Ground deformation monitoring at the Phlegrean Fields (Naples, Italy) from the exploitation of SAR data in the framework of CAT-1 and DUP activities Borgström S., Aquino I., Del Gaudio C., Ricco C., Siniscalchi V., Solaro G., Tizzani P., Ricciardi G.P. Istituto Nazionale di Geofisica e Vulcanologia-Osservatorio Vesuviano - Via Diocleziano 328, 80124 Naples (Italy) Abstract The presentation is focused on the main results obtained in the framework of CAT-1 (1065, Integration of SAR Interferometry with classical geodetic techniques for ground deformation monitoring in the Neapolitan volcanic area) and DUP (MINERVA, Monitoring by INterferometric SAR of Environmental Risk in Volcanic Areas) activities, aimed at the integration of EO (ERS/ENVISAT) data with geodetic data from ground networks (mainly GPS and levelling) for ground deformation monitoring in the Phlegrean Fields volcanic area, which gave bradyseismic activity in recent time. Differential SAR Interferometry is a powerful technique for the geodetical monitoring of the area, as the information retrieved has been always comparable, both in space and time, with terrestrial data. Moreover, the information from spaceborne sensors is more effective in comparison with ground data, as it covers a wide area with an higher temporal sampling, if compared to the mean repetition time of field measurements. Besides SAR data processing, also a network of Corner Reflectors (CR) in the whole Neapolitan Volcanic District has been partially set up, in order to increase SAR data resolution in areas with low or no coherence: in particular 8 CR (4 couples) were already located in the Solfatara crater, which is marked by a different dynamics in comparison with the whole Phlegrean system and therefore of remarkable interest from the scientific point of view. 1 INTRODUCTION Already in historical time the Phlegrean Fields were marked by uplift and subsidence phenomena, also with seismic activity. In recent time the strong bradyseismic crises of the years 1969-72 and 1982-84 shall be mentioned, with a maximum uplift for both up to about 180 cm. Since January 1985, the Phlegrean Fields area is undergoing a subsidence phase, with only short uplift events recorded in 1989, 1994 and, more recently, in the period from March to August 2000 [9], [1], [7]. The beginning of the 2000 uplift event was pointed out by the tiltmetric and GPS permanent networks, suggesting to carry out levelling measurements besides spaceborne DInSAR data processing; the availability of both SAR (ERS2) and geodetic data, has allowed an accurate comparison between different data sets besides a detailed evaluation of the space-temporal trend of the deformation. In fig.1 the layout of the geodetic monitoring system (permanent and periodic networks) of the Phlegrean Fields is shown. Fig. 1. The Phlegrean Fields (Campi Flegrei) geodetic networks
Interferometric data processing was carried out by the IREA-CNR (Istituto per il Rilevamento Elettromagnetico dell Ambiente - Consiglio Nazionale delle Ricerche) group applying the SBAS (Small BAseline Subset) algorithm, proposed by Berardino et al. [2] that extends the approach of Lundgren et al. [8]; it allows, besides the production of spatially dense deformation maps, to reconstruct ground deformation time-series for each coherent pixel of the scene. In the March-August 2000 deformation map, shown in fig.2a, a ground deformation of more than 4 cm into the radar line of sight (LOS) was clearly pointed out in Pozzuoli area. It must be remarked that the considered March-August data pair is part of a large data set produced by the ERS1-2 systems from descending orbits starting from June 1992. By processing the overall InSAR data set it is possible to investigate not only the single 2000 event but also the temporal evolution of the whole bradyseismic phenomenon in the period which the data set refers to. To clarify this point, in fig.2b the temporal evolution of the deformation is plotted for a point close to the ACAE GPS station (see fig.1). This plot clearly shows the inversion of the deformation, from subsidence to uplift, at the beginning of 2000 with a significant value from March to August. A comparison between the GPS measurements projected into the radar LOS and the DInSAR results is also shown in fig.2c and clearly shows the good agreement between the two measurements. Fig. 2. DInSAR results (a) deformation map from ERS2 descending images of March and August 2000; (b) ground deformation time-series from DInSAR data in the time interval 6/1992-9/2000 for a point close to the ACAE GPS station; (c) comparison between GPS measurements (ACAE site) projected into the radar LOS and DInSAR deformations for the time interval 3-8/2000 (from Lanari et al., 2004) 2 CAT-1 1065 Following the 2000 event, the research groups of the INGV-Osservatorio Vesuviano (INGV-OV) and IREA-CNR decided to start an joined activity for the space-temporal evaluation of ground deformation in the Neapolitan Volcanic District, in particular for the Phlegrean Fields area, to be carried out by integrating spaceborne SAR data with classical geodetic data from the terrestrial networks.sar data were acquired in the frame of a CAT-1 project (1065, Integration of SAR Interferometry with classical geodetic techniques for ground deformation monitoring in the Neapolitan volcanic area - P.I.: Dr. G. Ricciardi, INGV-OV); such an activity has allowed the team to set up a database of SAR scenes from both ascending (track 129, frames 809/819) and descending (track 36, frame 2781) orbits of the Neapolitan Volcanic District for the ERS1-2 systems. Recently also ENVISAT/ASAR data were acquired (mainly descending,
