Ofis Experiments at Camp Elliott: Paving the Way to Infrasonic Radar and a Portable Infrasonic Sensor Calibrator

Optical fiber infrasound sensors (OFIS) are long compliant tubes wrapped with two optical fibers that integrate pressure variation along the length of the tubes via laser interferometry at the speed of light. We have previously shown that several OFIS arms can resolve the back azimuth and elevation angle of infrasound signals due to the spectral fingerprint of the instrument response in the recorded signal and the time separation provided by separating the centers of each OFIS arm. In this paper, we present the recent results of calibration, coherence, wind-noise reduction, and fine-tuning experiments that we accomplished in route to completing our grant objectives. Specifically, we show that the sensitivity of the OFIS is temperature dependent, and develop a real-time calibration system that allows us to compensate for this variation. We also show that M-sequences transmitted via subwoofers can be used as portable calibration tools in the low-frequency audible range and can test wind noise reduction technologies for some sensors. Comparison of a 60-m OFIS with a B&K microphone at a signal frequency of 50 Hz for a source ~300 m away shows that the OFIS reduces wind and other types of undesirable noise by 12 dB over the B&K microphone with an attached sponge wind filter. We also present the layout of a six-arm OFIS array at Camp Elliott (CEL) in north San Diego county where our investigations are continuing. This array has an aperture of 150 m, and is collocated with a digital video camera that will provide confirmation of the back azimuth and elevation angle of aircraft infrasound signals that are tracked by a passive infrasonic radar system. 29th Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies