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Levelling and GPS networks to monitor ground subsidence in the Southern Po Valley
Journal of Geodynamics 30 (2000) 355±369
Levelling and GPS networks to monitor ground subsidence in the Southern Po Valley Gabriele Bitelli a,*, Flavio Bonsignore b, Marco Unguendoli a a
DISTART, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy b ARPA, Emilia Romagna Region, Via Po 5, 40139 Bologna, Italy
Received 1 April 1998; received in revised form 1 March 1999; accepted 1 May 1999 This paper is dedicated to Prof. Michele Caputo, in occasion of the 70th birthday, who is one of the leading scientists involved in the study of this problem
Abstract After a short historical introduction to the problem of the subsidence in the Po Valley, some results are shown of levelling campaigns carried out by various authorities or agencies in recent years, mainly in the southeast part of the Po Valley. We also present the results obtained along the coastline using the GPS technique both in a static and in fast-static modes. Attention is focused on the problem of the homogeneity of data from dierent sources, dierent periods and using dierent reference benchmarks. To overcome these problems, we propose a general levelling network covering the whole area of the valley, south of the Po river, connected with a GPS network of more than 50 points. This network will represent a spatial and temporal reference framework for all the measurements made in the past and for those planned for the future. A data information system has been implemented in order to organise and manage all available data in a well de®ned and homogeneous framework. This system will make it possible to store other related data, such as the pattern of the water table, geological evidences, land use, etc. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction In the frame of crustal movements studies, we are particularly interested in the phenomenon of subsidence in the Po Valley, because of its extreme importance as regards local problems, * Corresponding author. 0264-3707/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 4 - 3 7 0 7 ( 9 9 ) 0 0 0 7 1 - X
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such as the stability of buildings in historical cities (Venice, Bologna, Ravenna) or questions regarding agriculture, waterways, coastal erosion. We are also interested in the phenomenon because in the zone there are several important reference stations involved in large international projects, such as the VLBI, Laser and GPS permanent stations at Medicina (Bologna) and the important tide gauge at Porto Corsini close to Ravenna. It should also be said that the phenomenon has been recognised for a very long time, but a systematic control has only recently been set up. It should also be remembered that in recent years there has been a shift in the zone of maximum interest: originally the greatest attention was concentrated on the Po Delta and north up to Venice and its hinterland. Today, interest is focused on the southern part of the valley because of the presence of numerous wells for water and/or gas extraction, on the mainland and also oshore not far from the coast. In fact, it has been pointed out that, for example, in the zone of Ravenna the natural subsidence due to the natural Quaternary basin compaction and to tectonic movements is of the order of 2 mm/year, while the anthropic component of the subsidence reaches often several centimetre per year (Gambolati et al., 1991). Even if the ®rst evidence of the phenomenon date back to the 1930s, associated with the Po Delta and linked to water pumping and land drainage projects, in our opinion, the ®rst important studies were the papers of Salvioni (1957) and Boaga (1957). The interest of the University of Bologna in the study of the phenomenon goes back to 1965±1970, when a general study was made based on repeated levelling lines provided by some local authorities and agencies, and taking into account the above previous works and other historical evidence (Caputo et al., 1970). The ®rst attempt made was to draw up a map of equal movements for all the eastern part of the Po Valley in the period 1900±1957. The results are summarised in Fig. 1, which shows movements of more than 40 cm in the Po Delta with a considerable slope towards the coastline. For the period 1957±1967 the paper analysed in particular the zones of the Po Delta, revealing an increasing velocity of movements which reached something like 10 cm a year for the period 1957±1962 and a great decrease in the period 1962±1967 due to the closure of a lot of wells in 1961 and 1962. Another general study of the movements covering the entire north of Italy for the period 1897±1957 was made by IGMI Ð Istituto Geogra®co Militare Italiano (Arca and Beretta, 1985). The results of this study are in good agreement with those obtained by the above cited authors and they also give a better idea of the phenomenon in the zone of Bologna and Medicina and towards the west (Fig. 2). 2. Some recent results 2.1. Spirit levelling surveys It was only in the 1970s that many local authorities and agencies, such as the municipalities of Bologna, Ravenna and Modena, regional agencies and the oil company AGIP began to perform regular controls in the southern part of the Po Valley, setting up close-knit precise levelling networks and obtaining interesting but not homogeneous results. As an example, we present the levelling network of Bologna (Fig. 3), planned by the University of Bologna
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Fig. 1. Map of equal movements for all the eastern part of the Po Valley in the period 1900±1957 (from Caputo et al., 1970).
