Geology Reference
In-Depth Information
Pseudo-sections are nearly always plotted in relation to horizontal base-
lines, even in rugged terrain. Referencing them to topographic profiles
(using construction lines similar to those of Figure 7.5 but at 45
◦
to the
actual ground surface) has its dangers, since it might be taken as implying
much closer correlations with true subsurface distributions of resistivity and
chargeability than actually exist. However, steep and varied slopes do in-
fluence dipole-dipole results and it is arguably better that they be displayed
than ignored.
7.5.3 Negative IPs and masking
Negative IP effects can be caused by power or telephone cables or, as shown
by signal contribution sections (Figure 6.5), by lateral inhomogeneities.
Layering can also produce negative values, and can conceal deeper sources,
most readily if both the surface and target layers are more conductive than
the rocks in between. The penetration achieved in such cases may be very
small, and total array lengths may need to be ten or more times the desired
exploration depth.
Interactions between conduction and charge in the Earth are very complex,
and interpreters may need resistivity data that are more reliable than those
provided by the dipole-dipole array, which performs poorly in defining
layering. A small number of Wenner or Schlumberger expansions, carried
out specifically to map resistivity, can be invaluable. Also, any changes in
surface conditions that might correlate with changes in surface conductivity
should be noted. The response from an ore-body will be quite different
beneath a bare rock ridge and an adjacent swamp.
