None of the known resistivity borehole devices possesses azimuthal focusing properties whereas the unconventional coplanar coil array has, by design, a strong azimuthal focus. In order to understand in detail the influence of this property of the coplanar system, its electromagnetic responses in the varying boreholes conditons are obtained for a two coil array. Although simple, the solutions of a homogeneous conducting medium are exploited to understand the skin effects phenomena. The coplanar response of a nonhomogeneous medium, obtained through Sommerfeld boundary value problem, is then extended to the various borehole models, particularly (1) the invaded mud filtrate with gradational transition zones; (2) dipping multilayer sequences; (3) thinly laminated zones, and (4) gradational transition zone between two thick beds. Based on the comparative study between, the traditional coaxial and the unconventional coplanar coil responses we conclude that: 1. the skin effects are stronger in the coplanar responses than the coaxial but this disadvantage is partially compensated by applying the corrections for these effects; 2. the polarization "horns" are obtained in the coplanar profiles in front of bed boundaries, consequently, they are their high quality indicators; 3. the coplanar system is an important auxiliar tool to investigate the mud filtrate invasion and the presence of annulus zones which are direct indicators of movable hydrocarbons, and; 4. its azimuthal focussing properties can be explored in the borehole investigations of the axially assymetrical geological situations such as vugular or fracture zones and invasion zones in horizontal wells.
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