Examining the Fidelity of Seismic Data Recovered from Mircoformats

During 1960s and 1970s, substantial numbers of analog seismograms recorded on paper were converted into microfilm or microfiche film chips (Figure 1). Unlike paper records, these microforms are small, and require special equipment to visualize their contents. This project investigates the differences that arise when extracting high quality digital data stored on microfiche or microfilm as compared to the same process applied to their paper counterparts. The goal is to determine the best practices and strategies for the scanning of microforms as well as understanding limitations for time series data derived from them. Upon comparison of two scanned images of the same record -- one from microfilm and one from paper -- it seems likely that there are significant differences between the fidelity of the data recoverable from these two media types (Figure 2).

Figure 1: Flowchart showing the path of information from initial ground motion to digital waveforms. Gray boxes show type of information, and black arrows indicate tools for processing this information.

Figure 2: Zoomed in portions of the seismogram showing the seismic signal from the July 16, 1945 Trinity Nuclear test as observed at Tuscon, Arizona, with (a) a color scan from a paper record and (b) a bi-tonal scan from microform. Note the loss of the signal from (a) to (b). Courtesy of James Dewey, USGS.

Upon closer examination of the digitized versions of traces seen in Figure 2, we can see that not only do the waveforms have different shapes, but there also seem to be time differences offsetting the traces which are at least 0th order (have time dependence themselves) (Figure 3). The waveform shape differences are largely a result of resolution differences between the two media types, as the microfilm resolution is generally limited by the older viewing and scanning technologies. The timing differences are due to a horizontal distortion in the scanned image that may be non-linear (Figure 4).

Figure 3: Traces seen in Figure 2 digitized and overlaid upon one another (microform in red, and paper in blue), with a zoomed in inset focusing on the first arrival.

Figure 4: Overlays of portions of the microfilm and paper scans showing two columns of aligned time-marks (blue), and two columns of unaligned timemarks (yellow and red) in between those which are aligned. By examining the degree to which the alignment is off (insets), it can be determined that between the yellow and red columns there is stretching and between the red and rightmost blue column there is contraction.

These issues may be due to how the microform records were originally generated (e.g., uneven film feed-speed into the camera causing the horizontal distortion), in which case, any existing original paper records need to be preserved. Alternatively, the issues may be resultant of how the records are scanned (e.g., uneven film feed-speed into the scanner causing horizontal distortion), in which case, better scanning equipment may resolve some issues.

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