An Improved Method of Back Azimuth Determination with a Multi-arm Ofis

The detection of infrasound in the presence of wind is challenging. Increasing wind speeds lead to higher noise floors. While the exact nature of the noise is still a subject of investigation, several technologies and array configurations have been developed to maximize the signal to noise ratio and lower the detection threshold. Optical fiber infrasound sensors (OFIS) are long compliant tubes wrapped with two optical fibers that integrate pressure change along the length of the tube with laser interferometery. Spatially incoherent wind noise is naturally attenuated relative to spatially coherent infrasound. Because the signal pressure variation is integrated along the length of the tube, the instrument response is a function of the orientation of the OFIS arms relative to the orientation of the wavefront. We show with theory and real infrasound data recorded at Piñon Flat Observatory in southern California that this spectral property can be exploited with multiple OFIS arms in different ways to determine the phase velocity of infrasound signals. Based on these findings and unpublished results, we propose two OFIS configurations as alternatives to rosette pipe array infrasound stations. 29th Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies