Research Projects
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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 may lead to an 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 the following advantages:
- 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;
- 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
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.
People
Mark Zumberge
Jon Berger
Jose Otero
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