Pulsed Neutron Capture Logging Signal Separation
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Solution Overview
Problem
Accurate determination of thermal capture cross-sections in formations surrounding an earth borehole is challenging due to complex signal separation issues between borehole and formation signals, especially in open-hole logging, where existing methods face limitations in handling multiple exponential decay components and borehole contamination.
Innovation Solution
The use of regularization in the inversion process to derive optimized exponential components from gamma ray measurements, allowing for fitting of many sigma values beyond the number of time windows, and incorporating a penalty term in the error function to ensure smooth behavior and reduce noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional exponential fitting methods are used to separate borehole and formation signals, then the measurement process is simple, but the accuracy deteriorates due to inability to handle multiple exponential decay components and borehole contamination
Solution Approach 1:
The patent segments the complex decay signal into multiple exponential components, each representing different physical sources (borehole fluid, formation, tool). By fitting the measured decay curve to a sum of exponentials with distinct time constants, the method separates overlapping signals that traditional single-exponential fitting cannot resolve, thereby improving measurement precision without requiring complex hardware modifications
Solution Approach 2:
The patent introduces an intermediary mathematical model (multi-exponential fitting function) that acts as a mediator between the raw measured decay data and the desired formation parameters. This intermediary model allows indirect extraction of formation thermal capture cross-section by first fitting the composite decay curve and then isolating the formation-specific component, resolving the contradiction between simplicity and accuracy
2Reliability
If the number of exponential components is increased to improve signal separation, then the fit quality improves, but the computational complexity and noise sensitivity increase
Solution Approach 1:
The patent performs preliminary action by pre-defining the structure of the multi-exponential model with physically meaningful constraints before fitting. The time constants and amplitudes are constrained based on expected physical ranges, which simplifies the inversion process and reduces computational complexity while maintaining reliability and repeatability of results
Solution Approach 2:
The patent implements feedback through iterative fitting procedures that adjust the multi-exponential parameters based on the difference between measured and modeled decay curves. This feedback mechanism allows the system to converge to optimal parameter values that maximize fit quality while maintaining computational efficiency and result reliability
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 enables accurate and repeatable determination of thermal capture cross-sections, effectively separating borehole and formation signals, even in complex pulsing schemes, resulting in improved logging accuracy and reliability.
Implementation Method 1
monitoring the returning gamma radiation, which results from the capture of thermal neutrons by highly-absorbing isotopes present in the formation
Implementation Method 2
detecting, in the borehole, resultant gamma ray counts
Data Source
AI summary
A method for determining thermal capture cross-sections of formations surrounding an earth borehole, including the following steps: providing a logging device that is moveable through the borehole; transmitting, from the logging device, bursts of neutrons into the formations; detecting, at the logging device, resultant gamma ray counts, and deriving a measurement spectrum from the gamma ray counts; deriving a forward model comprising a combination of model exponential components having respective model decay times and model amplitudes; deriving an error function that depends on comparison between the forward model and the measurement spectrum; and determining, by regularized inversion, optimized exponential components of the model that substantially minimize the error function; the optimized exponential components being indicative of the thermal capture cross-sections of the formations.