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Fig. 2. Movements for the northeast part of Italy for the period 1897±1957 (from Arca and Beretta, 1985).
G. Bitelli et al. / Journal of Geodynamics 30 (2000) 355±369
359
(Barbarella et al., 1990). The zones in the ®gure indicate dierent average distances between the benchmarks along the lines, reaching a value of about 50±100 m in zone 4 (Bologna historical centre). Notwithstanding the lack of homogeneity of the data it is possible to highlight some interesting results. The ®rst example is related to the movements detected close to the tide gauge of Porto Corsini for the period 1970±1993 (Fig. 4), where there is an almost linear trend of about 1.6 cm/year. As regards Medicina, there is very little data, but, using some old data and a velocity model
Fig. 3. General schema of the Bologna levelling network (from Barbarella et al., 1990).
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deduced by interpolation of the surrounding levelling lines, we can estimate the movements of the last period at around an order of 4±5 mm/year, in good agreement with the results from the VLBI observations (Tomasi, 1997). On the contrary, very high movements are detected not far from Medicina (25 km), towards the west in the hinterland of Bologna, where subsidence in the period 1983±1992 has been estimated at 8.5 cm/year following a almost linear trend (Folloni et al., 1996). Another interesting result was obtained from the comparison of the velocity of movements over long periods in three zones close to the coastline between the Po river and the southern border of the Po Valley (Fig. 5). As we can see, in the last period considered there has been an inversion in the zone of maximum movement from north to south, probably due to the large number of wells for pumping water in the tourist area of Rimini. It should also be said that it is very dicult to model the phenomenon because of the high spatial variability in the velocity of movements: as example, we can consider the two values mentioned above (4±5 mm/year and 8.5 cm/year), or the velocity of movements detected on a levelling line along the coast for the period 1984±1993, where there have been dierences of the order of more than 1 cm/year over distances of a few kilometres (Fig. 6).
Fig. 4. Movements detected close to the tide gauge of Porto Corsini for the period 1970±1993.
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361
2.2. GPS surveys For the period 1993±1996 we have no data from repeated levelling lines but only some GPS results. In fact, in 1993 the ®rm AGIP asked the university of Bologna to set up a GPS network for the control of subsidence along the coastline in the area of Ravenna (Fig. 7). The net to be measured in static mode consists of 13 points, six of which located on the hills in geologically stable zones: three near Bologna, where AGIP has the fundamental point for its levelling networks and three not far from the area to be investigated. The net was measured in seven sessions using six instruments, two of which as semi-permanent stations, for a total number of 35 independent baselines. The results of the adjustment showed the major semidiameter of the planimetric error ellipses (95%) of the order of 1.2 cm and an error in height of about 1.5 cm. Starting from the six points located in the district of Ravenna, using the fast static method we also determined the ellipsoidal heights of 35 surrounding benchmarks
Fig. 5. Velocity of movement (mm/year) in three areas along the coastline.
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Fig. 6. Velocity of movement (mm/year) detected on a levelling line along the coast for the period 1984±1993.
belonging to levelling lines using a simple connection to the GPS antenna. In 1996 we repeated the measurements using the same operative schema and we recalculated the old measurements using the new software BERNESE 4.0 and the same ®xed point, the coordinates of which had been determined in 1996 in the IGS frame. The coordinates obtained in the two campaigns presented very high dierences, especially for height, probably due also to a dierent set of satellite ephemeris (in 1993 the IGS service was not operational, Beutler et al., 1995). So we made a 7-parameter transformation using the six points located in the stable zones, obtaining
Fig. 7. GPS network and fast-static points for the control of subsidence along the coastline in the district of Ravenna, measured in 1993 and 1996.
