Carbonell, Ramón

Dañobeitia, Juan José

Gràcia, E.

Guasch, L.

Lo lacono, Claudio

Mateo, M.Á.

Rodríguez, P.

UPCommons

Application of high-resolution seismo-acoustic methodologies in the study of the seagrass posidonia aceanica (L.) delile

UPCommons. Portal del coneixement obert de la UPC

INSTRUMENTATION VIEWPOINTSessions665. References[1] B. Alonso, G. Ercilla, Small turbidite systems in a complex tectonic setting (SW Mediterranean Sea): morphology and growth patterns, Mar Petrol Geol 19, 1225-1240, 2003[2] M. Garcia, B. Alonso, G. Ercilla, E. Gràcia, The tributary systems of the Alm-ería Canyon (Alboran Sea, SW Mediterranean): Sedimentary architecture, Mar Geol 226, 207-223, 2006[3] E. Gràcia, R. Pallàs, J.I. Soto, M. Comas, X. Moreno, E. Masana, P. Santanach, S. Diez, M. Garcìa, JJ. Dañobeitia, Active faulting off shore SE Spain (Alboran Sea): Implications for earthquake hazard assessment in the Southern Iberian Margin, Earth Planet. Sc. Lett. 241, 734-749,2006. [4] S. Duggen, K. Hoernle, H. van de Bogaard, Magmatic evolution of the Albo-ran region: the role of subduction in forming the western Mediterranean and causing the Messinian Salinity Crisis, Earth Planet. Sc. Lett., 218, 91-108, 2004APPLICATION OF HIGH-RESOLUTION SEISMO-ACOUSTIC METHODOLOGIES IN THE STUDY OF THE SEAGRASS POSIDONIA OCEANICA (L.) DELILEC. Lo Iacono (1), M.Á. Mateo (2), L. Guasch (3), E. Gràcia (1), P. Rodriguez (1), JJ. Dañobeitia (1), R. Carbonell (3)1 UTM, Unitat de Tecnologia Marina - CSIC, Centre Mediterrani d’Investigacions Marines i Ambientals, Pg. Marítim de la Barceloneta 37-49, 08003 Barcelona, Spain e-mail: loiacono@cmima.csic.es2 CEAB, Centre d’Estudis Avançats de Blanes - CSIC, 17300 Blanes, Girona, Spain3 IJA, Institut De Ciencies De La Terra Jaume Almera – CSIC, 08028 Barcelona, Spain1. IntroductionThe distribution of the Mediterranean seagrass Posidonia oceanica has been widely assessed using acoustic methodologies. However, informations on P. oceanica internal structure obtained with seismic methods are still very rare. In the framework of the multidisciplinary project CARBOMED (Proyecto Intramural de Frontera, PIF-2005, CSIC, Ref. 200530F0232), a seismo-acoustic survey was carried out in Port Lligat Bay (Catalonia, NE Spain) with the aim to assess the volume potentially occupied by P. oceanica organic deposits (known as “mat-te”), detect its internal structure and highlight the morphological and sedimentological characters of the study area (Fig. 1). A fi rst calcula-tion of the potential volume of P. oceanica deposits in Port Lligat Bay was estimated in about 175.000 m3 [1]. Figure 1: Navigation tracks of the seismo-acoustic records acquired in Port Lligat Bay during the Carbomed Project. Track in red: profi le 71 (Fig. 2). Track in yellow: profi le 8 (Fig. 3). Red dot: core site2. Data and MethodsThe high resolution parametric echosounder “Innomar SES 2000 com-pact” was used to acquire 75 seismo-acoustic records spaced from 5 to 40 m each others, for a total length of about 7.5 km. The position-ing system was a diff erential GPS (DGPS) Trimble AGP 132. The echo-sounder was characterized by a primary frequency of 100 kHz and secondary frequency ranging from 5 to 12 kHz. Pulse Repetition Rate was up to 30/sec and the beamwidth of ±1.8°. Each seismo-acous-tic record has been acquired twice, using secondary frequencies of 10 and 6 kHz. The lower secondary frequency (6 kHz) gave the best result because of its higher resolution and relatively higher penetra-tion, and the records obtained with this confi guration were used for this study. The shallow water of the study area (depth from 3 to 6m) produced high amplitude multiples.3. Results and discussionHere we present an interpretation of seismo-acoustic records and of data from a core collected in the matte of Port Lligat bay with the fi nal aim of reconstruct the dynamics of P. oceanica since it was estab-lished in the study area. The acoustic masking of the internal struc-ture of the seagrass and the presence of multiple refl ectors due to the reduced depth of the area (maximum 4 m) did not allowed to associ-ate a specifi c seismo-acoustic facies to the matte of P. oceanica. Nev-ertheless, in some sectors it was possible to detect a strong refl ector, from 2 to 6 m depth, that was interpreted as the initial substratum where the seagrass established for fi rst time (Fig. 2). Also, sporadic sandy areas, apparently no more than 4 m thick, were identifi ed in-terlayered with the P. oceanica matte (Fig. 3). Figure 2: Seismo-acoustic record N. º 71 showing the potential base of the matte, on which P .oceanica established.A core collected on the P. oceanica meadow showed the presence of a dense matte for the fi rst tens of centimeters downcore, degrading to sandy sediments with rhizomes and leaf sheaths for a total depth of 6 m. The base of the core presented gravely bioclastic sediments and its depth correlates, in a seismic record near the core location, to the refl ector interpreted as the base of the P. oceanica matte. Assum-ing a growth rate of P.oceanica of 1.3 to 4.1 mm y-1 [2,3], the gravel deposits observed at the base of the core may be aged between ca. 1500 and 4500 yrs BP.M7INSTRUMENTATION VIEWPOINTSessions67Figure 3: Seismo-acoustic record N. º 8 showing the diff erent seis-mic facies recognized in the area.  