ELECTROMAGNETIC WAVE RESISTIVITY TOOL WITH TILTED ANTENNA

A downhole method and apparatus for simultaneously determining the horizontal resistivity, vertical resistivity, and relative dip angle for anisotropic earth formations uses an antenna configuration in which a transmitter antenna (T1-TN) and a receiver antenna (R1, R2) are oriented in non-parallel planes such that the vertical resistivity and the relative dip angle are decoupled. Preferably, either the transmitter or the receiver is mounted in a conventional orientation in a first plane that is normal to the tool axis, and the other antenna is mounted in a second plane that is not parallel to the first plane. Also disclosed is a method and apparatus for steering a downhole tool during a drilling operation in order to maintain the borehole within a desired earth formation. The steering capability is enabled by computing the difference or the radio of the phase-based or amplitude-based responses of the receiver antennas (R1, R2) which are mounted in planes that are not parallel to the planes of the transmitter antennas (T1-TN).