1. 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

2. 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.

3. 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

4. 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

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

6. 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

7. 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

8. 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

9. 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

10. Deduced parameters fo = 11 Hz Q = 4 zero frequency = 44 Hz,
m, msus, mbot
seabed stiffness k = 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

11. 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.