How Does Wavefield Separation Affect Direction Estimates Using a Rotational Sensor?

An active experiment was held in Fürstenfeldbruck, Germany, to investigate and compare the performance of rotational sensors. The active sources were vibroseis truck sweeps and explosions. In this study we focused on monitoring the movement of a vibroseis truck using six-degree-of-freedom (6-DoF) measurements, which combine rotational sensors and seismometers. The investigation spanned from 20 November 2019, at 11:00 UTC, to 21 November 2019, at 14:00 UTC. Throughout this period, 480 sweep signals were emitted, each lasting 15 seconds and covering frequencies from 7 to 120 Hz. Sweeps were emitted at 160 different locations.

During the second day of measurements, SV-type of waves dominated at frequencies up to 60 Hz while SH waves were dominant between 60 Hz to 120 Hz. At lower frequencies, we estimate the direction based on a method that uses SV-type waves. The accuracy of estimates declined with increasing distance between the truck and sensors. To gain insights into this phenomenon, we scrutinized the wavefield itself. Upon separating the wavefield using a fingerprinting algorithm, we observed that only a small portion of the wavefield exhibited a strong dominance of SV waves. This is surprising as the source generated only SV waves. Thus, we were able to track the movement of the truck only after separating the wavefield and determine the portion corresponding solely on SV-type of waves.