AG_57.04.14_DOMINICI-MELONI_finalonline_Layout 6 ANNALS OF GEOPHYSICS, 57, 4, 2014, G0433; doi:10.4401/ag-6525 G0433 Revision of the Italian Magnetic Database for the Albegna basin (South Tuscany, Italy) Guido Dominici*, Antonio Meloni Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 2, Rome, Italy ABSTRACT A comparison between ground level total magnetic field intensity anom- aly map (F) of Italy and the total intensity aeromagnetic map by ENI/AGIP, had shown that an anomaly pattern for the Albegna basin (South Tuscany), quite evident from ground measurements, doesn’t show in the aeromagnetic map. Ligurian units, made of ophiolite blocks (metagabbros, basalts, serpentinites), intrusives and subordinate volcanic products, all able to trigger a strong magnetic signal, could not be excluded in the area, and for this reason the magnetic anomaly estimated by ground level measurements was not considered unreasonable. In this paper the result of a magnetic survey finalized to verify the authentic existence of such a large magnetic total intensity anomaly in the Albegna basin, is re- ported. On the basis of the new result, the suspected ground level total in- tensity anomaly in the Albegna basin, was demonstrated to be non-existent and then the Italian Magnetic Database corrected accordingly. Measure- ments and procedures that brought to the magnetic elements elaboration and new anomaly maps for Albegna basin, are shown here. 1. Ground level magnetic maps and the sea level magnetic anomaly map of Italy Magnetic field measurements are undertaken in different ways and for different reasons. Observatories monitor time variations of the field, while measure- ments carried out on land and sea by ships, helicopters, planes, and also onboard satellites at low orbital alti- tudes, provide information concerning the spatial struc- ture of the field and then give information on the main Earth’s magnetic field and on that of crustal origin. In particular, for crustal magnetism studies, ground meas- urements are widely used for their immediate proxim- ity to the magnetic sources. Information on Earth’s magnetism can be found, for example, in Campbell [2003], Lanza and Meloni [2006], Encyclopedia of Geo- magnetism and Paleomagnetism [2007]. In many nations, ground magnetic measurements are organized in networks of survey points. The so- called first order networks are devoted mainly to secu- lar variation monitoring, while second order networks, with higher spatial detail, are dedicated to geomagnetic elements map compilation and to crustal magnetism studies. As shown in Figure 1, the first order Italian magnetic network has an average stations spacing of about 100 km, while the second order network has an average spacing of about 5-10 km. Every five years, for their institutional roles, INGV (Istituto Nazionale di Geofisica e Vulcanologia) and IGM (Istituto Geografico Militare) carry out measure- ments (total intensity F, horizontal component H, ver- tical component Z, declination D and inclination I) over the about 120 repeat stations of the first order magnetic network (Figure 1). The aim of this periodic effort is to replicate magnetic measurements at repeat stations in order to monitor geomagnetic field secular variation and to compute updated coefficients of regional mod- els for representing the main geomagnetic patterns over Italy, at the various epochs. In Italy ground level magnetic measurements on the second order network were performed in an organized national effort only once between 1977 and 1981, within the framework of the Progetto Finalizzato Geodinamica of the Consiglio Nazionale delle Ricerche (PFG-CNR). All the work was coordinated by the then ING (Istituto Nazionale di Geofisica). At that time a database of 2552 magnetic data (total intensity F, horizontal component H, vertical component Z) was assembled with the objec- tive to produce magnetic field maps of Italy for intensive elements and to successively compile a magnetic crustal field anomaly map for Italy [Molina et al. 1986]. Magnetic anomaly maps are produced starting from field measurements, after the Earth’s core field and magnetic time variations, are removed. The re- moval of the core main field is generally undertaken by subtraction of IGRF (International Geomagnetic Ref- erence Field; Finlay et al. [2010]) or by