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偏远地区，海底的地震覆盖范围很少。在过去的十年中，开发出了宽带海底地震仪(OBS)，它可以从任何海洋学船只上部署，自动记录一年多的数据，并从海底恢复。虽然这些仪器的地面运动垂直分量的数据质量接近陆地上类似临时部署的数据质量，但记录横波的水平分量受到海底洋流倾斜造成的高噪声水平的困扰。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，阻碍了我们研究地球内部的动力学和发生在海底的重要构造过程的能力，这些过程产生了地震、火山和海啸。埋设传感器组件有效地降低了OBS水平组件上的噪声水平，但其成本高得令人望而却步，需要远程操作运载工具(ROV)，并增加了每次部署和恢复的船舶时间。更具成本效益的替代方案是通过减少传感器包与水柱接触的表面积来减少传感器包与水柱的耦合。这现在可能是可能的，因为纳米地震仪器公司已经生产出紧凑(直径90 mm，高度128 mm)、低功率(150 MW)、宽带(0.083-100赫兹)的地震仪组件，应该是宽带海底被动和主动震源地震学的理想选择。这种较小的传感器的体积是类似带宽型号的27%，需要的压力外壳和万向平衡系统要小得多。我们建议调查一个较小的外形流体动力学稳定的传感器组件或仪器“伪埋”是否可以显著改善水平分量噪声水平。在开发的第一阶段，我们将结合理论理解和流体流动的数值模拟，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，我们将构建传感器原型，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，我们还将通过模拟传感器在充满沉积物的袋子中的部署来减少传感器包与水柱之间的耦合，从而进行传感器包的“伪埋”实验。如果成功，这项研究将解决海底地震学中的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.