Abstract:

Presence of palaeochannels pose serious concern in development of hydropower schemes, the foreknowledge of existence of such buried channels being important in establishing secure and economic projects. Accordingly, it necessitates initiating a well-considered subsurface investigation plan, involving application of indirect technique of engineering geophysics towards optimizing the direct exploration program in view of restricted time-frame and finance. Of the various engineering geophysical methods, the refraction seismics is a prime cost effective exploration technique in furnishing continuous profile of the underground status. The paper aims to highlight utility of seismic refraction technique as an efficient subsurface exploratory tool for delineation of palaeochannel in hydropower project: Detailed account of utilization of seismic refraction survey in delineating palaeochannels through generation of 2-D/ 3-D model defining the bedrock disposition is presented herein, citing a couple of case studies demonstrating its advantageous usage.

1.
INTRODUCTION

Development of hydropower schemes, which involve large structures and underground works, knowledge of the subsurface is important. This demands a comprehensive program of investigation that would generate sufficient information in establishing safe and economic project. While developing hydropower projects in river valleys, presence of palaeochannel pose serious concern in planning and execution of the scheme. Broadly, palaeochannels are unconsolidated sediments deposited in ancient, currently inactive river and stream channel system. Due prior knowledge of existence of such buried channels is important in layout planning as well as in conceiving the requisite measures needful during construction of the project. In hydroelectric project site, delineation of palaeochannel calls for a planned approach for performing the needful exploration in view of the restricted time-frame and finance. It necessitates formalizing a well-considered investigation program, involving indirect technique of engineering geophysics towards optimizing the extensive direct explorations like drilling, pitting, etc. [1]. Seismic refraction is a cost-effective engineering geophysical method in furnishing continuous profile of the underground for demarcating bedrock interface efficiently [2]. In present time, technological advancements and development of computerized instrument systems have increased the versatility of application of refraction seismics. The paper highlights the utility of the technique in delineating palaeochannels in hydropower projects. A couple of case studies are presented which portrays the utility of seismic refraction technique in delineating palaeochannel: The case studies include application of refraction seismics at Nimoo Bazgo and Kotlibhel Stage-1B projects of NHPC, wherein the existence of palaeochannels had been anticipated. The geophysical study was effective in defining the palaeochannel and generating 2-D/3-D model depicting the bedrock disposition, illustrating the configuration of the buried channel. The study was helpful towards layout planning and developing secure hydropower schemes.

2.
THE TECHNIQUE

Geophysical methods rely on measuring the physical properties of soil/rock: it is the contrast in the physical properties between different materials that helps in interpreting the subsurface status. Of the various exploration geophysical techniques, the seismic refraction method is widely utilized in civil engineering applications for in-situ investigations of geotechnical problems, since the seismic velocity is controlled by the fundamental parameters of elastic strength and density.

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