Shocks or explosions within the earth' s crust are always accompanied by generation of elastic waves, which travel in all directions from the point or place of shock, the focus.
Velocity of these shock waves is related to the nature of the medium through which they travel. In nature these waves are produced during earthquakes. The seismic waves reveal a great deal of information about the internal constitution of the earth.
Although different types of waves are generated when a shock occurs, these are the P waves (longitudinal waves), which are the fastest and strongest. Their velocity, Vp, is related broadly to the medium (rocks) through the following equation:
where E is Modulus of elasticity, e is density and v is the Poisson' s Ratio of the medium.
The controlling factor is, obviously, the modulus of elasticity which itself is dependent upon nature of rock, its chemical and mineralogical composition, degree of freedom from structural discontinuities and degree of saturation with water and other fluids.
From experimental investigations, characteristic velocity values for P waves have been broadly established for different rock types.
As such, if the velocity of seismic waves travelling through a section of the ground is known, nature of the ground can be fairly assessed.
This is the underlying principle of all the seismic methods.
Ø The fundamental procedure in all seismic investigations for subsurface explorations is the same: a shock is created at a chosen point or location either by exploding a charge, of dynamite;
the waves so produced are recorded at different distances from the shot point with the help of geophones or special detectors.
Ø The instant of shot, that is the shot time and the first arrival are recorded very carefully from which time -d istance plots are prepared in a selected manner.
Ø A proper interpretation of these time-distance plots may reveal presence of unusually high or low velocity media at certain depths.
Reflection methods have been found especially useful for subsurface studies under! bodies of water (e.g. lakes, rivers, and estuaries) because in such surveys signals from surface and shear waves are obliterated by water and arrival times of only longitudinal waves are recorded clearly and easily.
Ø It may be pointed out that for correct inferences, it is imperative that reflection and refraction records are properly distinguished from each other as well as from records of other associated events.
These demand considerable skill and expertise.