IS2), as the CAT-1 is still in progress. By the integration with previous data, the availability of SAR scenes at present is in the order of about 200 images (ERS1-2, ENVISAT). During these years, the main problem was related to the Doppler centroid problems of the ERS-2 platform that prevent the team from using most of the data acquired after 2003. This is an important issue since the final lay-out of the GPS permanent network was set up in the area only after 2000 and therefore a short time interval to compare the two techniques is available. Also, more recent levelling measurements cannot be used for comparison. To overcome this problem, the team has extended the ERS deformation time series with new interferograms from ENVISAT data (IS2), acquired on descending tracks in the last years. In fact, the SBAS technique provides an easy way to merge multi-sensor data, while avoiding the use of cross-platform 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, upto-date ERS and ENVISAT acquisitions would be useful for the aims of this project. In fig.3 the DInSAR deformation map is shown for the period 1992-2003, with the ground deformation time-series for the benchmark (bm) 25 of the levelling line merging ERS1-2, ENVISAT and levelling data. Fig. 3. DInSAR deformation map (6/1992-8/2003) with the ground deformation time-series for bm 25 (ERS1-2, ENVISAT - blu triangles - and levelling - red stars - data) (by IREA-CNR/INGV-OV) 3 DUP ACTIVITIES The INGV-OV was recently involved in a DUP (Data User Programme) activity, together with the IREA-CNR and other Italian and foreign groups. The focal objective of the project (MINERVA, Monitoring by INterferometric SAR of Environmental Risk in Volcanic Areas ) financed by ESA was the design, development and assessment of a demonstrative small scale information service based on the interferometric processing of images from the SAR sensors on board ERS1-2 and ENVISAT. Such an activity has allowed to get a software code for DInSAR data processing at the INGV-OV. MINERVA is based on a new approach for DInSAR data processing, which allows to optimize the quality of interferograms spanning from 35 days up to several years, and to merge them to generate a single solution describing the temporal evolution of the ground deformation in the area. The software allows to update this solution each time a new SAR image is available, representing a powerful tool for ground deformation monitoring in active volcanic areas [10]. The code was adapted according with the INGV-OV needs in order to get a comparison between interferometric and geodetic data, by showing the ground deformation time-series from SAR and levelling data for the pixels in which both the information were available (fig.4): in this case the good temporal coverage of SAR data has allowed to retrieve the information not available from levelling in the time interval between the field measurements. Also considering slightly different geometries in the deformation measurement (LOS vs. vertical), a good agreement between the different techniques can be pointed out.
Fig. 4. Comparison between ground deformation time-series from levelling and SAR (ERS) data - bm25 The availability of the MINERVA code has moreover allowed to produce the results published for the first time in the INGV-OV Surveillance Reports starting from 2002. As an example, the differential 28/9/2000-3/10/2002 interferogram from MINERVA code is shown in fig.5 (left), where two fringes are clearly visible, showing a subsidence of about - 6 cm in the maximum deformation area (Pozzuoli area). Fig. 5. Differential interferogram 28/9/2000-3/10/2002 - ERS2, descending orbit (left); height variations from levelling for the period 10/2000-12/2002 (coast line) (top right); temporal variations of the Up component of the POSI-RITE GPS baseline (1/2001-10/2002) (bottom right) (from INGV-OV, 2002 Surveillance Report, modified) The joined analysis of SAR and classical data confirms the renewal of subsidence after the 2000 uplift event, as shown by the results of levelling measurements for the period October 2000-December 2002 (top right) with a maximum value of subsidence of about - 7 cm. In fig.5 (bottom right) the time-series of the vertical component of the POSI-RITE (Posillipo-Rione Terra) GPS baseline (1/2001-10/2002) is also shown; the trend of RITE station, located in the centre of