the removal of Article history Received March 6, 2014; accepted June 26, 2014. Subject classification: Geomagnetism, Magnetic survey, Reference field, Secular variation, Magnetic cartography. an equivalent regional magnetic field that can be de- rived directly from measured data. Removal of time variations of Earth’s magnetic field is done by using a Magnetic Observatory time measurements. In our case all field measurements are reduced to nighttime level (generally 02.00 UT) and corrected for diurnal and pos- sible perturbations in the field. Magnetic field maps of Italy for intensive elements (F, H and Z), from second order network data, were published for the land areas [Molina et al. 1985] and also a total field anomaly mag- netic map was produced and published, initially for the land area only, excluding the surrounding seas [Molina et al. 1994]. One important result of the five year surveys on the first order network, is to provide an analytical for- mula able to represent, at large, the Earth’s core mag- netic field over a certain area, this is the so-called regional magnetic field model. Regional fields can be formulated as a second order degree polynomial ex- pression of the form E(z,m) = a'0 + a1z + a2m + a3z 2 + a4m 2 + a5zm where E is the generic geomagnetic ele- ment, m the longitude, z the latitude (both expressed in degrees or minutes of arc), a'0 is the base level (in nT) for each element, an(n=1.5) are the coefficients, expressed in nT/degree or nT/minutes of arc in longitude and latitude. Even if regional field models can include some contribution of deep crustal origin, superimposed to the core field, it has been shown that at the spatial scale of Italy they are in a good fit to IGRF [Finlay et al. 2010] and effectively represent the core field (see for example, De Santis et al. [1997]). In Italy the regional polynomial expression called ItGRF (Italian Geomagnetic Refer- ence Field) is regularly updated by using the five years repeat station measurements: this model avoids the boundary effects that are commonly present in the re- gional fields and makes use of the secular variation as determined by Observatory measurements (for details, see De Santis et al. [2003]). Once regional models are computed at five year in- tervals, they are used to update all measurements of the Italian area, like for example second order network data points. Every five years the national maps of magnetic elements at the specific epoch are then redrawn and the magnetic database is updated. For the various proce- dures on this point see Dominici et al. [2007], and the last compilation for year 2010.0 in Dominici et al. [2012]. Between 1965-1972, 28,833 offshore total field in- tensity data were acquired by oceanographic ships by the Osservatorio Geofisico Sperimentale (OGS) in the seas surrounding Italy [Morelli et al. 1969, Morelli 1970a, 1970b]. At the end of the 90s of last century, on- shore and offshore total field data were merged in order to obtain a single, large total field data set. This data set was then reprocessed and all data were reduced to the common epoch 1979.0, using the ItGRF as main re- gional reference field. All data (2552 onshore and 28,833 offshore) elaborated by INGV, constitute the Italian Magnetic Database. Finally, data of the Italian database wase integrated for the compilation of a sea level shaded relief magnetic anomaly map of total intensity magnetic field for Italy and the surrounding seas, for the geomagnetic epoch 1979.0 [Chiappini et al. 2000]. The published map was considered the final product of a long time effort, al- lowing an unprecedented view of magnetic anomalies in Italy and showing their relations to the major tec- tonic elements in their regional setting, confirming a good correlation between known structural geology and magnetic anomalies. 2. Aeromagnetic anomaly map of Italy For what concerns aeromagnetic investigations, the Italian national Oil Company AGIP (Azienda Gen- erale Italiana Petroli, now part of ENI, Ente Nazionale Idrocarburi) commissioned an aeromagnetic survey of Italy to a private company in the early 70s of last cen- tury. AGIP collected all total field magnetic data at var- DOMINICI AND MELONI 2 Not surveyed heavily anomalous areas First order network Second order network Figure 1. Italian magnetic measurements network. Red stars refer to first order network (repeat stations) while black crosses refer to the second order network points, as undertaken by PFG-CNR for el- ements F, H and Z. All second order network data points were meas- ured between 1977-1981, while only first order network points are repeated every five years for secular variation monitoring. 