| 摘要 |
In a downhole oil well restivitity logging tool having a focused electrode arrangement, and in order to avoid an undesirable vertical current component which distorts the measurement, a vertical current is deliberately established in the adjacent formation and is varied, the voltage of vertically spaced electrodes (46,48) being monitored to detect zero crossing. Zero crossing is indicative of zero vertical current flowing in the formation. At this instant, the entire current flow from the electrode array is radially into the formation whereby measurements of the current flow and voltage of the current emitting electrode (41) enables calculation of the formation resistivity. |
