Hyderabad: The Srisailam Left Bank Canal tunnel may be just around 44 km long but for the detailed electromagnetic survey study that revealed the rock structure, weak zones, and associated faults, or geological folds, NGRI scientists flew a total 970 ‘line’ kilometres covering a total area of nearly 110 sq kilometres.
The geophysical survey using electromagnetic methods by way of a heliborne survey, covered 1.2 km on either side of the tunnel alignment to make sure all the required zones of the rocks along the tunnel on both its sides were studied properly.
“This was the first time ever that NGRI conducted a heliborne VTEM survey with a spacing of just 25 metres along an alignment. This has never been done before and was done this time as it was important to understand fully how the rocks are not just along the alignment but also on both its sides,” a senior NGRI scientist told Deccan Chronicle.
While the tunnel lies 250 metres below mean sea level, the study looked at the rock structure upto a depth of 800 metres from the surface.
The study used equipment carried inside the helicopter reading and recording the signals from an underslung transmitter and receiver along with ground stations along the multiple flight paths with the survey starting on November 2 with a test run and being completed on November 14, 2025.
According to the NGRI study report, the maps derived from the magnetic data effectively highlight the edges and boundaries of shallow structural features. The data analysis also revealed discontinuities, additional breaks along the untunneled portion of the SLBC canal.
The report said that such features may serve as potential pathways for groundwater circulation. Incidentally, seepages of water into the tunnel have been a persistent issue and could still be an issue when the tunnelling reaches the portion under the Nallavagu stream, a little over 6 km from the collapsed zone. The study pointed out that a major structural fault lies at this location.
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