The effect of low pass filters in the Molikpaq data acquisition system on "phase lock" interaction signals

During the deployment of the Molikpaq in the Canadian Beaufort Sea, in the 1980’s, various data signals were recorded by a digital data acquisition system. As part of this system an analog lowpass anti-aliasing filter was incorporated. The frequency response of this filter has been found to be far from ideal, resulting in significant changes in the amplitudes and wave shapes of data sampled at 50 Hz. Because of this distortion the severity and understanding of ‘phase lock’ interactions may have been in error. As an example, peak caisson accelerations have been reported in the literature to be up to about 10%g. The current investigation into the effect of the filter indicates that the peak accelerations are 50% to 75% larger. In this paper, the authors discuss the filter frequency response and its effect on the strain-gauge and acceleration data recorded in the 50 Hz (“Burst”) files with the emphasis on phase lock interactions. The authors also discuss aspects of the interpretation of the phase lock phenomena. It will be shown that there are consequences for the material contained in Section A.8.2.6, dealing with Dynamic Ice Actions, of the proposed International Organization for Standardization (ISO) 19906 Standard.