| 摘要 |
A resistivity tool for use in an LWD system includes a transmitter array with multiple transmitters positioned above a pair of receivers. The transmitters are selectively energized, causing current to be induced in the collar of the tool. The axial current in the tool collar is measured by the pair of receivers to determine the amount of current that flows into the formation between the receivers. From this, the formation resistivity can be measured for each transmitter to provide multiple depths of investigation. In addition, the lower receiver also can be used to selectively monitor the bit resistivity. A plurality of discrete electrodes are positioned between the two receivers to permit imaging of the borehole by obtaining azimuthally sensitive resistivity readings based upon the voltage level at each discrete electrode and the amount of current flowing into the formation. In one embodiment, the voltage level of each discrete electrode also is summed to provide a radial resistivity reading based on the summed voltage levels and the current flowing into the formation. In a second embodiment, a ring electrode also is provided from which a voltage level is obtained in the vicinity of the receiver array for use in computing the radial resistivity reading. According to another embodiment, the discrete electrodes are formed as lateral-logs with a conductive central disc surrounded by two concentric metal rings and a focusing metal ring. In this embodiment, the concentric rings are kept at the same voltage by changing the amount of current that is emitted from the focusing metal ring. By measuring the amount of current emitted, together with the voltage at one of the concentric rings, an azimuthally sensitive focused resistivity reading can be obtained. |
