Hausmann, M.

Moreno Lupiáñez, Manuel

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Instrumentation ViewPoint.Autumn 2005513. ConclusionsAt this moment, the system is almost finished.We only have to calibrate the sensors andensemble some parts of the microprocessor’sprogram that have already been testedseparately. As the main characteristics of thedevice, we can say it gets data from theseismological sensors every 4 ms, data fromthe environmental conditions every minute anddata from electrical signals every 4 ms. Nowwe are thinking about how to improve the deviceby incorporating new sensors as a tilt sensorand trying to get data from electrical  signalsmore often.Figure 2. OBS tester photographMiniDOBS Stability and FloatabilityM. Hausmann, M. Moreno  (1,2)(1) Dep. Física e Ingeniería NuclearEscuela Universitaria Politécnica de Vilanova i la GeltrúUniversidad Politécnica de CataluñaAvda. Víctor Balaguer, s/n 08800-Vilanova i la Geltrú (España)(2) SARTICentro Tecnológic de Vilanova i la GeltrúRambla Exposició, s/n 08800-Vilanova i la Geltrú (España)manuel.moreno@upc.eduIntroductionMarine Seismometers (OBS:Ocean BottomSeismometers) are instruments that are widelyused [1]. Their mission is to register and storedifferent data series of physical magnitudes, aswater pressure and ocean bottom vibrations,etc.. They are used for the geological study ofthe marine bottom structure and seismicity.Marine Technological Unit (UTM) of CSIC is incharge of the Spanish OBSs which are calledMiniDOBS (Mini Digital Ocean BottomSeismometer). The first ones were built in 1997in Bullard Labs of the Cambridge University(UK).The empirical knowledge of the mechanicalbehaviour of such equipment [2] has taken usto carry out a stability and floatability study andfinally a mechanical characterization. To do that,it was necessary to gather the related technicaldocumentation. Ten sketches were obtainedfrom the original ones drawn by T. Owen fromCambridge university. The rest were carried outfrom direct measurements of different elementsof the MiniDOBS.This study contains the stability and floatabilitycalculations of the MiniDOBS in 3 stages of itsoperation cycle: the immersion, ocean bottomdeployment and rise to the surface.2.  Results and DiscussionsIn order to analyse the mechanical behaviourof the MiniDOBS, a study of each componentwas carried out using the autoCAD softwaretool [3]. In table I, the mechanical features areshown: material type, density and mass of eachcomponent of the instrument.Once each element is identif ied andcharacterized, the geometrical centre (CG) thatdepends on the geometry, and centre of gravity(CM) that depends on the material, in theindicated situations (immersion, ocean bottomand rise), were calculated. Elements made ofone material were separated from the onescomposed of several types. A standard analyticalmethod (that is often used in elements withsimple geometrical shape, therefore the onesthat can be decomposed to simple figures: cube,sphere, cylinder, cone, etc.) was used, and whenneeded, a consistent manual method to find theintersection of vertical lines, perpendicular tofloor passing through different subjection points[4], [5].All the data relative to decomposition as well assketches drawn and the results of thecalculations are available at www.cdsarti.org.3 ConclusionsThe results obtained allow us to characterizethe MiniDOBS mechanically and obtain a betterknowledge of the behaviour during differentoperation cycles (immersion, ocean bottomdeployment, rise). The stability and floatabilityof the equipment are studied and analysed indetail, establishing a general method applicableto other volumetric bodies.Specifically, the results can be summarized inthe following way. At immersion stage, theequipment is in total equilibrium stability (CGand CM in the same line of action). When theinstrument is sitting on the ocean bottom, it isat total equilibrium. At the rising stage, theinstrument is at partial stability. There is a smallInstrumentation ViewPoint. Autumn 200552displacement between CG and CM. Thehorizontal force due to water current does notprovoke instability. It is found that the anglebetween the equipment base and the oceanfloor (considered as rocky) has to be over 6.3ºin order to be able to slide.The MiniDOBS floatability calculations [6] confirmthat during the immersion, it is negative as theweight is bigger than the drive and drag forces.The result is obtained when the equipment issitting on the bottom. During the emersion, theinstrument rises (positive floatability) as thedrive force is bigger than the weight and drag.The study carried out will be extended withcorrection of the stability of the equipment inthe rising stage, analysis of behaviour of differenttypes of surfaces in the ocean floor (sand,sediments, etc.) and the development oradaptation of a simulator that allows us tomanipulate an existing model of an equipmentand its working conditions. Between few existingsoftware, the one that better adapts to ourrequirements is Unigraphics NX. It is a simulatorthat models a body and introduces a movemenvariable.4. References[1] USGS, What is an Ocean BottomSeismometerwoodshole.er.usgs.gov/operations/obs/whatobs.pdf[2]  A. Mànuel, H. Toruella, S. Berga PalaciosJ.M. Fábrega, A. Bermúdez, Caracterizaciónautomatizada de sistemas energéticos ensismómetros marinoswww.convertronic.net/articulos/aplicacion5a.htm [3] F.J. Rodríguez, R. Galerraga, “Normalizacióndel Dibujo Industrial”, ed. Donostiarra, SanSebastián, 1993.[4]   J.L. Meriam, L.G., Kraige, “Mecánica paraIngenieros”, vol. 3, Estática, Reverté, Barcelona,1999.[5] J. Marks, “Manual del Ingeniero Mecánico”,McGraw-Hill, México D. F., 1995.[6] J.B. Franzini, E.J. Finemore, “Mecánica deFluidos con Aplicaciones en Ingenieria”,McGraw-Hill, Madrid, 1999. Table I. Material type, Density and mass ofdifferent MiniDOBS components considered inthe study. The numbers assigned to eachelement (first column) correspond to sketchesdrawn for each element. The data acquisitionand acoustic release systems have beenincluded in the same group. It is assumed thatthe volume occupied is the same as the glasssphere protection structure. The subjectionelement of the transducer is included. Theelements not included in the table  are:thegeophone support and release system, radarreflector, anchor fixing and damping systems

MiniDOBS stability and floatability

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