Abstract
For the last 30 yr, the India Meteorological Department (IMD) has been designing a torsion seismograph ( T s = 0.8 s, V max = 1000) and an electromagnetic seismograph ( T s = 1.0 s, T g = 0.2–1.0 s) in addition to micro-seismographs ( T g = 7.6 s, T s = 7.6 s). In view of the non-availability of sensitive galvanometers, IMD has installed helicorder-type recorders. However, it has been found that the detection capability for regional and distant events has considerably reduced, compared with the Benioff (World Wide Standardized Seismograph Network) and IMD electromagnetic seismographs. Also, these instruments at low-gain stations such as Calcutta and Madras, located on alluvium, pick up traffic noise. Similar problems have been shown by demultiplexing the short-period data recorded on magnetic tape at the high-gain Seismic Research Observatory (SRO), Shillong, in spite of the use of borehole seismographs. In India, microearthquake seismographs have been developed in recent years. Attempts have also been made to develop digital telemetered seismic arrays and digital seismographs. Although the field trials of the telemetry and digital system are still awaited, a preliminary comparison of the performance characteristics of the indigenously designed microearthquake seismographs shows performance comparable with that of the imported systems.
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