Becker Matthew W, Coleman Thomas I
Department of Geological Sciences, California State University Long Beach, 1250 Bellflower Boulevard, Long Beach, CA 90840, USA.
Silixa LLC, 16203 Park Row, Suite 185, Houston, TX 77084, USA.
Sensors (Basel). 2019 Apr 27;19(9):1975. doi: 10.3390/s19091975.
The solid Earth strains in response to the gravitational pull from the Moon, Sun, and other planetary bodies. Measuring the flexure of geologic material in response to these Earth tides provides information about the geomechanical properties of rock and sediment. Such measurements are particularly useful for understanding dilation of faults and fractures in competent rock. A new approach to measuring earth tides using fiber optic distributed acoustic sensing (DAS) is presented here. DAS was originally designed to record acoustic vibration through the measurement of dynamic strain on a fiber optic cable. Here, laboratory experiments demonstrate that oscillating strain can be measured with DAS in the microHertz frequency range, corresponding to half-day (M) lunar tidal cycles. Although the magnitude of strain measured in the laboratory is larger than what would be expected due to earth tides, a clear signal at half-day period was extracted from the data. With the increased signal-to-noise expected from quiet field applications and improvements to DAS using engineered fiber, earth tides could potentially be measured in deep boreholes with DAS. Because of the distributed nature of the sensor (0.25 m measurement interval over kilometres), fractures could be simultaneously located and evaluated. Such measurements would provide valuable information regarding the placement and stiffness of open fractures in bedrock. Characterization of bedrock fractures is an important goal for multiple subsurface operations such as petroleum extraction, geothermal energy recovery, and geologic carbon sequestration.
固体地球会因月球、太阳和其他行星体的引力作用而产生应变。测量地质材料对这些地球潮汐的挠曲情况，可以获取有关岩石和沉积物地质力学特性的信息。此类测量对于理解坚硬岩石中断层和裂缝的扩张尤为有用。本文介绍了一种利用光纤分布式声学传感（DAS）测量地球潮汐的新方法。DAS最初设计用于通过测量光纤电缆上的动态应变来记录声学振动。在此，实验室实验表明，利用DAS可以在微赫兹频率范围内测量振荡应变，该频率范围对应半日（M）月球潮汐周期。尽管实验室中测量的应变幅度大于地球潮汐预期产生的应变幅度，但仍从数据中提取出了清晰的半日周期信号。随着在安静野外应用中预期的信噪比提高以及使用工程光纤对DAS的改进，利用DAS有可能在深钻孔中测量地球潮汐。由于传感器的分布式特性（在数公里范围内测量间隔为0.25米），可以同时定位和评估裂缝。此类测量将提供有关基岩中开放性裂缝的位置和刚度的宝贵信息。基岩裂缝的表征是石油开采、地热能回收和地质碳封存等多种地下作业的重要目标。
