Hydrogeophysical investigations in a typical khondalitic terrain to delineate the kaolinised layer using resistivity imaging

摘要

Investigation for high yielding water wells in the khondalitic terrain (graneti ferrous silliminite gnesiss) is mostly faced with the problem of identification of the extent of the depth of kaolinisation of the aquifer. The traditional Vertical Electrical Sounding survey, Seismic Refraction survey and Very Low Frequency Electromagnetic survey could not identify the kaolinisation of the aquifer in the present investigations. The Two Dimensional (2D) Resistivity and Induced Polarization (IP) Imaging surveys are attempted for the identification of kaolinised layer and depth of kaolinisation. Number of 2D Resistivity and IP Imaging profiles were conducted near Chipurupally in Vizianagaram district of Andhra Pradesh, India along successful and failed wells located within short distances. Resistivity and IP measurements were carried out using an ABEM SAS 4000 Terrameter. The resistivity and I.P. images have provided a clear view of the thickness of the highly weathered zone (kaolinised zone) at both successful and failed wells. The highly weathered zone is identified with the resistivity values below 25 ohm.m. The depth of highly weathered material at failed well is extended about 8-10 m more deeper than the successful wells at some places to as much as 20 m more deep at some other places. This extended deeper kaolinisation of the aquifer is responsible for failure of wells. Layers having resistivities between 25-65 Ohm.m are identified as aquifer layers which are composed of moderately weathered and fractured khondalitic suit of rocks (Garnti ferrous sillimanite/biotite gneiss). Layers with resistivities greater than 65 Ohm.m are interpreted to have basement characteristics belonging to the granite gneiss. Interestingly IP imaging has not provided any greater insights in delineating the kaolinistion of the aquifer when compared to resistivity Imaging, in fact resitivity imaging has shown greater depths of kaolinisation than IP Imaging.