A Portable Infrasonic Sensor Calibrator Down to at Least 8 HZ

An optical fiber infrasound sensor (OFIS) is a long compliant tube wrapped with two optical fibers that integrate pressure variation along the length of the tube at the speed of light via laser interferometry. Here we only present the initial results of research conducted under this contract due to delays in the construction of the Camp Elliott OFIS array. All the field work has been completed, but the remainder of the data analysis will be completed in the next two months and presented in the associated poster. In this paper, we show that M-sequences transmitted via an array of 18-inch subwoofers comprise a portable infrasound sensor calibrator, which also permits in situ evaluation of wind noise reduction filters. For example, a comparison of a 60-m OFIS with a Bruel & Kjaer (B&K) microphone at a signal frequency of 50 Hz for a source ~240 m away shows that the OFIS reduced wind and other types of undesirable noise by 18 dB over the B&K microphone with a standard sponge wind screen. An 8-subwoofer array at a range of ~240 m yields a reliable calibration signal that smoothly decreases in power by a rate of 2.5 dB/Hz with decreasing frequency to 8 Hz, which is fairly remarkable given the 20 Hz rolloff frequency of the amplifier and speakers.
Presented at Proceedings of the Monitoring Research Review (30th): Ground-Based Nuclear Explosion Monitoring Technologies, held at Portsmouth, VA on 23-25 Sep 2008. Published in Proceedings of the Monitoring Research Review (30th): Ground-Based Nuclear Explosion Monitoring Technologies, p902-911, 2008. The original document contains color images. Proposal no. BAA06-41.