Abstract

Near surface geophysical surveys commonly involve determination of seismic velocities and hence subsoil characterization. Refraction seismic methods have been conventionally used since times immemorial for mapping the velocity structure. Advances in geophysical survey techniques resulted in the emergence of Multi-Channel Analysis of Surface Waves (MASW) as one of the latest techniques used for shallow subsurface profiling. It is based upon the dispersive nature of surface waves. The sampling depth of a particular frequency component of surface waves is in direct proportion to its wavelength which makes the surface wave velocity frequency dependent, i.e. dispersive. The shear wave velocity structure can therefore be obtained by the inversion of surface-waves dispersion curve. A meticulous and comparative study of both these methods in terms of time-efficiency, accuracy of results, reliability, range of validity and cost factors reveal that MASW is a more universal method. The fact that it puts to use surface wave component of seismic waves and not body waves plays a vital role in giving it an upper-hand over refraction methods. Signal to noise ratio (S/N ratio) is maximized in MASW. Also refraction seismic methods require as a requisite condition that the earth under survey should be made up of layers of material that increase in seismic velocity with each successively deeper layer. This constraint again adds to the advantages of MASW which has no such limitations. Besides, refraction methods can give erroneous results when a lowvelocity layer underlies a high-velocity layer. Hence MASW comes across as a powerful, accurate and cost- effective tool for imaging the subsurface and estimating accurate shear-wave velocity of structures