Seismogram Scanning Project

Robert A. Uhrhammer

Introduction

This aim of Seismogram Scanning Project (SSP) is to scan selected analog seismograms kept on store in the Berkeley Seismological Laboratory (BSL) Seismogram Archive and generate digital image files for studying microseismic source areas and distribution in relation to wave climate. The Berkeley Seismogram Archive, where approximately 1.2 million analog seismograms dating back to 1910 are stored, contains seismograms recorded at the seismic stations located on the UC Berkeley Campus (BRK and BKS), dating back to 1930, which are crucial for this project. This scanning project is being undertaken in collaboration with Dr. Peter Bromirski of the University of California, San Diego/Scripps Institution of Oceanography on his project funded by the California Department of Boating and Waterways. The corresponding digitization of the scanned seismograms is being done at the Scripps Institution of Oceanography.

Background and Motivation

The photographic and smoked paper seismograms kept on store in the BSL Seismogram Archive are gradually deteriorating over time, and scanning the seismograms into a computer readable format is essential if they are to be preserved for the future. BSL has a wide format scanner, and whenever the opportunity and funding allows, we make an effort to scan seismograms.

Initial Effort

The initial effort was to systematically digitize the November-March vertical component Sprengnether seismograms and also the November-March Berkeley vertical component Wilip-Galitzin seismograms for selected "El Nino and La Nina" winters. The seismograms were scanned at a resolution of 400 dots per inch on an Contex Scanning Technology Ideal FSS 18000 DSP Full Scale Scanner using their interactive WIDEimage scanning software package (availabe via URL: http://www.contex.com). The scanned images are stored as Tag Image Format (TIF) bitmap image files. To substantially reduce the storage requirements, a histogram analysis is employed to interactively set an appropriate threshold and the scanned seismograms are stored as 1-bit resolution line-art images. An example of a scanned SPR seismogram is shown in Figure 16.1.

Current Effort

During the past year we have completed the task of systematically scanning the available November-March Berkeley vertical component Sprengnether seismograms for the winters of 1962 through 1991 and also the available November-March Berkeley vertical component Wilip-Galitzin seismograms for the winters of 1930 through 1964 (a total of approximately 9,300 seismograms). We are also finalizing a paper "Comparison of Wilip-Galitzin, Strengnether and Streckeisen Seismographic Recordings at Berkeley" to be submitted to the Bulletin of the Seismological Society of America.

Figure 16.1: A comparison of: A) the scanned BKS Sprengnether Z-component seismogram; B) the corresponding digitized waveform, and; C) the corresponding waveform synthesized from the co-sited Streckeisen STS-1 broad band seismograph. The seismic signal is from a M 5.7 teleseism which occurred 4000 km SE of Berkeley (1991/09/18,09:48:13; 14.65N, 90.99W) in Guatemala. All three plots are at the same scale with the horizontal axis spanning 511.25 seconds and the vertical axis spanning 175 mm. That all three waveforms are highly similar is verification of the accuracy of the digitization procedure and the accuracy of the Sprengnether transfer function.
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Digitization Procedure

Digitization of the photographic seismograms has two components. The seismograms are first scanned at a resolution of 400 dots per inch on an Contex Scanning Technology Ideal FSS 18000 DSP Full Scale Scanner. The scanned images are stored as Tag Image Format (TIF) bitmap image files. To substantially reduce the storage requirements, a histogram analysis is employed to interactively set an appropriate threshold, and the scanned seismograms are stored as 1-bit resolution line-art images. The traces on the scanned seismogram image are then digitized at nominally 4 samples per second using the SeisDig software package (Bromirski and Chuang, 2003) available for download at URL: hppt://www.ucsg.edu/ bromirski. The resulting digitized seismogram has an amplitude resolution of 0.0635 mm (i.e. the pixel resolution and equivalent to a data logger sensitivity of 15748 DU/M) and a time resolution of 0.254 seconds for the SPR seismograms recorded at 15 mm/min and 0.127 seconds for the W-G seismograms recorded at 30 mm/min (also the pixel resolution). The effective dynamic range is approximately 64 dB (20-log10(100mm/0.0635mm)). An example of a digitized SPR seismogram is shown in Figure 16.1.

Calibration of Wilip-Galitzin, Sprengnether and Streckeisen Seismographs

As seismic instrumentation has evolved, three different three-component sets of long-period/broadband seismographs have been installed and operated, with overlapping intervals, at Berkeley since the 1930's. Wilip-Galitzin (W-G) seismographs were operated on the northern most seismic piers in the basement of Haviland Hall on the Berkeley Campus (BRK) from August 28, 1930 until February 1, 1965. World Wide Standardized Station (WWSS) Sprengnether (SPR) long-period seismographs were operated in the Byerly Seismic Vault (BKS), located in Strawberry Canyon behind the Botanical Garden, from June 8, 1962 until September 30, 1991. Streckeisen (STS-1) broadband seismographs began recording in the Byerly Seismic Vault (BKS) on May 11, 1987 with a 16-bit PC-based recording system (Bolt et al., 1988) and a 20 second pendulum configuration and by August 8, 1991 they had evolved into the current 24-bit Quanterra Q680 data logger and 360 second very-broadband (VBB) pendulum configuration. The W-G and SPR seismographs operated concurrently, but not co-sited, from June 8, 1962 until February 1, 1965. The SPR and STS-1 seismographs operated concurrently at BKS from August 8, 1991 until September 30, 1991. Comparison of selected seismograms from these intervals allow us to verify the calibration and response of these seismographs, and to demonstrate that, within appropriate passbands, the earlier instrument seismograms can be reliably synthesized from later instrument seismograms, and that absolute ground motions can be reliably estimated. An example of a SPR seismogram, synthesized from the corresponding STS-1 seismogram, is shown in Figure 16.2. A Streckeisen STS-2 seismograph has operated at BRK since January 1, 1993 and a comparison of selected seismograms from the BKS STS-1 and the BRK STS-2 is used to quantify differences in their site responses. A comparison of the responses of these seismographs is shown in Figure 16.2.

Figure 16.2: A comparison of the velocity responses of the BRK W-G and STS-2 and the BKS SPR and STS-1 seismographs operated at Berkeley. The W-G and SPR instruments recorded galvanometrically on photographic paper, and the paper seismograms are scanned at a resolution of 400 dots per inch. The STS-1 and STS-2 instruments record digitally via a high-resolution data logger. The velocity sensitivity is given in digital units per meter per second (DU/(M/S)). Two responses are given for the SPR because its natural period (Ts) was changed from 30 seconds to 15 seconds on May 12, 1965.
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Acknowledgements

UC Berkeley student Kevin Lee participated in this project during the past year, and we thank him for his efforts.

This project was funded by sub-award of the California Department of Boating and Waterways Contract 03-106-105.

References

Bolt, B. A., J. E. Friday and R. A. Uhrhammer, A PC-based Broadband Digital Seismograph Network, Geophysical Journal, 93, 565-573, 1998.

Bromirski, P. D. and S. Chuang, SeisDig Software to Digitize Scanned Analog Seismogram Images: User's Manual, Scripps Institution of Oceanography Technical Report, Scripps Institution of Oceanography, Integrative Oceanography Division, UC San Diego, 28 pp., July 2, 2003.

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