The free shock wave as a seismic source
The free shock wave has particular advantages as a source for seismic survey. Its physical parameters can be precisely defined and its interaction with the rock environment predicted. In particular, the wave mode conversion which occurs leads to the simultaneous generation of both strong compression and shear waves in the rock matrix. The first part of the present study outlines the basic principles of design for a source that will generate, repetitively, free shock waves with associated pressure fields that range over two orders of magnitude (measured in bars). The second part of the paper describes the wave system which develops when shocks, generated by such a source, interact with water in, for example, a water-filled surface bore-hole. The multiplication of incident shock pressure in water, which is characteristic of the operational performance of the shock-wave source, is shown to be a consequence of the complex interactions that take place between wave systems transmitted and reflected at the gas-water interface and those that are reflected at the water-solid interface. The third part of the paper illustrates the behaviour of compressional and shear waves generated by the shock-wave source in both sedimentary rock and granite. It is shown that the mode conversion to compressional and shear waves in granite leads to near-ideal behaviour in terms of the ratio of velocities of propagation for the two types of wave in the rock.