Sensitive instruments that make the earth a 'living being'

By A.A Harichandan

BANGALORE, JULY 22. India's finest seismological facility is located at Kotaladinna village, Gauribidanur taluk, Kolar district. It is a part of the Bhabha Atomic Research Centre's (BARC) Seismology Division.

The station was commissioned 25 years ago, when the legendary scientist and visionary, the late Homi Bhabha, laid the first seismometer in its pit at the station. "At that time, we used bullock carts to lay the cables. Today, we have sophisticated wireless equipment," said Dr. G.Jayachandran Nair, Head, Seismology Division, BARC.

When presspersons visited the facility recently, they found that the last time an organised media visit to the facility was made was over 15 years ago. The purpose of this visit was to obtain a rudimentary understanding of seismology, and the work done at the station.

The quiet, far-from-tectonic-noise location of the station belied the excitement of seeing the violent upheavals as the earth rearranged its outer shell (the tectonic plates), and that was not all. Here lay the infrastructure to accurately determine the location of any significant seismic event, natural or man made, anywhere on the earth at any given time. The sensitivity of the instruments, which Dr. Nair informed the visitors, compared with the best in the world, makes the earth a "living being."

The scientists at the Gauribidanur station monitor and analyse seismic signals produced by earthquakes and underground nuclear explosions occurring the world over. They routinely record, for all seismic events detected, parameters such as onset time, signal amplitude, event magnitude, and source location. These are compiled to form seismic bulletins which represent valuable data for further research.

From the excitement of seeing the first waves of Pokhran II, to the horror of Bhuj appearing on their monitors as impersonal waveforms, these dedicated and disciplined scientists have seen much.

How it works:

Modern day seismometers (a device that converts ground motion into an electrical signal) can record ground motion with frequency as low as one cycle per 300 seconds, and as high as 300 cycles per second (300 Hz). In Gauribidanur, they have been placed in an L shaped array, with 10 seismometers along each arm. The distance between any two seismometers on an arm is 2.5 km. Signals from the 20 seismometers reach the central station through cables and in wireless form.

A frequency-modulated digital converter (FMDC), from the U.K. is used to store this multi-channel data on high-end computers. "The array offers the advantage of being linear, as methods of ``beaming'' can be employed for the waves from a particular seismic event, for example an earthquake. The signals can then be ``superposed'' (added up, by shifting the signal over the delay between one seismometer and the next) to get the best beam for the event," Dr. Nair explained.

PC based digital data acquisition:

After the collaboration with the U.K. was discontinued in the early Nineties, scientists at Gauribidanur developed a PC- based alternative to the FMDC, using a software package called VISIDAQ. While this software, which is used to monitor various parameters (temperature for example) in industrial installations, was developed in the U.S., the modifications required to use it for seismometry were made indigenously. Dr. Nair gave full credit to Mr. Dinesh, one of the scientists at the facility, for the feat. This system was commissioned in January 2001.

The VISIDAQ takes frequency modulated (FM) digital signals and ``demodulates'' them. A digitiser has been set up at the location of each of the seismometers itself to minimise distortion of the signal. The existing array has been converted to transmission of analog (continuous signals) FM signals. The digital (transmission of signals in periodic bursts) transmission using the digitisers is under field trial. Soon, these too will be optimally utilised, taking the degree of self-reliance one notch higher.

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