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Geophysical survey of the offshore microtunnel

route in Delimara Peninsula (Malta)

Assessing the seafloor integrity and the possible deteriorations that
might arise from the bedrock collapse during the nearshore
microtunnelling, and identifying and avoiding cavities during the work

Vanessa Barcaglioni, Giuseppe Canna, Giuseppe Vecchio
Techfem S.p.A.
Maurizio Ponte, Università della Calabria
Giuseppe Ferraro, Geofisica Misure s.n.c.

his paper reports the main result of the cavities along the pathway analysed and, thus, to
geophysical survey performed in the distinguish the karst lithologies (limestone and marly
Delimara peninsula (Malta) along the limestone) from the non-karst ones (marls and clays).
offshore route of the microtunnel
realized for the shore approach of the
T 22” Melita Transgas Pipeline. The main Introduction
goals of the survey were to assess the seafloor integrity The geophysical investigations aim to support the
and the possible deteriorations that might arise from design of a gas pipeline (Melita Transgas Pipeline) in the
the bedrock collapse during the nearshore section interested by the microtunnelling technology.
microtunnelling, and then to identify and avoid cavities The microtunnel crosses under the Delimara peninsula
during the work. for 600 m and extends under the seabed for other 600
The preliminary geognostic investigations carried out m. The purpose of the investigations was reconstructing
on this segment through several boreholes highlighted the lithostratigraphic structure of this nearshore section,
the presence of fractured limestone and possible up to a depth of 50-60 m from the seabed, also
cavities. A following tomographic analysis (Electrical assessing the fracturing conditions and any karst
Resistivity Tomography) allowed the detailed processes that affect the lithological formation present.
reconstruction of the lithostratigraphic and
geomechanics characteristic of the entire corridor. The “The geophysical investigations aim
geoelectric prospect also allowed to verify and evaluate to support the design of the Melita
the risk of intercepting cavities along the corridor during Transgas Pipeline in the section
the drilling phase. interested by the microtunnelling
The installation of seven multielectrode lines were technology
performed: one of them was longitudinal to the
microtunnel pathway, two parallel and the rest The activities are subsequent to a first phase of
transversal to it. Six of them were installed on the investigations carried out “on the land” (drilling, refraction
seabed, with the help of divers, to better mitigate the seismic and geoelectric tomography prospecting). In the
effects of the marine depth. The post-processing of the nearshore area, two continuous drilling operations and a
data, optimized through the precise knowledge of the morpho-bathymetric survey of the seabed have already
electrode positions and the resistivity values of the been carried out, the latter used as a support for the
seawater, led to the definition of a two-dimensional and programming and processing activities carried out in this
a three-dimensional predicting model, able to work up phase. Just starting from the acquired data, it was
to 50 m depth from the seabed. Such models link the possible to evaluate the most suitable geophysical
values of the resistivity measured and the lithologies of methodology for the achievement of the expected
the seabed (known from the stratigraphic logs) to its objectives. The seabed is featured by a very variable
stratigraphy, allowing to exclude the presence of depth between the coast and the outermost part, which

IndustrIal Plants - May 2022
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