Introduction to the resistivity surveying method. The resistivity of ...

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2.7.4 Landslide - Cangkat Jering, Malaysia
A recent problem faced in Malaysia is landslides on hill slopes. The landslides are
often triggered by water accumulation within part of the slope which leads to weakening of a
section of the slope. Figure 19 shows the results from a survey conducted on the upper part of
a slope where a landslide had occurred in the lower section. Weathering of the granitic
bedrock produced a clayey sandy soil mixed with core boulders and other partially weathered
material. The image obtained from this survey shows a prominent low resistivity zone below
the centre of the survey line. This is probably caused by water accumulation in this region
which reduces the resistivity to less than 600 Ohm•m. To stabilise the slope, it would be
necessary to pump the excess water from this zone. Thus, it is important to accurately map the
zone of ground water accumulation. This data set also shows an example with topography in
the model section.
2.7.5 Old Tar Works - U.K.
A common environmental problem in industrial countries is derelict industrial land.
Before such land can be rehabilitated, it is necessary to map old industrial materials (such as
metals and concrete blocks) that are left buried in the ground. Another problem in such areas
is chemical wastes that had been stored within the factory grounds. Due to the nature of such
sites, the subsurface is often very complex and is a challenging target for most geophysical
methods. The survey for this example was carried out on a derelict industrial site where
leachate was known, from a small number of exploratory wells, to be moving from a surface
waste lagoon into the underlying sandstones (Barker 1996). Eventually the leachate was seen
seeping into a nearby stream. However, the extent of the subsurface contamination was not
known.
An electrical imaging survey was carried out along an old railway bed between the
lagoon and the stream. The metal railway lines had been removed except for short lengths
embedded in asphalt below a large metal loading bay. In the apparent resistivity
pseudosection (Figure 20a), the area with contaminated ground water shows up as a low
resistivity zone to the right of the 140 metres mark. The metal loading bay causes a prominent
inverted V shaped low resistivity anomaly at about the 90 metres mark. In the inversion
model (Figure 20b), the computer program has managed to reconstruct the correct shape of
the metal loading bay near the ground surface. There is an area of low resistivity at the right
half of the section which agrees with what is known from wells about the occurrence of the
contaminated ground water. The plume is clearly defined with a sharp boundary at 140 metres
along the profile. The contaminated zone appears to extend to a depth of about 30 metres.
2.7.6 Holes in clay layer - U.S.A.
This survey was carried out for the purpose of mapping holes in a clay layer that
underlies 8 to 20 feet of clean sand. The results from the electrical imaging survey were
subsequently confirmed by boreholes.
The pseudosection from one line from this survey is shown in Figure 21a. The data in
the pseudosection was built up using data from horizontally overlapping survey lines. One
interesting feature of this survey is that it demonstrates the misleading nature of the
pseudosection, particularly for the dipole-dipole array. In the inversion model, a high
resistivity anomaly is detected below the 200 ft. mark, which is probably a hole in the lower
clay layer (Figure 21b). This feature falls in an area in the pseudosection where there is an
apparent gap in the data. However, a plot of the sensitivity value of the blocks used in the
inversion model shows that the model blocks in the area of the high resistivity body have
higher sensitivity values (i.e. more reliable model resistivity values) than adjacent areas at the
Copyright (1999-2001) M.H.Loke