NMR Free Induction Decay Signal Measurement via RF Parameter Optimization
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Solution Overview
Problem
NMR well logging instruments face challenges in measuring free induction decay signals due to static magnetic field inhomogeneity, which shortens decay time and limits effective logging speed, and struggle to efficiently determine T1 properties with existing multiple pulse sequences.
Innovation Solution
The method involves inducing a static magnetic field and a radio frequency magnetic field with specific parameters to minimize inhomogeneity effects, allowing for the detection of free induction decay signals and enabling the measurement of T1 properties using a single pulse sequence, thereby increasing logging speed and efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a strong static magnetic field is induced in the formation volume, then the NMR signal strength is improved, but the static magnetic field inhomogeneity shortens the free induction decay time making measurement impracticable
Solution Approach 1:
The patent changes the parameters of the RF magnetic field, specifically using a low-power RF field with a duration of at least 100 milliseconds. This parameter change allows the system to measure FID signals even in the presence of static magnetic field inhomogeneity, resolving the contradiction between achieving sufficient signal strength and maintaining measurable decay time.
2Measurement precision
If multiple pulse sequences are used to determine T1 properties, then measurement accuracy is improved, but logging speed decreases
Solution Approach 1:
The patent combines multiple measurement objectives into a single pulse sequence. By measuring both FID signals and spin echo signals within one sequence, the system determines both T2 and T1 properties simultaneously, eliminating the need for separate measurement sequences and thereby improving logging speed while maintaining measurement accuracy.
Solution Approach 2:
The patent enables continuous measurement by using a single pulse sequence that continuously collects data for both T1 and T2 characterization. This continuous action approach replaces discrete, sequential measurements, maintaining precision while significantly improving productivity.
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 allows for accurate measurement of free induction decay signals and T1 properties with improved logging speed, enabling the determination of subsurface formation characteristics such as porosity and relaxation times without the need for multiple pulse sequences.
Implementation Method 1
inducing a static magnetic field in a sample volume
Implementation Method 2
A radio frequency magnetic field is then induced in the sample volume. The radio frequency magnetic field has parameters selected to minimize the contribution of inhomogeneity in the static magnetic field to a free induction decay time
Implementation Method 3
The free induction decay signal is then detected from the sample volume
Data Source
AI summary
A method to obtain a free induction decay signal using includes inducing a static magnetic field in a sample volume. A radio frequency (RF) magnetic field is then induced in the sample volume. The RF magnetic field has parameters selected to minimize the contribution of inhomogeneity in the static magnetic field to a free induction decay time. The free induction decay signal is then detected from the sample volume. In one example, prior to inducing the RF magnetic field, a reorienting radio frequency magnetic field is induced in the sample volume to reorient magnetic spins by a first selected angle. The inducing the RF magnetic field in this example has parameters selected to reorient spins by a second angle. The inducing the RF magnetic field and detecting the free induction decay signal are repeated until nuclear magnetic equilibrium is substantially attained.