Floating Sensor Nodes for Directional Seismic Imaging
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Solution Overview
Problem
Current ocean bottom seismic data acquisition methods using grounded nodes are limited by their inability to accurately distinguish between upgoing and downgoing wavefields, leading to artifacts in 3D seismic imaging, particularly with two-way wave equation algorithms like RTM, which fail to recognize wave directionality.
Innovation Solution
The use of floating sensor nodes equipped with pressure sensors and three-axis motion sensors allows for the recording of collocated pressure and pressure gradient data, enabling the creation of directional receivers that can backpropagate waves in the reverse direction of reception, improving imaging algorithms by incorporating directional information.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional grounded nodes with pressure sensors and linear motion sensors are used, then both p-waves and s-waves can be recorded, but the ability to distinguish upgoing and downgoing wavefields is lost due to node coupling with the ocean bottom
Solution Approach 1:
The patent extracts the harmful coupling effect by transitioning from grounded nodes to floating nodes. The floating node configuration removes the node from contact with the ocean bottom, thereby eliminating the coupling that causes linear motion sensors to respond to both p-waves and s-waves indiscriminately. This extraction of the harmful element (coupling) allows the pressure gradient information to cleanly distinguish upgoing from downgoing wavefields.
Solution Approach 2:
The patent introduces floating nodes as an intermediary configuration between traditional grounded nodes and the water column. These floating nodes act as mediators that maintain acoustic coupling with the water for pressure sensing while avoiding mechanical coupling with the bottom that contaminates linear motion sensor data. The floating node serves as an intermediate state that preserves the useful pressure gradient information while eliminating the harmful bottom coupling effects.
2Device complexity
If scalar pressure wavefield is used for imaging, then the imaging process is simpler, but directional information of propagating waves is lost leading to artifacts in 3D seismic images
Solution Approach 1:
The patent combines pressure data and pressure gradient data to create a composite wavefield representation that retains both scalar simplicity and vector directional information. By fusing these two types of measurements, the system achieves a composite data product that enables directional receivers without requiring complex processing of multiple independent sensor types. The composite approach maintains imaging simplicity while recovering lost directional information.
Solution Approach 2:
The patent transitions from scalar pressure field (0-dimensional magnitude only) to vector pressure gradient field (3-dimensional directional information). By incorporating the gradient measurements that provide spatial derivative information in multiple directions, the system adds dimensional information about wave propagation direction to the imaging process, enabling directional sensitivity while maintaining a unified imaging framework.
3Reliability
If two-way wave equation algorithms like RTM are used, then comprehensive wave propagation is modeled, but artifacts appear because the algorithms cannot recognize wave directionality from scalar pressure data
Solution Approach 1:
The patent performs preliminary separation of upgoing and downgoing wavefields using pressure gradient data before the two-way wave equation imaging process. By pre-processing the data to extract directional information and create directionally-filtered inputs, the algorithm receives clean, directionally-separated wavefields as initial conditions. This preliminary action eliminates the need for the imaging algorithm to recognize directionality during processing, thereby preventing artifacts while maintaining comprehensive wave propagation modeling.
Solution Approach 2:
The patent uses pressure gradient measurements to provide feedback about wave propagation direction to the imaging algorithm. The gradient data continuously informs the imaging process about the directionality of incoming waves, allowing the system to adjust the backpropagation direction accordingly. This feedback mechanism ensures that the two-way wave equation algorithm receives accurate directional information, eliminating artifacts caused by inability to recognize wave direction from scalar pressure data alone.
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 enhances the accuracy of seismic imaging by providing precise directionality of propagating waves, reducing artifacts and improving the quality of 3D subsurface images without increasing costs, by modifying the initial conditions of wavefield propagation in two-way wave equation methods like RTM.
Implementation Method 1
a pressure sensor and a three-axis motion sensor... the pressure sensor responds to a propagating acoustic wavefield
Implementation Method 2
a 3-component accelerometer measures the gradient of the propagating pressure wavefield scaled by −1/ρ where ρ is the density of the sea water
Implementation Method 3
recording in an acoustic medium, as is the case with water... records the linear motion of the node in the three orthogonal directions
Data Source
AI summary
Embodiments herein describe techniques for performing acoustic imaging when collocated pressure and three-directional pressure gradient measurements are available. Such measurements become available through the use of a hydrophone and a 3-component geophone or accelerometer when the containing node is neutrally buoyant, or nearly neutrally buoyant, and is coupled to the water column, rather than grounded and thus coupled to the ocean bottom sediments.