We can obtain the performance of the geophone in the sediment
to know its coupling in the bottom sea. This paper is about the
parameters of coupling in order to obtain the response of geophone
through the frequency and the amplitude of the vibrations. The use
of the shake table permits to obtain the transfer function of coupling
between the geophone and the sediment sea without using a detailed
model of interaction OBS/seabed

Carbonell Ventura, Montserrat

Roset Juan, Francesc Xavier

We can obtain the performance of the geophone in the sediment

to know its coupling in the bottom sea. This paper is about the

parameters of coupling in order to obtain the response of geophone

through the frequency and the amplitude of the vibrations. The use

of the shake table permits to obtain the transfer function of coupling

between the geophone and the sediment sea without using a detailed

model of interaction OBS/seabed

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Coupling of geophone in the ocean bottom

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INSTRUMENTATION VIEWPOINTSessions42lights and radio beacons allow an easy recovery of the instrument at sea.The development of the CMG40T sensor and the release mechanism has initiated a strong cooperation between Guralp System Ltd and Geoazur. We are pleased to announce the development of an OBS based on the Hippocampe design using a Guralp CMG-DM24/4 data (4 channel, 24bit Digitiser, 8 channels, 20bit slow rate environmental channel, IEEE 1349 (fi rewire) data port, Real time clock drift 0.8*10-8 drift over –3 to 14 degrees of temperature, 8 Gb Flash memory stor-age – accessible via fi rewire port.Deployment of an Hippocampe OBS . The sensor (on the right) will be released from the arm when the OBS lay at bottom of the sea.Geoazur has a strong experience in marine operation and data inter-pretation. Guralp System Ltd is one of the leaders for sensor and data logger in seismology. The new OBS commercialized by GSL will be soon accessible for the whole scientifi c community.More improvements will take place in this new OBS. Guralp System Ltd and Geoazur are studying also a LTOBS (Long Term OBS), which will allow continuous observation for a period of 3 years. Acoustic communication will allow checking the instrument operations and shuttles retrieving data from time to time. LTOBS is an alternative to future cabled observatories such as discussed in Esonet. This instru-ment would also be an answer to remote parts of the Oceans.  1 · Institut de Recherche pour le Développement 2 · Centre National de la Recherche Scientifi que 3 · Université Pierre et Marie Curie (Paris) 4 · Université de Nice Sophia AntipolisCOUPLING OF GEOPHONE IN THE OCEAN BOTTOMXavier Roset (1) and Montserrat Carbonell (2)(1) SARTI Technological Centre Vilanova i la Geltrú, xavier@eel.upc.edu, Rbla.Exposició S/N 08800 Vilanova i la Geltrú (2)Department of fluids mechanics, Polytechnic University of CataloniaAbstract: We can obtain the performance of the geophone in the sed-iment to know its coupling in the bottom sea. This paper is about the parameters of coupling in order to obtain the response of geophone through the frequency and the amplitude of the vibrations. The use of the shake table permits to obtain the transfer function of coupling between the geophone and the sediment sea without using a de-tailed model of interaction OBS/seabed.IntroductionThe main problem when someone presents a marine geophone design is to know the performance of the geophone in the bottom sea in order to obtain good response to vibrations of seabed. This interaction between the OBS (Ocean Bottom Seismometer) and the sediment or rocks is not usually good because the geophone is de-ployed only by surface contact, without penetration into the bottom sea. The response to forced oscillations of OBS with the seabed is the coupling ratio. Suppose an OBS of mass m suspended in water moving in response to a sinusoidal force F. The relation between this force and the result-ing velocity vsus follows from Newton’s law: in which msus is the hydrodynamic added mass. If we now consider the OBS on the seabed the equation of motion for the bottomed OBS is:where mbot denotes