P.o: Posidonia oceanica; sp: sand patches;  m: multiple4. ConclusionsThe application of seismo-acoustic methodologies revealed a reli-able method in the study of the internal structure of P. oceanica. However, the dispersion and masking of the acoustic signal through the P. oceanica did not allow to totally detect the base of the matte. Preliminary results allow us to conclude that in Port Lligat Bay P. oce-anica probably developed on gravely sediments, mainly constituted by less fragmented gastropods and bivalves, and probably deposited in a very shallow coastal setting, characterized by moderate hydro-dynamic energy. Given the high spatial variability of matte accretion rates observed in the bay, the forthcoming radiocarbon dating of the bioclastic gravel deposits will be essential to relate their actual posi-tion and age with the geological moment during which the P. oce-anica meadow established in the bay as the sea begun to invade it. 5. AcknowledgementsThe authors acknowledge the support of CARBOMED Project (Mag-nitud del sumidero de Carbono asociado a la fanerógama marina Posidonia Oceánica y potencial en Paleoecología - Proyectos Intra-murales de Frontera (PIF)-2005, CSIC, Ref. 200530F0232). We thank Oscar Serrano and Laura Serrano for their support during the seismic survey. We also thank the captain of the R/V “Dolores” Ferran Crespo and the INNOMAR technician Hans Jorn Korsgaard.6. References:[1] L. Guasch, M.A. Mateo, C. Lo Iacono, E. Gràcia, R. Carbonell, First attempt to evaluate the size of the peat-like deposits formed by the seagrass Posidonia Oceanica using high resolution seismics, European Geosciences Union (EGU) General Assembly, Wien, 15 – 20 April, 2007[2] MA Mateo, J. Romero, M. Pérez, MM. Littler, DS. Littler, Dynamics of millena-ry organic deposits resulting from the growth of the Mediterranean seagrass Posidonia oceanica, Estuarine Coastal and Shelf Science, 44: 103-110, 1997[3] MA. Mateo, P. Renom, R. Julià, R. Michener, Carbon under the meadow, Oral presentation. 7th International Seagrass Biology Workshop. Zanzibar, Tanza-nia. 10-16 September, 2006.FIRST HIGH RESOLUTION MULTICHANNEL SEISMIC EXPERIMENT ON THE RV HESPERIDES: PROCESSING AND APPLICATION TO THE CARBONERAS FAULT ZONE OFFSHORE (ALBORAN SEA)Ximena Moreno (1,2), Eulàlia Gràcia (1), Rafael Bartolomé (1), Eduardo Rubio (1),Abel Zahinos (1), Manuel J. R. Alpiste (1), Eulàlia Masana (2), Juanjo Dañobeitia(1) and IMPULS Team1Unitat de Tecnologia Marina, Centre Mediterrani d’Investigacions Marines i Ambientals, CSIC, 08003 Barcelona, Spain2 Grup de Tectónica Activa i Paleoseismología, Facultat de Geología, Universitat de Barcelona, 08028 Barcelona, Spain1. IntroductionThe IMPULS project is based on a multidisciplinary and integrated onshore-off shore neotectonic and paleoseismological study of the Carboneras Fault Zone (CFZ) at the SE margin of the Iberian Penin-sula. Recently published marine geophysical data from the HITS 2001 cruise [1] reveals the seafl oor morphology along the off shore Carbon-eras Fault segment as an upwarped 5-10 km wide deformation zone bounded by subvertical faults. With the purpose of studying the pale-oseismological parameters of the CFZ off shore we carried out a ma-rine cruise in 2006 onboard the RV Hespérides. The main goals were to image the internal geometry of the off shore continuation of the Carboneras Fault Zone, to estimate a Late Quaternary slip rate of the marine segments and to calculate the recurrence interval. High-reso-lution marine geophysical methods are crucial in paleoseismological studies because of the need to focus on the upper meters below the seafl oor to accurately calculate the paleoseismological parameters of the fault during the Late Quaternary. In order to obtain the resolution required to study the vertical geometry of active faults in South Iberi-an Margin (e.g the Carboneras Fault) we tested a new high-resolution multi-channel seismic system onboard the RV Hespérides. 2. The IMPULS 06 cruise: High-Resolution MCS experimentDuring the IMPULS cruise, we implemented new technologies on board the RV Hespérides, such as a high-resolution multi-channel seismic acquisition. The source, a 10 m long airgun array, was spe-cially designed to favor high frequencies, comprising eight guns, four Bolt, model 900LLX-T, and four Sleeve Guns I. The confi guration (2x40, 2x55, 2x30 and 2x20 cubic inches) equals an array of 4.75 liters of volume. Gundalf software was used to specially design the gun array to provide with an adequate signal for the IMPULS survey. The energy peak obtained is larger than the one obtained with the classic cluster Sleeve Gun array, and the band width reaches even higher frequencies, up to 300 Hz. To trigger and synchronize the gun array, we have used the Minipulse gun controller (Hydrasystems) able to work with a total of 8 guns.To record the high-resolution multichannel seismic data, a “GeoEel” Geometrics (USA) digital streamer was rented funded by the Acción Especial STREAMER (MEC). The streamer is 573 m long, with 300 me-tres of active section (6 active sections of 50 metres each). Each of the active sections is confi gured to form 8 channels, totalizing 48 chan-nels separated of 6.25 m. The distance from the aft of the vessel until the fi rst active section is of 93 m. M7

Application of high-resolution seismo-acoustic methodologies in the study of the seagrass posidonia aceanica (L.) delile

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