Geology Reference
In-Depth Information
Geological features and man-made structures such as buried pits are three-
dimensional in nature. Pole-pole, pole-dipole and dipole-dipole arrays are
typically used for 3D surveys and continue to achieve good data coverage
even near survey grid boundaries. Until recently, the collection and process-
ing of 3D resistivity data was prohibitively costly, but with the development
of multichannel resistivity meters to collect data faster and more powerful
microcomputers to invert very large data sets, 3D surveys have become
almost routine.
The advantages of ERI are only fully realised if purpose-designed equip-
ment is used to reduce the time taken to collect the necessary large volumes
of data, and with a modern 61-channel system the ground can be covered
about 50 times faster than with a single-channel system.
6.5.3 Topographic effects
Topography can mask features in apparent resistivity data, and inversion can
produce resistivity sections with spurious structure in areas of high relief.
In these cases it is important to measure the relative heights of electrodes in
the field and model the topographic effects.
Even where the topography does not significantly affect the resistivity
values, it is important that it be recorded, because it provides additional
information about the site being investigated. The pseudo-section in Figure
6.11d, which is referenced to the actual topography, is far more informative
than the simpler version in Figure 6.11c.
6.5.4 Time-lapse measurements
Time-lapse ERI has many applications where situations change with time.
These include, but are not limited to, seepage pathways in earth dams,
leachate egress from landfills, remediation progress at clean-up sites, sink
hole activity, seasonal variations in permafrost, movements of fresh water/
salt water interfaces in tidal areas, embankment stability and landslide risk.
The standard approach is to separately model data obtained at different
times and produce difference plots to highlight changes. The initial inverted
model from the first data set obtained at the site may be used for reference
or, if appropriate, comparisons may be made with models based on data
from areas where the subsurface is, and remains, in a relatively unperturbed
state.
Remote monitoring of permanent ERI installations using mobile phone
or internet technologies can be a powerful and cost-effective alternative to
regular site visits. It is likely that this technique will grow significantly in
popularity over the next decade.
