Shallow Water CSEM Gradient Analysis for Airwave Suppression
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Solution Overview
Problem
Conventional controlled source electromagnetic (CSEM) surveying techniques are limited in shallow water due to the dominance of the 'airwave' component, which reduces the sensitivity to subterranean resistivity structures like hydrocarbon reservoirs, leading to reduced detectability and increased requirements for signal-to-noise ratio.
Innovation Solution
The method involves analyzing horizontal electric or magnetic field data by determining and combining horizontal gradients along orthogonal directions to generate combined response data, which is less sensitive to transverse electric (TE) mode components that propagate through air, allowing for more effective detection in shallow water without relying on vertical electric field measurements prone to noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional CSEM surveying is performed in shallow water, then the survey can be conducted in accessible water depths, but the airwave component dominates the signal reducing sensitivity to subterranean structures
Solution Approach 1:
The electromagnetic field signal is segmented into TE mode and TM mode components. The method selectively processes TM mode components which are less affected by airwave contamination, while separating them from the dominant TE mode airwave signals through gradient operations and mode decomposition techniques.
Solution Approach 2:
The method changes the measurement parameters by computing horizontal gradients of electromagnetic field components and combining them to generate TM mode decomposition data. This parameter transformation converts the raw field measurements into a form that suppresses airwave dominance and enhances sensitivity to subterranean structures in shallow water.
2Device complexity
If horizontal electric or magnetic field measurements are used, then the measurement setup is simpler and less noisy, but the data is contaminated by TE mode airwave components
Solution Approach 1:
The method extracts the TM mode signal component from the total electromagnetic field by computing horizontal gradients and combining field components. This extraction process separates the useful TM mode information (sensitive to subterranean structures) from the contaminating TE mode airwave components in the horizontal field measurements.
Solution Approach 2:
The horizontal gradients of electric and magnetic field components serve as intermediaries that mediate between the raw horizontal field measurements and the final TM mode decomposition data. These gradient operations act as a filtering mechanism that reduces airwave contamination while preserving the signal of interest.
3Power
If TE mode components are present in the signal, then the signal strength is higher, but the detectability of subterranean structures is reduced due to airwave dominance
Solution Approach 1:
Instead of using the dominant TE mode components directly for detection, the method inverts the approach by computing TM mode decomposition data from horizontal gradients of field components. This inversion strategy uses the weaker but structurally-sensitive TM mode information that can be extracted even from horizontal field measurements, overcoming the limitations of direct TE mode usage.
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 analysis of survey results in shallower water than previously possible, providing data that is insensitive to TE mode components and independent of transmitter orientation, thus enhancing the detection of subterranean resistive or conductive bodies like hydrocarbon reservoirs.
Implementation Method 1
propagation occurs by diffusion of electromagnetic fields
Implementation Method 2
The rate of decay in amplitude and the phase shift of the signal are controlled both by geometric spreading and by skin depth effects
Implementation Method 3
The horizontal electric dipole detectors are sensitive to horizontal components of the electric fields induced by the HED transmitter
Implementation Method 4
The horizontal magnetic field detectors are sensitive to horizontal components of the magnetic fields, for example the magnetic flux density, induced by the HED transmitter
Data Source
AI summary
A method of analyzing electromagnetic survey data from an area of seafloor 6 that is thought or known to contain a resistive or conductive body, for example a subterranean hydrocarbon reservoir 12, is described. The method includes providing horizontal electromagnetic field data obtained by at least one receiver 125 from at least one horizontal electric dipole transmitter 22. Horizontal gradients in the electromagnetic field data are determined for a first component of the electromagnetic field data along a first direction and for a second component of the electromagnetic field data along a second direction. The first and second components can be the electric field along the first and second directions, or the magnetic field perpendicular to the first direction and second directions. The gradients are then combined to provide combined response data. Because the combined response data are relatively insensitive to the transverse electric (TE) mode component of the transmitted signal, the method allows hydrocarbon reservoirs to be detected in shallow water where the TE mode component interacting with the air would otherwise dominate.