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363
very small residuals (few millimetres). After this we applied the parameters obtained to the whole network, obtaining for the points on the coastline residuals (movements) ranging from 3 to 6 cm, in good agreement with those expected on the basis of the trend detected in the past with spirit levelling surveys. This good result is not unexpected because it has been demonstrated (Al-Bayari and Unguendoli, 1997) that repeated measurements of the same networks present very large dierences in height but after a 7-parameter transformation between two measurements the residuals in height are the smallest (Fig. 8). It should be pointed out that in 1990 we set up a small GPS network connected to the levelling network of Bologna and in 1995 a large GPS network for the control of some rivers (Reno and its tributaries), but we have not repeated measurements on this network. The ®rst measurements in the three networks were carried out together with levelling measurements, so it was possible to make an interesting comparison with the gravimetric geoid in the districts of Bologna and Ravenna (Bitelli et al., 1993a, 1993b, 1994; Al-Bayari et al., 1996).
Fig. 8. (a) Dierences of the coordinates obtained for the same network observed in two subsequent days. (b) Residuals and scale factor of a 7-parameter transformation of the previous solution (scale factor: ÿ0.689 2 0.270 mm/km).
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3. Regional network project It is obvious that we now have a great deal of data, but we also have a lot of problems because each authority or agency used dierent speci®cations and, therefore, dierent precision for their levelling networks, dierent observation periods, dierent reference benchmarks. There is also a bad spatial distribution of the levelling lines, and in this confusion it is very dicult, if not impossible, to make a general study of the subsidence phenomenon. For this reason, last year the Regional Agency for Environmental Protection (ARPA) decided, in collaboration with Bologna University, to set up a project for a levelling network covering the whole of the Po Valley belonging to the Emilia Romagna region, that is almost all the southern part of the valley. The ®nal project consists of a levelling network of about 2000 km (Fig. 9) with more than 2000 benchmarks, three quarters of which already monumented, to be measured in a very short lapse of time (2 months) following well de®ned speci®cations. We also planned a large GPS network, connected to the levelling one, of 53 points, nine of which located on the hills in geologically stable zones, to be measured in static mode following compulsory speci®cations (Unguendoli, 1990; Bitelli and Unguendoli, 1996). A scheme of such speci®cations is reported in Table 1, and Fig. 10 shows the design of the independent baselines to be measured. As it can be seen in Fig. 9, the ®xed points of the GPS network are located on the Apennines, all in the south of the network. This is not the best situation to anchor the overall network to stable points: for this purpose points to the north of the network are
Fig. 9. The project of the regional integrated levelling and GPS networks.
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Table 1 Speci®cations for the GPS static survey Number of points Number of independent baselines Maximum baseline length Receivers Minimum number of instruments Minimum session length Every point should be occupied at least twice using dierent instruments Every point should be connected at least by three independent baselines Software for data processing: scienti®c packages (i.e. BERNESE 4.0)
53 122 40 km Double frequency of the last generation 5 4h
also necessary. The closest geologically stable places to the north are the Euganean hills and the Pre-Alps in the zone of Brescia, quite some distance away. Therefore, some separate very long measuring sessions will be carried out to connect up two±three points that will be set up on these zones. If the data on the stability of the permanent station at Padua are con®rmed (Caporali et al., 1997), this point will also be included in the network. In addition, some receivers will remain for several days on some points (semipermanent stations) in order to make possible a good connection to the IGS frame. The aim of the GPS network is two-fold: we have the possibility to control and improve the
Fig. 10. Independent baselines in the project of the regional GPS network.