the bottomed added mass, the vbot is the bot-tomed velocity and Z is the interaction impedance between an OBS and the seabed which accounts for the seabed stiff ness k and damp-ing R. (3)Coupling RatioThe geophone was placed on clayed sediment of density 2005 kg/m3 and dampness 29 %. It is characterized by a liquidity limit wL (41 %), a plastic limit wP (23%) and a plastic index IP (18%). Laboratory studies indicate this material behaves theoretically as a non-Newtonian substance. Under low to intermediate shear stresses (2 - 121 Pa) and shear rates (0,46 – 500,2 s-1) it behaves as shear thin-ning (pseudo plastic), the apparent viscosity decreases when shear stresses increases. The relationships have been defi ned as:M3INSTRUMENTATION VIEWPOINTSessions43where τ is the shear stress in Pa, γ is the rate of shear strain in s-1, and η is the apparent viscosity in Pa•s.Figure 1.Response of bottomed OBS to horizontal seabed motionThe coupling ratio, r, between bottomed and suspended velocities is:When Z/ω approaches zero as frequency increases, the high-frequen-cy limit of the coupling ratio is simply a ratio of masses (4).As shown in fi gure 1, if  the seabed moves with horizontal velocity vo and the OBS responds by moving with horizontal velocity v while the water remains at rest, (considering Z, msus, mbot and r no distin-guished between vertical and horizontal motion) the transfer func-tion for horizontal seabed motion is Th (5). If the coupling ratio can be measured, the Th can be inferred, without recourse to detailed model of the OBS/sediment interaction. Substituting the Z value (3) in the Th expression we obtain:In the expression 5 the resonance frequency ωo and the resonance quality factor Q is:gMeasures in the lab The measures in the shaker table with transducer vibration calibrator BERAN in the SARTI lab and the geophone upon the sediment basin and all together upon the shaker table fi gure 1, we can obtain the sensibility of the sensor under test, related to a reference sensor.This function is the voltage output of the geophone regarding the velocity vibration of the table (7), and is equal to the sensibility of the geophone by the ratio of velocities of the geophone regarding the The fi gure 2 shows the sensibility of the geophone through the fre-quency obtained from the sediment by calibrator (squares) and the geophone sensibility from the shaker table directly (triangles).Figure 2 Sweep frequency graf for the sensibility (square points represent the geophone upon the clay and triangle points upon the shake table)The transfer function for horizontal seabed motion Th can be ob-tained from fi gure 2, by subtracting both graphics, with a zero and a double pole according to the expression (5) and according to the Os-ler reference (1). By identifying parameters the pole frequency is 11,2 Hz quality factor Q=4 and zero frequency is 44 Hz, in the present case. For low frequencies the function is practically the unity. For frequen-cies higher than the resonance the slope descends. The performance of the coupling is pseudo elastic due to the box walls containing the geophone.  References(1) J.C.Osler and D.M.Chapman “Quantifi ying the interaction of an ocean bot-tom seismometer with the seabed” Journal of Geophys.res. Vol. 103 nº B5 pp 9879-9894, may 1998(2) E.Wielandt “Seismic sensors and their calibration” in New Manual of Ob-servatory Practice” Ed P.Borman E. Bergmann, oct. 2002F.K. Duennebier and G.Sutton “Fidelity of Ocean Bottom seismic observations” Marine Geophys. res. 17, pp 535-555, oct.1995(3) A.Trehu “Coupling of Ocean Bottom seismometer to sediment” BSSA vol 75 nº1 pp 271-289 feb.1985(4) X.Roset, S.Shariat, A.Manuel, J.del Rio  “Calibration and modelling of a bot-tom sea geophone based on virtual instrument” Instrum. Measur.Technolo.Confe. Poland may, 2007.sediment and the ratio of velocities to the sediment regarding the table motion. Considering that the expression (7), vo/vT is near to the unit, the transfer function Th can be deduced. The Th function is the sensibility of the velocity geophone to the velocity sediment regard-ing the table vibrations.M3

Coupling of geophone in the ocean bottom

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