A New Technology for Seismographic Observations · 2011. 6. 29. · A New Technology for...
Transcript of A New Technology for Seismographic Observations · 2011. 6. 29. · A New Technology for...
The Optical Seismometer:
A New Technology for Seismographic Observations
Mark ZumbergeJonathan Berger
Presentation to CTBTO S&T 2011
The Earthquake Spectrum
• Resolve Noise Model:
• 10-12 m /√Hz at 10 Hz
• 10-11 ms-2/octave at 200 seconds
• Several mm maximum displacement
From Prospects for Low frequency Seismometry, 2005
• K is the spring stiffness (N/m)
• γ Viscous friction (Ns/m)
• X(t) time dependent ground motion
• Y(t) absolute mass motion
• Z(t) mass to frame (ground) displacement
• z(t) = y(t) - x(t)
As seismologists, we are interested in x(t), the actual ground motion!
The Seismometer Equation
Response of Seismometer Suspension
Force-Balance Seismometer
New Vertical Component Optical Seismometer
Mechanical geometry by Wielandt - leaf spring design
Optical system by Zumberge
Michelson InterferometerDisplacement Transducer
Ellipse Pattern
Fringe:- 3 x 10-7 m
Desired Resolution:- 10-6 fringe
Ellipse Fitting Algorithm• X(t) = X0 + A*Sin(p(t) + p0); Y = Y0 + B*Cos p(t)
• Sample X and Y at ~105 sps
• Represent ellipse in nonparametric form by eliminating pfrom these equations.
• Use linear least squares to find the 5 ellipse coefficients over a ~ 100 second epoch.
• Solve for optical phase, p(t), at 105 sps
• Decimate to desired sample rate (~400 sps)
Femtometer
Functionality
1. Digitize x, y at 100,000 sps
2. Time stamp3. Fit ellipse4. Solve for optical
phase (100,000 arctancalculations/s
5. Filter6. Format, store, and
forward data
Example of Raw Output
Comparison of iSTS1 with Standard STS-1
Long-Period Noise comparison of iSTS1 and STS1
New DesignIn-line Vertical Component
Monolithic, electrical discharge machined (EDM)suspension springs
Comparison of in-line Optical Seismometer with Standard Trillium-240
Comparison of in-line Optical Seismometer with Standard Trillium-240
Horizontal-Component Optical Seismometer
Monolithic, electrical discharge machinedFlexure
Comparison of iTilt with Standard Trillium and STS-1
Variance Ratio = 1.05e-3
Comparison of iTilt with Standard Trillium 240
Some Advantages of the Optical Seismometer
1. The optical sensor is a linear, high-resolution displacement sensor, providing about a 30-bit resolution digital output without a high-resolution analog-to-digital converter.
1. Bandwidth and resolution sufficient to resolve the GSN low noise model from DC to > 15 Hz.
2. Dynamic range sufficient to record the largest teleseisms and most regional and local earthquakes.
3. No electronics in the sensor package. Only optical fiber connections to the seismometer. Eliminates heat and noise pickup from connecting electrical cables.
Some Advantages of the Optical Seismometer
5. Smaller package — our design is applicable to both vault and borehole installations and should be relatively easy to manufacture.
6. It measures absolute displacement referenced to the wavelength of light.
5. Unlike standard feedback seismometers whose response and calibration are dependent upon numerous electronic and mechanical components, the calibration and response the optical seismometers are simple using only three free parameters that can be determined at any time through examination of the data.
Absolute Calibration with tides
Comparison of iTilt with Standard Trillium 240
Example of Ellipse Fits
Two-Component Horizontal Borehole Optical Seismometer