Marine CSEM Anisotropy Correction via Segmented Field Data
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Offshore controlled-source electromagnetic surveys face challenges in accurately determining earth resistivity due to electrical anisotropy, which distorts signals and can lead to false positives or negatives in hydrocarbon detection, as existing methods fail to effectively account for the difference between horizontal and vertical resistivity measurements.
Innovation Solution
A computer-implemented method that uses specific electromagnetic field data from online and offline receiver locations to solve Maxwell's equations, distinguishing between horizontal and vertical resistivity components, and calculating vertical electrical anisotropy by combining measurements from HED and HMD sources, including vertical and horizontal electric and magnetic field components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If HED controlled source is used for marine CSEM surveys, then operational advantages and superior coupling of energy into the earth are achieved, but signal distortion due to electrical anisotropy occurs leading to false positives or negatives in hydrocarbon detection
Solution Approach 1:
The invention segments the measurement process by separately determining horizontal resistivity (using offline Hz data) and vertical resistivity (using online Ez data) components. This segmentation allows the anisotropic effects to be isolated and corrected, resolving the contradiction between maintaining operational efficiency with HED sources and improving the reliability of hydrocarbon detection by eliminating signal distortion.
2Device complexity
If electrical anisotropy is not accounted for in signal interpretation, then simpler interpretation methods are used, but accurate determination of earth resistivity and hydrocarbon detection are compromised
Solution Approach 1:
The invention applies preliminary action by first determining the anisotropy parameters (horizontal and vertical resistivity ratios) before interpreting the CSEM data for hydrocarbon detection. This preliminary characterization of the subsurface anisotropic properties allows subsequent interpretation to be corrected for these effects, improving measurement precision without requiring complex real-time adjustments during hydrocarbon detection.
3Device complexity
If only horizontal resistivity measurements are used, then measurement process is simpler, but vertical resistivity information needed for accurate anisotropy determination is lost
Solution Approach 1:
The invention achieves multi-functionality by using the same HED CSEM survey data to simultaneously determine both horizontal and vertical resistivity components. By processing different field components (offline Hz for horizontal, online Ez for vertical) from the same survey, the method extracts multiple types of resistivity information without requiring separate measurement campaigns, thus avoiding information loss while maintaining practical complexity levels.
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 method allows for accurate determination of electrical vertical transverse anisotropy, enabling precise prediction of reservoir resistivity and reducing false indications in hydrocarbon detection by accounting for anisotropic effects in offshore electromagnetic surveys.
Implementation Method 1
controlled-source electromagnetic surveys... in which a controlled electromagnetic transmitter is towed above or positioned between electromagnetic receivers on the sea floor
Implementation Method 2
the resistivity in the earth can be strongly dependent upon the direction of the electrical current flow used to make these measurements... This phenomenon is called electrical anisotropy
Implementation Method 3
the vertical electrical resistivity can be much (two or more times) larger than the horizontal electrical resistivity, especially in finely layered rocks such as shales
Data Source
AI summary
A method is disclosed for determining earth vertical electrical anisotropy from offshore electromagnetic survey measurements. The method requires both online and offline data, which includes at least one electromagnetic field component sensitive at least predominantly to vertical resistivity and another component sensitive at least predominantly to horizontal resistivity. Using a horizontal electric dipole source, online EZ and offline HZ measurements are preferred. For a horizontal magnetic dipole source, online HZ and offline EZ data are preferred. magnetotelluric data may be substituted for controlled source data sensitive to horizontal resistivity. Maxwell's equations are solved by forward modeling or by inversion, using resistivity models of the subsurface that are either isotropic or anisotropic.