From the raster image to the numeric format tracing
By the term vectorialisation we mean the process by which the tracing recorded in raster format is transformed into a series of segments connected between them such as to be reproduced. This procedure requires several concatenated passages having a specific order: the identification of the tracing of the event on the raster image, cut-out of the tracing, cleaning up of the image, follow-up, correction and saving procedure.
Having selected and recovered the raster of the seismogram to be elaborated by the database, in order to work more fluidly, it is convenient to cut out only the relevant part of the seismic signal. Seeing the different quality of conservation and the different types (smoked paper, photographic paper, etc.) of the historical seismograms being dealt with, at times, before starting the vectorialisation, it is necessary to clean up the cut part and to make the tracing more visible (e.g. by modifying the contrast and the brightness).
In the following figure we report before (1) and after (2) the cleaning of a cut-out of a smoked paper seismogram dating from the early 20th century.
Teseo 2 and the Bézier instrument
Teseo2 software has been developed by SISMOS for the digitalisation and vectorialisation of the seismograms. Inserted inside GIMP, an image elaboration programme, Teseo2 allows for the manual and automatic follow-up of the earthquake tracing (using the Bézier curve) and to save the result of the vectorialisation in different formats.
The calque of the seismic tracing is made automatically in small sections, compete signal and well contrasted, and in general for low frequency signal parts. For the other cases it is preferable to perform manual follow-up.
The digitalisation thus performed is at the pace of variable sampling and the set of the resulting data need to be converted into a signal at a constant sampling pace. The same procedure allows us to correct the curvature of the tracing consequent to the length of the writing nib and any straying of the consequent signal at non–optimal recording conditions. After these corrections the wave forms thus accrued are saved in different formats (SAC, ASCII, etc), ready for seismological elaboration.
Graziano Ferrari(person in charge from 2015)