| abstract |
In accordance with an illustrative embodiment of the present invention, methods and apparatus are disclosed for measuring the average conductivity and heterogeneity of formations surrounding a borehole. To accomplish this, certain phase components of the electrical signal produced by an induction logging tool are measured and combined in accordance with given mathematical relationships to produce indications of the average conductivity and heterogeneity of a formation. Moreover, the average conductivity and heterogeneity indications can in turn be combined to give indications of the conductivity of different radial regions of the formation under investigation. Additionally, to measure these phase components, a technique is disclosed for eliminating the mutual coupling between the transmitter and receiver coils of an induction logging tool. In this technique, the transmitter coil(s) is excited at two different frequencies and the mutual coupling component of the voltage induced in the receiver coil(s) at the lower frequency is used to cancel out the mutual coupling component at the higher frequency. The phase components used for computational purposes are then measured at the higher frequency. |