The small but clear gravity signal above the natural 5 GPS OBSERVATIONS 6 DTM B699 Positions of gravity stations have been observed using different tech- G4.1/GD8.3 “Grotta Gigante” ( ) niques and instruments: Carla Braitenberg1, Daniele Sampietro2,4, David Zuliani3, Tommaso Pivetta1, Alfio Barbagallo3, Paolo Fabris3, (1) Geodetic GNSS receivers in Real Time Kinematic (RTK); (2) Geodetic Julius Fabbri1, Ahmed H. Mansi4, Lorenzo Rossi4 GNSS receivers in fast static mode, 15 minutes of 1Hz observations, pro- cessed with double differences (DD) with respect to permanent sta- 1) DMG, Università di Trieste 2) GReD s.r.l., Como 3) IINOGS, Udine 4) DICA, Politecnico di Milano, Milano (Italy) tions; (3) Total station (TS) combined with GNSS data using the GeoNet software (Rossi et al. 2012); 1 HAZARD OF NATURAL 2 GROTTA GIGANTE SCIENTIFIC SHOW CASE Planar N° obs Mean [mm] Std [mm] Min [mm] Max [mm] DD vs TS 10 -0.4 3 -1.1 1.2 !Potential hazard for construction and heavy loading. !Ultra-Broad Band horizontal pendulums. Continuous since 1964. Topo IngressoEffect DD vs RTK 42 0 17 -10 69 !Traffic: Busses, cars. !Length: 100 m. Period: 6 minutes. !5062520 Heights Full topographic reduction requi- mGal !! res a DTM across three countries 5062500 Stability difficult to model Secular term deformation recorded. Identified signals: Earth tides, Karstic water DD vx TS 10 -12 16 -17 28 35.2 5062480 35.15 flow, Ocean Loading, Free oscillations and covering bathymetry. 35.1 ! 35.05 How much can top be loaded without creating failure? DD vs RTK 42 0 13 131 68 5062460 35 Presently 10m DTM available only 34.95 !Depends on faulting, thickness 5062440 34.9 The standard deviation in the up direction is always smaller than 2 cm over Italian part and not offshore. 34.85 34.8 5062420 Uscita 34.75 ! thus assuring an accuracy for the free-air correction at a level of Palma Presently no awareness of potential hazard A systematic gravity survey 34.7 5062400 34.65 10microGal. Pendoli ! 34.6 Nomemory of collapsing cave would benefit from a complete 5062380 34.55 34.5 34.45 DTM. Model must be pieced toget- 5062360 ! 34.4 Colonna No routine observations are made Ruggero DD RTK TS 34.35 her due to National Border and 5062340 34.3 ! Gravity useful in identifying caves !easy to be !easy to be performed !fast (1/2 minutes per performed !fast (½ minutes per point) point) Adriatic sea. 5062320 !!not robust for not optimal sky !does not require sky 5062300 robust for not 403750 403800 403850 403900 optimal sky visibility visibility visibility !slow (15 minutes per !requires Internet connection !laborious to be 8 CONCLUSIONS point) and permanent GNSS performed 3 KNOWN CAVES DEVELOPPED ALONG !requires a master network !Cave clearly visible in gravity signal with 1.5 mGal amplitude WATER FLOW. TODAY UNDERGROUND. 4 CAVE POSITION BELOW VILLAGE 7 RESULTS !Systematic coverage of gravity measurement of would FreeIngresso Air BouguerIngresso require spacing of 30m and precision of 0.2 mGal to detect 5062520 5062520

mGal 5062500 mGal 5062500 caves of similar size. 13.75 -21.15 13.7 -21.2 13.65 5062480 5062480 -21.25 13.6 !Caves bare potential hazard due to possible failure of top after 13.55 -21.3 13.5 -21.35 5062460 5062460 13.45 -21.4 13.4 -21.45 13.35 -21.5 overload or resonance effects 5062440 13.3 5062440 13.25 -21.55 13.2 -21.6 13.15 -21.65 5062420 Uscita 5062420 Uscita 13.1 -21.7 ! Palma Palma Problem is presently completely neglected. 13.05 -21.75 13 -21.8 5062400 12.95 5062400 Pendoli Pendoli 12.9 -21.85 ! 12.85 -21.9 Database is benchmark for gravity inverse and forward mode- 5062380 12.8 5062380 -21.95 12.75 -22 12.7 -22.05 12.65 ling. 5062360 5062360 -22.1 12.6

Colonna Colonna -22.15 Ruggero 12.55 Ruggero 12.5 -22.2 5062340 12.45 5062340 -22.25 5062320 5062320 REFERENCES 5062300 5062300 403750 403800 403850 403900 403750 403800 403850 403900 !Above Fig., Left: vertical section of !Braitenberg et al., 2013. Boll. Geof.Teor., (open access) !Gravimeter drift-curves in one day up to 0.1 mGal empirically recove- cave. Bottom of vertical channel 7m !Tenze et al., 2011. Boll. Geof.Teor., (open access) red. Solid Earth Tide correction. Loading Tide in future- model mis- above sea level. Undergrond water !Rossi et al., 2012. Proceedings Asita (open access) found only during greater floods (co- sing. !Above Fig.: A number of caves along theun- !Battaglia et al., 2003, EOS. urtesy Grotta Gigante). !Cave seen clearly in F A anomaly. derground water-flow. Water variations over !Right: Civil constructions and infra- ! 40m in the caves.New caves are discovered !Bouguer field marks NS-elongation of cave better than FA. Acknowledgements: The Grotta Gigante, Soc. Alpina delle Giulie, is than- structure indepent of cave positio- by chance. Red dot: Grotta Gigante (Tenze et !Gravity together with Laser-Scan of cave is a useful dataset to ked for assistance during the gravityand GPS campaign and for handing ning. Outline of cave in yellow. al., 2011) the Laser Scan data. Contact: [email protected] Constructions directly above. benchmark inversion algorithms.