Mapping hidden near-surface karsts (guphas) in Pokhara (Nepal) using Geophysical Methods

What is it about?

The Pokhara city and its sorrounding suburb within the Pokhara valley sits on quaternary sediments made of clastic sediments such as clay, silt, sand and gravel (conglomerate), which were transported the Seti River from the Annapurna mountain range and deposited there. The uppermost layer of gravel veneer (cover) is rather hard, but the sediments below contain abundant calcium carbonate that dissolves in water. So, areas penetrated/drained by water (rain water or river water) for prolonged duration (tens and hundreds of years) long time will have underground cavities and channels that ultimately lead to caving and land subsidence. Sometimes these channels might have been filled naturally or by people inhabiting these areas without giving them serious considerations. This research paper describes the geological aspects of these sediments, the nature of the cavities and karsts. Then, it demonstrates how geophysical methods are used to measure physical (electrical, magnetic and radiometric) properties of the underground material using specialized portable scientific equipment and without causing any disturbance to the land and provide information about the location (depth), size (approximate diameter), extent (length), type of filling material etc. of the cavities (caves, channels either open or filled).

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Why is it important?

This study was carried out long before the widespread damage due to subsidence in the Armala area (in Pokhara) that attracted attention of local people as well as explorers. Geologists and engineers need to be aware of the techniques for rapidly mapping the karstic features and also inform the inhabitants about the danger posed by extensively developed subsidence features/channels and also measures related to proper management (of water drains) and precautions required during construction of any structures in areas having potential for occurrence of such features.

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