Pozzuoli, shows a relative subsidence of - 5.8 cm with respect to the POSI reference station. Again, the agreement between different kind of data is satisfactory. MINERVA project was completed in 2003. 4 THE CORNER REFLECTORS NETWORK From the analysis of different coherence maps, it can be pointed out how some areas in the Phlegrean Fields are marked by temporal decorrelation phenomena, which allow only a partial investigation of the area. In order to increase the resolution of the SAR method in areas with no or low coherence, a network of passive reflectors (Corner Reflectors, CR) is going to be set up: in particular 8 CR (4 couples) were already located in the Solfatara crater. The availability of a couple for each site allows the visibility from both ascending and descending orbits, with an inclination of the CR of 23 on the zenith axis in order to be detected from both ERS2 and ENVISAT (IS2) SAR sensors. The Solfatara crater is marked by a spatial and temporal dynamics quite different from the rest of the Phlegrean system and therefore of remarkable interest from the scientific point of view. A recent improvement is the fulfilment of a new type of CR (fig.6), conceived to easily manage the reflecting system, besides reducing its cost. Moreover, recent studies have also shown that a mutual interference could be pointed out, when dealing with two CR one strictly behind the other and therefore the use of only one CR can avoid this problem. On the contrary, the one shall be moved each time, according to the different tracks of the SAR sensor. 5 DISCUSSION AND CONCLUSIONS Fig. 6. A recent improvement of the Corner Reflectors network The comparative analysis between classical and DInSAR data for the Phlegrean Fields area 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 phase, when data sampling is strongly increased. Also the differences among the components of ground motion recorded by SAR and by classical techniques have to be taken into account: by splitting SAR data into the EW component and the vertical one, when data from both ascending
and descending orbits are available, allows a comparison with levelling data or with GPS data when the latter, for instance, are projected into the radar LOS. In conclusion, the exploitation of DInSAR for volcano monitoring represents 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 availability of both interferometric and geodetic data allows moreover to decide which kind of surveying technique should be used depending on the deformation rate and the volcanic risk level in the area of interest: as an example, classical techniques could be employed with no or low dynamics, whereas spaceborne techniques during pre-eruptive and eruptive phase, when approaching the volcano is difficult and/or the traditional monitoring networks are not available due to blackout. This approach may be successfully used in the next future to also investigate other active volcanic areas, as it was already done in the case of Mt. Vesuvius [6], [3]. 6 REFERENCES [1] Achilli V., Aquino I., Berardino P., Borgström S., Cecere G., Del Gaudio C., De Martino P., Fabris M., Fusco A., Galluzzo D., Lanari R., Menin A., Ricciardi G.P., Ricco C., Salemi G., Sansosti E., Sepe V., Siniscalchi V., Tesauro M. (2001) - Integration of SAR Interferometry with classical geodetic techniques for ground deformation monitoring at the Phlegrean Fields (Naples, Italy). Proceedings of the Italy-Canada 2001 Workshop on 3D Digital Imaging and Modeling Applications of: Heritage, Industry, Medicine & Land. Padova. On CD-ROM. [2] Berardino P., Fornaro G., Lanari R., Sansosti E. (2002) - A new algorithm for surface deformation monitoring based on small baseline differential SAR interferograms. IEEE Trans. Geosci. Rem. Sens., 40: 2375-2383. [3] Borgia A., Tizzani P., Solaro G., Manzo M., Casu F., Luongo G., Pepe A., Berardino P., Fornaro G., Sansosti E., Ricciardi G.P., Fusi N., Di Donna G., Lanari R. (2005) - Volcanic spreading of Vesuvius, a new paradigm for interpreting its volcanic activity. Geophysical Research Letters, 32, L03303, doi:10.1029/2004gl022155. [4] Istituto Nazionale di Geofisica e Vulcanologia - Osservatorio Vesuviano, Surveillance Report of the year 2002 (2004). Internal Report. [5] Istituto Nazionale di Geofisica e Vulcanologia - Osservatorio Vesuviano, Surveillance Report of the year 2003 (2005). Internal Report. [6] Lanari R., De Natale G., Berardino P., Sansosti E., Ricciardi G.P., Borgström S., Capuano P., Pingue F., Troise C. (2002) - Evidence for a peculiar style of ground deformation inferred at Vesuvius volcano. Geophysical Research Letters, Vol. 29, No. 9, 10.1029/2001GL014571. [7] Lanari R., Berardino P., Borgström S., Del Gaudio C., De Martino P., Fornaro G., Guarino S., Ricciardi G.P., Sansosti E., Lundgren P. (2004) - The use of IFSAR and classical geodetic techniques for caldera unrest episodes: application to the Campi Flegrei uplift event of 2000. Journal of Volcanology and Geothermal Research, 133: 247-260. [8] Lundgren P., Usai S., Sansosti E., Lanari R., Tesauro M., Fornaro G., Berardino P. (2001) - Modeling surface deformation observed with synthetic aperture radar interferometry at Campi Flegrei caldera. Journal of Geophysical Research, 106,19,355-19,366. [9] Osservatorio Vesuviano, Surveillance Report - Crisi bradisismica della Caldera dei Campi Flegrei 2000. (2000). Internal Report. [10] Tampellini L., Sansosti E., Usai S., Lanari R., Borgström S., Van Persie M., Ricciardi G.P., Maddalena V., Cicero L., Pepe A. (2003) - Minerva: an INSAR monitoring service for volcanic hazard. Proceedings of the Fringe Workshop of European Space Agency. Frascati. Available at: http://earth.esa.int/fringe03/proceedings/posters/77_tampe.pdf