Common Mode Rejection in Electromagnetic Induction Logging
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Solution Overview
Problem
Existing common mode rejection methods for electromagnetic induction logging instruments are inadequate in effectively filtering out interference signals, leading to inaccurate and fluctuating measurements due to incomplete rejection of common mode interference, lack of reliability, and high production and maintenance costs, especially in high-temperature downhole environments.
Innovation Solution
A method involving a receiving coil, switch module, differential amplifier, and processor that alternates the receiving coil between forward and reverse connection modes to separate and subtract common mode interference from the formation signal, using phase-sensitive detection to calculate and eliminate the interference, thereby enhancing measurement accuracy and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If traditional coil structure improvement methods (center tap grounding) are used, then common mode interference is partially reduced, but the rejection is incomplete and residual common mode signals remain large
Solution Approach 1:
The patent applies inversion by reversing the connection polarity of the receiving coil between forward and reverse modes. This polarity reversal causes the formation signal to invert while the common mode interference remains unchanged, enabling complete separation and rejection of common mode signals through differential processing, thereby achieving accurate measurement despite strong interference
Solution Approach 2:
The patent implements periodic action by alternately switching the receiving coil between forward and reverse connection modes. This periodic polarity reversal allows the system to collect two sets of measurements (forward and reverse) that can be processed differentially to eliminate common mode interference and extract the formation signal
2Measurement precision
If amplifying power is increased to enhance signal amplitude, then the weak measured signal is strengthened, but noise and interference signals are also amplified causing excessive fluctuations
Solution Approach 1:
The patent converts the harmful common mode interference into a useful component for differential processing. By reversing the coil polarity and measuring twice, the system captures the interference in both measurements. Subtracting the reverse measurement from the forward measurement eliminates the interference while preserving and doubling the formation signal, transforming the harmful interference into a means for signal enhancement
3Object-affected harmful factors
If coil shielding by metal structures is used, then common mode signal strength is lowered, but complete sealing is impossible and low-frequency signals below 100 KHz cannot be eliminated
Solution Approach 1:
The patent replaces the mechanical/physical shielding approach (metal structures) with an electrical/differential processing approach. Instead of physically blocking interference with shields that fail at low frequencies, the system uses electrical polarity reversal and differential measurement to eliminate common mode interference across all frequencies, including low-frequency signals below 100 KHz
4Object-affected harmful factors
If traditional common mode rejection methods are used, then some interference is reduced, but the methods are prone to temperature and vibration effects and subject to aging in high-temperature downhole environments
Solution Approach 1:
The patent implements self-service by having the system measure itself in two different states (forward and reverse polarity). The differential processing uses the system's own measurements to automatically eliminate common mode interference without requiring external calibration or adjustment. This self-referential approach makes the system inherently resistant to temperature, vibration, and aging effects
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 approach effectively reduces common mode interference, providing accurate and stable formation signal measurements even in the presence of strong interference, across various frequencies, and is applicable to both wireline and logging while drilling operations.
Implementation Method 1
an electromagnetic induction logging instrument excites strong electromagnetic fields in a geological formation through a transmitting coil and acquires parameters of the formation (such as resistivity and dielectric constant) by measuring the phase and amplitude of the receiving coil signal
Implementation Method 2
by adding a center tap and connecting it to ground, the common mode interference can be partially reduced by utilizing the symmetry on both sides of the tap and the common mode rejection characteristic of the differential amplifier
Implementation Method 3
The processor controls the switch module switch to cause the receiving coil to be in the forward or reverse connection mode
Implementation Method 4
The processor conducts phase-sensitive detection processing on the data output by the signal acquisition module
Data Source
AI summary
A method of reducing or rejecting common mode interference and a circuit for electromagnetic induction logging with improved common mode interference reduction or rejection are disclosed, which are useful in the field of electromagnetic induction logging technology. Accurate formation signals are calculated based on phases and amplitudes of common mode signals, which do not change with the coil access mode, and phases of differential signals, which are reversed with amplitudes unaltered when the coil is connected reversely. This provides a mechanism to effectively eliminate the influence of common mode interference even when the common mode interference is greater in amplitude than the signals from a geological formation.