Borgia L. 1, C. Colombero 2, C. Comina 2, F. Del Bianco 3, L. Gasperini 3, F. Priore 4, L. Sambuelli 5, G. Stanghellini 3, S.

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Trippetti 6 1 Assessore alla cultura, Comune di Tuoro sul Trasimeno 2 DST - Università di Torino 3 ISMAR - CNR 4 DiFeST -

1 around the Malpasso site (Tuoro sul Trasimeno, Italy) for geological and archaeological characterization Borgia L. 1, C. Colombero 2, C. Comina 2, F. Del Bianco 3, L. Gasperini 3, F. Priore 4, L. Sambuelli 5, G. Stanghellini 3, S. Trippetti 6 1 Assessore alla cultura, Comune di Tuoro sul Trasimeno 2 DST - Università di Torino 3 ISMAR - CNR 4 DiFeST - Università di Parma 5 DIATI - Politecnico di Torino 6 Archeologa, Rotta dei Fenici/Cammino di Annibale, Itinerari Culturali del Consiglio d'europa

2 SUMMARY Geological setting of the survey area Seismic reflection survey Continuous Vertical Electrical Soundings Data integration Further surveys Conclusions

3 TRASIMENO LAKE Tuoro sul Trasimeno A 128 km 2 h a.s.l. = 258 m Max depth 6 m Average depth = 4.3 m Origin related to tectonic processes Complex geological evolution from a marine gulf (Early Pliocene) to a subsiding tectonic depression (from Middle Pleistocene to present)

evolution from a marine gulf (Early Pliocene) to a subsiding

GEOLOGICAL SETTING b: depositi alluvionali (Olocene) c: depositi lacustri e palustri emersi e sommersi (Olocene) SUPERSINTEMA DELLA VAL DI CHIANA (Pliocene sup. - Pleistocene sup.

4 GEOLOGICAL SETTING b: depositi alluvionali (Olocene) c: depositi lacustri e palustri emersi e sommersi (Olocene) SUPERSINTEMA DELLA VAL DI CHIANA (Pliocene sup. - Pleistocene sup.) SINTEMA DEL TRASIMENO (Pleistocene medio -sup.) TMO2: Subsintema di S. Feliciano - Limi argilloso-sabbiosi e sabbie limose di ambiente lacustre TMO1: Subsintema di Macchie - Sabbie medio-fini e sabbie limose di piana alluvionale/lacustre-palustre DOMINIO TOSCANO UNITA TETTONICA FALDA TOSCANA MACIGNO (Eocene Miocene) (MAC2: Membro di Poggio Belvedere - MAC3: Membro di Lippiano) Areniti feldspatiche e litofeldspatiche sequenza torbiditica Malpasso site: sandstone formation protruding towards the lake Historical reconstructions suggest a positive identification of this site as a probable location of the Trasimeno battle (217 B. C.) Level fluctuation in the Roman times the lake level was lower than present Carta Geologica d Italia alla scala 1: PROGETTO CARG Foglio Passignano sul Trasimeno

Feliciano - Limi argilloso-sabbiosi e sabbie limose di ambiente lacustre TMO1: Subsintema di Macchie - Sabbie medio-fini e sabbie limose di piana alluvionale/lacustre-palustre DOMINIO TOSCANO UNITA

5 WATERBORNE SURVEYS Seismic reflection survey Continuous Vertical Electrical Soundings Magnetic survey with the aim of: reconstructing the geological setting of the area around the Malpasso site eventually localizing archaeological remains of the Trasimeno battle

reconstructing the geological setting of the area around the

SEISMIC SURVEY Seismic reflection profiles were collected using a Benthos Chirp III system with 4 transducers mounted on board of a small catamaran towed

6 SEISMIC SURVEY Seismic reflection profiles were collected using a Benthos Chirp III system with 4 transducers mounted on board of a small catamaran towed by a boat (ISMAR-CNR, Bologna) Data were collected using a pulse length of 5 ms, a frequency sweep from 2 to 7 khz, and a digitally sampled at 16 khz rate

towed by a boat (ISMAR-CNR, Bologna) Data were collected using a pulse

7 SEISMIC SURVEY Seismic data were stored in SEGY files, after deconvolution and instantaneous amplitudes computation. Data processing was carried out using SeisPrho (Gasperini and Stanghellini, 2009) and included static correction, filtering and automatic gain control. Acoustic images with vertical resolution < 10 cm Maximum penetration depth 10 m

Data processing was carried out using SeisPrho (Gasperini and Stanghellini, 2009) and

8 SEISMIC SURVEY RESULTS Upper fine-layered unit Lower more reflective unit, showing high-amplitude internal reflectors The boundary is a dome-shape reflector (U1), which marks a relatively high acoustic impedance contrast U0 upper unit U1 lake bottom lower unit Strong reflector (U0): presence of gas in the sediment hamper the penetration of the signal and might indicate early diagenesis due to fluid circulation

impedance contrast U0 upper unit U1 lake bottom lower unit Strong reflector (U0): presence of gas in

9 CONTINUOUS VERTICAL ELECTRICAL SOUNDINGS Multichannel georesistivimeter (Syscal-pro in Sysmar upgrade Iris Instruments) Array of 9 electrodes fixed on a floating cable dragged by a small boat

The resistivity meter was connected to a GPS device, in

10 CVES SURVEY Survey area 2 km 2 20 CVES profiles Total length of acquisition 17 km On average, one VES every 6 m The resistivity meter was connected to a GPS device, in order to record the exact spatial position of the acquired traces.

