Application of gamma-ray spectrometry and electrical resistivity
imaging technique to detect subsurface karst structure in Pokhara Valley,
central Nepal
Gautam Pitambar 1,2), Pant Surendra R. 2) and Ando Hisao 3)
1) Department of Earth and Planetary Sciences, Graduate School of Science,
Hokkaido University, N10 W8, Kita-ku, Sapporo 060-0810, Japan. E-mail: gautam@cosmos.sci.hokudai.ac.jp
2) Central Department of Geology, Tribhuvan University, Kirtipur, Kathmandu,
Nepal.
3) Suimonchishitsu Kenkyuusho (Institute of Hydrogeology Co.Ltd.), Kita-4
bldg., N4 W6, Chuou-ku, Sapporo, Japan.
Abstract
Pokhara valley measuring c. 50km x 5km represents an intermontane fluvial
basin which offers a panorama of the Himalayan mountain peaks such as
Annapurnas (several peaks exceeding 8000m) and Machhapuchhre (6993m). It is
also the site of a fast growing city of Pokhara - a popular tourist
destination. Spread around the midstream of the Seti river, the Pokhara
basin is filled with a large volume of layered clastic deposits (gravel with
subordinate silt and clay with a maximum thickness of about 100m), of
Quaternary age, brought from the Annapurna mountain range by a series of
catastrophic debris flows. Due to the dominance of easily soluble calcareous
material (1/4 to 2/3 by volume) in the clastic sediments, the Seti river
and its tributaries have carved splendid river terraces and deep gorges.
Karst structures (subsurface flow-channels, solution cavities, sink-holes,
pinnacles, solution chimneys etc.) are widely developed both at the surface
and underground and pose serious threat to houses, farmlands and public work
of virtually any scale.
Dipole-dipole electrical images and gamma-ray profiles obtained, under a
joint JICA-CDG TU program, at 4 locations, viz. Powerhouse area, Gupteshwar
Cave, Mahendra Cave and Chamero Cave, effectively reveal known as well as
unknown karst features. Total gamma-ray intensity anomalies of up to 100
cps are detected over the subsurface openings. In Powerhouse area, a network
of at least 3 linear NNE-SSW oriented subparallel zones with elevated
radiometric counts corresponding to subsurface channels made by underground
streams can be traced. In an inverted electrical resistivity image profile,
contours of elevated resistivity values reflect the subsurface
cross-sectional geometry of elliptical or cylindrical cavities. The
gamma-ray method is very sensitive to near surface cavities. The electrical
image very effectively locates the void spaces at intermediate (up to 15m)
depths. We recommend a detailed radiometric mapping first followed by
electrical image profiling along selected lines for future exploration over
possible karst-prone areas.
raeg98@tansa.kumst.kyoto-u.ac.jp
Last modified: Mon Oct 19 15:40:26 1998