Method and apparatus for azimuthal electromagnetic well logging using shielded antennas

An improved well logging tool for making azimuthally sensitive resistivity measurements within a formation surrounding a drill string positioned within a borehole, said improved well logging tool comprising:

• a plurality of transmitting antenna coils positioned on said drill string, said transmitting antenna coils for generating primary electromagnetic fields within said surrounding formation, thereby inducing currents to flow within said formation which in turn generate secondary electromagnetic fields within said formation, said borehole, and said drill string;
• a plurality of receiving antenna coils positioned on said drill string in spaced relationship with said transmitting antenna coils and with each other, said plurality of receiving antenna coils for measuring characteristics of said secondary electromagnetic fields through induction of a current within said receiving antenna coils indicative of said characteristics;
• a conductive shield positioned on said drill string in such a manner as to partially shield at least one of said transmitting antenna coils;
• and
• a conductive shield positioned on said drill string in such a manner as to partially shield at least one of said receiving antenna coils;
• wherein said conductive shields define windows through which said primary electromagnetic fields may be generated or said secondary electromagnetic fields may be detected by said transmitting or receiving antennas.

An improved method for well logging, the improvement being a method for making azimuthally sensitive resistivity measurements within a formation surrounding a drill string positioned within a borehole, said improved method comprising the steps of:

• positioning at least one transmitting antenna coil on said drill string;
• directing a current to flow within said transmitting antenna coil so as to generate a primary electromagnetic field within said surrounding formation, thereby inducing currents to flow within said formation which currents in turn generate a secondary electromagnetic field within said formation, said borehole, and said drill string;
• positioning at least one receiving antenna coil on said drill string in spaced relationship with said at least one transmitting antenna coil;
• positioning at least one conductive shield over said transmitting antenna coil or said receiving antenna coil, said conductive shield defining at least one window through which said secondary electromagnetic field may be detected by said receiving antenna coil or said primary electromagnetic field may be generated by said transmitting antenna coil;
• measuring characteristics of said secondary electromagnetic field through induction of a current within said receiving antenna coil indicative of said characteristics;
• and
• orienting said at least one window of said conductive shield in plurality of azimuths and repeating said step of measuring characteristics of said secondary electromagnetic fields at each of said azimuths.