Direction finding in an active source experiment in Fürstenfeldbruck, Germany using 6 DoF measurements

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SUMMARY The seismic wavefield is fully described by translation, rotation and strain. Until recently, the seismological community has not been able to measure rotation directly with portable sensors with satisfactory resolution. Portable blueSeis-3A (Exail) sensors allow measuring three components of rotational motions. Colocated with conventional seismometers, one can locally observe six degrees of freedom (6 DoF) of ground motion. To test the performance of the rotational sensors, an active source experiment was carried out in Fürstenfeldbruck, Germany, in 2019 November. Five explosions with different yields and distances from our sensors, were fired. In a first stage, eight rotational sensors were deployed inside a bunker next to a seismometer. In a second stage, the rotational sensors were installed in two clusters of four sensors each. We compare the backazimuths derived using two different methods: (i) a method using horizontal rotational components and (ii) a standard polarization analysis using only 3C translational data. Backazimuths calculated using rotational data for five explosions have an average $10.2^\circ$ deviation from the theoretical backazimuths. Estimates using 3C translational data for the first stage of the experiment have a $1.8^\circ$ deviation from the theoretical backazimuth. For the second stage, we found a $29.4^\circ$ deviation using the seismometer from stage 1. We conclude that within our distance range from 50 to 1070 m, all rotational sensors provide reliably backazimuths of explosive sources when using only horizontal rotational components. For future applications of rotational sensors in other environments, this is promising as backazimuths can be derived reliably.