions, and the di-
rections of apparent dip are not necessarily the directions of dip of the charge-
able bodies. Even qualitative interpretation requires considerable experience
as well as familiarity with model studies.
Figure 7.5
Pseudo-section construction. The three different positions of the
current dipole correspond to three different multiples of the basic spacing.
Measured values [of induced polarisation (IP) or resistivity] are plotted at
the intersections of lines sloping at 45
◦
from the dipole centres. The plotting
'point' often doubles as a decimal point for IP values. The 'pant's leg'
anomaly shown is typical of those produced by small, shallow bodies.
