Ground deformation monitoring at the Phlegrean Fields (Naples, Italy) from the exploitation of SAR data in the framework of CAT-1 and DUP activities

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1 S. Borgström, I. Aquino, C. Del Gaudio, C. Ricco, V. Siniscalchi, G. Solaro, P. Tizzani & G.P. Ricciardi I.N.G.V. Osservatorio Vesuviano - Via Diocleziano 328, Naples, Italy INTRODUCTION CAT (in progress) DUP MINERVA (completed in 2003) REMARKS AND CONCLUSIONS

2 Geodetic monitoring networks Spirit levelling network (322 benchmarks on 120 km of leveling line); GPS network (35 3D vertices, 8 of which as permanet stations); Tiltmetric network (permanent) 4 sensors + 2 borehole; Tide gauge network (4 stations); Gravimetric network (19 stations)

3 The deformation history of the Phlegrean Fields Height variations of benchmark 25, levelling network (coast line, up)

4 GPS data ( ) QUAR MORU ARFE IPPO 5200 RITE 5180 BAIA ACAE POSI GPS data from the permanent network (RITE and ACAE stations, see above) (2005 Surveillance Report, INGV-OV, in press)

5 Tiltmetric data ( ) Tilt inversion Tilt inversion

6 Levelling data (5-8/2000) A 30 +2σ 4B 7 9 2σ

7 Why DIfSAR?...a comparison with classical techniques A good space-temporal coverage in space: - high coverage from spaceborne sensors; - information on the deformation field outside the ground networks (i.e. possible migration); in time: good temporal coverage (ERS2: 35 days - ENVISAT: 35 days/swath) vs. the mean repetition time of field measurements (tipically once or twice a year, with no or low dynamics). An optimization of the monitoring system integration of SAR data with geodetic ground data; the choice of the geodetic surveying techniques more suited as a function of the deformation rate (volcanic risk level) of the area: tipically classical ground techniques with no or low dynamics, geodetic surveying from spaceborne sensors during pre-eruptive/eruptive phase. A (drastically) lower cost 25 /scene (ERS2/ENVISAT, CAT-1 projects) vs. some to many thousand for each geodetic field measurement.

8 CAT Integration of SAR Interferometry with classical geodetic techniques for ground deformation monitoring in the Neapolitan volcanic area ***

9 CAT

10 CAT

11 SAR/GPS: a comparison (3-8/2000) a) DIfSAR deformation map from ERS2 descending images of March and August 2000; b) ground deformation time-series (6/1992-9/2000) from SAR data for the ACAE GPS location; c) comparison between GPS measurements (ACAE site) projected into the radar line-ofsight (LOS) and SAR measurements, for the period from March to August (from Lanari et al., 2004)

12 SAR/levelling: a comparison (3-8/2000) c a, b) DIfSAR deformation map (3-8/2000) with the deformation time-series, bm 25 (IREA-CNR); c) vertical displacements for bm 25 (1/1985-5/2005) (INGV-OV)

13 CAT (Progress Report 2/11/2004) - Problems The main problem encountered is related to the Doppler centroid problems of the ERS-2 platform that prevent us from using most of the data acquired after This is an important issue since continuous GPS networks have been installed in the test area only after 2000 and therefore we have a short time interval to compare the two techniques. Also, more recent levelling campaigns cannot be used to make comparisons. To overcome this problem, we have extended the ERS deformation time series with 7 interferograms from 6 ENVISAT/ASAR data (swath IS2), acquired on descending orbits in the last two years. In fact, the SBAS technique provides an easy way to merge multi-sensor data, while avoiding the use of crossplatform interferograms, which could be critical. However, for this combination to be reliable, the time overlap between the data from the two sensors must be as large as possible; as a consequence, up-to-date ERS and ENVISAT acquisitions would be useful for the aims of this project

14 DIfSAR deformation map ( ) DIfSAR deformation map (6/1992-8/2003) from ERS1-2 and ENVISAT (blu triangles) with the ground deformation time-series for bm 25 (by IREA-CNR/INGV-OV)

15 ENVISAT data (in progress ) a) b) SAR focusing of ENVISAT data (IS2) (by IREA-CNR) a) 24/08/2005 (ascending), b) 22/09/2005 (descending)

16 DUP MINERVA Monitoring by INterferometric SAR of Environmental Risk in Volcanic Areas Design, development and assessment of a demonstrative small scale information service based on processing of images from the ASAR instrument on board ENVISAT MINERVA Team (User)

17 Levelling/SAR: a comparison ( ) Comparison of ground deformation time-series from SAR (ERS) and leveling data: the good temporal coverage of SAR data allows to retrieve the information not available from levelling in the time interval between field measurements.

