A great deal of our knowledge of the Earth's interior comes from seismology: the analysis of elastic waves propagating through the Earth. Seismometers measure these waves, and the science of building more and more sensitive seismometers extends back for many years. Almost all seismometers consist of a mass suspended by a spring. Usually an electronic circuit senses the displacement of the mass (with respect to the seismometer's frame) when a seismic wave produces a local vibration. Under continuing funding from the National Science Foundation, we are developing a new seismometer which uses optics instead of electronics to sense mass motion. New signal-processing electronics allow us to analyze laser light reflected from the seismometer mass and detect displacements as small as a trillionth of a meter. A variety of advantages are afforded from this new technique, including a high dynamic range and wide bandwidth, a calibration based on the wavelength of laser light, a connection to the sensor via fiber optic cables rather than electrical ones, and elimination of electronics in the sensor package. Our research over the next two years will include further noise-reduction efforts and independently monitored tests of our new seismometer at seismic observatories other than our own.
PROJECT OUTCOMES Development of a New Broadband Optical Seismometer: Phase 2. Principal Investigators: Mark Zumberge and Jonathan Berger Scripps Institution of Oceanography, University of California, San Diego Our knowledge of the Earth’s interior structure and the mechanics of earthquakes is critically dependent upon the observations of seismic signals. But the mainstay observatory-class seismometer used by global networks for the past two decades is obsolete and is no longer manufactured. In this project, we set out to develop a new observatory-class seismometer that is fundamentally different from those of the past in that we employ laser interferometry rather than traditional electronics to accomplish significant improvements. The interferometer provides a linear, high-resolution displacement sensor— the optical sensor includes the functionality of a digitizer providing about a 30-bit resolution digital output; Absolute displacement measurements referenced to the wavelength of light; Range of the instrument is sufficient to record the largest teleseisms and most regional and local earthquakes; Only optical fiber connections to the seismometer are required, eliminating heat from electronics in the sensor package and noise pickup from connecting electrical cables; Our new seismometer has shown that our optical technique works and that it has improved upon current technology. These improvements are important for advancing our knowledge of Earth because it will ultimately result in clearer seismic images of the interior. It will also provide for better observations from within boreholes, where the inherent seismic signal-to-noise ratio is better. In the process, we have developed a displacement sensor of sufficient sensitivity, dynamic range and bandwidth not only to satisfy the requirements of observatory-class seismology but with applications to other fields as well. It is our intention that this research will lead to a renewed supply of observatory-quality instruments and that the resulting enabling technology will lend itself to commercialization. In effect, this program will reduce the development time and risks required to bring such quality instruments to the commercial marketplace and thereby encourage suppliers into this small market. This work will help provide essential sensor technology for developing seismic networks such as EARTHSCOPE, IRIS GSN and PASSCAL, the Advanced National Seismic System, and the NSF’s Ocean Observatories Initiative.
