may 25, 2004csmip processing, shakal et al1 csmip strong motion data processing anthony shakal, moh...
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May 25, 2004 CSMIP Processing, Shakal et al 1
CSMIP Strong Motion Data Processing
Anthony Shakal, Moh Huang and Vladimir Graizer
California Strong Motion Instrumentation Program
California Geological Survey (was CDMG)
Sacramento, California
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CSMIP Processing Development • CSMIP began joint processing project with USGS
in late ‘70s (film scanning by Towill Co.- software devel. and processing at Lawrence Berkeley Lab)
• In early ‘80s standalone processing at CSMIP– Scanning system installed patterned after that
developed at Univ. Southern Calif. by Trifunac & Lee– Processing software of Caltech Bluebook project
(Hudson et al) as modified by Trifunac & Lee
• Software upgraded for production, with noise level improvement at CSMIP
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Uniform Processing – Guiding filter period selection based on Signal and Noise Spectrum (Trifunac, 1977)
Digitized accelerogram as sum of desired acceleration and background noise
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• Signal spectrum moves up and to the right with increasing magnitude
• Noise spectrum controlled by – digitization (by film scanner,
or by A-to-D converter)– sensor properties
• Initial filter corner estimate - above & left of junction, an SNR 2 or 3
• Final corner guided by time domain output of suite of runs having filter near this period
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Current CSMIP procedure is to use one filter corner for all components
• Pro: Multi-dimensional aspects can be studied by end user– Particle motion – Torsional response in structures
• Con: Period controlled by the lowest signal/highest noise channel (often vertical, or lowest n building)
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Steps in processing (analog)1. Baseline correction – minimal (remove mean;
perhaps remove slope) 2. Instrument correction3. High-frequency filtering (25 Hz Ormsby
classically)4. Initial integration & long period filtering5. Maximum-bandwidth response spectra6. Time-history suite for long-period filter
selection7. Final product preparation
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Example – Whittier analog record
• Digitize at 200 points/cm
• Digitize two fixed traces for reference-trace subtraction to remove film shift (earthquake) and film drift (canister) problems
• Use 2 pulse/sec time trace to correct film-speed change errors
• Digitize fiducial marks placed on film to control multiple-panel concatenation
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Example – Digitized (Vol. 1)
• Digitized, time-corrected, 200 pts/sec, scaled by sensitivity • Match-test to film image, check for offsets, drifts, panel-
junction effects
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Response Spectrum
• Response spectrum (Nigam-Jennings) of Vol.1 data
• ‘Wide-open’ bandwidth• Compare the long period
decay of signal spectrum with long-period noise
• Initial corner estimate
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Suite of time-histories
• Range of corners from 12 second to 2.5 sec period• Period chosen was 3.5 second
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Final Spectrum
• Filter corners given on plot
• Plotted only out to corner selected
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Usable Data Bandwidth
• 3 dB (half-power) points (whether Ormsby, Butterworth or other filter) define UDB for user
• User assumed to be knowledgeable, but not necessarily in data processing
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Digital Records• Frequency domain processing• Noise level controlled by A-to-D
converter’s effective number of bits (last bits often noise)
• In general, more dynamic range (72, 96 dB, or more vs ~50-60 dB)
• Sensor noise/drift more critical – the next focus in getting the most from recorded data
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Automatically Processed Record
Record processed automatically at the time of the earthquake (2 am Sunday morning, May 9, M4.4 off Santa Barbara).
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CISN Internet Quick Report – tied to automatically generated ShakeMap
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Summary (1)• CSMIP processing evolved from the Caltech
Bluebook project as extended at USC• Nearly 1000 records digitized/processed (1000s of
traces)• General approach goal is to release as much signal
as possible, with as little noise accompanying the signal as practical
• Using one filter corner per record means that results may be: – more conservative than another policy would yield; but – less difficult to use, for most users
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Summary (2)• No acausal filters routinely used; processing of special
problem or offset records done by hand, on case-by-case basis, or restricted band pass provided
• Automatic processing done by straightforward processing, with diagnostic checks/set-asides if potential problems (DC shifts, electronic noise events, etc)
• Automated processing and Internet Quick Report providing rapid release for response and post-earthquake engineering evaluations.