Method and apparatus for well logging resistivity image tomography

An apparatus for evaluating an earth formation penetrated by a borehole, comprising:

• a plurality of azimuthally spaced-apart pads each having at least two electrodes thereon, said at least two electrodes on each pad being vertically spaced apart from one another, said at least two electrodes on each of said plurality of pads configured to be positioned proximal to a wall of said borehole;
• operational circuitry for driving electrical current from a transmitting electrode on a first of said plurality of pads to at least one receiving electrode on at least a second one of said plurality of pads, resulting in a separate measurement signal being produced on said at least one receiving electrode;
• wherein said transmitting electrode is azimuthally displaced from said at least one receiving electrode, defining an azimuthal path between said transmitting electrode and said at least one receiving electrode;
• and
• one electrode on each one of said plurality of pads is separately selected, in a predetermined sequence of said pads, as said transmitting electrode.

A method for evaluating an earth formation penetrated by a borehole, comprising:

• positioning a plurality of azimuthally spaced-apart pads each having at least one electrode thereon proximal to a wall of said borehole;
• operating a first one of said electrodes as a first transmitter electrode and at least a second electrode as a first return electrode, such that current is driven along a first current path between said first transmitter electrode and said first return electrode, resulting in a first measurement signal being detectable at said first return electrode;
• further operating third one of said electrodes as a second transmitter electrode and at least a fourth electrode as a second return electrode, such that current is driven along a second current path between said second transmitter electrode and said second return electrode, resulting in a second measurement signal being detectable at said second return electrode;
• determining a first property of said earth formation based at least in part on said current on said first current path and said first measurement signal, and determining a second property of said earth formation based at least in part on said current on said second current path and said second measurement signal;
• and
• selecting in a predetermined sequence a plurality of said pads, and when each pad is selected, transmitting current conducted from the selected pad to at least one other one of said plurality of pads.