Marine Technology Workshop (Martech05), 17-18 November 2005, Vilanova i la Geltrú, Barcelona.-- 2 pages, 2 figuresPeer Reviewe

Dañobeitia, Juan José

Mànuel, Antoni

Sallarès, Valentí

Mànuel-Làzaro, Antoni

Digital.CSIC

Instrumentation ViewPoint.Autumn 2005471. IntroductionOcean bottom seismometers (OBS) can bepotentially used in a wide number of fields withinthe Earth Sciences: from seismic monitoringand analysis, hazard evaluation, awarenessand stability control of margins and thecontinental slope, to the study of the structureand physical properties of the crust and thestress state in civil engineer works. However,there is a number of technical andmethodological issues that must be addressedbefore being able to exploit all the possibilitiesoffered by this type of instruments.The Sigsensual Project aims at activelyprogressing in the improvement and adjustmentof the existing portable underwater sensorsacquired by UTM-CSIC to the Bullard Labs.(Univ. of Cambridge) in 1996 to state-of-the arttechnology (Figure 1). Therefore, we havecrit ical ly considered al l  the previousmeasurements achieved with the availableequipment. From these preceding results, wehave already learnt many crucial aspects relatedwith the reliability and robustness of theinstrument, including the acquisition system,geophone, and mechanical structure design [1].In addition, an effort have been made to developsoftware allowing systematic data acquisition,reading, representation and processing [2],focussing in the optimisation of signal/noiseratio and the attenuation of spurious signal [3],as well as improving and accounting for clockstability and time precision. Another interest isto include systems for on-line control of thewhole system including the recording packagesand the conditions within the glass sphere whenthe OBS is at the seafloor.2. Technical-methodological issuesAt present day, the technological developmentof the new OBS generations at internationallevel is closely linked with the scientific needsand to theadvance in the signal processing and datamodelling methodologies. Therefore, OBS havedeserved growing attention from the geoscientificcommunity during the last twenty years, and anincreasing number of ongoing internationalprogrammes in order to develop, built, and shareboth permanent and autonomous OBS networkshave came up. The main working axesconcerning the development of portable,autonomous OBS are the fol lowing: ·Increasing the storage capacity and theautonomy of the instruments up to > 1 year incontinuous recording, which is key for seismicmonitoring in risk areas. ·To make the OBS capable of handling differenttype of sensors (from short period to broad band)depending on the objectives of the experiment(active/passive seismics, local/regionalseismicity, teleseisms). ·Improving the coupling system between thegeophone and the seafloor to improve signal tonoise ratio in the seismic records. ·To incorporate low consumption  A/D converterswith wide dynamic range in order to respond tothese necessities. ·To include fast data transmission interfaces(e.g., USB 2.0, FireWire, etc) to make faster therecuperation and re-deployment process of alarge number of instruments. ·To make more robust the release system inorder to minimize instrument loses. ·To incorporate shipboard transducers to beable to communicate with the instruments fromthe ship, detecting performance anomalies, andpositioning the OBS at any time. ·To reduce OBS size and to increase its numberin order to have denser receiver coverage inseismic experiments, taking into account thelimited storage capacity of scientific vessels.3. Data processing of active seismic dataThe technological development of OBS isconcomitant with that of the signal processingtechniques.In contrast to multichannel seismics (MCS),where a wide number of tools have beendesigned by oil industry to adequately processacquired data, the development of Wide-Anglereflection/refraction seismics (WAS) pre-processing tools has deserved only minorattention to date. Most existing tools have beendeveloped by research institutions in order toread raw data from their own instruments,construct and display the record sections, andSigsensual: A Project  to Develop an Ocean Bottom SeismometerPrototype and its Associated Signal Processing SoftwareV. Sallarès (1), J.J. Dañobeitia (1), A. Mànuel (2)(1) Marine Technology Unit – CMIMAConsejo Superior de Investigaciones Científicas (CSIC)Pg. Marítim de la Barceloneta 37-49, 08003 – Barcelona, Spain(2) Vilanova i la Geltrú Technological Center - SARTITechnical University of Catalonia (UPC)Rambla Exposició s/n, 08800 - Vilanova i la Geltrú, SpainFigure 1: Original Micro-DOBS acquired in 1996by the Marine Technology Unit.Instrumentation ViewPoint. Autumn 200548Characterization of the water optical properties using hyperspectralE. Torrecilla (1,2), J. Piera (1,2), R. Quesada (2), I. Fernández (1), S. Pons (2)(1) Marine Technology Unit (CMIMA-CSIC).932309500 torrecilla@utm.csic.es.