Advantages and Limitations of Seismic Refraction Method Using Hammer Sources

Abstract

The seismic refraction surveying method uses seismic energy that returns to the surface after traveling through the ground along refracted ray paths. The first arrival of seismic energy at a detector offset from a seismic source always represents either a direct ray or a refracted ray. This fact allows simple refraction surveys to be performed in which attention is concentrated solely on the first arrival (or onset) of seismic energy, and timedistance plots of these first arrivals are interpreted to derive information on the depth to refracting interfaces. this simple approach does not always yield a full or accurate picture of the subsurface. In such circumstances more complex interpretations may be applied. The method is normally used to locate refracting interfaces (refractors) separating layers of different seismic velocity, but the method is also applicable in cases where velocity varies smoothly as a function of depth or laterally.Despite the numerous limitation of hammer seismic refraction survey, the technique is still very much in use due to the various aspects in which it is found advantageous. The most pronounce advantage of the refraction method is that it gives directly the velocities of the refracting beds. Which are needed in the depth interpretation. This this work reveals that even though there are several problem of this technique, there are also a lot of advantages, one of which is that it is the most readily available seismic energy source we can lay hands on.Hammer source is a low energy source whose use is restricted to shallow survey. Due to weak energy generated,only the first few layers of the ground subsurface can be detected. It provides only the picture of the bedrock configuration. When there arises problems such as noise, irregularities in the shallow bedrock surface or eroded and weathered rock surface, anisotropic surface, existence of blind zone, velocity reversal with depth, and when the boundary between the interfaces are non-parallel, then correction measures are necessary to improve on the results obtained. Ambiguities do occur in this technique hence it requires thought, care, a high degree of skill and a good sense of judgment.These shortcomings are not enough to dismiss the use of the hammer source in seismic refraction method as its results has been shown to be useful in solving civil engineering and hydrogeological problems. The method is economical in cost, time and area of coverage. It poses no danger to the environment to which the work is done, or neither personnel nor damage to instruments used. This work has not only enumerated the limitations and advantages of seismic refraction method using hammer source but has also made suggestions to aid in the basic problem of the method, like dipping layers, blind zone, velocity inversion occurrence and noise control.