18 SAR/levelling/GPS: a comparison ( ) a) b) POSI RITE U (m) /01/01 02/03/01 01/05/01 30/06/01 29/08/01 28/10/01 dd/mm/yy a) Differential interferogram 28/9/2000-3/10/ ERS2, descending orbits; b) height variations from levelling 10/ /2002 (coast line); c) variations of the up component of the POSI-RITE baseline (1/ /2002) c) 27/12/01 25/02/02 26/04/02 25/06/02 24/08/02

19 ENVISAT data (2003) Differential interferogram 6/4/ /7/2003 (bottom left) with the master scene of the interferometric pair (top left); on the right the coherence map ENVISAT data - swath 6 - descending orbits

20 ENVISAT data ( ) Ground deformation time-series from ENVISAT data (11/2002-8/2003) swath 2 - descending orbits. From levelling: height variation on bm. 25 (7/ /2003) = -1.3 cm.

21 ERS2 data ( ) From levelling: height variations on bm. 25 (7/2002-5/2004) =-1.9cm Deformation map, 3/2002-5/2004 (ERS2, descending orbits)

22 The Corner Reflectors network at the Solfatara crater a) b) c) a) SAR focusing (by IREA-CNR) of the ERS2 descending pass of 29/07/2004 on the Solfatara crater; b) the Digital Terrain Model of the area with the 4 couples of CR; c) the S4 couple

23 The Corner Reflectors network Some recent improvements (1)

24 The Corner Reflectors network Some recent improvements (2)

25 Some remarks (1) Ground deformations measurement at the Phlegrean Fields is a crucial point for two main reasons: for monitoring purposes, as ground deformation seems to precede the beginning of seismic activity (also in 2000); to define the deformation field of the area and therefore to propose a source modelling, taking into account that not only a vertical component of ground motion was recorded in time, but also horizontal displacements (see GPS and SAR data).

26 DIfSAR deformation maps The 2000 uplift event (Courtesy of IREA-CNR)

27 Levelling measurements ( ) Variation in ground elevation along EW and NS profiles of single benchmarks during a given time interval, normalized to the maximum value of deformation in the same time interval (from Orsi et al., 1999)

28 Some remarks (2) According to GPS and tiltmetric data, a very low uplift was measured by the end of 2004 ( Surveillance Reports), but the DIfSAR technique was not able to record the event; the reason lies in the resolution of the SAR sensor (in C band) when dealing with a very low deformation rate. This limitation can be relieved by processing SAR data spanning a larger time interval, but only with a clear and continuous trend of ground deformation; as to the geometric resolution of SAR data, no more resolution is needed for the geodetic study of the Phlegrean Fields, as we deal with a large deformation field (but it could be a problem for other volcanic areas!).

29 From our experience As the geodetic networks (GPS, levelling) need a stable reference point (zero deformation point) to retrieve afterwards the information over critical areas, DIfSAR can easily indicate the area in which the reference point can be located; the CAT-1 team carried out also DIfSAR measurements on the island of Ischia (M. Manzo et al., 2005, in press): the problem in this case was the choice of a stable reference point for DIfSAR data processing in a small area. Such a point was selected from GPS measurements, using a short GPS baseline between a (stable) vertex located on the land and the one on the island: no variation of the baseline was recorded in time, therefore indicating the stable point on the island to be used for DIfSAR data processing; it is unlikely to validate SAR data with only another satellite geodetic technique like GPS, because they both suffer of the same limitations (i.e. the atmospheric artefacts): to overcome the problem, validation must be carried out with terrestrial networks (levelling) which do not suffer the limitations of DIfSAR. When the comparison between DIfSAR and GPS was carried out, the team always had the information from levelling, indicating precisely the trend of ground deformation in the area of interest.

30 Conclusions (1) The exploitation of DIfSAR for volcano monitoring is a strong improvement of the surveillance system, as it allows to retrieve an information on wide areas, with a good temporal coverage at a very low cost; the comparative analysis between classical and DIfSAR data has shown a clear agreement between the different techniques in both space and time; the main limitations toward a more precise comparison are due to the poor spatial coverage of the geodetic permanent stations vs. the high spatial coverage of SAR data and the poor temporal sampling of the periodic networks, unless in case of pre-eruptive or eruptive phase; the difference between the component of ground motion recorded by SAR and by classical techniques has also to be taken into account: by using SAR data from both ascending and descending tracks, the EW component of the horizontal deformation can be retrieved.

31 Conclusions (2) DIfSAR proved to be a very powerful technique to retrieve ground deformation time-series over critical points in volcanic areas and it could be successfully used for monitoring purposes with no or low dynamics. Problems arise when dealing with an higher deformation rate, when continuous monitoring must be carried out: in that case systematic data acquisition becomes the priority and the 35 days time interval (ERS2) to get the geodetical information is not sufficient, especially with an increasing dynamics. The problem can be partially overcome by using ENVISAT data, considering different image swaths (IS). Also the time for data delivery from the Agency to the User has to be changed and fast data delivery (via FTP) shall become the standard: it was already done in some cases in the recent past, demontrating its feasibility in case of an emergency. Thank you for your attention...