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ESTABLISHMENT OF A LEVELLING NETWORK FOR THE MONITORING OF POSSIBLE VERTICAL
MOVEMENTS IN THE AREA OF PISA - SAN GIULIANO TERME-PONTASSERCHIO-MIGLIARINO PISANO (ITALY)
G. Caroti (*), E. Mengali (*), A. Rossi (**), C. Scalese (*)
(*) DIEIT, University of Pisa, Italy
(**) HRG - CNR, Pisa, Italy
ISPRS Commission VI, Working Group 3
KEYWORDS: Compaction, Geothermics, Levelling, Subsidence
ABSTRACT
One of the wavs of inspection expected for the monitorig of the possible environmental alterations caused by the utilization of
thermal underground waters in the heating of the buildings and laboratories of the CNR Research Campus in Pisa is the supervision
of the surroundig district subsidence.
Taking into account the well-known information about the extension and location of reservoir - rocks to be exploited we have located
the area to be monitored: it consists of a broad part of Pisa, opening like a fan towards the north stretches to reach Migliarino Pisano
and San Giuliano Terme, through a benchmark network by meshes which are narrower near the CNR complex and wider in more
distant areas.
1. INTRODUCTION:
GEOTHERMAL SPACE-HEATING OF THE CNR
RESEARCH CAMPUS IN PISA:
PROBLEMS IN ENVIRONMENTAL MONITORING
The joint venture project by ENEL and CNR proposes to heat
the buildings and laboratories of the CNR Research Campus in
Pisa, utilising thermal waters (60-70°C) by means of a doublet
of wells. The project has been partly sponsored and funded by
the European Union (Research Project U.E., ЕЮ XVII), the
Regional Council of Tuscany, and the Italian Ministry for
Industry.
Right from the start the proponents were aware that they would
have to overcome public distrust and suspicion for the use of
this type of non-conventional energy'.
Therefore the project is aimed at fostering the development of
similar initiatives, in order to save fossil fuel and reduce the
amount of air pollutants discharged into the atmosphere.
The promotional effect of the Pisa CNR project could be further
enhanced by demonstrating the benefits that can be achieved
with the direct use of low-temperature geothermal energy, also
in the residential part of such an old town as Pisa.
Арап from its technical feasibility and innovative solutions, the
most challenging aspect of the Pisa project will be to show the
emironmental benefits, and to verify' the sustainability of
geothermal development with time, provided an effective
system of integrated networks is set up all over the Pisa area to
monitor environmental natural parameters and improbable
induced effects of this activity.
The reservoir of the geothermal fluid is expected to he at a
depth of 800-1200 m below ground level, with temperatures
around 70°C, within the Mesozoic carbonate complex that is
present throughout a large pan of Tuscany (Bellani et al., 1995).
A production/reinjection doublet of directional wells will be
drilled from the same drilling pad. The distance between the
two well-bottoms will be about 1000 m. The geothermal fluid
will circulate in a "closed loop" (carbonate formations,
production well, heat exchangers, reinjection well, carbonate
formations) as all the extracted fluid will be reinjected after use.
There will be no direct contact with shallow aquifers, surface
waters and atmosphere, thus preventing any pollution.
The Pisa geothermal project is based on a very conservative
analysis of the safety parameters, in order to prevent any risk.
Nevertheless the Pisa plain cannot be considered a safe area in
steady-state condition from the environmental point of view. It
is a flat lowland area (2-3 m a.s.l.) that was flooded a few times
by the Amo river in the past, and affected by natural historical
subsidence. Precision levelling data show negative ground-level
variations at a maximum rate of 1 cm per year during the period
1969-1983 (Palla, 1988). This high rate of subsidence can be
explained by a combination of natural and man-induced effects.
The Pisa plain rests on a thick sequence of soft Quaternary
unconsolidated sediments, and has been a marsh land since the
Holocene; its reclamation was not complete until the first half of
this century. Drainage of surface waters and pumping from
shallow aquifers, in particular, increased the compaction rate of
the shallower layers. The recent intensive urbanisation has
further compromised the delicate equilibrium of the foundations
of the city' buildings.
Subsidence is commonly caused by the settlement and
compaction of recent sediments and does not affect the deep
Mesozoic carbonate substratum, at least on a human time-scale.
The production-reinjection process will be developed through a
closed loop between the surface heat exchangers and the deep
carbonate formations without interacting with the cover
formations, from which the wells will be completely isolated. It
must be stressed that hydrothermal exploitation will not entail
any extraction of mass, but only of heat.
The draft programme of an integrated system of monitoring
networks has therefore been carefully studied to answer the
demanding questions on the various environmental aspects of
the geothermal development The svstem consists of:
1. A topographic network for monitoring the vertical ground
movements by means of periodical surveys with precise
levelling and Global Positioning System methods.
2. A microgravity network, utilising about 30 % of the
benchmarks of the topographic network, will help to
detect, with periodical surveys, any crustal mass variation,
fluid displacements, and water level changes in surface
acquifers.
3. A hydrogeochemical monitoring network of the
unconfined and confined shallow aquifers in 5 existing
