Reduction of noise on broadband ocean-bottom seismographs through sensor design optimization using numerical and laboratory studies

地震学是了解地球内部结构和动力学的主要工具。然而，由于深海盆地的极端环境和地处偏远，海底的地震覆盖范围很有限。在过去的十年里，宽带海底地震仪（OBS）已经被开发出来，可以在任何海洋研究船上部署，自动记录超过一年的数据，并从海底回收。虽然这些仪器对地面运动的垂直分量的数据质量接近陆地上类似的临时部署，但记录剪切波的水平分量受到海底水流倾斜引起的高噪音水平的困扰。水平分量的数据质量差是宽带海底地震学的一个严重限制，并妨碍了我们研究地球内部动力学和海底发生的产生地震、火山活动和海啸的重要构造过程的能力。传感器封装的埋藏有效地降低了OBS水平组件的噪音水平，但成本过高，需要远程操作车辆（ROV），并且每次部署和回收都增加了船舶时间。一种更具成本效益的替代方案是通过减少与水柱接触的传感器封装的表面积来减少传感器封装与水柱的耦合。这现在可能成为可能，因为Nanometrics地震仪器公司已经生产出一种紧凑的（直径90毫米，高128毫米），低功率（150兆瓦）宽带（0.083-100赫兹）地震仪包，它应该是宽带海底被动和主动地震学的理想选择。这种较小的传感器的体积是类似带宽模型的27%，并且需要一个小得多的压力外壳和平衡系统。我们建议研究是否一个更小的剖面流体动力稳定传感器封装或仪器“伪埋”可以产生显著改善水平分量噪声水平。在开发的第一阶段，我们将利用理论知识结合流体流动的数值模拟来提出一种传感器封装形状，该形状有助于控制下降姿态、着陆，并在传感器就位时将流动引起的噪音降至最低。在第二阶段，我们将构建原型传感器，并使用模拟流体实验比较和对比宽带地震传感器的保真度。在模拟实验中，我们还将通过模拟将传感器部署在充满沉积物的袋子中来减少包与水柱之间的耦合，从而对传感器包的“伪埋藏”进行实验。如果成功，这项研究将解决海底地震学中一个长期存在的问题，并将是我们理解地球内部剪切速度结构能力的巨大飞跃。这些实验是将NERC海底仪器设备（OBIF）能力扩展到宽带海底地震学的第一步。

Seismology is the main tool for understanding the structure and dynamics of the Earth's interior. Yet, due to the extreme environment and remoteness of the deep ocean basins there is a dearth of seismic coverage on the ocean bottom. Within the past decade broadband ocean-bottom seismographs (OBS) have been developed that can be deployed from any oceanographic vessel, record data autonomously for periods of over a year, and be recovered from the seafloor. While the data quality for these instruments for the vertical component of ground motion approaches that of similar temporary deployments on land, the horizontal components, which record shear waves, are plagued by high noise levels caused by tilting due to ocean-bottom currents. The poor data quality of the horizontal components is a serious limitation of broadband ocean-bottom seismology and hinders our ability to study both the dynamics of the Earth's interior and important tectonic processes that occur on the ocean floor, which produce earthquakes, volcanism and tsunamis. Burial of the sensor package effectively reduces noise levels on the OBS horizontal components, but it is prohibitively expensive, requiring an remotely operated vehicle (ROV) and increased ship time for each deployment and recovery. A more cost effective alternative is to reduce the coupling of the sensor package with the water column by reducing the surface area of the sensor package in contact with it. This may now be possible because Nanometrics Seismological Instruments Inc. has produced a compact (90 mm diameter, 128 mm height), low power (150 mW) broadband (0.083-100 Hz) seismometer package that should be ideal for broadband ocean-bottom passive and active source seismology. This smaller sensor is 27% the volume of similar bandwidth models, and would require a substantially smaller pressure housing and gimbaling system. We propose to investigate whether a smaller profile hydrodynamically stable sensor package or instrument 'pseudo-burial' can yield significant improvement in horizontal component noise levels. In the first stage of development we will use theoretical understanding coupled with numerical simulations of fluid flow to propose a sensor package shape that helps controls descent attitude, landing, and minimises the flow induced noise when the sensor is in place. In the second phase we will construct prototype sensors and compare and contrast the fidelity of a broadband seismic sensor using analogue fluid experiments. In the analogue experiments, we will also experiment with 'pseudo-burial' of sensor packs by simulating a deployment of the sensors in sediment-filled bags to reduce coupling between the pack and the water column. If successful, this research will solve a long-standing issue in ocean-bottom seismology, and will be a huge leap forward in our ability to understand the shear velocity structure of the Earth's interior. These experiments are first steps towards expanding the NERC Ocean-Bottom Instrumentation Facility (OBIF) capabilities into broadband ocean bottom seismology.