Seismic and Electrical resistivity methods of soil exploration :
These are the indirect methods of soil exploration.
- Seismic methods:
In impact or explosion that generate sound waves are usually of three types, namely:
- Surface waves ( Rayleigh waves).
- Compression waves.
- Shear waves.
The second and third type of waves usually travel outward of the epicenter. The compression is arrived at any point first because it is fastest. The velocity with which the wave will pass through a medium depends upon the density of the material of the medium. If the material is dense and rigid then the velocity of the traveling wave is higher, while if the medium is loose and flexible then the velocity of the wave traveling will be comparatively low. Thus we can say that the change in velocity determines the boundaries of different materials. The method we use may be seismic refraction. It can not be used when the velocity of traveling of lower stratum is less than that of upper material.
- Electrical resistivity methods:
These are also the indirect methods of soil exploration. The difference of electrical resistivity between the various strata is the base of this method. Rocks or dense soil can be differentiated on the basis of electrical resistivity from the loose soil, as the value of electrical resistivity in rocks is higher than the value of electrical resistivity in loose soil. It can also be observed as the difference in electrical resistivity above and below GWT. One of the effective methods is Wenner’s four electrode method, ASTM G57.
In this method of soil exploration generally two electrodes are buried into the ground and a potential is applied across these electrodes, the measurements are made on the basis of current flowing through the two electrodes or the potential drop across the intermediate electrodes. The changes in the spacing between the inner electrodes depends upon the condition of the site and the requirement of the data ( i.e. the intensity of investigation). The spacing between the electrodes should be constant.