Geology Reference
In-Depth Information
Figure 6.12
Electrode layout patterns for three popular electrical resistiv-
ity imaging (ERI) arrays, illustrating the different switching considerations
for multicore cables. With the Wenner spread, the original current elec-
trodes can be used, after two expansions, as voltage electrodes. With the
Schlumberger array, the voltage electrodes may not need to be switched at
all, whereas switching for the dipole-dipole array must keep voltage and
current electrodes in constant-separation pairs but can implement lateral
shifting as well as expansion.
6.5.2 Choice of array and depth of investigation
Identifying the array that best meets the survey objectives is a critical part
of ERI survey design, but the basic principles are the same as those that
govern the selection of arrays for profiling or depth-sounding - that is, the
depth of investigation is largely controlled by the maximum length, the
vertical resolution is controlled by the type of array, and lateral resolution is
controlled by electrode separation. However, the choice also depends on the
type of structure to be mapped, the sensitivity of the resistivity meter and
the background noise level, and the ease with which automatic switching
can be implemented via a multicore cable (Figure 6.12).
As a general rule, the Wenner array provides better vertical resolution of
layers, and the dipole-dipole array better lateral resolution of steep bound-
aries. Because of the greater signal strength for a given current input, the
Wenner array would be most appropriately deployed in noisy conditions and
the dipole-dipole in quiet conditions where increased instrument sensitivity
can be used to record relatively low signal strengths. The Schlumberger
array is a good compromise for targets requiring both vertical and lateral
resolution.
All inversion methods are attempts to find models for the subsurface with
responses that agree with the measured data. Unsurprisingly, there are likely
to be many models that produce calculated apparent resistivity values that
agree equally well with a given set of measured values.