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gravimetric geoid in the area and we have the possibility to make quick and not expensive control of some points in order to decide, for example, the repetition of some levelling lines or the repetition of the whole network. The whole project has been set-up in order to obtain a common and well-de®ned spatial and temporal reference system for all the measurements collected for particular reasons or in particular zones but always connected to the general network, which is linked to all local networks. A crucial question was represented by the choice of the reference point(s). Our decision, supported by temporal analysis of existing levelling lines along some valleys in the Apennines hills and by geological surveys in the same zones, was to take as reference point a benchmark belonging to an IGMI line south of Bologna. After the connection to this point, other benchmarks located on the hills can be used as reference for the repetition of some local networks or of some lines of the general network. In this way it would be also possible to homogenise the data coming from historical levelling lines, connected to the new network, in order to elaborate some velocity models in particular zones. To achieve this goal, a relational data information system has been implemented for the management of the enormous amount of data and to render the historical datasets more homogeneous (Fig. 11). The system, partially based on previous experiences (Bitelli et al., 1994; Bitelli and Vittuari, 1997) handles dierent kinds of data of geodetic interest for network maintenance and subsidence monitoring. Among the entities handled there are levelling and GPS surveys, benchmarks, adjusted heights, data processing documentation, homogeneisation parameters, agency data, and so on. Fig. 12 illustrates a form related to a levelling benchmark, together with its ancillary graphical data. The system, created with Access97 data management system, will be interfaced with a Geographical Information System and is also designed for the future recording of other kinds of information, such as changes in the water table, location of active wells, geological evidence, to provide a working instrument for geologists and geophysicists interested in the subsidence problem.
Fig. 11. Main functions of the information system.
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Fig. 12. Forms for a levelling benchmark from the information system.
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4. Conclusions The Po Valley is aected by a subsidence phenomenon that in some areas reaches signi®cant values and, due to the important economic and social implications, requires continuous monitoring of the entire area. The geometric characteristics have been measured in recent decades by various authorities using dierent speci®cation and dierent reference benchmarks. This makes it very dicult to obtain a homogeneous interpretation or to make a basic synthesis even for the de®nition of appropriate models. On the basis of some literature and on recent levelling surveys and GPS, some interesting aspects were highlighted and some of the more important questions for the study of the phenomenon have been described. To overcome the fragmentary nature of the data and the lack of homogeneity between the studies carried out in the past, a project has been presented for the setting up of two integrated regional networks for spirit levelling and GPS surveys. The overall proposal is a large project, very costly, but it could represent the only possibility for a real control of the subsidence phenomenon in the whole southern part of the Po Valley. The success of the operation may ®nally be guaranteed if at the same time an information system is developed to collect all the information, not just geodetic±topographic, of interest for the study of this phenomenon.
References Al-Bayari, O., Bitelli, G., Bonini, C., Capra, A., Dominici, D., Ercolani, E., Gandol®, S., Pellegrinelli, A., Unguendoli, M., Vittuari, L., 1996. A local geoid in the south-eastern Po Valley (Italy). Proceedings European Geophysical Society Ð XXI General Assembly, De-Hague, May 1996, Published in Reports of The Finnish Geodetic Institute, 96, 2. Al-Bayari, O., Unguendoli, M., 1997. Experiences in GPS control networks computation. Proceedings 5th Bilateral Meeting Poland±Italy, Monselice, Reports on Geodesy, 5, 28. Arca, S., Beretta, G.P., 1985. Prima sintesi geodetico-geologica sui movimenti verticali del suolo nell'Italia settentrionale. Bollettino di Geodesia e Scienze Ani XLIV, 2. Barbarella, M., Pieri, L., Russo, P., 1990. Studio dell'abbassamento del suolo nel territorio bolognese mediante livellazioni ripetute: analisi dei movimenti e considerazioni statistiche. INARCOS, 506, Bologna. Beutler, G., Kouba, J., Springer, T., 1995. Combining the orbits of the IGS analysis centers. Bulletin Geodesique 69, 4. Bitelli, G., Gatti, M., Russo, P., 1993a. An information system for