Seismic methods record the movement of vibrations through the ground with their speed and path telling us something about the structure, strength and/or stability of the subsurface. The analogy is; if the subsurface layers are rigid or well intact, the velocity of the waves will increase, but if the layers are fractured, the particles are not closely spaced, as a result, the velocity of the wave will slow down. Thus, geophysicists measures the time the waves comes back to the detector. The faster the waves, it means the rocks below are solid. Thus the area can be good for construction purposes, roads and many others.