3 ious altitudes, and drew a first aeromagnetic anomaly map for the Italian territory in 1981 and a later contour version superimposed to regional geological features in 1994 [AGIP 1981, Cassano et al. 1986, Servizio Geo- logico Nazionale 1994]. An inspection of the two maps, the one produced by INGV at ground level, and the one produced by AGIP, has shown differences that could not be attrib- uted to measurement problems. This was noticeable es- pecially in the long wavelength magnetic anomaly features. It was realized that the discrepancy could be attributed to an incorrect removal of the reference field from measurements in the AGIP aeromagnetic compi- lation map; in fact, neither the IGRF or other regional magnetic models were used for reference. Only in 2005, after a revised version of the aeromagnetic map of Italy and surrounding seas was undertaken (also including the addition of some new surveys), AGIP company allowed a re-processing of all the aeromagnetic data set. The result of this work was a new map that used IGRF as main reference field [Caratori Tontini et al. 2004]. The detailed reprocessing of aeromagnetic data at the national scale, the inclusion of the new data, the subtraction of an appropriate reference level, are the main reasons for the tangible enhancement of the anomaly definition emerging from this new aeromag- netic anomaly compilation. Finally the two anomaly maps, ground level and aeromagnetic, could be fruitfully used for magnetic crustal studies. The two maps are in fact comparable, provided that INGV map is referred to the sea level and the AGIP-ENI map to an average elevation of 2500 m and, consequently, the resulting anomalies are en- hanced or flatted in the two maps. Figure 2 shows the maps from Italian database (panel A) and ENI/AGIP database (panel B). One of the local striking features that comes to the eye in the com- parison of the two maps at the local scale, is the case of the Albegna basin (South Tuscany), that is the area delimited by the red square. In fact, the two maps dis- play a different anomaly structure in this area. In detail, a strong magnetic anomaly circularly shaped that can be seen in the ground level map (panel A), does not show in the aeromagnetic map (panel B). 3. Albegna basin: geological characteristics and mag- netic implications The study area of interest for this paper (Figure 3) is located in the Neogene Albegna basin, a region char- acterized by a complex tectonic and sedimentary evo- lution. This area is delimited to the north by the volcanic complex of the Mount Amiata and to the east by the volcanic complex of the Vulsini Mountains. In the compressive phase (ended in the Early Miocene), the crust thickening resulted in an emplacement of tec- tonic units originating by different paleogeographic do- mains. Various authors [Bettelli 1980, Bonazzi et al. 1992, Bonciani et al. 2005] worked out a dynamic pre- and syn-collisional tectonic model for the area and a post-collisional extension event framework. ALBEGNA BASIN MAGNETIC DATA REVISION -450 -350 -250 -150 -50 50 150 250 350 450 550 650 750 850 A B M a g n e ti c A n o m a ly [ n T ] M a g n e ti c A n o m a ly [ n T ] Figure 2. Total intensity magnetic anomaly maps: A) ground level from Italian database; B) aeromagnetic from AGIP/ENI database. In both maps, the red rectangles show the area referred to in this paper. From the bottom to the top, the Nappe (Nappe is a large sheetlike rock that has been moved from origi- nal position for tectonic movements) superposition is represented by: - the metamorphic units of the Tuscan basement that crop out in the south-east margin of the area with the “Verrucano Formation”; - the Tuscan Nappe Unit, a sedimentary succes- sion, that ranges from the evaporites of “Anidriti di Bu- rano Formation” to the sandstone and siltstone of “Macigno Formation”, with wide outcrop in this area; - the Ligurian Unit, a tectonic unit derived from ex- ternal oceanic domain (the “Canetolo Formation”, the “Pietraforte Formation”) up to the internal oceanic Lig- urian Domain of “Ofioliti Formation”. Then, in the extensional events, the sedimentation is represented by continental-lacustrine deposits con- trolled by a system of normal faults NNW-SSE with crustal thinning and after, in the Lower Pliocene, by a transgressive sequence where an unconformity sedi- mentation of shallow-marine deposits controlled by a different system SW-NE takes place; the sedimentary evolution ended with Pleistocenic sequences. The presence, in the Ligurian units, of ophiolite blocks (metagabbros, basalts, serpentinites) which trig- ger a strong magnetic signal, is a possible source of magnetic crustal anomaly. Ophiolites are not detected in the study area (they are certainly present only at north) but the hypothetical occurrence of ophiolite blocks below the sedimentary successions which filled the Albegna basin, could not be excluded. Another im- portant magnetic inference is given by the magmatic activity (7.0-4.0 Ma) related to the extensional process [Carmignani et al. 1994, Jolivet et al. 1998, Acocella et al. 2002], mainly through anatectic intrusives and sub- ordinate volcanic products, as in the near Tuscan Mag- matic Province of Mount Amiata, or the younger (1.7 Ma-present) potassium alkaline volcanism of the Roman Province, as in the near volcanic complex of the Vulsini Mountains. Moreover, a possible uplift of a magmatic batholith can also be a cause of magnetic anomaly, although not supported by other data. For all reasons above reported the magnetic anom- aly found in the ground level map in the Albegna basin, was considered reasonable and only after the final com- pilation of the new aeromagnetic map, the area was taken in consideration for deeper investigation. 4. Magnetic survey and elaboration Figure 4 shows the second order network onshore data (red crosses) and the contour values of the total field magnetic anomaly for the region of interest. The mean distance between the stations is of about 9 km. The definitive answer to the question whether the anomaly under investigation was real or not could come only from a new set of measurements. For this reason a new dedicated magnetic survey was per- formed in area: in Figure 4 the 19 stations of the new survey are marked in blue points. The distribution of the new measurement points DOMINICI AND MELONI 4 42.4 42.5 42.6 42.7 11.2 11.4 11.6 longitude E la ti tu d e N Figure 3. Geological sketch map of Albegna Basin. Legend: 1=Tuscan units; 2=Ligurian units; 3=Mt. Amiata and Vulsini volcanic units; 4=Miocene clastic deposits; 5=Pliocene-Holocene sedimentary units; 6=normal faults; the rectangle on the map refers to the area re- ferred in this paper. 5 was focused to obtain a concentric geometry and the doubling of the density of stations around the anom- aly centre. Some stations of the new survey are very near to the stations of the old network, i.e. stations number 1, 13, 16, 17, 18, 19. At these stations the total field F was measured with an Overhauser magne- tometer, and also the magnetic inclination I with a DIM, Declination Inclination Magnetometer. At the other stations only the total field F was measured. All new measurements were reduced to a common day-time, at 02UT hour (considered as the magnetic quite time level) and, using L’Aquila Observatory data (Aq), to a mean epoch. The general procedure requires that the value of element E (i.e., F, H or Z) at station s, reduced at 02UT of day d (for this survey the day is Sep- tember 3rd, 2007), is calculated following the formula: Es (02UT)d = EAq (02UT)d + ⎣ Es (t)d − Es (t)d ⎦ where Es (02UT)d is the value of E at station s reduced at time 02UT of day of measure d, EAq (02UT)d is the value of E at Observatory at time 02UT of day of measure d, Es (t)d is the value of element E observed at station s at time t at that day d, EAq (t)d is the value of element E at Observatory at time t at that day d. In Table 1 the geo- graphic coordinates of the stations with the values at the ground level of Es (02UT)09/03/2007, where E is the magnetic element, are reported. The Element E at station s computed at epoch 2005.0 ( January 1st, 2005) is obtained according to Es(2005.0) = EAq(2005.0) + ⎣ Es(02UT)d − EAq(02UT)d ⎦, where Es(2005.0) is the value of E at station s reduced at epoch 2005.0, EAq(2005.0) is