Inductive Downhole Sensor Center Tap Common Mode Rejection
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Solution Overview
Problem
Electromagnetic well logging in boreholes faces inaccuracies due to common mode coupling, particularly in non-conductive mud and highly conductive formations, leading to distorted measurements and low Common Mode Rejection Ratio (CMRR), which affects the estimation of formation properties like resistivity.
Innovation Solution
The use of a tool with a receiver assembly featuring induction antennas with overlapping windings and a balun transformer to minimize common mode coupling, along with a capacitive shield to reduce eddy currents, enhances the Common Mode Rejection Ratio (CMRR) and improves measurement accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional receiver assemblies are used in non-conductive mud and highly conductive formations, then electromagnetic measurements can be obtained, but common mode coupling causes measurement distortion and low CMRR
Solution Approach 1:
The receiver assembly is segmented into multiple induction coils with separate windings (first winding and second winding) that are processed independently. Each winding captures the electromagnetic signal separately, allowing differential processing to reject common mode interference while preserving the formation response signal.
Solution Approach 2:
The patent inverts the conventional single-winding receiver design by using multiple windings with opposite polarities. The windings are configured to produce signals that are equal in magnitude but opposite in phase for the formation response, while common mode interference appears identical in both windings. This inversion allows the differential amplifier to cancel common mode signals while preserving the formation response.
2Measurement precision
If standard induction antennas are used, then electromagnetic signals can be received, but eddy currents in conductive structures cause additional interference and reduce measurement quality
Solution Approach 1:
A capacitive shield is introduced as an intermediary element between the induction coils and the conductive tool body. This shield acts as a mediator that blocks the coupling path for eddy currents while allowing the electromagnetic signal from the formation to pass through to the receiver coils, thereby reducing interference without compromising signal quality.
3Reliability
If simple receiver assemblies are used, then device complexity is low, but Common Mode Rejection Ratio is insufficient for accurate formation property estimation
Solution Approach 1:
Multiple functional elements are merged into a single integrated receiver assembly: multiple induction coils with separate windings, capacitive shields, and differential amplification circuitry are combined in one compact unit. This merging achieves high CMRR and measurement reliability while maintaining a manageable device structure suitable for downhole deployment.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration significantly reduces common mode signal interference, leading to more accurate electromagnetic measurements and enhanced real-time estimation of formation properties during drilling operations.
Implementation Method 1
creating electromagnetic (EM) excitation in the formation with at least one transmitter
Implementation Method 2
receiving related signals at one or more receiver antennas
Data Source
Figure 1A
Figure 1B~2A
Figure 2B~3
AI summary
Systems, methods, and devices for evaluation of an earth formation intersected by a borehole using a logging tool. Methods include generating an excitation in the formation with an electromagnetic (EM) signal from a transmitter assembly at at least one frequency; making EM measurements using an EM tool on a tool string in the borehole by receiving a signal at at least one receiver assembly responsive to the excitation; wherein the at least one receiver assembly includes an induction antenna comprising a first winding and a second winding that share a common center tap, and wherein the average potential on the induction antenna is substantially the same as a ground potential. The receiver assembly or transmitter assembly may comprise a receiver or transmitter electrically connected to the induction antenna by a balun transformer, and a capacitive shield encompassing the induction antenna, the capacitive shield comprising a meander shaped isolation gap.