Coupling ratio of geophone in the sea bed

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Coupling ratio of geophone in the sea bed
X.Roset , M.Carbonell A.Manuel Universitat
Politècnica de Catalunya International Summer
Course of Non-homogeneous Turbulence08
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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.

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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

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Coupling ratio

• The response to forced oscillations of OBS with
  the seabed is the coupling ratio gt 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
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Transfer function for horizontal seabed motion
of geophone
resonance frequency
quality factor
m
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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
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Rotary-oscillatory reometer Haake
elastic module component G is always higher than
the viscous module G
1 Pascal stress varying the frecuency
frequency 1Hz varying the shear stress.
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Shake table measurements
The measures in the shaker table with transducer
vibration calibrator BERAN
About 1
Deduced Th Transfer function
In the table
Sediment ? Geofone on top
Geophone sensibility
Measured
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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 Q4 zero frequency 44 Hz msus
0,588 kg mbo t 0,78 kg
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Deduced parameters
seabed stiffness k 19380 kg/s2 damping R
69,2 kg/s
fo 11 Hz Q 4 zero frequency 44 Hz, m,
msus, mbot
shear wave velocity of sediment Cs 2,97m/s
Considering the Poisson coefficient s
0,49, geophone radius 0,1 m density of the
material of geophone 2830kg/m3
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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.