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

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

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.