Page 33 - Industrial Plants

Fig. 5 - Laying of the
cables, arrangement
of the electrodes on
the seabed and data
acquisition

faster data acquisition. See Figure 4. or deficit in conductance in the water model by the
For each of the longitu dinal lines, 635 m long, 4167 corresponding increase or decrease in the resistivity. In
measurements were made, while in the transverse this case, the bottom topography varies greatly; hence,
ones, with a length of 315 m, the measurements were it is of utmost importance to measure both bottom
1565. The spatial resolution of the survey depends not topography for each survey line to avoid misleading
only on the spacing of the electrodes and the number results. The water depth in the study area was mapped
of measures, but on a series of factors in a system to using multibeam sonar, already carried out in a previous
be considered complex. In correspondence of possible survey; the data acquired by this measure are of good
cavities, being filled with water, there is to expect lower overall quality, with their degradation in the area close to
values of electrical resistivity than the carbonate rock in the shore line. This situation has created some problems
which they are contained. The possibility of solving of elaboration in proximity of the coast. The value of the
them is linked to their dimensions in relation to their resistivity of sea water was measured in a laboratory on
depth, as well as to the contrast of resistivity between a taken sample (0.19 Ωm at 15°C).
rock and the filling marine water. At a depth of 20-25 m
from the seabed, a resolution (ability to detect a cavity)
of 4-6 m can be assumed; below this size the electrical Processing and Interpretation

anomaly produced, is considered too weak to be The TR00 survey was carried out on the land, with a Electrical Resistivity
perceived by measurements performed on the seabed. length of 315 m, between the elevation of 10 and 13 Tomography – TR00.
See Figure 5: (fig. 5a,
5b, 5c, 5d)
Water depth and
resistivity: In order to
obtain useful estimates of
resistivity in soil and rock,
it has been necessary to
integrate the water depth
and the resistivity of the
water in the interpretation
model. Errors in water
depth or resistivity lead to
artefacts in the model, as
the inversion program
compensates for surplus

INDUSTRIAL PLANTS - May 2022
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