Patent ID: 7256582

Claim:
An apparatus for determining a resistivity parameter of an earth formation, the apparatus comprising: (a) a logging tool in a borehole in the earth formation, the logging tool having at least one measurement electrode configured to convey a measure current into the earth formation; (b) at least one guard electrode associated with the at least one measurement electrode configured to focus the measure current; (c) a shielding electrode interposed between the at least one guard electrode and the at least one measurement electrode; and (d) circuitry that minimizes a current flow between the at least one measurement electrode and the shielding electrode, the circuitry including a processor configured to alter at least one of (i) a potential of the at least one measurement electrode, (ii) an impedance between the at least one guard electrode and the at least one measurement electrode, and (iii) a potential of a least one guard electrode.