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Research Projects

Optical Seismometer

Experiments with an Optical Seismometer

Mark Zumberge, Jonathan Berger, and Jose Otero;

Scripps Institution of Oceanography, University of California, San Diego

Erhard Wielandt;

Institute of Geophysics, Stuttgart University, Stuttgart, Germany

ABSTRACT

Modern seismometers rely on electronic displacement transducers to sense the motion of an inertial mass suspended by a spring. The more sophisticated systems use electrostatic or electromagnetic force-feedback on the inertial mass to ameliorate the shortcomings of the spring and the displacement transducer.

Recent advances in optical fiber technology and digital signal processing offer an alternative to the modern observatory seismometer. We have recently developed an optical fringe resolver to replace the electronic displacement transducer that promises to lead to a greatly improved seismometer. The use of optical fiber interferometry in place of electronics adds other important benefits, including immunity to noise pickup, simplification of remote deployment (in a borehole, for example), the elimination of a heat source in the seismometer-an important cause of noise in the best existing systems, and elimination of electrical connections between the seismometer and the recording system.

Our first test of this concept was to apply it to a standard STS-1 seismometer. For this experiment, we added interferometric components to the seismometer frame and a retroreflector to the seismometer's mass. We removed the feedback electronics and recorded the STS-1 mass displacement with our new interferometric system. Simultaneously we recorded the output of a standard STS-1 set up on the same pier. The results, which include observations of large teleseisms and microseisms, indicate that the new technique is promising. In our second experiment, we measured the inherent noise floor of the displacement transducer. In a 100 Hz bandwidth, the RMS noise was approximately

This, when applied to a mass-spring suspension having a 5.4 s period and a Q of 7.4 will resolve the USGS ground noise model up to at least 15 Hz.

The use of optical fiber interferometry rather than traditional electronic displacement transducers affords significant advantages, including:
  • A linear, high-resolution displacement detector - the proposed optical sensor includes the functionality of a digitizer providing about a 30-bit digital output;
  • Absolute displacement measurement referenced to the wavelength of light;
  • Bandwidth sufficient to resolve the USGS Low Noise Model from DC to > 15 Hz;
  • Dynamic range sufficient to record the largest teleseisms and most regional and local earthquakes;
  • Minimum electronics in package - only optical fiber connection to the seismometer, minimizing heat from electronics in the sensor package and noise pickup from connecting electrical cables;
  • Smaller package - our design will be applicable to both vault and borehole installations and should be relatively easy to manufacture.

IRIS Design Goals

  • Clip level of 5.8 x 10-3 m s-1 RMS over the band 10-4 Hz to 15 Hz.
  • Bandwidth of 10-4 Hz to about 15 Hz.
  • Seismometer linearity of 90 dB or greater.
  • Calibrations good to 1% and gain stability of 1% between calibrations.
  • Sensitive axis orientation accurate to 0.6 degrees (minimum)

    • We believe that our new desing will substantially meet most if not all these goals. At the very least, the design will offer an alternative to the existing very-broadband seismometers.


    Figure 1: Conceptual mechanical design for vertical component Optical Seismometer.


    Design Considerations




    Optical Fringe Resolver




    Figure 3. The quadrature fringe signals. In (a) we plot the two signals vs. time. In (b) we plot x and y against each which yields an ellipse. Increasing (decreasing) the path length difference causes the x-y ordered pair at any instant to move clockwise (counterclockwise) around the ellipse. It is this position on the ellipse that we want to record.

    Fringe Resolver Specifications





    Figure 4. Smoothed interferometer and electronic noise spectra compared with the USGS Low Noise Model acceleration spectrum passed through Equation (1) with T= 5.4 seconds and Q = 7.4, which corresponds to the STS-1. Spikes in the spectra are believed to be an artifact of the fringe-processing scheme.


    Figure 5. There are two limits to the new design. First, the mass stops limit mass motion to about ± 1 cm. Second, the optical fringe resolver will lose track of the mass when the velocity exceeds 7.5 cm/s. The red curve shows how these limits translate to large accelerations (examples of which are shown with the green curves) with our prototype suspension.

    Pre-Prototype: A Modified STS-1



    Figure 6. Photographs of the experiment we carried out comparing a standard STS-1 to one in which we replaced the electronics with optics. Both seismometers were situated on the same pier. The first was operated normally. In the second, the electronic position sensor was disconnected, and the forcing coil was shorted internally to provide damping. No electrical connections were made to the modified STS-1; laser light entered and exited through a window in the seismometer's vacuum jar to provide the position information.


    Figure 7. Seismograms recorded with both the modified Optical STS-1 and a standard STS-1 from a magnitude 6.7 event off South Georgia Island on 15 Nov. 2002 (about 115° away from San Diego). The signal at IGPP reached an amplitude of ±2.5 ¥ 10­p;4 m of mass motion, very close to the clip level of the STS-1 seismometer but well within the dynamic range of the Optical Seismometer.


    Figure 8. We have new facilities to test the prototype seismometer. We have precision laboratory shake tables in our lab, and we are constructing an underground seismic test vault at Piñon Flat Observatory.

    Prototype Vertical Component Optical Seismometer


    Figure 9. Prototype Vertical Optical Seismometer. This unit has a mass of 360 grams and a free period of a few seconds. The spring is a single strip of "NiSpan-C", a trade name for a particular alloy of iron-nickel with small amounts of chromium and titanium.


    Figure 10. (a) displays the ring-down of the prototype vertical seismometer. The shape of the curve is governed by the damping and the restoring force of the spring. Numerically correlating the computed acceleration with position yields the spring constant; correlating acceleration with velocity yields the damping. (b) shows the restoring force (after correcting for damping) as a function of mass position. The slope of this is the spring constant k over the mass m;
    period
    (c) shows the non-linear portion that results from the geometry of the leaf-spring suspension.