levelling data in land deformation surveys. In: Proceedings 7th International FIG Symposium on Deformation Measurements, Ban. Bitelli, G., Capra, A., Dominici, D., Folloni, G., Gubellini, A., Radicioni, F., Vittuari, L., 1993b. GPS levelling for ground subsidence monitoring and geoid computation in the Bologna area. In: Proceedings 7th International FIG Symposium on Deformation Measurements, Ban. Bitelli, G., Capra, A., Dominici, D., Folloni, G., Radicioni, F., Unguendoli, M., Vittuari, L., 1994. GPS measurements for ground subsidence monitoring and geoid computation in the Ravenna area. In: Proceedings XX FIG Congress, Melbourne. Bitelli, G., Unguendoli, M., 1996. Alcune indicazioni pratiche per l'esecuzione di reti GPS. Bollettino SIFET 1, pp. 105±119. Bitelli, G., Vittuari, L., 1997. A data management approach for Antarctica GPS campaigns. In: Proceedings VII International Symposium on Antarctic Earth Sciences 1995. Terra Antartica Publication, Siena. Boaga, G., 1957. Sugli abbassamenti del delta padano. Metano, Petrolio e Nuove Energie, 6. Caporali, A., Della Corte, V., De Perini, V., Galgaro, A., 1997. The activities at the permanent IGS station UPAD
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of the University of Padova. Proceedings 5th Bilateral Meeting Poland±Italy, Monselice, Reports on Geodesy, 5, 28. Caputo, M., Pieri, L., Unguendoli, M., 1970. Geometric investigation of the subsidence in the Po Delta. Boll. Geodesia Teorica e Applicata, 47. Folloni, G., Russo, P., Radicioni, F., 1996. La subsidenza del territorio bolognese dal 1983 al 1993. INARCOS, 571, Bologna. Gambolati, G., Ricceri, G., Bertoni, W., Brighenti, G., Vuillermin, E., 1991. Mathematical simulation of the subsidence of Ravenna. Water Resources Research 27, 11. Salvioni, G., 1957. I movimenti del suolo nell'Italia Centro-Settentrionale. Boll. di Geodesia e Scienze Ani, XVI. Tomasi, P., 1997. Geodetic VLBI in Europe: the recent results. In: Fifth Bilateral Meeting Poland±Italy, Monselice. Unguendoli, M., 1990. A rational approach to the use of a large number of GPS receivers. Bulletin Geodesique 64, 4.
Levelling and GPS networks to monitor ground subsidence in the Southern Po Valley Gabriele Bitelli a,*, Flavio Bonsignore b, Marco Unguendoli a a
DISTART, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy b ARPA, Emilia Romagna Region, Via Po 5, 40139 Bologna, Italy
Received 1 April 1998; received in revised form 1 March 1999; accepted 1 May 1999 This paper is dedicated to Prof. Michele Caputo, in occasion of the 70th birthday, who is one of the leading scientists involved in the study of this problem
Abstract After a short historical introduction to the problem of the subsidence in the Po Valley, some results are shown of levelling campaigns carried out by various authorities or agencies in recent years, mainly in the southeast part of the Po Valley. We also present the results obtained along the coastline using the GPS technique both in a static and in fast-static modes. Attention is focused on the problem of the homogeneity of data from dierent sources, dierent periods and using dierent reference benchmarks. To overcome these problems, we propose a general levelling network covering the whole area of the valley, south of the Po river, connected with a GPS network of more than 50 points. This network will represent a spatial and temporal reference framework for all the measurements made in the past and for those planned for the future. A data information system has been implemented in order to organise and manage all available data in a well de®ned and homogeneous framework. This system will make it possible to store other related data, such as the pattern of the water table, geological evidences, land use, etc. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction In the frame of crustal movements studies, we are particularly interested in the phenomenon of subsidence in the Po Valley, because of its extreme importance as regards local problems, * Corresponding author. 0264-3707/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 4 - 3 7 0 7 ( 9 9 ) 0 0 0 7 1 - X
356
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such as the stability of buildings in historical cities (Venice, Bologna, Ravenna) or questions regarding agriculture, waterways, coastal erosion. We are also interested in the phenomenon because in the zone there are several important reference stations involved in large international projects, such as the VLBI, Laser and GPS permanent stations at Medicina (Bologna) and the important tide gauge at Porto Corsini close to Ravenna. It should also be said that the phenomenon has been recognised for a very long time, but a systematic control has only recently been set up. It should also be remembered that in recent years there has been a shift in the zone of maximum interest: originally the greatest attention was concentrated on the Po Delta and north up to Venice and its hinterland. Today, interest is focused on the southern