砂土含水率对DAS振幅响应影响的试验研究

Experimental Study on Impact of Moisture Content in Sandy Soil on DAS Amplitude Response

利用分布式声波传感(DAS)技术和地下通讯光纤网进行周界安防(入侵)监控、管道泄漏监测、交通状况评估等是城市安全动态监测的新方向。为探究DAS监测过程中暗光纤周围岩土介质含水率变化对DAS振幅响应特性的影响,设计了小球撞击圆盘与小球直接撞击砂土面二种激振方式下五种质量含水率0%、5%、10%、15%和20%砂土的DAS振幅响应试验。试验结果表明:(1)小球自由下落撞击砂土面激发的振动信号和传感光纤圆环布设可提高DAS信噪比,试验装置可靠性高、试验结果重复性好;(2)受砂土似黏聚力和声波传播衰减两个因素共同影响,DAS信号振幅随砂土含水率变化存在一个临界含水率。当砂土含水率小于临界含水率时,DAS信号振幅随含水率增大而减小,而当砂土含水率大于临界含水率时,DAS信号振幅随含水率增大而增大;(3)因砂土似黏聚力作用和小球撞击时能量转化的差异,小球与砂土的接触形式对DAS振幅响应有显著影响。研究结果为城市地下工程安全动态DAS精细监测提供理论依据。

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.