Methods and apparatus for imaging earth formation with a current source, a current drain, and a matrix of voltage electrodes therebetween

An apparatus for electrically imaging the wall of a borehole in an earth formation having a plurality of beds separated by bed boundaries, said apparatus comprising:

• a) current generating means for generating a current flow through a portion of the formation such that current density near the wall of the borehole adjacent to said portion of the formation is substantially non-parallel to bed boundaries;
• b) voltage measuring means for measuring voltage at the wall of the borehole adjacent to said portion of the formation, said voltage measuring means comprising at least six electrodes including at least three equally spaced pairs, with each of a first group of three electrodes of said at least six electrodes being horizontally displaced relative to the others of said first group of three, and each electrode of a pair being vertically displaced relative to another electrode of said pair;
• and
• c) signal processing means coupled to said voltage measuring means for generating a resistivity image of the wall of the borehole based on voltage measured by said voltage measuring means.

A method for electrically imaging the wall of a borehole in an earth formation having a plurality of beds separated by bed boundaries utilizing a tool having a plurality of electrodes, said method comprising:

• a) generating a current flow through a portion of the formation such that current density near the wall of the borehole adjacent to said portion of the formation is substantially non-parallel to bed boundaries;
• b) using the electrodes in pairs, measuring a plurality of voltages at the wall of the borehole;
• and
• c) generating a resistivity image of the wall of the borehole based on a plurality of differences in voltages measured by electrodes in each pair, wherein
• said step of generating a current flow includes generating current flow through the portion of the formation in two substantially non-parallel directions.

An apparatus for use in a borehole investigating tool which is movable through the borehole for investigating the wall of the borehole, said apparatus comprising:

• a) a non-conductive pad adapted to be pressed against the borehole wall;
• b) a matrix of voltage electrodes carried on said non-conductive pad and adapted to sense voltages at the borehole wall;
• c) a first current source electrode adapted to inject a current into the borehole wall;
• d) a first current return electrode adapted to drain current from the borehole wall at a location spaced apart from said first current source electrode, said matrix of voltage electrodes being located between said first current source electrode and said first current return electrode;
• e) a second current source electrode and adapted to inject a current into the borehole wall;
• and
• f) a second current return electrode adapted to drain current from the borehole wall at a location spaced apart from said second current source electrode, said second current source electrode and said second current return electrode being located substantially non-parallel to said first current source electrode and said first current return electrode.

An apparatus for electrically imaging the wall of a borehole in an earth formation having a plurality of beds separated by bed boundaries, said apparatus comprising:

• a) current generating means for generating a current flow through a portion of the formation such that current density near the wall of the borehole adjacent to said portion of the formation is substantially non-parallel to bed boundaries;
• b) a plurality of voltage measuring means adjacent to said portion of the formation for measuring a plurality of voltages at the wall of the borehole;
• and
• c) signal processing means coupled to said plurality of voltage measuring means for generating a resistivity image of the wall of the borehole based on the plurality of voltages measured by said plurality of voltage measuring means, wherein
• said current generating means includes means for generating current flow through said portion of the formation in two substantially non-parallel directions.

An apparatus for electrically imaging the wall of a borehole in an earth formation having a plurality of beds separated by bed boundaries, said apparatus comprising:

• a) current generating means for generating a current flow through a portion of the formation such that current density near the wall of the borehole adjacent to said portion of the formation is substantially non-parallel to bed boundaries;
• b) a plurality of voltage measuring means adjacent to said portion of the formation for measuring a plurality of voltages at the wall of the borehole;
• and
• c) signal processing means coupled to said plurality of voltage measuring means for generating a resistivity image of the wall of the borehole based on the plurality of voltages measured by said plurality of voltage measuring means, wherein
• said plurality of voltage measuring means includes first, second, third, and fourth electrodes, and means for comparing the voltage difference between said first and second electrode with the voltage difference between said third and fourth electrode,
• said first electrode lies in a first row of electrodes,
• said second electrode lies in a second row of electrodes,
• said third and fourth electrodes lie in a third row of electrodes, and
• said third row is located in between said first row and said second row.

A method for electrically imaging the wall of a borehole in an earth formation having a plurality of beds separated by bed boundaries utilizing a tool having a plurality of electrodes, said method comprising:

• a) generating a current flow through a portion of the formation such that current density near the wall of the borehole adjacent to said portion of the formation is substantially non-parallel to bed boundaries;
• b) using the electrodes, measuring a plurality of voltages at the wall of the borehole;
• and
• c) generating a resistivity image of the wall of the borehole based on the plurality of voltages measured, wherein
• said step of generating a current flow includes generating current flow through the portion of the formation in two substantially non-parallel directions.