 |
| Titel |
Investigation of subsidence in the Manfredonia Gulf (Southern Italy) through multitemporal DInSAR techniques |
| VerfasserIn |
M. Triggiani, A. Refice, D. Capolongo, F. Bovenga, M. Caldara |
| Konferenz |
EGU General Assembly 2009
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250025572
|
|
|
|
|
|
| Zusammenfassung |
| We present results of an experiment aimed at detecting possible displacements due to
subsidence in the coastal area of the Tavoliere plain, Puglia Region, in Southern Italy, through
analysis of remotely sensed data.
The Tavoliere is the second largest Italian plain. Its coastal area, between the
urban centers of Manfredonia and Barletta, is composed of a 50 km long sandy
beach (Manfredonia gulf), linking the Gargano massif at north with the Murge
plateau in the south-east. Both areas belong to the carbonate Mesozoic Apulian
platform.
The current configuration of the Gulf was reached recently, as a consequence of the
Holocene sea level rise. During the Neolithic age the plain was occupied by an elongated
lagoon (Salpi Lagoon) [1]. During the 2nd century B.C., alluvial deposition caused the
lagoon to be separated into two basins: the so-called Salpi lake at south and the Salso lake at
north. To cope with the increasing demand of arable lands and with the necessity to
make unhealthy areas accessible to humans, some reclaims were accomplished by
diverting and channelling the rivers crossing the Tavoliere plain, and by levelling
dune belts. At present, the beach is separated by low artificial dunes from the areas
already reclaimed and intensely cultivated or exploited as evaporation basin for salt
production.
In the last decades, the coastal area has been retreating due to a reduction in sediment
input necessary for coastal equilibrium. The levelling of dunes and the decrease of fluvial
turbid discharge due to dam constructions are probably the cause of this deficit in sediment
supply.
During highly intense hydrodynamic and meteorological events, sea waters often
penetrate deeply inland, flooding intensively cultivated areas. These events are occurring with
growing frequency and rates. They are an indication of the possibility that those areas are
subject to subsidence at a faster rate in comparison to the surroundings.
An example is the salt marsh located inland of the tourist sea village "Ippocampo". Here,
unpublished studies based on ground data indicate average subsidence rates of the order of
0.20 mm/y in the last 125 ka for the inland area next to the village.
More recently, height maps issued by the Italian Military Geographic Institute (IGM) in
the 1950s report heights a.s.l. of the order of a few m. Observing that today the area is
practically at sea level, an average subsidence of the order of tens of mm/y can be inferred for
the last 50 years.
To gain insight into the recent evolution of these phenomena, we investigate vertical
movements on the coastal Tavoliere area through multitemporal differential Interferometric
synthetic aperture radar (DInSAR) techniques. We use a persistent scatterers interferometry
(PSI) processing methodology [2] to estimate subsidence displacement rates from long
temporal series of SAR acquisitions. PSI techniques, first developed at POLIMI [3], allow to
retrieve phase information from stacks of co-registered SAR interferograms spanning
many years and taken from different directions with large baselines, by restricting
the analysis to selected image pixels containing single objects with strong radar
backscatter returns. Exploiting the high temporal stability of radar returns from these
targets, it is possible to correct the images from spurious phase contributions such as
atmospheric phase artefacts and errors in the digital elevation models used to account for
topographic InSAR phase. Such stable objects typically coincide with man-made
features, so successful applications of PSI techniques are mainly reported over urban
centers.
We processed a total of 105 SAR images acquired from the ERS-1/2 and ENVISAT
satellites, organized in 3 stacks related to both descending (50 ERS-1/2 scenes) and ascending
(25 ERS-1/2 and 30 ENVISAT scenes) acquisition geometries. The acquisitions refer to
the temporal periods from 1995 to 2000 (ERS) and 2003 to 2008 (ENVISAT),
respectively, with a temporal repetition frequency of roughly 1 acquisition every 35
days.
The 3 stacks, covering approximately the same ground area centred on the Tavoliere
coastal plain, were processed independently. Reliable phase measurements were
obtained over small urban centers and anthropogenic features scattered along the
coast.
Results from all 3 stacks indicate the presence of displacements occurring through the
entire temporal interval of observation. In particular, displacements appear spatially
organized as a subsidence "bowl" centered approximately around the area of Zapponeta, with
maximum subsidence rates exceeding 20 mm/y.
The detected displacements appear consistent with the average rates deduced heuristically
from analysis of the environmental settings as exposed above. Moreover, they also
qualitatively agree with other investigations performed using analogous techniques and data
over the region (e.g. [4]).
Possible interpretation of these results can be attempted by considering that the area has
been repeatedly subject to reclaiming through filling, and that the deposited sediments are
most exposed to compaction. Moreover, the area is subject to intense water extraction, which
further enhances the effects of sediment compaction.
Validation of the obtained measurements is in progress through extended data analysis
and in situ activities. However, these preliminary analyses and comparisons between InSAR
and ground data hint to the possible presence of two co-existing subsidence phenomena in the
area: a natural subsidence due to tectonics or isostatic rebound, with slow subsidence effects
occurring over geologic time scales, with an additional, more pronounced subsidence
phenomenon on the recent sediment deposits due to sediment compaction under
lithostatic loading, and an anthropogenic local, accelerated subsidence on the lowest
areas, due to intensive draining mostly for irrigation purposes, which adds to recent
land remediation actions to cause a worrisome lowering of the water table in the
area.
References
[1] F. Boenzi, M. Caldara, M. Moresi, L. Pennetta 2002, "History of the Salpi
lagoon-sabhka (Manfredonia Gulf, Italy)". Il Quaternario, 14, 93-104. 2001
[2] F. Bovenga, A. Refice, R. Nutricato, L. Guerriero, M.T. Chiaradia, "SPINUA: a
flexible processing chain for ERS / ENVISAT long term interferometry", Proceedings of
ESA-ENVISAT Symposium, Salzburg, Austria, 6-10 September, 2004.
[3] Ferretti, A., Prati, C., Rocca, F. "Permanent Scatterers in SAR Interferometry". IEEE
Transactions on Geoscience and Remote Sensing 39, 8–20, 2001.
[4] S. Salvi, S. Atzori, C.A. Brunori, F. Doumaz, G.P. Ricciardi, G. Solaro, S. Stramondo,
C. Tolomei, R. Lanari, A. Pepe, A. Ferretti, S. Cespa, "The VELISAR initiative for the
measurement of ground velocity in italian seismogenic areas", EGU General Assembly,
Wien, Austria, 15-20 April, 2007. |
|
|
|
|
|
|