EFFECT OF TOOL ECCENTRICITY ON SOME ELECTRICAL WELL LOGGING TOOLS.

An algorithm to compute efficiently the propagation of nonaxially symmetric waves in a multicylindrically layered geometry is presented. One of the applications of such a program is in electrical well logging, where oil exploration tools operate in a borehole drilled into the ground for geophysical subsurface sensing. The borehole with the invasion zone around it can be modeled as a multicylindrically layered geometry. The sensing tool, which is in the borehole, may not be exactly at the center, even though the center position has traditionally been thought to be the best place to operate the tool. An algorithm to study the effect of this eccentricity is considered. It uses a system of 2 multiplied by 2 reflection and transmission matrices to determine, in each homogeneous region, the ratio of standing waves to outgoing waves. Given this ratio (also a 2 multiplied by 2 matrix) in the center region, an infinite system of equations relates the geometry of the tool to that of the borehole. The solution of this system gives the amplitudes of the fields everywhere, from which the signals received by the tool can be computed.