Subsurface Resistivity Modeling Using Equivalent Current Dipoles
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Solution Overview
Problem
Current electromagnetic (EM) geophysical soundings face challenges in accurately calculating subsurface current flow and resulting EM fields due to the presence of conducting casings in boreholes, which complicates the computation and increases computational resources, especially when dealing with varying resistivity and complex subsurface structures.
Innovation Solution
A method is developed to efficiently calculate subsurface EM fields by representing the conducting casing as an equivalent electromagnetic source within a subsurface model, allowing for the decoupling of primary fields produced by the casing from subsurface anomalies, using a series of current dipoles and separating the problem into DC and AC components to reduce processing time and complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conducting casings are included in the subsurface model for EM field calculation, then measurement precision is improved, but device complexity and computational resources increase significantly
Solution Approach 1:
The patent extracts the casing from the subsurface model and represents it as an equivalent electromagnetic source (current dipole) at the borehole location. This separation allows the casing to be accounted for in the calculation without being included as a physical object in the subsurface model, thereby reducing model complexity while maintaining measurement precision.
Solution Approach 2:
The patent introduces an equivalent electromagnetic source (current dipole) as an intermediary to represent the casing's effect. Instead of directly modeling the complex conducting casing structure, the current dipole serves as a simplified mediator that captures the essential electromagnetic behavior of the casing, reducing computational burden while preserving accuracy.
2Measurement precision
If conducting casings are included in the subsurface model, then measurement precision is improved, but processing time increases
Solution Approach 1:
By extracting the casing from the subsurface model and representing it as an equivalent current dipole, the patent eliminates the need for computationally intensive modeling of the casing structure within the subsurface mesh, significantly reducing processing time while maintaining the ability to account for casing effects on measurements.
Solution Approach 2:
The patent changes the representation parameter of the casing from a detailed geometric structure with material properties to a simplified current dipole moment parameter. This parameter transformation reduces the computational complexity from solving Maxwell's equations with complex boundary conditions to using a closed-form or simplified numerical solution.
3Measurement precision
If the subsurface model includes extensive regions to capture all EM field effects, then measurement precision is improved, but the computational resources and processing time increase
Solution Approach 1:
The patent extracts the casing effect from the extended subsurface modeling region and represents it as a localized current dipole source. This allows the EM field calculation to focus on the subsurface anomalies without requiring extensive modeling of the casing structure, improving calculation efficiency while maintaining accuracy in capturing the essential physics.
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 faster and more cost-effective calculation of subsurface resistivity distributions, allowing for accurate modeling of EM fields without being limited by the extent of the subsurface region, and is applicable to various borehole configurations and orientations.
Implementation Method 1
a significant fraction of the electric current produced by the source flows along the casing of a borehole
Implementation Method 2
EM geophysical soundings probe electrical resistivity in the earth as a function of depth, where 'earth' generally refers to any region in the subsurface
Data Source
AI summary
A method for efficiently calculating a subsurface distribution of electrical resistivity or conductivity generated by an electromagnetic (EM) source is provided wherein a significant fraction of the electric current produced by a source flows along a casing of a borehole. The method is comprised of two steps: calculating EM fields produced by the casing in the background lithology; and calculating EM fields caused by a resistivity anomaly in the presence of an EM field produced by the casing within a subsurface or survey model that does not include the casing.