Optimal coil design for well-logging applications

Transmitting and receiving antennas used in many well-logging tools are made up of ferrite-backed coils wound on a metallic cylinder. A critical requirement in the design of these coils is to produce a desired spatial variation of the magnetic field. In some other applications, MRI for instance, in addition to a specified spatial variation, it is also desirable to have low inductance (L) of coils to reduce switching time constants that depend on L/R where R is the coil resistance. In well-logging tools, it is often required to have uniformity of electromagnetic field along the axial direction over a specified range at a given radial distance from the tool-face. This requirement is met by placing several coils along the length of the tool. In this paper we discuss an application of genetic algorithms to determine coil position and the direction of current. A combination of FEM and superposition method is used to solve the forward problem. Numerical and experimental results are presented.