Page 31 - Industrial Plants

These terraces are interpreted as coastal platform considered as an ideal technique for mapping the
formed by subaerial atmospheric agents during the saline and fresh water interface in coastal areas.
exposure at the bottom and the erosion of the waves The apparent resistivity measurements in the field
(hydraulic pressure and abrasion) that guided the have been carried out using an eight-channel
coast towards the ground during the post-glacial sea georesistivimeter with galvanic isolation (ABEM
level rise. As a consequence, the ground edge of Terrameter LS2 System) for resistivity, induced
these platforms, known as the corner of the coast, polarization and spontaneous potential, with the
provides an indicator of the maximum sea level at the following characteristics: Automatic or manual
time of formation of the platform. The shallower selection of the current input from 0.20 to 2500 mA;
paleoshoreline, on the other hand, correspond to the Energization with max voltage 600 V (1200 V peak-
notches identified by divers at depths of 25-30 m and peak); Maximum power to 250 W; Current
33-40 m, in the study area. transmission accuracy to 0.4 %; Current pulse
length 0.1%; Impedance 30 MΩ min; Resolution 3
nV; dV/I accuracy to 0.2 % and dV/I accuracy to
Electrical Resistivity Tomography 0.1%.

(ERT) To perform the measurements in multielectrode
configuration, the system is equipped with an
The electrical resistivity tomography (ERT) surveys automatic selector, capable of managing 64
aims to determine the subsurface resistivity electrodes simultaneously. The electrodes were
distribution through measurements on the ground placed manually on the seabed, then connected to
surface. The ground resistivity is related to various the selector via multicore cables. For sea operations,
geological parameters, such as the mineral and fluid a boat with a length of 12 m was used, engine power
contents, porosity and degree of water saturation in of 340 KW (“Simo” dive boat).
the rock. Electrical resistivity surveys have been used
for many decades in hydrogeological, mining and
geotechnical investigations. The fundamental Delimara ERT Survey

physical law used in resistivity is Ohm’s Law that Seven ERT surveys were carried out using a
governs the flow of current in the ground. The multielectrode method. Three of these were carried
equation for Ohm’s Law in vector form for current out in parallel with the pipeline design route, for a
flow in a continuous medium is given by: J= δ * E. length of about 630 m. In particular, one along the
Where δ is the conductivity of the medium, J is the pipeline axis (TR02) and the other two at a distance
current density and E is the electric field intensity. In of 10 m from each side. Other four ERT were carried
practice, what is measured on field the electric field out in the transverse direction, approximately NE-SW
potential. We note that in geophysical surveys the for a length of 315 m each one. One of them was
medium resistivity ρ, which is equals to the executed on land (TR00), it develops between the
reciprocal of the conductivity (ρ=1/δ), is more altitudes of 10 and 13 s.l.m. The other three have
commonly used. been realized offshore, so as to interest transversely
The resistivity values are largely dependent on the the track plan of the microtunnel path and the valley
porosity of the rocks and the salinity of the contained submerged valley present north of the pipeline track.
water. Unconsolidated sediments generally show The longitudinal measurement lines (TR01, TR02 and
even lower resistivity values than sedimentary rocks, TR03), were carried out by laying 4 cables with 32 x
with values ranging from about 10 to less than 1000 5.0m-spaced electrodes, for a length of 155 m each.
Ωxm. The resistivity value is dependent on the The measurements were carried out at the centre of
porosity (assuming all the pores are saturated) as each pair of cables (simultaneous acquisition on 64
well as the clay content. Clayey soil normally has a electrodes 315m in length), proceeding from offshore
lower resistivity value than sandy soil. However, an to the mainland, with a roll-along procedure,
overlap in the resistivity values of the different providing an overlap of 155 m, equal to the length of
classes of rocks and soils can be observed, as the a single cable. In this way, for each line, three
resistivity of a particular rock or soil sample depends stations will be required (acquisition step).
on a number of factors such as the porosity, the The acquisition points, coinciding with the central
degree of water saturation and the concentration of terminations of the cable pairs, have been
dissolved salts. The resistivity of groundwater varies materialized on the surface by buoys positioned
from 10 to 100 Ωxm depending on the concentration using a GPS-RTK (GR-3 - RTK GPS System from
of dissolved salts. The low resistivity (about 0.2 Ωxm) Topcon), anchored to the seabed with special
of seawater due to the relatively high salt content. ballasts (“dead bodies”). Both the positioning of the
For this reason, the resistivity method can be buoys and their anchorage, and the laying of the

IndustrIal Plants - May 2022
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