可探测地震前兆次声波的光纤声传感装置的研究

Earthquake prediction has always been an international subject. In order to improve the prediction success rate, the development of multi-field measurements and new seismic precursory instruments is needed. Among them, it has been attract many attentions by using the pre-earthquake infrasound signal to predict earthquake. Literature shows that most of the frequency components of pre-earthquake infrasound signals concentrate on the range of 0.001-0.01Hz, with the intensity of 10-200pa. Recently，though capacitance infrasonic sensor has been developed to measure these signals, it has been believed that fiber-optic acoustic sensors are more promising since they have many advantages over the electrical acoustic sensors. These advantages include a higher sensitivity based on optical interference, smaller damping in long distance transmission , an easier realization for remote sensing，a higher stability from EM interference, a smaller size and a lighter weight，and an easier operation for distributed measurements. Therefore, this subject will focus on the development of a fiber-optic infrasound sensor senstive to the earthquake precursory infrasonic wave. In this device, the infrasonic signal is first converted to changes in spectral or light intensity by using optical interference, whose spectrum characteristics is then extracted for signal identification. The expected technical indicators of this sensor include a 3dB frequency range from 0.001Hz to 1 Hz, a resolution of the sound pressure change smaller than 1 pa, a dynamic range greater than 30dB, further more, the device should be not sensitive to vibration and air disturbance. The technical difficulties of this subject is structure and size design of the sensitive film to satisfy the demands of extremely low frequency detection.

对地震进行预测预报是一个国际性课题。为了提高预测成功率，需要进行多场测量，发展和研制新的地震前兆仪器。其中，利用震前次声信号进行地震预测受到研究人员的关注。资料显示：震前次声信号频率成分集中在0.001-0.01Hz区间，强度一般可以达到10-200pa。在该频段，目前国内已有电容型次声传感器问世。与基于电器件的声传感器相比，光纤声传感器分辨率高，对信号的远距离传输衰减小，易于实现遥感遥测；对电磁干扰不敏感；重量轻体积小，便于实现分布式测量，极具发展前景。本课题致力于研制针对地震前兆次声波的光纤声传感器。研究方案：利用F-P光学干涉原理，将次声波动转换为光谱变化或光强变化并进行信号特征提取和识别。研究目标为：3dB频响范围 0.001Hz-1Hz，分辨率<1pa，动态范围>30dB，且对振动和风动不敏感。研究难点：对敏感薄膜进行结构和尺寸设计，使其满足极低频探测的需求。

地震预测的低成功率使之成为世界公认的科学难题，随着学科之间的交叉融合，学科边界也越来越模糊，各国都在寻找新方法对地震前兆信号进行多场测量，获取高精度、高覆盖率、近实时的观测数据，为“大地”诊脉。近年来，各台站和研究机构观测到了很多震前次声信号，其频率成分大部分集中在 0.001-0.01Hz 区间，强度可达 10-200pa。这些观测资料引发了我们利用次声信号进行地震预报的设想。 . 本项目的目标是研制出能探测到极低频地震前兆次声波的光纤声传感器，具体研究内容为：针对光纤FP腔结构，设计制作出对极低频气压信号敏感的高反射率压力传感薄膜，并进行传感器的性能测试。目前已开展的研究工作有：压力传感薄膜的理论模拟与设计加工，探头设计与加工，极低频气压变化舱（模拟次声引起的气压变化）的设计与制作。. 本课题的研究围绕薄膜设计与制作展开，分析表明将弹性模量大且反射率高的金属薄膜（硬膜）置于弹性模量小的薄膜（软膜）中心处做成双层复合薄膜结构，可以同时提高薄膜形变量和光纤FP腔的q值。取天然橡胶为软膜材料对复合膜进行了ANSYS模拟，结果显示：薄膜形变集中在外侧软膜层处，中心金属膜处形变极小。对薄膜中心形变量起决定作用的是软膜厚度，金属膜材料、厚度与直径变化产生的影响则很小，另外橡胶膜厚度超过4um时，在较大的气压范围内，中心形变量与气压为线性关系，灵敏度在nm/pa量级，且在低频0.001~1Hz区间有极其平坦的频响特性。在理论模拟基础上，我们取PDMS为软膜材料，采用MEMS技术加工了内嵌式和外凸式两种结构的薄膜，从工艺过程来看，内嵌式薄膜更容易控制软膜层的均匀性以及金属层的平整度以提高器件质量。. 本课题的创新点在于薄膜的结构设计与制作，模拟结果对实际加工时的尺寸选择有一定的指导意义。另外，摸索出金属——PDMS两层复合薄膜的制作工艺与传感器探头制作方法，为进一步研究打下基础。