Geophysical Investigations to Deal with Uncertainties in Difficult Ground Conditions

Any major construction work like power plants, high rise buildings, dams, tunnels, bridges etc., calls for a well-considered approach in view of the restricted timeframe and finance. In planning and development of project adequate knowledge of the geotechnical conditions at site is important besides the other factors that are involved. If not adequately investigated, geological ‘surprises’ can lead to failures and accidents at the time of excavation.

Geotechnical investigation through drilling is a time tested approach, yielding accurate and reliable results on the sampled area. The area sampled, however, is miniscule compared to total volume to be excavated. In urban settings and areas with complex geology, this become even further challenging, necessitating use of methods which can provide continuous and comprehensive information on underground conditions.

Engineering geophysics is an efficient means of subsurface investigation. The merit of application of this low cost aid lies in its ease of deployment and rapidity in providing a reliable knowledge of the underground over a large area, substantiating the requisite geotechnical evaluation studies thereby. Technological advancements and development of portable digital data acquisition instrument systems have increased the versatility in evaluating underground conditions and site characterization.

The state-of-the-art subsurface geophysical investigations are helpful towards minimizing involvement of the conventional direct exploration methods, aiding in accelerated and economical development of the construction projects. The continuous information provided by these methods, as against discrete point information provided by conventional means like drilling, further makes project execution safer, faster and free of ‘geological surprises’.

It is an established fact that a single geophysical method cannot resolve all the problems associated with subsurface investigations. As an example, seismic refraction cannot ‘see’ low velocity zones under rock interface, which might be present under certain geological conditions. There is therefore need for integrated application of various geophysical techniques like seismic refraction, resistivity imaging, ReMi, Crosshole/downhole/uphole seismic and seismic reflection, to determine various properties of subsurface like bedrock quality and depth, low velocity zones (even under rock interface), fault/fracture/shear zones, water lenses, tunnel route geology etc. Present paper discusses various geophysical techniques with emphasis on their integration to provide unique solutions to subsurface challenges. The paper also presents few case studies where integration of two or more geophysical techniques helped resolve complex problems.