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Re-design of a geophone for marine seismicsÀlvar Mánuel, P. Soler

AbstractThe present paper is based on the design of ageophone of a deep ocean marine seismometer,an instrument that collects passive and activeseismicity data. The present structure of thegeophone has been analyzed in 600atm pressureenvironment by using a finite element software.Simulations have been carried out using differentmaterial observing the maximum stress at differentpressure for each of them. To conclude, a re-design is implemented to optimize the mechanicalbehavior.

1. IntroductionThe first Spanish deep ocean marine seismometerwas developed by SARTI group in UPC withcollaboration of Cambridge University [1]. Thisautonomous instrument is deployed on the oceanbottom where collects data series (acousticwavefronts) generated artificially by anoceanographic vessel. This allows deducing thecortical distribution and rocks and sub-layergeological properties [2]. This paper is based onthe design of the accelerometer sensor housing(geophone).

As a previous work, the Figure 1 design has beenused testing its correct functionality at 4000mdepths. A study of the mechanical behavior athigher depth up to 600atm, is needed. This leadsto the design of the mechanical structure in twostages. In a first stage, a geometrical design iscarried out in order to optimize the stress at purepressure. In the second stage, a study usingdifferent material is carried, analyzing themaximum operating pressure.

2. Results and discussionsIn order to evaluate the stress, finite elementssoftware Unigraphics 2.0 has been usedgenerating a scenario of the study using thegeophone initial design and construction material[3].

1st stageThe design of the geophone is carried out on thetop cover, which is under more stress. The materialused is Aluminum_3005 H18. In Figure 2, througha finite element analysis, with a 400atm pressureload, we can see in red colour, the parts whichhave maximum stress load.

The first proposed modification is the additionof a verve that will join the central wall to theside wall, decreasing the point of maximumload stress. The next modification will be theincrease in rounding of the edge situated in thedeepest part of the cavity that holds the sensors,from R7 to R12 mm, smoothing this change ofdirection. We can observe all this in Figure 3.The points with most tension concentration arevery localized, therefore a study of an increasein thickness between the mentioned cavity withexternal part of the geophone is taken intoaccount.

Figura 1. Geophone initial design

Figure 2. Finite elements analysis of the initialdesign at 400atm pressure.

Figure 3. Finite elements analysis of modifieddesign at 400atm pressure.

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2nd stageIn this stage, we have chosen a series ofmaterials that would bear the marineenvironment, however some of them do notcomply the needed mechanical features [4].The study of the material has been carried outusing the initial design of the geophone anddifferent materials: metallic, non-ferrite metallicand plastic.

3. ConclusionsThe use of plastic material is completely rejectedfor this kind of application. Through this designtechnique, the maximum stress of the geophonehas been increased by 20%. In the seconddesign, for over 500atm pressure, a change ofmaterial is recommended.

4. References[1] A. Mànuel, G. Olivar, J. del Rio, H. Torruella,J. Dañobeitia, A. Bermúdez, J. Díaz, T. Owen,New Generat ion o f Ocean Bot tom,Seismometers. Preamplifier System, IEEEInstrumentation and Measurement TechnologyConference IMTC’2002, Anchorage (Alaska),21-23 Mayo 2002.

[2] F. Michaud, J.J. Dañobeitia, R. Carbonell,R. Bartolomé, D. Córdoba, and L. Delgado-Argote. 2000. New insights about the oceaniccrust entering the Middle American Trench offwestern Mexico (17-19ºN). Tectonophysics, 318,Vol. 1-4, 187-200.

[3] A. Mànuel, J. del Rio, H. Torruella, X. Roset,J.J. Dañobeitia, T. Bermúdez, T. Owen,Caracterización y diseño de un geófono parasísmica marina.

[4] C. Riba i Romera, Disseny de màquines IV,Selecció de materials 1 i 2. TEM – UPC, 1997.

Table I. Comparison of different materials witha high resistance to corrosion, their maximum

load stress and yield strength.

Model Characterization of Geophone SensorX.Roset , A.Mànuel

SARTI Technological Centre of Vilanova i Geltrú. Polytechnic University of Catalonia SPAIN

1.IntroductionIn applied instrumentation to oceanography andseismic prospecting, the equipment acquiresthe vibrations of the seabed. The waveformscan be either artificially generated at anoceanographic vessel on board or the OBS(Ocean Bottom Seismometer) can record naturalseismicity. With appropriated mathematicalalgorithms, the cortical distribution can bededuced (speed, deepness), and also geologicalproperties of the rocks and constitutive layerscan be studied [1]. The OBS measures thevibrations refracted of the seabed withgeophones in three orthogonal axis andfrequency range from 0,1 to 100 Hz, in order toinvestigate the composition and stratification ofoceanic subsoil. In order to characterise the underwatergeophone we need a precise model to obtainthe correct simulated answer. The work is anapproach to obtain a correct model of ageophone by measuring the sensor in a shaketable and extracting the parameters that define

the frequency performance in the model andsimulate the equivalent circuit. The similar resultsof the two ways validate the model. The problemsappear in the geophone with amplification whenthe ratio signal/noise is very low [2]. In order toprepare measure equipment with wantedspecification and performances in range offrequency, ratio S/N [3] and good couplingseabed we need a simulation of a proposedmodel, the subsequent validation in the lab andthe final test.

938967200 xavier@eel.upc.es.

Figure 1. Structure of the coupling with triaxialgeophones