Apparatus and Method for Investigating Boreholes filled with Conductive and Non-Conductive Fluids

摘要

An apparatus for investigating a formation surrounding a borehole, comprising:
- a first electrode (10) for injecting a current into the formation;
- a guard electrode adjacent the first electrode, the guard electrode being held at the same voltage as the first electrode;
- a return electrode (6), insulated (12) from and spaced from the first electrode and guard electrode (14), for returning current passing through the formation from the first electrode (10) and guard electrode (14); and
- a signal source (18) connected between the first electrode (10) and the return electrode (6) and operable to generate an injection current at either a first, lower frequency suitable for measurements when the borehole is filled with a substantially conductive fluid, or a second, higher frequency for measurements when the borehole is filled with a substantially non-conductive fluid.