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 using their interactive WIDEimage scanning software package (URL: http://www.contex.com/software/wideimagenet
/default.htm). 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 13.24.
Berkeley Seismological Laboratory
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