Weighted sum method for calculating ground force: an evaluation by using a portable vibrator system |
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| Authors: | Michiel van der Veen Jan Brouwer & Klaus Helbig |
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| Institution: | Swiss Federal Institute of Technology, Institute of Geophysics, ETH-Hönggerberg, CH-8093 Zürich, Switzerland.,;OYO-Center of Applied Geoscience, Archimedesbaan 16, 3639 ME Nieuwegein, The Netherlands.,;Kiebitzrain 84, 30657 Hannover, Germany. |
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| Abstract: | Using a lightweight portable vibrator, we have evaluated the accuracy of the 'weighted sum' method for calculating ground force. Experiments in which the vibrator was suspended elastically have shown that, contrary to expectations based on standard theory, the amplitude of the weighted sum ground force was significantly above zero at high frequencies (> 500 Hz). Complementary investigations with load cells confirmed these results. If not accounted for, these deviations may introduce significant 'vibroseis-correlation noise' in processed records. Furthermore, we have demonstrated that ground force and base-plate velocity can be used to estimate the radiation impedance, which describes the interaction of (vibratory) sources with the ground. Using the mechanical characteristics of the system (i.e. maximum displacement, maximum velocity and maximum acceleration of the base-plate) and the radiation impedance, the behaviour of the portable vibrator on typical Dutch soil types was evaluated. We found that for the same sweep, more high-frequency energy could be generated on hard grounds (e.g. concrete) characterized by a higher radiation impedance than on softer grounds (e.g. clay or sand). Knowledge of this behaviour may provide important information for use in data interpretation. |
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