Mapping of subsurface karst structure with gamma ray and electrical resistivity profiles: a case study from Pokhara valley, central Nepal

摘要

Electrical resistivity (sounding with Schlumberger array and dipole-dipole imaging) and natural gamma ray intensity measurements were made over the karst features (subsurface flow-channels, solution cavities, sinkholes) in the Pokhara valley, central Nepal. In the Powerhouse area, the upper 60-80 m section of the basin-filling Quaternary sediments is represented by layered elastic sediments (gravel, silt, clay) that are represented by KQ-type (rho(1) < rho(2) > rho(3) > rho(4)) electrical sounding (ES) curves. The true electrical resistivity of the layers has a wide range of variation (a few hundreds to several tens of thousands of Omega m) such that it is possible to determine both the vertical and lateral subsurface geological variations by integrating the electrical resistivity profiling and sounding techniques. Total gamma ray intensity profiles measured over various karstified locations reveal significant anomalies (up to 100 counts per second, cps) over the known or unknown subsurface openings. In the Powerhouse area, presence of a network of at least three linear NNE-SSW oriented subsurface channels, made by past and present underground flow-channels, is inferred. In interpreted electrical image profiles, contours of elevated resistivity reflect the cross-sectional geometry of cavities. The gamma-ray method is sensitive to near-surface cavities while the electrical image effectively locates the void spaces at intermediate (up to 5-20 m) depths. An exploration program involving rapid radiometric mapping followed by selective electrical imaging is recommended for future exploration of karst-prone areas in the valley. (C) 2000 Elsevier Science B.V. All rights reserved. [References: 19]