1 / 28

Data Processing and Recording at University of Southern California

Data Processing and Recording at University of Southern California. M.I. Todorovska and V.W. Lee Civil Engineering Department, University of Southern California Los Angeles, CA 90089-2531 www.usc.edu/dept/civil_eng/Earthquake_eng/. Laboratories.

kobe
Download Presentation

Data Processing and Recording at University of Southern California

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Data Processing and Recording at University of Southern California M.I. Todorovska and V.W. Lee Civil Engineering Department, University of Southern California Los Angeles, CA 90089-2531 www.usc.edu/dept/civil_eng/Earthquake_eng/

  2. Laboratories • Strong Motion Data Processing Laboratory (established in 1976) • Strong Motion Recording Laboratory (established in 1978, in support of Los Angeles and Vicinity Strong Motion Network)

  3. People • Faculty: M.D. Trifunac, V.W. Lee, M.I. Todorovska • Graduate students and post doctoral associates who took interest in this topic (E.I. Novikova).

  4. SM Data Processing Laboratory-Purpose • Software development for routine and specialized processing of analogue and digital strong motion accelerograms. • Routine processing of large accelerogram data sets and database organization for use in regression analyses, • Large scale regression analyses for empirical scaling of strong ground motion.

  5. Activities later extended to: • Advanced calibration of strong motion instruments. • Ambient vibration surveys of full-scale structures. • Structural health monitoring and damage detection.

  6. Presentation outline • The network • Digitization of accelerograms recorded on film (LeFilm software package) • Processing of digitized and digital accelerograms (Lebatch software package)

  7. Operating since 1980. 80 analog stations with absolute time (SMA-1). First urban SM network. Sensitivity calibrated in 1996. Supported by NSF. The Los Angeles and Vicinity SM Network

  8. Processed data Earthquake name Date M H Records 1  Santa Barbara Island  09/04/1981 5.557 2  North Palm Springs  07/08/1986 5.91015 3  Oceanside  07/13/1986 5.396 4  Whittier-Narrows  10/01/1997 5.91468 5  Whittier-Narrows aft. (1)  10/01/1997 3.81516 6  Whittier-Narrows aft. (2)  10/01/1997 1 7  Whittier-Narrows aft. (3)  10/01/1997 4.41433 8  Whittier-Narrows aft. (4)  10/01/1997 3.5143 9  Whittier-Narrows aft. (5)  10/01/1997 3.91341 10  Whittier-Narrows aft. (6)  10/01/1997 3.1153 11  Whittier-Narrows aft. (7)  10/01/1997 4.01535 12  Whittier-Narrows aft. (8)  10/01/1997 3.3152 13  Whittier-Narrows aft. (9)  10/01/1997 3.81512 14  Whittier-Narrows aft. (10)  10/01/1997 3.3162 15  Whittier-Narrows aft. (11)  10/01/1997 3.4151 16  Whittier-Narrows aft. (12)  10/04/1997 5.31360 17  Whittier-Narrows aft. (13)  02/11/1998 4.71737 18  Sierra Madre  06/28/1991 5.81265 19  Landers  06/28/1992 7.5561 20  Big Bear  06/28/1992 6.5550 21  Northridge  01/17/1994 6.71865 22  Northridge aftershocks01/17/ to 03/23/1994 >5115 23   Hector Mine 10/16/1999 7.1 6 39

  9. Digitization of film records • Hardware: PC, flatbed scanner and printer • Software: LeAuto system of interactive menu driven programs • Analog to digital image conversion • Automatic trace following • Editing • Trace concatenation

  10. Remarks • Trace following is a highly nonunique estimation process and depends on threshold level. • Scanning resolution: 600 dpi is optimal. • Depth: minimum 256 level of gray is recommended. • High contrast image enhancement should be avoided unless it is essential.

  11. Remarks (cont.) • Limit to recovering high frequency is not scanner resolution but finite width of light beam • Quality of digitized data critically depends on operators experience and quality control. • We digitize the entire recorded length.

  12. Illustrations

  13. Digital Signal Processing • Software package: LeBatch (Lee and Trifunac, 1990). • Programs: Volume 1, 2 and 3

  14. Volume 1 processing • Scaling of digitized image to time-acceleration units. • Initial baseline correction (subtraction of fixed trace and removal of linear trend). • Correction for transducer misalignment and cross-axis sensitivity. • Output: unequally spaced “uncorrected” acceleration

  15. Volume 2 processing • Instrument correction – standard and higher order. • Band-pass filtering – to ensure SNR>1 • Ormsby filter - minimum phase distortions. • Filtering - by convolution, in time domain. • End conditions - even extension beyond the domain of the data.

  16. Volume 2 processing (cont.) • High frequency cut-off: fixed at 25-27 Hz in automatic mode (for standard processing). • Low frequency cut-off: in automatic mode, determined by the program, based on representative noise spectrum, and SNR >1. Component specific. Checked by operator by visual inspection of velocity and displacement time histories, considering earthquake magnitude, distance, etc.

  17. Volume 2 processing (cont.) • For specialized applications that require linear combination of different traces, the record is filtered with the same low frequency cut-off (the highest of the low frequency cut-offs chosen by the program). • Necessary e.g. for analyses of building records, and radial and transverse motions.

  18. Volume 2 processing (cont.) • Output: “corrected” acceleration, velocity and displacement, equally sampled at 100 points per second.

  19. Remarks • High-pass filtering is a form of baseline correction (proposed by Trifunac in 1971). • Necessary for analog records to remove a “wavy” baseline. • We do same baseline correction for digital records. Piecewise baseline offsets, apparently instrument related, are not uncommon. At very long periods, recorded linear acceleration is “contaminated” by contributions from rotations to the transducer response (we can call it “noise”).

  20. Remarks (cont.) • Permanent displacement cannot be computed reliably from recorded linear accelerations unless rotations are measured independently (Trifunac and Todorovska, 2001).

  21. Conclusions • Digitization and signal processing are both art and science. • There is no exact answer. • Can be viewed as estimation processes, of a signal contaminated in noise.

  22. Conclusions (cont.) • Permanent displacement cannot be recovered reliably from 3-comp. translational transducers. • High-pass filtering is still the most reliable method for baseline correction.

  23. Conclusions (cont.) • Peak displacement is meaningful only if the frequency band is also specified. • Best data processing methods depend on the application. • Uncorrected acceleration and instrument constants should always be supplied for custom processing by advanced users.

  24. Conclusion (cont.) • The profession would benefit from larger spatial resolution of recording (Trifunac and Todorovska, 2001). • We need more affordable instruments.

More Related