the average value of E at ALBEGNA BASIN MAGNETIC DATA REVISION 11 11.2 11.4 11.6 42.4 42.5 42.6 42.7 7-27 2 -2 -10 -16 -15 9 -13 364 -43 -25 -47 -33 3 -745 -15-12 -15 -1 -73 -36 -23 -16 -400 -350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400 nT longitude E la ti tu d e N Figure 4. Total intensity anomaly map over Albegna Basin: crosses show the onshore stations from Italian database with the corre- sponding values of anomaly, and blue stars indicate the new meas- urements. Station Latitude Longitude Elevation Fs(02UT)09/03/2007 Hs(02UT)09/03/2007 Zs(02UT)09/03/2007 1 42.53350 11.36221 128 46251.4 2 42.52823 11.36488 122 46262.9 3 42.52625 11.36918 105 46258.1 24040.5 39520.4 4 42.56002 11.39479 105 46266.5 5 42.56148 11.33409 64 46260.4 6 42.50813 11.42970 109 46260.8 7 42.46215 11.51502 190 46291.3 8 42.51749 11.46162 88 46332.7 9 42.51301 11.47297 110 46279.3 10 42.54005 11.30206 12 46261.6 11 42.51400 11.32638 16 46236.0 12 42.49473 11.37053 34 46301.2 13 42.46282 11.39454 45 46199.0 24026.6 39459.7 14 42.58813 11.41664 89 46281.5 15 42.59515 11.30521 66 46267.8 16 42.54303 11.25712 21 46254.3 24002.2 39539.3 17 42.49373 11.28276 71 46222.8 24021.3 39490.8 18 42.62627 11.49241 224 46288.9 23946.9 39613.2 19 42.55345 11.51112 235 46265.7 23986.4 39562.2 Table 1. Coordinates of stations with the values of magnetic elements at 02UT hours of day September 3rd, 2007. L’Aquila Observatory at epoch 2005.0, EAq(02UT)d is the value of E at L’Aquila Observatory at time 02UT of the day of measure d, Es(02UT)d is the value of E at station s at time 02UT. Considering that each station F value had its own elevation above sea level, a reduction to the sea level for all stations data has been made considering only the Earth’s centered dipole contribution, according to the equation ΔE=3Eh/R, where E is the generic geomag- netic element, h is the station elevation a.s.l., R is the average Earth radius. Finally, in order to make this survey exploitable for a comparison with the published anomaly maps, all data were reduced at the epoch 1979.0, using the secu- lar variation computed by the regional main reference field ItGRF. In Table 2 the expressions with the coeffi- DOMINICI AND MELONI 6 F (nT) = 46063.1 +5.72335 { +1.22044 m −0.00184 {2 −0.00003 m2 −0.00047 {m 2005.0 F (nT) = 45388.4 +5.709 { +1.111 m −0.00153 {2 +0.00049 m2 −0.00068 {m 1979.0 H (nT) = 24283.9 −9.38524 { −0.09122 m +0.00009 {2 −0.00008 m2 +0.00002 {m 2005.0 H (nT) = 24104.2 -9.043 { +0.110 m +0.00036 {2 +0.00004 m2 −0.00042 {m 1979.0 Z (nT) = 39133.1 +12.68257 { +1.60346 m −0.00493 {2 −0.00016 m2 −0.00122 {m 2005.0 Z (nT) = 38451.7 +12.467 { +1.259 m −0.00444 {2 +0.00060 m2 −0.00069 {m 1979.0 Table 3. Values of magnetic element at 1979.0: EINGV(1979.0) = values of element E at 1979.0 from Italian database; ° = values of element E on station s reduced at 1979.0 with the ItGRF; °° = values of anomaly of the element E on station s respect ItGRF; °°° = values of anom- aly of the element E on Italian database with respect to ItGRF. ° ; °° ; °°° . Table 2. Coefficients of the Italian Geomagnetic Reference Field (ItGRF) for the epochs indicated at the end of each row, for the magnetic elements F, H, Z. Latitude { and longitude m are referred to 42°N and 12°E, respectively, with values expressed in degrees. F, H, Z are in nT. St at io n F I N G V (1 97 9. 0) H IN G V (1 97 9. 0) Z IN G V (1 97 9. 0) 1 45490.8 −21 2 45502.3 −8 3 45891.0 45497.5 −12 381 24538.0 23863.2 8 −317 38780.0 38747.9 −9 23 4 45505.8 −17 5 45499.8 −20 6 45500.1 −8 7 45530.5 34 8 45571.9 58 9 45518.5 6 10 45501.1 −11 11 45475.5 −29 12 45540.6 40 13 45457.0 45438.4 −53 −34 23906.0 23848.7 −42 16 38663.0 38687.3 −26 −50 14 45520.7 −14 15 45507.3 −22 16 45511.0 45493.9 −16 2 23883.0 23823.1 −23 37 38741.0 38768.3 6 −21 17 45465.0 45462.5 −31 −29 23902.0 23841.9 −31 29 38675.0 38719.8 −7 −52 18 45549.0 45527.9 −21 0 23796.0 23773.5 −27 −4 38839.0 38838.3 1 2 19 45537.0 45504.7 −21 11 23863.0 23812.1 −28 23 38784.0 38787.6 3 −1 A n Z IN G V It G R F ^ h 1 2 3 44 44 19 79 .0 Z s It G R F ^ h 1 2 3 44 44 A n H IN G V It G R F ^ h 1 2 3 44 44 19 79 .0 H s It G R F ^ h 1 2 3 44 44 A n F IN G V It G R F ^ h 1 2 3 44 44 19 79 .0 F s It G R F ^ h 1 2 3 44 44 ( . )E 1979 0s Itgrf 1 2 344 44 ( )An E ItGRF INGV1 2 344 44 7 cients of the reference field ItGRF are listed. ItGRF 