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Fig. 9 - Electrical
resistivity tomography
– TR01-TR02-TR03

in the SE direction, has a thickness of 20-22 m and a In the attached models, some areas with lower resistivity
resistivity between 20 and 40 Ωm, correlated with the are observed within the limestone horizons, which can
lithologies of the Upper Globigerina Limestone Member, therefore be interpreted as more fractured. One of them
as found in coring, consisting of limestone, weak to coincides with the escarpment between the platform
very weak, with grey marly. An underlying horizon, and the underlying bottom, from 10 m up to the
whose base is located at about 35 m depth, shows a progressive 150 m where the limestone mass is
reduction in the value of electrical resistivity, which is probably more disarticulated due to the morphological
between 12 and 18 Ωm, attributable to the increase in condition. Other areas where the resistivity decreases
marly intercalations. are between the progressive 260 m and 450 m, in
Below 35-38 m of depth, there is a sharp decrease in which the rock mass is presumably affected by fracture
resistivity values, between 2 and 6 Ωm, which can be due to tectonic causes. There are no localized sectors
attributed to a lithological variation, where pelites (marls with resistivity lower than 0.5 Ωm, so there is no
and clayey marls) are prevalent, such as also found in indication of detectable cavities.
coring.
For the purpose of further evaluations on the geometric
Regarding the evaluation of the state of fracture and of distribution of the values of the real electrical resistivity
karst forms that may affect subsoil sectors, it must be of the sector investigated with the longitudinal ERT
considered that the increase in the state of fracture and (TR01, TR02 and TR03 (See Fig 9:)), the data of the
karst have the effect of increasing the porosity of the three survey lines have been merged into a volume (3D)
rock mass. This condition determines, from the included between the bottom of the sea where the
electrical point of view, a sudden reduction in the electrode lines fall to a depth of about 60 m from the
resistivity values, as the conduction of the electric seabed. The geoelectric data thus placed in space
current is made easier by the interstitial water, even have been subject to 3D inversion through the use of
more so if it is high in salt content, being sea water. This the Res3DInv64 software. The inversion parameters
condition will be particularly evident when the lithology have remained the same as those used in the 2D
is of higher resistivity (limestones), in which the contrast inversion (L1 norm robust and 0.19 Ωm for sea water
with the fractured bands and, even more, in the karst resistivity).
areas filled with water, is higher. In particular, the
detectable cavities will be characterized by particularly As seen also in 2D models, and presumably for the
low resistivity values, close to those of sea water (<0.5 same reasons, an artifact near the coast is produced in
Ωm). In this perspective, therefore, the lateral variations the calculation. This is characterized by an area,
of low resistivity within the electric/lithological horizons, between progressive 450 and 500 m, with an abrupt
can be interpreted as more fractured and possibly karst lateral passage between a low-resistive and a high-
areas. resistive side by side, with unrealistic values and
distribution not consistent with the local lithostratigraphic

IndustrIal Plants - May 2022
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