Alpiste, Manuel J. R.

Bartolomé, Rafael

Dañobeitia, Juan José

Gràcia, Eulàlia

Masana, Eulàlia

Moreno, Ximena

Rubio, Eduardo

Zahinos Ruiz, Abel

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First high resolution multichannel seismic experiment on the RV Hesperides: processing and application to the carboneras fault zone offshore (ALBORAN SEA)

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INSTRUMENTATION 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. M7INSTRUMENTATION VIEWPOINTSessions68Figure 1 (next page): Images of the high-resolution multi-chan-nel seismic acquisition on board the RV Hespérides. From top to bottom and from left to right: 1) Depth controls (bird) on GeoEel streamer, 2) GeoEel streamer, 3) Acquisition system and quality control of seismic data, 4) View of the gun array spatially designed for the IMPULS cruise. The streamer was towed at 2 m depth. The GeoEel acquisition sys-tem is based on portable equipment. The opening of the acquisition window is generated from the gun controller triggering, with a very small delay. The data from each digitizing system arrive to the system trough an ethernet protocol, in which each digitizer is identifi ed by a private IP address. The acquisition PC gathers and saves the data simultaneously in a hard disk and DAT tapes. The acquisition software is GeoEel Marine NX Controller. During the IMPULS cruise, sampling rate was at 1 ms (1 KHz), source. In few profi les, mainly corresponding to deeper areas, triggering was every 10-12 s (25 m) with a record length of 8000 ms. An amplifi cation of 18 dB was applied to each channel. The signal is recorded in a SEG-D Revisión 8058 format. 3. High-Resolution MCS processingA total of 47 high-resolution MCS profi les were acquired and proc-essed using the PROMAX system. In a fi rst processing step, and after re-sampling from 1 to 2 ms in order to half the size of the datasets and pick the top mutes, a FK fi lter (between 20 and 200 Hz) was per-formed with the aim of reducing spatial aliasing, and a bandpass fi lter minimum phase (20-25-170-200) was imposed. The NMO correction was applied with a constant velocity of 1700 m/s as there were no appreciable diff erences compared with a velocity change on depth NMO due to the shortness of the streamer [Fig 2] In a second phase, a preliminary constant velocity Stolt migration was applied to stack profi les for better constrains the geometry on depth. Three main seismostratigraphic units were identifi ed: Neo-gene basement, lower, and upper sedimentary units, in coherence with the previous works. These units have been distinguished from the geometrical relationship and seismic facies of the refl ectors. An analysis of commercial seismic lines velocities helped to assign the velocity range to each of the seismic facies for the fi nal Fast Explicit migration.4. Interpretation and Future workHigh-resolution multichannel seismic profi les illustrate the shallow geometry and structure variability along the CFZ. Preliminary obser-vations of the succession of seismic profi les from shelf to the basin denotes morphostructural changes along the main trace of the fault: positive fl ower-structure morphologies in the shelf zone, underlap-ping restraining step-over in the central segment, and buried pres-sure ridges towards the south segment. The CFZ aff ects all the seis-mic units, as observed in most of the profi les. The exception is with the upper unit, which, on the southern profi les is not displaced by the fault.Sampling of the recentmost units (Holocene to Late Pleistocene) will give us a sediment rate and a fi ne chronology for specifi c horizons. This will allow us to calculate a Late Quaternary vertical slip rate of the marine segments of the CFZ and estimate a recurrence interval of past earthquakes. These parameters are of paramount importance to assess seismic hazard models in the Iberian Peninsula, especially when considering high magnitude earthquakes and long recurrence intervals (104 years).Figure 3: High-resolution multichannel seismic profi le IM-08 cross-ing the Carboneras Fault5. AcknowledgementsiFigure 2: Swath-bathymetry compilation of the Almeria Margin and location of the IMPULS high-resolution seismic profi les This work is funded by the Spanish national Project IMPULS (2003-05996MAR), the Acción Especial STREAMER (CTM2004-21203-E) and the Spanish national project EVENT (CGL-2006-12861-C02-02). We thank the captain, crew and technical staff  on board the R/V Hespérides for their assistance throughout the data collection.6. References[1] 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.M7

First high resolution multichannel seismic experiment on the RV Hesperides: processing and application to the carboneras fault zone offshore (ALBORAN SEA)

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