part of the valley because of the presence of numerous wells for water and/or gas extraction, on the mainland and also oshore not far from the coast. In fact, it has been pointed out that, for example, in the zone of Ravenna the natural subsidence due to the natural Quaternary basin compaction and to tectonic movements is of the order of 2 mm/year, while the anthropic component of the subsidence reaches often several centimetre per year (Gambolati et al., 1991). Even if the ®rst evidence of the phenomenon date back to the 1930s, associated with the Po Delta and linked to water pumping and land drainage projects, in our opinion, the ®rst important studies were the papers of Salvioni (1957) and Boaga (1957). The interest of the University of Bologna in the study of the phenomenon goes back to 1965±1970, when a general study was made based on repeated levelling lines provided by some local authorities and agencies, and taking into account the above previous works and other historical evidence (Caputo et al., 1970). The ®rst attempt made was to draw up a map of equal movements for all the eastern part of the Po Valley in the period 1900±1957. The results are summarised in Fig. 1, which shows movements of more than 40 cm in the Po Delta with a considerable slope towards the coastline. For the period 1957±1967 the paper analysed in particular the zones of the Po Delta, revealing an increasing velocity of movements which reached something like 10 cm a year for the period 1957±1962 and a great decrease in the period 1962±1967 due to the closure of a lot of wells in 1961 and 1962. Another general study of the movements covering the entire north of Italy for the period 1897±1957 was made by IGMI Ð Istituto Geogra®co Militare Italiano (Arca and Beretta, 1985). The results of this study are in good agreement with those obtained by the above cited authors and they also give a better idea of the phenomenon in the zone of Bologna and Medicina and towards the west (Fig. 2). 2. Some recent results 2.1. Spirit levelling surveys It was only in the 1970s that many local authorities and agencies, such as the municipalities of Bologna, Ravenna and Modena, regional agencies and the oil company AGIP began to perform regular controls in the southern part of the Po Valley, setting up close-knit precise levelling networks and obtaining interesting but not homogeneous results. As an example, we present the levelling network of Bologna (Fig. 3), planned by the University of Bologna
G. Bitelli et al. / Journal of Geodynamics 30 (2000) 355±369
357
Fig. 1. Map of equal movements for all the eastern part of the Po Valley in the period 1900±1957 (from Caputo et al., 1970).
358 G. Bitelli et al. / Journal of Geodynamics 30 (2000) 355±369
Fig. 2. Movements for the northeast part of Italy for the period 1897±1957 (from Arca and Beretta, 1985).
G. Bitelli et al. / Journal of Geodynamics 30 (2000) 355±369
359
(Barbarella et al., 1990). The zones in the ®gure indicate dierent average distances between the benchmarks along the lines, reaching a value of about 50±100 m in zone 4 (Bologna historical centre). Notwithstanding the lack of homogeneity of the data it is possible to highlight some interesting results. The ®rst example is related to the movements detected close to the tide gauge of Porto Corsini for the period 1970±1993 (Fig. 4), where there is an almost linear trend of about 1.6 cm/year. As regards Medicina, there is very little data, but, using some old data and a velocity model
Fig. 3. General schema of the Bologna levelling network (from Barbarella et al., 1990).
360
G. Bitelli et al. / Journal of Geodynamics 30 (2000) 355±369
deduced by interpolation of the surrounding levelling lines, we can estimate the movements of the last period at around an order of 4±5 mm/year, in good agreement with the results from the VLBI observations (Tomasi, 1997). On the contrary, very high movements are detected not far from Medicina (25 km), towards the west in the hinterland of Bologna, where subsidence in the period 1983±1992 has been estimated at 8.5 cm/year following a almost linear trend (Folloni et al., 1996). Another interesting result was obtained from the comparison of the velocity of movements over long periods in three zones close to the coastline between the Po river and the southern border of the Po Valley (Fig. 5). As we can see, in the last period considered there has been an inversion in the zone of maximum movement from north to south, probably due to the large number of wells for pumping water in the tourist area of Rimini. It should also be said that it is very dicult to model the phenomenon because of the high spatial variability in the velocity of movements: as example, we can consider the two values mentioned above (4±5 mm/year and 8.5 cm/year), or the velocity of movements detected on a levelling line along the coast for the period 1984±1993, where there have been dierences of the order of more than 1 cm/year over distances of a few kilometres (Fig. 6).
