Coupling ratio of geophone in the sea bed X.Roset , M.Carbonell & A.Manuel Universitat Politècnica de Catalunya International Summer Course of Non-homogeneous Turbulence’08

Objectives of the work Get the performance of the geophone in the sediment in order to know its coupling in the bottom sea Obtain the transfer function of coupling between the geophone and the sediment sea by shaker table without using a detailed model of interaction OBS/seabed.

Automatic Calibration The LabVIEW program obtain first the frequency response to the sensitivity of sensor in acceleration units, and in a second seep we can detailed the parameters of sensor for her characterization completely. We show one of the pages of the process program of LabVIEW in the figure 3, when the second sweep is beginning. (1) Fig.3 One of the visual program panel An acceleration model can characterize the geophone sensitivity with the expression (1). We can express the transfer function of the magnetic accelerometer according to the voltage output in function to the acceleration input in one axis

Coupling ratio The response to forced oscillations of OBS with the seabed is the coupling ratio > r The coupling ratio between bottomed and suspended velocities follows Osler and Chapman equation : hydrodynamic added mass bottomed velocity seabed stiffness damping bottomed added mass interaction impedance between an OBS and the seabed

Transfer function for horizontal seabed motion of geophone resonance frequency quality factor mbot v vo vT m

MEASURES IN THE LAB Material of the bottom seabed shear stress in Pa rate of shear strain in s-1 Laboratory studies have been carried out using co-axial cylindrical reometer Haake which indicate this material performs reologically as a non-Newtonian substance

Rotary-oscillatory reometer Haake elastic module component G’ is always higher than the viscous module G’’ frequency 1Hz varying the shear stress. 1 Pascal stress varying the frecuency

Shake table measurements The measures in the shaker table with transducer vibration calibrator BERAN mbot v vo vT About 1 Deduced Th Transfer function In the table Sediment  Geofone on top Geophone sensibility Measured

Shake table measurements Sweep frequency of 1 to 100Hz for measure the sensibility (amplitude 3mm/s) Transfer function [Th] geophone versus sediment SensBeran HG fo= 11 Hz ; Q=4 ;zero frequency = 44 Hz msus = 0,588 kg ; mbo t= 0,78 kg

Deduced parameters fo = 11 Hz Q = 4 zero frequency = 44 Hz, m, msus, mbot seabed stiffness k = 19380 kg/s2 damping R = 69,2 kg/s Considering the Poisson coefficient σ = 0,49, geophone radius = 0,1 m density of the material of geophone 2830kg/m3 shear wave velocity of sediment Cs= 2,97m/s

Conclusions We have inferred valuable parameters related to the coupling in the geophone-sediment interaction and the shear wave velocity of sediment. They have been obtained from a reology and vibration laboratory test. These parameters allows to perfectly characterize the coupling between the sensor and the sediment, and how the geophone performs when recording the ground and seabed vibrations data, what the expected dynamic range is and its accuracy level.