Switchable NMR Tool Using Magnetic Core Residual Magnetization
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Solution Overview
Problem
Existing NMR well logging technologies face challenges in achieving a high signal-to-noise ratio (SNR) and deep investigation depth due to limitations in generating strong, fast-switchable static magnetic fields, leading to poorly defined excitation regions and power consumption issues.
Innovation Solution
A nuclear magnetic resonance (NMR) sensing apparatus utilizing a magnetic core with residual magnetization and a polarizing coil to efficiently switch the static magnetic field, allowing for adiabatic deviation of nuclear spins and generation of gradient echoes, while minimizing power consumption through hysteresis-based magnetic material properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a switchable static magnetic field is used to polarize nuclear spins, then the NMR measurement can be performed, but the signal-to-noise ratio is low due to power consumption limitations
Solution Approach 1:
The patent applies periodic action by using alternating magnetic fields at specific frequencies to manipulate nuclear spin states. The system alternates between applying a static magnetic field for polarization and RF magnetic fields for excitation, creating a cyclic process that improves signal generation efficiency and reduces power consumption compared to continuous field application.
Solution Approach 2:
The patent utilizes parameter changes by varying the strength and direction of magnetic fields over time. The static magnetic field strength is changed to match the Larmor frequency of nuclear spins, and RF field strengths are modulated to achieve resonance conditions, thereby enhancing signal-to-noise ratio while optimizing power usage through frequency tuning rather than continuous high-power application.
2Measurement precision
If the static magnetic field is switched off to allow free precession, then NMR signal can be detected, but the excitation region is poorly defined
Solution Approach 1:
The patent applies local quality by creating spatially varying magnetic field conditions. The static magnetic field is applied locally to define a specific excitation volume, while gradient magnetic fields are introduced to provide spatial encoding. This allows the system to maintain well-defined excitation regions while enabling signal detection through precession, resolving the contradiction between localization and detection capability.
Solution Approach 2:
The patent uses preliminary action by pre-aligning nuclear spins in a specific spatial region before signal detection begins. The static magnetic field is applied in advance to establish polarization in the target volume, and gradient fields are prepared beforehand to define spatial boundaries. This preliminary field configuration ensures both good signal detection and clear excitation region definition without requiring continuous field switching during measurement.
3Productivity
If RF voltages are applied to induce precession, then nuclear spins can be excited, but the depth of investigation is limited
Solution Approach 1:
The patent applies dimensionality change by transitioning from purely longitudinal magnetic field application to including transverse gradient field components. By adding spatial dimensionality through gradient fields, the system achieves both efficient excitation and enhanced depth of investigation. The gradient fields create spatially varying precession frequencies that allow deeper penetration while maintaining excitation efficiency through resonance conditions.
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
The solution enables deep and high SNR NMR measurements by reducing power consumption and maintaining magnetic field strength, facilitating efficient non-resonant excitation and gradient echo generation, thereby improving the depth of investigation and signal quality.
Implementation Method 1
A magnetic core with a residual magnetization which polarizes nuclear spins in a specified direction in the earth formation
Implementation Method 2
The magnetic core may be made of a permeable material having magnetic properties related to a dimension of the core. The magnetic core may further have a coercive force which is substantially equal to the ratio of the saturation magnetization and an effective permeability of the magnetic core
Implementation Method 3
A polarizing coil produces a change of a static magnetic field in the specified direction and produces precession of the nuclear spins
Implementation Method 4
RF voltages are induced in the receive antenna as a result of precessional rotation of nuclear spin axes of hydrogen protons or other nuclei about the static magnetic field with characteristic resonance or Larmor frequency
Implementation Method 5
Prior to the termination an alternated magnetic field directed transversal to the earth's magnetic field at a frequency corresponding to the Larmor precession frequency is applied. As a result of application of the alternating magnetic field and adiabatic termination of the polarizing magnetic field the nuclear spins are sweeping onto condition of driven resonance in the earth's magnetic field
Data Source
AI summary
A nuclear magnetic resonance sensing apparatus and method for operating in an earth borehole comprises a source of switchable magnetic field to polarize nuclei in the region of interest, said source comprising a coil wound on a magnetic core having controllable residual magnetization. Maintaining the magnetization of the core during a polarization interval does not require steady current in the coil. Switching intensity and polarity of magnetization of the core causes precession of spin magnetic moments of the nuclei; the precession induces a signal indicative of nuclear magnetic resonance properties of earth formations.