Experimental Study on Impact of Moisture Content in Sandy Soil on DAS Amplitude Response
Using distributed acoustic sensing(DAS)technology in conjunction with underground tele-communication fiber networks for dynamic urban security monitoring,including perimeter security(in-trusion)monitoring,pipeline leakage detection,and traffic condition assessment,represents a new ap-proach.This research aimed to investigate the influence of moisture content variations in geotechnical mediums around unlit fibers on DAS amplitude response characteristics.To achieve this,the paper de-signed DAS amplitude response tests using sandy soils with varying moisture contents(0%,5%,10%,15%,and 20%)and two excitation modes:a small ball impacting a disc and directly on sandy surface.The test results revealed that:(1)Vibration signals generated by a free-falling ball impacting the sandy surface,coupled with the setup of a ring sensing optic fiber,could enhance the DAS signal-to-noise ratio,thereby increasing the reliability of the testing apparatus and the reproducibility of the results.(2)This study also identified a critical moisture content level that affected the DAS signal am-plitude,which was caused by the combined effects of the sand's pseudo-cohesion and the attenuation of the acoustic wave propagation.Below this critical level,the DAS signal amplitude decreased with increasing moisture content,whereas above it,the amplitude increased with higher moisture levels.(3)The contact method between the balls and the sand had a significant impact on DAS amplitude re-sponse due to the differences in sand's pseudo-cohesion and the energy conversion upon impact.The findings provide a theoretical basis for the application of DAS technology in precise monitoring of ur-ban underground engineering safety.
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