kristoffer t. walker, mark zumberge, michael hedlin, jon berger, and peter shearer

Resolving infrasound signals with arrays of optical fiber infrasound sensors (OFIS): Low wind noise, superb back

azimuth (and elevation angle) resolution, and a compact design

Kristoffer T. Walker, Mark Zumberge, Michael Hedlin, Jon Berger, and Peter Shearer

Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego

This work is funded by the U.S. Army Space and Missile Defense Command

See also OFIS poster (different paper).

DISTRIBUTION A. – Approved for public release; distribution unlimited

Motivation: Why an OFIS?

• Last 4 years: a. further development research to optimize the sensor. b. Signal direction determination with field experiments at

PFO, synthetic experiments, and theoretical calculations.

• We seek to improve wind noise reduction, instrument response, phase velocity direction resolution, and more compact array designs

• Summary:• OFIS technology has matured• Phase velocity direction resolution is better• Instrument response is always better• Wind noise reduction: more research needed• More compact array designs than rosette pipe arrays• Real time calibration permissible• No resonance issues• Well suited for permanent arrays

What is an OFIS – Optical Fiber Infrasound Sensor

OFIS is a Directional Microphone (similar to satellite dish)

Laser Interferometry Base: Low self noise, high dynamic range (real*64)

OFIS – Optical Fiber Infrasound Sensor


60- m OFIS Recording of Atlantis Reentry at Camp Elliott (2007)

Typical N-wave imprinted on top of long-period calibration signal


Vandenberg Delta II recorded at I57US

Three sensors: - I57US L2 pipe array - porous hose - buried 90-m OFIS (perpendicular to back azimuth)

The OFIS and hose array recorded the signal well. The pipe array had a resonance issue.


Low Self Noise, High Dynamic Range: ~1 m/sOFIS “self noise” lower than MB2K.

Microbaroms apparent.

OFIS “Acoustic noise floor” lowest.

Ambiguity with L2 porous hose and MB2K noise floor?

OFIS might be quieter; TBD with calibration/noiseexperiment.


Wind Noise Reduction: ~3 m/s

Wind noise increases at all frequencies.

Microbaroms not apparent anymore.

OFIS and hose “Acoustic noise floor” match.

OFIS is at least just as good at wind noise reduction.


Acoustic/Infrasound Sensing Directionality



Time delays lead to omnidirectional attenuation at

grazing elevation angles.

70 m rosette (I57US L*)


OFIS versus Pipe Array Response

70 m OFIS vs. 70 m rosette filter (I57US L2)

In terms of signal detection for 5-36 array, “worst” positioned OFIS always

attenuates about the same as rosette.

Best positioned OFIS has response at least as good as

the blue dashed line.

Same response for all pipe array BAZ. Must go to H*

pipe arrays for less attenuation, but at cost of

wind noise reduction.

All exploit “spectral resolution” and can include time-separation resolution

1.Water Level Deconvolution Beamforming (WLD)a. Typical decon. Yields BAZ, ELE, signal. Moderate speed.

2. Simultaneous Array Deconvolution Beamforming (SAD)a. Least-squares inversion decon. Yields BAZ, ELE, signal. Slowest.

3. Common Spectrum Identification Beamforming (CSI)a. Convolution-only approach. Yields BAZ, ELE. Fastest.

All these techniques are fast enough to run in real time.

3 Instrument-Response-Dependent Beamforming Techniques:

Pinpointing Phase Velocity Direction


Water Level Deconvolution Beamforming (WLD)

Simultaneous Array Deconvolution Beamforming (SAD)


Common Spectrum Identification (CSI)


Real Data Tests: PFO 3-120º OFIS versus 3-element I57US H* subnet

CSI has smallest 95% uncertainty region, greatest range, and smoothest misfit.CSI likely more accurate than TDB (3 elements of I57US H* subnet).

Reason for the consistent offset: airplane signal and systems not collocated.


CSI yields results that are more consistent with I57US H* subnet.CSI OFIS BAZ has a mean deviation from TDB of 0º.



81 Signals with elevation angles < 60º

Airplane Signal at PFO

SNR ~ 3 (10 dB)

CSI works well at high elevation angles

as well.

Reason for offset: not collocated sensors.



Synthetic Data Results

Time separation is not needed for the 5-72º config


Flexibility and Compactness

5-72º OFIS Configurations: Same Spectral Resolution Power

a-d: (5-72º) same spectral and time separ. res.

e: (5-36º) no time separ. res., but most compact design (75% less area)

Three Possible OFIS Array Configurations

OFIS Array Layout

Data Flow and Operational Management

• The details of the interferometer are transparent to the data processer, although are available if desired.