(2) Dept of Signal Theory and Communications. Technical School of Castelldefels (UPC).1. IntroductionHyperspectral instrumentation has opened a newdoor for optical oceanography and related fieldsthat make use of optical remote and in situ sensingof the oceans. Hyperspectral information providesoptical oceanographers the potential to accuratelyquantify and classify complex oceanicenvironments, finer-scale features (e.g. bottomtype and characteristics and phytoplanktonblooms), depth-dependent inherent opticalproperties (IOPs) and specific chemicalcompounds [1]. For instance, the hyperspectralinstrumentation has made possible the remoteidentification of different taxonomic groups ofphytoplankton due to that some pigments areunique to individual phytoplankton group orspecies [2].The incorporation of hyperspectral sensors toautonomous sampling platforms of anoceanographic observing system makes essentialpick the seismic phases. Alternatively, WASprocessing is also done using existing tools chieflydesigned for MCS geometries and not all thetechniques are therefore well-suited for properWAS processing. One of the objectives ofSigsensual has been to design a new adaptedsoftware tool allowing to perform all theseoperations using a single, easy to use, modularplatform [2].In contrast to MCS, wide-aperture WAS acquisitionsystems are specifically designed to record boththe reflections and the continuous refractions ofthe seismic waves propagating through themedium. This makes that the obtained recordsections (Figure 2) do not yield directlyinterpretable images of the sub-seafloor, beingnecessary to built models accounting for thepropagation velocity of the seismic waves in orderto interpret the data.Until very recently, seismic velocity models wereobtained either by forward modelling or by traveltime inversion, and therefore the basic data pre-processing consisted of allow identifying andpicking the most prominent seismic phasesrecorded by the system (chiefly first arrivals).However, the fast increasing on computingfacilities have allowed developing tomographytechniques that use not only first arrivals butthe full wavefield. It is therefore necessary todesign  more elaborate processing sequencesto make WAS data amenable to this typemethodology.Useful seismic phases present in the recordsections are usually masked by different typesof noise, which may be caused by the instrument(electronic noise, quantification …), by theenvironment (ship noise, cetacean, sea wavecourse, currents …), or even by the signal itself(higher order reverberations and scatteredsignals which obscure later arriving prominentphases). Another of the objectives of Sigsensualproject is to design filters and processingsequences allowing to obtain as much of thevaluable information contained in the recordsections as possible, by improving the signalto noise ratio, as well as removing or attenuatingsome well characterized phases, such as thewater wave or its reverberations [3].References[1] Sh. Shariat Panahi et al., Characterizationof a High Resolution Acquisition System forMarine Geophysical Applications, Mar. Technol.Workshop, Vilanova i la Geltrú, 2005.[2] I. Rodríguez et al., “A new software tool forWide-Angle reflection/refraction Seismic dataProcessing and Representation (WASPAR),Mar. Technol. Workshop, Vilanova i la Geltrú,2005.[3] S. Ventosa et al., Signal processingtechniques applied to seismic signal detection,Mar. Technol. Workshop, Vilanova i la Geltrú,2005.Figure 2: Record section (offset-travel timediagram) corresponding to the recording of airgunshots by an OBS.to develop new spectral algorithms and techniquesfor the analysis of the hyperspectral data obtained[3] [4].The SAMPLER project [5] coordinates thedevelopment of an oceanographic sonde in orderto measure physical and biological parametersat small scale, and an integrated software packageto analyse all the data obtained at small and largerscales. As it has been described, this integratedinstrumental system will include a newhyperspectral  sensor based on LIGAmicrosystems technology (figure 1). This sensorwill be able to measure upwelling and downwellingirradiance spectra within the water column.Apparent and inherent optical properties (AOPs,IOPs) will be estimated from these spectral values.2. Hyperspectral AnalysisThe development of new spectral techniques for

Sigsensual: A Project to Develop an Ocean Bottom Seismometer Prototype and its Associated Signal Processing Software

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