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

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)

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).

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.

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

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

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

Processed data Earthquake name Date M H Records 1 Santa Barbara Island 09/04/ North Palm Springs 07/08/ Oceanside 07/13/ Whittier-Narrows 10/01/ Whittier-Narrows aft. (1) 10/01/ Whittier-Narrows aft. (2) 10/01/ Whittier-Narrows aft. (3) 10/01/ Whittier-Narrows aft. (4) 10/01/ Whittier-Narrows aft. (5) 10/01/ Whittier-Narrows aft. (6) 10/01/ Whittier-Narrows aft. (7) 10/01/ Whittier-Narrows aft. (8) 10/01/ Whittier-Narrows aft. (9) 10/01/ Whittier-Narrows aft. (10) 10/01/ Whittier-Narrows aft. (11) 10/01/ Whittier-Narrows aft. (12) 10/04/ Whittier-Narrows aft. (13) 02/11/ Sierra Madre 06/28/ Landers 06/28/ Big Bear 06/28/ Northridge 01/17/ Northridge aftershocks 01/17/ to 03/23/1994 > Hector Mine 10/16/

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

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.

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.

Illustrations

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

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

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.

Volume 2 processing (cont.) High frequency cut-off: fixed at 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.

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.

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

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”).

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

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.

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.

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.

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