The effect of wave amplitude on S-wave velocity in porous media: an experimental study by Laser Doppler Interferometry

Rock physics models are widely used for reservoir characterisation in seismic studies. By improving the models, especially for reservoir rocks, more realistic characterisations can be achieved. To calibrate such models the elastic properties of reservoir porous rocks have been measured by ultrasonic techniques for many years. In conventional ultrasonic methods the local strain inside the wave is much higher than that in field experiments. Up until now, it has not been clear how the velocity of the ultrasonic wave depends on the strain, however this knowledge is important for interpretation of the results of the experiments. This work is an extension of our previous study on the effect of the strain amplitude on wave velocity. This paper focuses on S-wave velocity and its dependency on strain-amplitude effect for porous media (Bentheimer sandstone) and reference media (aluminium). In this study we: (1) directly measured the particle displacement in the ultrasonic wave using a Laser Doppler Interferometry (LDI) and (2) measured the changes in P and S-waves velocities by variations of increasing amplitudes applied to a source transducer from 43 to 400 volts. We measured the velocity using a conventional ultrasonic method and linked the changes of velocity to the measured local strain in the wave which we measured by LDI. The study indicated that for the unconfined Bentheimer sandstone, increasing the local strain produced by an ultrasonic wave from (3.6) 10(?7) to (5.7) 10(?6) resulted in the S-wave velocity decreasing by 4.75%. Using the LDI technique we also investigated the strains at different points on the surface produced by an S-wave transducer.