Apparatus and method for wellbore resistivity imaging using capacitive coupling

An apparatus conveyed in a borehole for obtaining a resistivity parameter of an earth formation penetrated by the borehole, the apparatus comprising:

• (a) at least one measure electrode at a first electrical potential capacitively coupled to said formation though a substantially non-conducting fluid in the borehole, said at least one measure electrode connected to a source of electrical current and conveying a measure current into the formation wherein the measure current further comprises a modulated electrical current having a carrier frequency and a modulating frequency substantially less than said carrier frequency.
• (b) at least one focusing electrode at a second potential in proximity to said at least one measure electrode for focusing the measure currents into the formation, said second potential having a magnitude greater than a magnitude of the first potential;
• (c) at least one pad carrying said at least one measure electrode and said at least one focusing electrode, said at least one pad at a third electrical potential substantially equal to the first potential for maintaining the focusing of the measure currents in the formation.

An apparatus conveyed in a borehole for obtaining a resistivity parameter of an earth formation penetrated by the borehole, the apparatus comprising:

• (a) at least one measure electrode at a first electrical potential coupled to said formation through a substantially conducting fluid in the borehole, said at least one measure electrode coupled through a capacitor to a source of a modulated electrical current and conveying a measure current into the formation;
• (b) at least one focusing electrode at a second potential in proximity to said at least one measure electrode for focusing the measure currents into the formation, said second potential having a magnitude greater than a magnitude of the first potential;
• (c) at least one pad carrying said at least one measure electrode and said at least one focusing electrode, said at least one pad at a third electrical potential substantially equal to the first potential for maintaining the focusing of the measure currents in the formation.

A method of obtaining a resistivity parameter of an earth formation penetrated by a borehole, the method comprising:

• (a) conveying a modulated electrical current through at least one measure electrode at a first electrical potential into the formation;
• (b) using at least one focusing electrode in proximity to said at least one measure electrode, and focusing the measure current into the formation, said at least one focusing electrode having a first potential with a magnitude greater than a magnitude of the second potential of the measure electrode;
• (c) maintaining a guard at an electrical potential substantially equal to the potential of the measure electrode and maintaining the focusing of the measure currents in the formation.

An apparatus conveyed in a borehole for obtaining a resistivity parameter of an earth formation penetrated by the borehole, the apparatus comprising:

• (a) at least one measure electrode in electrical contact with a fluid in the borehole, said at least one measure electrode operatively connected to a source of a modulated electrical current having a carrier frequency, said electrode conveying a measure current into the formation;
• and
• (b) a device responsive to least one of (i) the current in the at least one measure electrode, and, (ii) a voltage of the at least one measure electrode, said device producing a measure signal representative of the resistivity parameter.

A method of obtaining a resistivity parameter of an earth formation penetrated by a borehole containing a fluid, the method comprising:

• (a) using a measure electrode for conveying a measure current into the formation through the fluid, the measure current comprising a modulated electrical current having a carrier frequency;
• (b) determining at least one of (i) the current at the at least one measure electrode, and, (ii) a voltage of the at least one measure electrode for producing a measure signal representative of the resistivity parameter.