Transient electromagnetic waves applied to prospecting

An argument based on a high-frequency filter can be used to show that the later stages of a transient are strongly controlled by the low-frequency component of its spectrum. In particular for a sphere of radius 50 m and conductivity 2 mho/m, frequencies greater than 3100 Hz are unimportant for times greater then 3 ms. For the later stages of the transient, where only low frequencies are important, it is possible to provide solutions for the transient response of spheres, cylinders, and dykes. These solutions have the attractive feature that the singular part of the Green´s function is the most important term in this function for these calculations. The early stages of the transient are useful for depth estimations and for these times, different methods of calculation must be used. The various approaches to the problem of calculating transient electromagnetic responses utilize various singularities of the transfer function of the ground. This has shown that the contributions from the integration about poles as well as branch cuts of the transfer function must be considered. Alternatively, if the singularities are seen as specifying the structure, then deriving transient responses for frequency-domain data is likely to be a very ill-conditioned problem. A further conclusion is that care must be given to the choice of pulse shape and measuring time.