f Classification of borehole sources

The first explicit investigation of the physics of downhole sources was carried out by Heelan (1953). Since then, and in particular during the past 10 years, the onderstanding has grown and seems to have reached a mature state. This seems to be valid both as far as pure mathematical onderstanding is concerned, bot more important the validity of the idealized models for real sources. The reason for the current interest for source physics is the growth of crosswell data acquisition for tomography and crosswell reflection surveys. In all these applications the wavelengths are much longer than the borehole radius which makes it possible to obtain analytical solutions. It appears to be difficult to design good downhole sources suited for every situation and geometry. My proposition is that with proper use of currently available theory one can at least explain why this is so, and draw some practical conclusions with respect to design of such sources. This paper aims at summarizing the present state of the art in terms of theory, interpret some observation data and set up some design guidelines