Logging of earth formations using electromagnetic (EM) propagation resistivity is typical accomplished by instruments that form part of a drill string. Such instruments are commonly referred to as “logging while drilling” (LWD) instruments. Typical EM propagation LWD instruments include one or more transmitter antennas, and a plurality of receiver antennas disposed on a drill collar. Radio frequency (RF) power, usually at a frequency in a range of 0.1 to 10 MHz is passed through the one or more transmitter antennas, and an amplitude and phase of RF voltages induced in the receiver antennas are measured. Generally, the conductivity of earth formations proximate any pair of receiver antennas is related to the amplitude ratio and phase difference of the induced voltages between the receiver antennas.
In most EM propagation resistivity LWD instruments, the antennas are formed as loops or coils wound around the exterior of the instrument or drill collar so that they form magnetic dipoles having moments substantially parallel to the axis of the instrument. Such a configuration makes the instrument mainly responsive to the formations disposed laterally around the wellbore proximate the antennas. Various combinations of transmitter and receiver antennas are known in the art which provide responses in particular selected axial positions along the drill collar, and at selected lateral depths from the wellbore, but the sensitivity of most EM propagation LWD instruments is primarily laterally around the instrument. Such sensitivity is most useful when the instrument axis is substantially perpendicular to the attitude (“dip”) of the various layers of the earth formations penetrated by the wellbore.
Many conventional EM induction and propagation logging tools have transmitter and receiver antennas that are mounted with their axes along the longitudinal axis of the tool. Thus, these tools are implemented with antennas having longitudinal or axial magnetic dipole moments. A “transverse” antenna or coil has a magnetic dipole moment that is perpendicular to the tool axis, and a “tilted” antenna has a magnetic dipole moment that is neither parallel with nor perpendicular to the tool axis. Many recent patents disclose methods and apparatus to make directional measurements and obtain resistivity anisotropy and bedding orientation, involving tilted or transverse antenna.
Various antenna designs beyond traditional transverse and tilted antennas are possible, and can be used to achieve directional measurements and to obtain resistivity anisotropy and formation dip and azimuth. Each such design has respective advantages and disadvantages. Certain novel antenna designs and related systems and methods are described herein.