Marine Seismic Source Ghost Elimination via Wavefield Separation
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Solution Overview
Problem
Marine seismic data acquisition is limited by source ghosting, which causes interference and reduces the accuracy of inferring subsurface rock formations, as the energy emitted by air guns is attenuated and the ghost signal interferes with the direct wave-field, especially at low frequencies, making it difficult to achieve high-resolution imaging of shallower targets without increasing the energy source strength or depth.
Innovation Solution
Deploying at least two marine seismic energy sources at different depths and actuating them with varying time delays during seismic signal recording, allowing for the extraction and constructive summation of up-going and down-going wave-fields after phase shifting, thereby eliminating the ghost signal and maximizing the energy utilization across the frequency band.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the air gun or air gun array is actuated at greater depth to maximize low-frequency energy, then the source ghost notches occur at lower frequencies, but this limits the high-frequency parts of the spectrum needed for high-resolution imaging of shallower targets
Solution Approach 1:
The patent extracts and separates the up-going wave-field (ghost signal) from the down-going wave-field (direct signal) using wavefield separation techniques. By isolating the ghost signal component, the system can eliminate its harmful interference effects while preserving the useful direct signal, thereby resolving the contradiction between maximizing low-frequency energy and maintaining high-resolution imaging capability
Solution Approach 2:
The patent converts the harmful ghost signal into a beneficial component by extracting it and applying 180-degree phase shifting. The previously destructive interference pattern is transformed into constructive interference, allowing the ghost signal to be added to the direct signal rather than subtracted from it, thereby enhancing the overall signal quality across the frequency spectrum
2Use of energy by moving object
If the fundamental frequency of the air gun increases with increasing depth, then the energy in the low frequency end increases, but this counteracts the benefit of deeper towing for maximizing low-frequency energy
Solution Approach 1:
The patent applies 180-degree phase shifting to the extracted up-going wave-field, converting the phase relationship from destructive to constructive interference. This allows the ghost signal to reinforce the direct signal across the entire frequency spectrum, including the low-frequency end, thereby maximizing energy utilization without being constrained by the air gun's fundamental frequency characteristics
3Power
If traditional air gun arrays are used to impart acoustic energy, then the same or different vessel tows streamers for detection, but the source ghosting effect limits the accuracy of inferring subsurface rock formations
Solution Approach 1:
The patent extracts the up-going wave-field (ghost signal) from the recorded seismic data using wavefield separation techniques. By isolating this component, the system can remove its harmful interference effects from the final processed data, thereby improving the accuracy of subsurface structure inference while maintaining the full acoustic energy output of the air gun array
Solution Approach 2:
The patent converts the harmful ghost signal into a beneficial component by extracting it and applying 180-degree phase shifting. The previously destructive interference pattern is transformed into constructive interference, allowing the ghost signal to be added to the direct signal, thereby enhancing the overall signal quality and improving measurement precision across the frequency spectrum
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 effectively doubles the primary energy level for a given source, enhancing the seismic signal quality and allowing for improved high-resolution imaging by eliminating the source ghost and boosting energy around ghost notches, including the 0 Hz notch, without the need for increasing the energy source strength or depth.
Implementation Method 1
acoustic energy radiates generally outwardly from the air gun or array
Implementation Method 2
most of this energy reflects from the water surface whereupon it travels downwardly
Implementation Method 3
Hydrophones, as is known in the art, are sensors that generate an optical or electrical signal corresponding to the pressure of the water or the time gradient (dp/dt) of pressure in the water
Data Source
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AI summary
A method for acquisition and processing of marine seismic signals to extract up-going and down- going wave-fields from a seismic energy source includes deploying at least two marine seismic energy sources at different depths in a body of water. These seismic energy sources are actuated with known time delays that are varied from shot record to shot record. Seismic signals from sources deployed at different depths are recorded simultaneously. Seismic energy corresponding to each of the sources is extracted from the recorded seismic signals. Up-going and down-going wave-fields are extracted from the sources deployed at different depths using the extracted seismic energy therefrom. A method includes the separated up-going and down-going wave-fields are propagated to a water surface or a common reference, the up-going or the down-going wave -field is 180 degree phase shifted, and the signals from these modified up-going and down-going wave-fields are summed.