Seismic Refraction Survey in Kenya & East Africa.
Seismic refraction investigates the subsurface by generating arrival time and offset distance information to determine the path and velocity of the elastic disturbance in the ground. The disturbance is created by shot, hammer, weight drop, or some other comparable method for putting impulsive energy into the ground. Detectors laid out at regular intervals, measure the first arrival of the energy and its time. The data are plotted in time–distance graphs from which the velocities of the different layers and their depths can be calculated. This is possible because rays (a continuum points on the expanding wavefront) of the disturbance wave follow a direct route and is the first arrival energy at the close–in geophones. The rays are refracted across layer boundaries where there is a difference in elastic and density properties. The critically refracted ray travels along with the layer interface, at the speed of the lower layer, and continuously “feeds” energy back to the surface, to be successfully detected by the line of geophones. At some distance, the refracted ray becomes the first arrival.