Fig. 4. Movements detected close to the tide gauge of Porto Corsini for the period 1970±1993.
G. Bitelli et al. / Journal of Geodynamics 30 (2000) 355±369
361
2.2. GPS surveys For the period 1993±1996 we have no data from repeated levelling lines but only some GPS results. In fact, in 1993 the ®rm AGIP asked the university of Bologna to set up a GPS network for the control of subsidence along the coastline in the area of Ravenna (Fig. 7). The net to be measured in static mode consists of 13 points, six of which located on the hills in geologically stable zones: three near Bologna, where AGIP has the fundamental point for its levelling networks and three not far from the area to be investigated. The net was measured in seven sessions using six instruments, two of which as semi-permanent stations, for a total number of 35 independent baselines. The results of the adjustment showed the major semidiameter of the planimetric error ellipses (95%) of the order of 1.2 cm and an error in height of about 1.5 cm. Starting from the six points located in the district of Ravenna, using the fast static method we also determined the ellipsoidal heights of 35 surrounding benchmarks
Fig. 5. Velocity of movement (mm/year) in three areas along the coastline.
362
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Fig. 6. Velocity of movement (mm/year) detected on a levelling line along the coast for the period 1984±1993.
belonging to levelling lines using a simple connection to the GPS antenna. In 1996 we repeated the measurements using the same operative schema and we recalculated the old measurements using the new software BERNESE 4.0 and the same ®xed point, the coordinates of which had been determined in 1996 in the IGS frame. The coordinates obtained in the two campaigns presented very high dierences, especially for height, probably due also to a dierent set of satellite ephemeris (in 1993 the IGS service was not operational, Beutler et al., 1995). So we made a 7-parameter transformation using the six points located in the stable zones, obtaining
Fig. 7. GPS network and fast-static points for the control of subsidence along the coastline in the district of Ravenna, measured in 1993 and 1996.
G. Bitelli et al. / Journal of Geodynamics 30 (2000) 355±369
363
very small residuals (few millimetres). After this we applied the parameters obtained to the whole network, obtaining for the points on the coastline residuals (movements) ranging from 3 to 6 cm, in good agreement with those expected on the basis of the trend detected in the past with spirit levelling surveys. This good result is not unexpected because it has been demonstrated (Al-Bayari and Unguendoli, 1997) that repeated measurements of the same networks present very large dierences in height but after a 7-parameter transformation between two measurements the residuals in height are the smallest (Fig. 8). It should be pointed out that in 1990 we set up a small GPS network connected to the levelling network of Bologna and in 1995 a large GPS network for the control of some rivers (Reno and its tributaries), but we have not repeated measurements on this network. The ®rst measurements in the three networks were carried out together with levelling measurements, so it was possible to make an interesting comparison with the gravimetric geoid in the districts of Bologna and Ravenna (Bitelli et al., 1993a, 1993b, 1994; Al-Bayari et al., 1996).
Fig. 8. (a) Dierences of the coordinates obtained for the same network observed in two subsequent days. (b) Residuals and scale factor of a 7-parameter transformation of the previous solution (scale factor: ÿ0.689 2 0.270 mm/km).
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3. Regional network project It is obvious that we now have a great deal of data, but we also have a lot of problems because each authority or agency used dierent speci®cations and, therefore, dierent precision for their levelling networks, dierent observation periods, dierent reference benchmarks. There is also a bad spatial distribution of the levelling lines, and in this confusion it is very dicult, if not impossible, to make a general study of the subsidence phenomenon. For this reason, last year the Regional Agency for Environmental Protection (ARPA) decided, in collaboration with Bologna University, to set up a project for a levelling network covering the whole of the Po Valley belonging to the Emilia Romagna region, that is almost all the southern part of the valley. The ®nal project consists of a levelling network of about 2000 km (Fig. 9) with more than 2000 benchmarks, three quarters of which already monumented, to be measured in a very short lapse of time (2 months) following well de®ned speci®cations. We also planned a large GPS network, connected to the levelling one, of 53 points, nine of which located on the hills in geologically stable zones, to be measured in static mode following compulsory speci®cations (Unguendoli, 1990; Bitelli and Unguendoli, 1996). A scheme of such speci®cations is reported in Table 1, and Fig. 10 shows the design of the independent baselines to be measured. As it can be seen in Fig. 9, the ®xed points of the GPS network are located on the Apennines, all in the south of the network. This is not the best situation to anchor the overall network to stable points: for this purpose points to the north of the network are
Fig. 9. The project of the regional integrated levelling and GPS networks.
