We have been developing a new type of seismometer for several years. We use a new technology that relies on optical interferometry in place of the traditional electronic displacement transducer to measure the motion of an inertial mass. This technology offers significant advantages over the conventional electronic-feedback seismometers in wide use today. The most important ones are: simple and stable calibration, good performance well below seismic frequencies (for tidal observations, for example), the possibility to operate at very high temperature, and the capability to operate in a borehole without the need for down hole electronics.
The new seismometers use optical fibers to carry laser light to a sonde housing a pendulum (for horizontal sensors) or a mass suspended from a spring (for vertical sensors). A polarization sensitive Michelson interferometer with one of two reflectors mounted on the pendulum (or spring-mounted mass) tracks the displacement. Two optical fibers carry fringe signals in quadrature to photodetectors and a signal processor which transforms the optical signals to mass displacement. This signal is further analyzed to yield ground acceleration.
In what we expect will be the final phase of the sensor development, we plan to build a new vertical suspension, increase the dynamic range, improve the optical signal analysis, add laser wavelength control, and investigate thermal compensation. In addition we will study noise sources and establish corrections for non-linearity and cross coupling.
(2) Non-technical description
A seismometer is a very simple device ? it is a mass on a spring. Ground shaking is detected by recording the position of the mass relative to its housing. However the level of interesting ground shaking varies over an enormous range, from nearby earthquakes, which can cause shaking with an amplitude of many centimeters, to the minute motions (less than an atomic diameter) caused by the ringing of the entire Earth. A global seismic network (GSN) consists of over a 100 stations worldwide to record these tiny seismic signals. Analysis of them is the most effective means we have to study Earth?s interior.
Most seismometers use electronics to record the mass motion. While this has been generally successful, electronics have some limitations and many difficulties come with electronic systems operating continuously in remote and rugged environments. We have invented an alternative means using laser light and optical fibers to measure the minute mass motions.
The research proposed here will further advance this new technology into a practical, well understood, fully characterized, reliable, borehole and vault deployable set of broadband seismometers. Minimization of size, power requirement, and cost will be emphasized while maintaining the performance goals of the GSN.
A new technology has been developed and tested that uses interferometry to measure the motion of a seismometer’s inertial mass instead of the traditional electronic displacement transducer. This technology offers significant advantages over the conventional feedback seismometers in wide use today. These advantages include: -- The optical sensor used to measure the seismometer mass’s displacement is a linear, high-resolution digital displacement detector, which measures displacement referenced to the wavelength of the laser light and provides about a 30-bit resolution digital output without the requirement of a high-resolution analog-digital converter; -- The bandwidth and resolution are sufficient to resolve the Global Seismic Network (GSN) low noise model from DC to > 15 Hz with a dynamic range sufficient to record the largest teleseisms and most regional and local earthquakes; -- The technology allows the laser and other electronic elements to be located hundreds of meters from the seismometers with the only connection made by optical fiber, thus eliminating heat from electronics in the sensor package, noise pickup from connecting electrical cables, and susceptibility to lightning strikes; -- Unlike standard feedback seismometers whose response and calibration are dependent upon numerous electronic and mechanical components, the calibration and response of seismometers utilizing the new technology are simple using only three free parameters that can be determined at any time through examination of the data. The Intellectual Merit is the investigation of this new technology and its efficacy. The Broader Impact is the determination of the suitability of this methodology to support the Global Seismic Network and other broadband borehole seismic networks. The research proposed here has advanced this new technology into a borehole three-component seismometer that is currently being recorded in a 60-m-deep borehole and appears to meet the goals of the Global Seismic Network’s requirements.
