Development of an integrated Borehole Geodetic and Seismic Sensor: Project Completion

地震仪测量地震引起的地面震动，强度从人类无法检测到的微小震动到对建筑物和人构成危险的破坏性大地震。很难捕捉到的是来自附近震源的非常小的震动，或者是大事件之后的超低频振动，导致地球在之后的许多天里像钟声一样响起。我们一直在努力改进地震仪，以便能够记录如此广泛的信号。研究它们可以提供更多关于地球内部情况的信息。地震波是让我们看到地球内部的“光”，而地震仪是让我们创建地球内部结构图像的“眼睛”。除了震动，地震和相关现象还会导致地壳缓慢、逐渐的变形。要检测这些变形，需要极其精确的传感器。通常会发现两种变形：一种是地面相对于局部垂直方向的微小变化，称为“倾斜”；另一种是地面的伸长或压缩，称为“应变”。这些并不是无关的，但构造活动引起的信号类型通常非常小--以十亿分之一的倾斜度和应变来测量。埃拉斯科学仪器和设施计划部门奖支持开发一种高灵敏度的光学检测方法和仪器，以测量由悬挂在井底的外壳中的钟摆或弹簧悬挂的质量的运动。光纤将激光向下发送到传感器外壳，并在光线从连接到质量块的镜子反射后返回。通过用电子学和位于井眼外的一台小型计算机分析这种光，研究人员可以检测到与单个原子直径相当的质量运动，从而检测到与地震相关的活动引起的微小地震震动和地面变形。随着研究人员对地震过程了解得越多，有一天就越有可能对地震发生的地点和时间做出准确的预测。这项合同的支持将允许制造和安装一个钻孔系统，该系统利用光纤干涉在一个钻孔中提供：(A)宽带垂直地震仪/重力仪，(B)宽带双分量水平地震仪/倾斜仪，以及(C)低噪声垂直长基线应变仪。该组合系统将能够测量垂直和水平地面速度、重力、倾斜和应变，其灵敏度在从10赫兹到许多天的时间范围内与任何现有系统相比都是有利的；井下组件完全是无源的，使仪器寿命更长，并能耐受井下高温。该仪器将安装在Pinon Flat天文台的一个现有钻孔中，以进行测试，并与那里已经运行的地震和应变系统进行比较。组合仪器有望替代独立地震仪、GPS接收器、重力仪、倾斜仪和应变仪的多仪器观测。预计地震观测将满足全球地震台网目前的要求，大地测量将在短于几周的时间内提供比全球定位系统更低的噪声观测。目标应用可包括研究地壳形变的动力学，包括缓慢滑动事件、连续和间歇性滑动，以及已知发生但不会产生破坏性地震的其他地球运动。部署的仪器可以帮助了解岩浆和地下流体动力学(例如，碳氢化合物提取和二氧化碳封存)对地壳变形的影响。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Seismometers measure the shaking of the ground caused by earthquakes, ranging in intensity from tiny tremors undetectable by humans to large damaging earthquakes that pose a hazard to structures and people. It is the very small shaking from nearby sources, or the ultra-low frequency vibration following large events that cause the Earth to ring like a bell for many days afterward, that are difficult to capture. We are constantly trying to improve seismometers to be able to record this wide range of signals. Studying them can provide much information about what is inside the Earth. Seismic waves are the "light" that lets us see inside the Earth and seismometers are the "eyes" that let us create images of the internal Earth structure. In addition to shaking, earthquakes and related phenomena cause slow, gradual deformation of Earth's crust. To detect these deformations, extremely precise sensors are needed. Two types of deformation are often searched for: a very slight change in the position of the ground with respect to local vertical, called "tilt," and elongation or compression of the ground, called "strain." These are not unrelated, but the types of signals caused by tectonic activity are normally extremely small – measured by parts per billion in both tilt and strain. This Division of Erath Science Instrumentation and Facilities Program award supports development of a highly sensitive optical detection method and instrument to measure the motion of masses suspended by pendulums or a spring in a housing cemented into the bottom of a borehole. Optical fibers send laser light down to the sensor housing and return the light after it has been reflected from mirrors attached to the masses. By analyzing this light with electronics and a small computer located outside the borehole, the researchers can detect motions of the masses comparable to the diameter of individual atoms, and therefore detect both small seismic shaking and deformation of the ground caused by earthquake-related activity. As the investigators learn more about earthquake processes, the better are the chances of one day making precise forecasts of where and when they might occur.Support from this award will allow for fabrication and installation of a borehole system that utilizes optical-fiber interferometry to provide in one borehole: (a) a broadband vertical seismometer/gravimeter, (b) a broadband two-component horizontal seismometer/tiltmeter, and (c) a low-noise vertical long baseline strainmeter. The combined system will be able to measure vertical and horizontal ground velocities, gravity, tilt, and strain with sensitivities that compare favorably with any existing system over time scales from 10 Hz to many days; the downhole components are entirely passive, giving a long instrument lifetime and resistance to high downhole temperatures. The instrument is to be installed in an existing borehole at Pinon Flat Observatory for testing and comparison with the seismic and strain systems already operated there. The combined instrument promises an alternative to multi-instrument observations from independent seismometers, GPS receivers, gravimeters, tiltmeters and strain meters. The seismic observations are anticipated to meet the current requirements of Global Seismographic Network stations and geodetic measurements would offer lower noise observations than GPS at periods shorter than weeks. Target applications could include studies of the dynamics of crustal deformation including slow slip events, continuous and episodic slip, and other Earth movements that are known to occur but which do not generate damaging earthquakes. A deployed instrument could help to understand the effects of magmatic and subsurface fluid dynamics (e.g., hydrocarbon extraction and CO2 sequestration) on crustal deformation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.