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Semi-Differential Antenna Design for High-Frequency Electromagnetic Measurements

IP.com Disclosure Number: IPCOM000246777D
Publication Date: 2016-Jun-29

Publishing Venue

The IP.com Prior Art Database

Abstract

Downhole electromagnetic measurements used for formation resistivity and/or permittivity measurements are usually interpreted by modeling the associated antennas as magnetic dipoles. The validity of this approach, however, depends on the correct antenna feed mode with the differential mode being the textbook solution. This becomes even more important for high-frequency tools operating above ~100MHz. A true differential feed is sometimes not desirable or practical due to space or component constraints for all associated antennas. This invention demonstrates the practicability of a mixed-mode design using either single-ended transmitter and differential receiver antennas or vice versa.

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Title:  Semi-Differential Antenna Design for High-Frequency Electromagnetic Measurements

Abstract:  Downhole electromagnetic measurements used for formation resistivity and/or permittivity measurements are usually interpreted by modeling the associated antennas as magnetic dipoles. The validity of this approach, however, depends on the correct antenna feed mode with the differential mode being the textbook solution. This becomes even more important for high-frequency tools operating above ~100MHz. A true differential feed is sometimes not desirable or practical due to space or component constraints for all associated antennas. This invention demonstrates the practicability of a mixed-mode design using either single-ended transmitter and differential receiver antennas or vice versa.

Description: 

THE NOVEL FEATURES OF THE INVENTION

·         Being much less complex and reasonably resistant to misalignment errors, the semi-differential antenna design is a good alternative to the fully-differential antenna design for creating magnetic dipole sources at high frequency.

BACKGROUND AND PRIOR ART

Even with the help from modern computers, fast interpretation and inversion of resistivity logs still rely on forward modeling software developed based on reasonable simplifications of both the resistivity logging instrument and its surrounding earth formation. The earth formation is often simplified to one-dimensional layer cakes or sometimes even homogeneous whole space. The transmitter and receiver coils (aka loop antennas) of the tool are normally reduced to infinitesimal current loops (i.e. magnetic dipole sources) placed at their corresponding center locations, while the rest of the tool body is simply omitted. From experience the tool body at relatively low frequencies (less than a few MHz) normally affects only the moment of the equivalent magnetic dipole, so the above simplification is mostly valid with the inclusion of a moment scaling factor in final results. Nevertheless at higher frequencies the antenna symmetry and geometry of the tool body start to play a much more important role in modifying the shape of the electromagnetic field which makes it less straightforward, and sometimes impossible, to simplify the tool response to dipole equivalent models. In other words, for high-frequency applications, the tool must be designed with the idea of “reducible to magnetic dipoles” up from the beginning.

Loop Antenna Feed Modes

Loop antennas can be fed in three ways

  • Differential (symmetric) mode
  • Single-ended (asymmetric) mode
  • Common mode

Only the first two modes are practical for downhole tool operation. Here, the common-mode excitation is also explained as we will make the point that the single-ended mode is a superposition of common-mode and differential mode excitation.

Any common-mode response of the sensor is clearly unwanted, as it represents E-field rather than H-field coupling between TX and RX antennas of the sensor and as such cannot be model...