Development of a new seabed coupled shear wave source:: VibroPile

Abstract

This thesis describes the development of a new seismic shear wave source - VibroPile. The source has been developed to be installed on the sea oor. It can however also be used as an onshore shear wave generator, e.g. in soft sediments where conventional shear wave sources may cause problems. The thesis includes results from both modeling of the pile, evaluation of time-varying ltering techniques and experimental data analysis from a two week long onshore eld-test in France in 2002.

The local behavior of the VibroPile seismic source is modeled by both 1D nite dierence modeling and 3D nite element modeling. The seismic source is based upon the Vibroseis technology. Time-varying ltering is needed to study recorded data. Dierent methods for time-varying ltering of synthetic and real harmonic distorted vibroseis sweeps are compared. The following methods have been used for time-varying ltering: non-stationary combination, non-stationary convolution and Gabor multiplier ltering using tight window functions.

The VibroPile SH-wave source was tested at an onshore eld-test in April 2002. The purpose of the work was to collect data that could be used to study the local behavior of VibroPile and also collect data containing the signals in the far- eld. The thesis includes a quantitative study of the VibroPile source signature and pile behavior during the eld-test. Time-frequency domain techniques are developed and used in this work.

Signals from VibroPile seismic source were collected in a zero-oset VSP experiment. Two methods for sweep removal are evaluated in the thesis: Sweep removal using crosscorrelation and sweep removal using Wiener deconvolution. The processing revealed strong SH-waves, P-waves and SV-waves. Wave eld separation was performed to nd the down- and up-going SH-waves and down- and up-going P-waves.