• Real-time system inherited from the IDA global seismic network data hub (ISI – Ida System Interface).

• Pressure ISI packets stream to any local unix/linux OS from remote locations in real time (< 2 s delay) and can be converted in real time to other formats (e.g. CSS3.0).

• Data acquisition control is through a simple web interface that also contains a real-time JAVA waveform plotting option. Single/double precision, sampling rate, output channels are all options.

• State of health and log files can be queried/investigated with the same web interface.

• 100% uptime with a Sabeus Inc. wavelength-locked laser that is remotely controllable.

OFIS Array JAVA Control Interface

Has been tested for a year now and is stable.

Works with Data Translation Inc. 8-channel DSPs.

Can control up to 4 OFIS arms.

Uses security certificates instead of passwords.

System is also used for an optical seismometer and strainmeter.

Conclusions

• OFIS technology has matured in the last 3 years and is cable of operational use in a permanent array setting.

• A 3-120º OFIS array is just as capable as the I57US H* subnetwork in resolving phase velocity direction (baz and elev) with CSI for all SNR. Could ignore CSI altogether.

• The optimum OFIS array has at least 5 linear arms at carefully positioned azimuths.

• A 5-36º OFIS array can record signals with a much better signal to noise ratio than the same diameter rosette arrays.

• A single 90-m buried OFIS has the same wind noise reduction capabilities as an entire 70-m rosette array (I57US, L2, 3.5 m/s wind).

• OFIS can be used in many ways for a permanent array depending on the monitoring frequency range of interest, required wind noise reduction, and site logistics.

• OFIS now has a professional real-time data flow system and remote control interface.

~1 m/s

What’s Next?

Optimizing codes for real-time event detection experiment with video ground truth: Camp Elliott 5-72º array next to Miramar USMC Air Station (2007-2008)

Array consists of buried 60-m OFIS arms with real-time calibrators.

[also see poster]

M-code signal generation to further investigate wind noise reduction capabilities of OFIS and other technologies at Elliott.

M-code with a portable speaker array can generate 8 Hz that is detectable at a range of 250 m.

What’s Next?

• M-sequence research at Elliott to measure instrument response at various angles and frequencies.

• Real-time beamforming at Elliott with collocated video cam.

• Wind noise reduction experimentation with Chaparral sensors at Elliott and PFO with a very long OFIS.

• Seek remote locations and/or other researchers to independently evaluate/monitor an OFIS array.

• Continue measurements and experiments that lead to further optimization and ruggedization of the OFIS design and array configurations.

What’s Next?

• We installed a 2-90º OFIS array at PFO during 2003-2004. This array only worked well with signals of excellent SNR.

• We installed a 3-120º OFIS array at PFO during 2005-2006 (see this presentation). This array worked very well for signals of all SNR and was equivalent to the I57US rosette H* subnetwork.

• OFISs are inherently delicate at the end where the fibers attach to the electronics. The black external tube has been kicked, pulled, and stepped on by horses and humans—no significant damage occurred. However, just as rabbits can damage cables, mice have been known to damage OFIS fibers if they are exposed.

• A buried 5-72º OFIS array with 180-m aperture for streaming data to IGPP costs ~$50-60k in materials and labor including DSP, lasers, and electronics (not incl. UCSD 52% overhead).

• OFISs can have 100% uptime, are protected from rodents, and are quieter if they are buried beneath 6” of coarse gravel. They can still be used above ground though (e.g. WSMR II at station BACA).

• OFIS sensor technology has matured to the point that it can be used for general infrasound research in an operational setting.

Additional Notes

• We are not sure if calculating the array responses for a string of microphones is appropriate to evaluate OFIS array designs, but…

Additional Notes

• We are not sure if calculating the array responses for a string of microphones is appropriate to evaluate OFIS array designs.

Additional Notes



No time delays with OFIS arrays.

Each OFIS records a unique measurement of the signal phase

velocity direction (spectral resolution).

Able to obtain direction from an OFIS array the size of a single

rosette.

At least one arm records the signal just as well as H* rosettes, yet the

OFIS 90 m length gives better wind noise reduction.

90 m 5-36 OFIS array

kristoffer t. walker, mark zumberge, michael hedlin, jon berger, and peter shearer

Documents

ofis technology

azimuththe ofis

ofis perpendicular

best positioned ofis

worst positioned ofis

mswind noise

mb2k noise floor

low wind noise