An oil based mud wellbore imaging tool for measuring formation permittivity and resistivity includes a reflectometry-type microwave scanning system. In some embodiments, The system contains at least one high-Q microstrip resonator with one or several slots on the top surface of it. This micostrip resonator is excited by one port, which is fed with electromagnetic signals by a coaxial cable. From the phase detection of signal at the port, the formation resistivity and permittivity can be determined and corresponding borehole images can be generated.