11 CVES RAW DATA Statistical analysis of more than 2500 apparent resistivity curves Apparent resistivity maps over the survey area ρw = 7,2 7,8 Ωm lacustrine deposits water ancient coastline deposits sandstones (MAC)

over the survey area ρw = 7,2 7,8 Ωm lacustrine

LATERALLY CONSTRAINED INVERSION A three layer conceptual model has been implemented in a LCI inversion scheme: (Auken and Christiansen, 2004; Wisén and Christiansen, 2005) Fixed water resistivity ρ 1

12 LATERALLY CONSTRAINED INVERSION A three layer conceptual model has been implemented in a LCI inversion scheme: (Auken and Christiansen, 2004; Wisén and Christiansen, 2005) Fixed water resistivity ρ 1 (mean value of 7.5 Ωm obtained from the resistivity of the nearest potential dipoles); Fixed water depth h 1 (from bathymetry); Lateral variability constraints on the basis of raw data lateral variability at the same pseudo-depth.

13 CVES - LCI RESULTS sandstones (MAC) ρ3 < ρ2 ρ3 ρ2 ρ3 < ρ2

14 CVES - LCI RESULTS Similar approach can be used in a Spatially Constrained Inversion (SCI) of the whole dataset. sandstones (MAC)

15 CVES 3D MODEL sandstones (MAC) possible ancient shore line lacustrine deposits water ancient coastline deposits sandstones (MAC)

16 DATA INTEGRATION Picking of the reflection interfaces evidenced from the seismic campaign Laterally Constrained Inversion (LCI) of the CVES data with the a priori constrained interfaces

contrasts at the same depth Constraints: ρ 1 h 1 h 2 from

17 DATA INTEGRATION Constraints: ρ 1 h 1 ρ 2 lateral variability simply evaluated from row data lateral contrasts at the same depth Constraints: ρ 1 h 1 h 2 from the picking of the seismic reflector (between VES n ; m)

18 FURTHER SURVEYS Magnetic survey to detail the investigation in the lake area in front of the Malpasso site, using GEM GSM19V Overhauser proton magnetometer 1 reading/s 1 reading/1.5 2 m 23 magnetic profiles, for a total length of 16 km

site, using GEM GSM19V Overhauser proton magnetometer 1

TMF [nt] grad [nt/m] MAGNETIC SURVEY Some localized magnetic gradient anomalies will be further analyzed in order to verify their possible causes 46840 46820 46800 46780 46760 46740 46720 46700 46680

19 TMF [nt] grad [nt/m] MAGNETIC SURVEY Some localized magnetic gradient anomalies will be further analyzed in order to verify their possible causes ,000 50, , , , , , , ,000 distance [m] Profiles showing TMF and magnetic gradient anomalies will be processed with an inversion approach for further investigation TMF [nt] grad [nt/m]

150,000 200,000 250,000 300,000 350,000 400,000 distance [m] 50 40 30 20 10 0-10 -20-30 -40-50 Profiles showing TMF

20 CONCLUSIONS Preliminary results of both seismic reflection and CVES evidenced good potentiality in characterizing the sediments of the submerged area around the Malpasso site which is interesting both from the geological and archeological point of views. The results appears to match in first approximation. It is possible to evidence that both of the surveys show the presence of a submerged interface within the sediments that can be related to more compacted/resistive deposits probably associable to the ancient shore line. Electrical profiles have also locally evidenced the presence of a sandstone formation protruding down the lake bottom near the shore. Further studies are necessary to allow for an unique reconstruction of the geological setting respectful of both the surveys evidences. In this respect an accurate data integration must be undertaken. The magnetic survey, with waterborne profiles very close to the Malpasso site, evidenced some localized TMF and magnetic gradient anomalies, which will be further analyzed for possible archeological findings.

It is possible to evidence that both of the surveys show the presence of a submerged interface within the sediments that can be related to more compacted/resistive deposits probably associable to the

PALEOENVIRONMENTS OF THE LAKE BALATON AREA

Eötvös Loránd University Department of Geophysics HUNGARY 1117 Budapest Pázmány Péter sétány 1/C Tel: +36-1-3812191 Fax: +36-1-3812192 E-mail: geofizika@ludens.elte.hu PALEOENVIRONMENTS OF THE LAKE BALATON