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Table 1 Speci®cations for the GPS static survey Number of points Number of independent baselines Maximum baseline length Receivers Minimum number of instruments Minimum session length Every point should be occupied at least twice using dierent instruments Every point should be connected at least by three independent baselines Software for data processing: scienti®c packages (i.e. BERNESE 4.0)
53 122 40 km Double frequency of the last generation 5 4h
also necessary. The closest geologically stable places to the north are the Euganean hills and the Pre-Alps in the zone of Brescia, quite some distance away. Therefore, some separate very long measuring sessions will be carried out to connect up two±three points that will be set up on these zones. If the data on the stability of the permanent station at Padua are con®rmed (Caporali et al., 1997), this point will also be included in the network. In addition, some receivers will remain for several days on some points (semipermanent stations) in order to make possible a good connection to the IGS frame. The aim of the GPS network is two-fold: we have the possibility to control and improve the
Fig. 10. Independent baselines in the project of the regional GPS network.
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gravimetric geoid in the area and we have the possibility to make quick and not expensive control of some points in order to decide, for example, the repetition of some levelling lines or the repetition of the whole network. The whole project has been set-up in order to obtain a common and well-de®ned spatial and temporal reference system for all the measurements collected for particular reasons or in particular zones but always connected to the general network, which is linked to all local networks. A crucial question was represented by the choice of the reference point(s). Our decision, supported by temporal analysis of existing levelling lines along some valleys in the Apennines hills and by geological surveys in the same zones, was to take as reference point a benchmark belonging to an IGMI line south of Bologna. After the connection to this point, other benchmarks located on the hills can be used as reference for the repetition of some local networks or of some lines of the general network. In this way it would be also possible to homogenise the data coming from historical levelling lines, connected to the new network, in order to elaborate some velocity models in particular zones. To achieve this goal, a relational data information system has been implemented for the management of the enormous amount of data and to render the historical datasets more homogeneous (Fig. 11). The system, partially based on previous experiences (Bitelli et al., 1994; Bitelli and Vittuari, 1997) handles dierent kinds of data of geodetic interest for network maintenance and subsidence monitoring. Among the entities handled there are levelling and GPS surveys, benchmarks, adjusted heights, data processing documentation, homogeneisation parameters, agency data, and so on. Fig. 12 illustrates a form related to a levelling benchmark, together with its ancillary graphical data. The system, created with Access97 data management system, will be interfaced with a Geographical Information System and is also designed for the future recording of other kinds of information, such as changes in the water table, location of active wells, geological evidence, to provide a working instrument for geologists and geophysicists interested in the subsidence problem.
Fig. 11. Main functions of the information system.
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Fig. 12. Forms for a levelling benchmark from the information system.
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4. Conclusions The Po Valley is aected by a subsidence phenomenon that in some areas reaches signi®cant values and, due to the important economic and social implications, requires continuous monitoring of the entire area. The geometric characteristics have been measured in recent decades by various authorities using dierent speci®cation and dierent reference benchmarks. This makes it very dicult to obtain a homogeneous interpretation or to make a basic synthesis even for the de®nition of appropriate models. On the basis of some literature and on recent levelling surveys and GPS, some interesting aspects were highlighted and some of the more important questions for the study of the phenomenon have been described. To overcome the fragmentary nature of the data and the lack of homogeneity between the studies carried out in the past, a project has been presented for the setting up of two integrated regional networks for spirit levelling and GPS surveys. The overall proposal is a large project, very costly, but it could represent the only possibility for a real control of the subsidence phenomenon in the whole southern part of the Po Valley. The success of the operation may ®nally be guaranteed if at the same time an information system is developed to collect all the information, not just geodetic±topographic, of interest for the study of this phenomenon.
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