1979.0 epoch values are calculated by linear interpola- tion between neighboring sets 1975.0 and 1980.0. The value of the generic element E at the epoch 1979.0 is deduced by Es(1979.0) = Es(2005.0) − ⎣EItGRF (2005.0) − EItGRF (1979.0)⎦ where EItGRF (epoch) is the value of element E calculated with the expressions of Table 2 at the two epochs, and ⎣EItGRF (2005.0) − EItGRF (1979.0)⎦ represents the value of secular variation between the two epochs. In Table 3 all values of measurements reduced at the epoch 1979.0 with the secular variation calculated by ItGRF and the anomalies values with respect the ItGRF, both for this survey and for Italian database are reported. 5. Conclusions In this paper we have considered two total mag- netic anomaly field maps compiled for Italy in the last years. The first is the sea level shaded relief magnetic anomaly map of total intensity of the Earth’s magnetic field for Italy and the surrounding seas, for the geo- magnetic epoch 1979.0 [Chiappini et al. 2000]. The sec- ond is the aeromagnetic map [Caratori Tontini et al. 2004] compilation obtained after a revision of an older version of the aeromagnetic map of Italy, and sur- rounding seas originally undertaken by AGIP, and re- ferred to the same epoch. One of the striking features that emerged from the comparison of the two maps, was a discrepancy in the area of the Albegna basin (South Tuscany). In Figure 2 the dissimilar anomaly structure in the area is delimited by red squares. A strong magnetic anomaly, circularly shaped, can be clearly seen in the ground level map but not in the aeromagnetic map. ALBEGNA BASIN MAGNETIC DATA REVISION 42.4 42.5 42.6 42.7 7-27 2 -2 -10 -16 -15 9 -14 -14 -30 -25 -51 -33 3 -745 -15-12 -21 -19 -73 -36 -23 -16 -400 -350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400 nT longitude E la ti tu d e N 43900 44100 44300 44500 44700 44900 45100 45300 45500 45700 45900 46100 46300 46500 46700 46900 47100 47300 47500 47700 47900 nT 46217 46223 46200 46190 46198 46178 46559 46132 46136 46125 46108 46149 4612046167 46218 46206 46137 46142 46124 46129 43900 44100 44300 44500 44700 44900 45100 45300 45500 45700 45900 46100 46300 46500 46700 46900 47100 47300 47500 47700 47900 nT 46217 46223 46200 46190 46198 46171 46175 46140 46136 46116 46108 46149 4612046167 46207 46183 46137 46142 46124 46129 Figure 5. New total intensity anomaly map over Albegna Basin: crosses indicate the onshore stations by Italian database now cor- rected. Figure 6. Two maps for comparison. Left: map of total field F at 2005.0 by Italian database with the enlargement of Albegna basin and val- ues. Right: map of F at 2005.0 by Italian database corrected after new data from survey described in this paper were included (red points in the enlargement). A magnetic survey made on purpose in 2007, with the data reduction to 1979.0 for a common reference epoch with the two above mentioned maps, was un- dertaken. By means of an independent new data set, a new evaluation of the situation is now possible. The new survey shows clearly that that there is no inconsis- tency between the two sets of ground level data except for the large anomaly value reported in the Chiappini et al. [2000] map, and not at all present in the new survey results. This holds true not only for total field F but also for horizontal intensity H (not shown here), computed after also new inclination data I were used in this new elaboration. With this last consideration we assume that the old ground level measurements set included a wrong measurement value for F. After this revision of the total field data set, the incorrect station value in the Italian database was corrected and, as reported in Fig- ures 5 and 6 the total intensity anomaly map for this area and the geomagnetic field map were redrawn. In the Albegna area possible sources of magnetic crustal anomaly are actually present, Ligurian units of ophiolite blocks (metagabbros, basalts, serpentinites), subordinate volcanic products, as in the near Tuscan Magmatic Province of Mount Amiata, or the younger (1.7 Ma-actual) potassium alkaline volcanism of Roman Province, as in the near volcanic complex of the Vulsini Mountains. For this reason the magnetic anomaly de- tected in the Albegna area was initially considered fea- sible. 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