Blurred Free-Surface Reflectivity for Seismic Deghosting
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Solution Overview
Problem
Current seismic surveying methods face limitations in bandwidth due to ghost reflections from the sea surface, which interfere with primary reflections and reduce the usability of seismic data, particularly in marine surveys, where the sea surface reflectivity is complex and frequency-dependent.
Innovation Solution
The method involves defining a 'blurred' free-surface reflectivity function, using either uniform or Gaussian distributions to model the sea surface, which simplifies the deconvolution of ghost reflections and improves the deghosting process by providing time and frequency domain expressions for ghost functions, allowing for easier attenuation of ghost effects in seismic data processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional deghosting methods are used to remove ghost reflections, then ghost effects are attenuated, but the process becomes unstable due to spectral notches and frequency-dependent reflectivity
Solution Approach 1:
The patent introduces a wave height distribution function as an intermediary model to represent the complex sea surface reflectivity. This function serves as a mediator between the actual variable sea surface and the deghosting algorithm, providing a stable mathematical representation that can be consistently applied across different frequency bands and sea conditions, thereby stabilizing the deconvolution process while effectively attenuating ghost reflections
Solution Approach 2:
The patent transforms the frequency-dependent reflectivity problem into a time-domain wave height distribution representation. By changing the parameter domain from frequency to time and using statistical distribution parameters (mean, standard deviation) to characterize sea surface variability, the method achieves stable deghosting that accounts for frequency-dependent effects without the instability of traditional frequency-domain approaches
2Measurement precision
If high-resolution seismic data is acquired to characterize thin hydrocarbon reservoirs, then bandwidth is increased, but ghost reflections create spectral notches that reduce usable bandwidth
Solution Approach 1:
The patent converts the harmful spectral notches caused by ghost reflections into beneficial information by modeling the wave height distribution that causes them. Instead of treating spectral notches as mere obstacles, the method uses them to infer sea surface conditions and apply appropriate deghosting filters, thereby recovering usable bandwidth while maintaining the high-resolution characteristics needed for characterizing thin hydrocarbon reservoirs
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 moderates the spectral notches and phase reversals associated with ghost reflections, leading to more stable deconvolution and improved high-resolution seismic data acquisition, enhancing the characterization of subsurface hydrocarbon reservoirs by reducing the interference from ghost reflections.
Implementation Method 1
Acoustic signals travelling upwardly in the water layer will be reflected from this interface and undergo a reversal in polarity because the reflection coefficient of the air/water interface is approximately −1. Such reflected signals are termed ghost reflections.
Implementation Method 2
Ghost reflections destructively interfere with the primary reflection of interest resulting in 'notches' in the frequency spectrum of the detected acoustic signal at particular frequencies.
Implementation Method 3
Methods known in the art for reducing the effect of ghost reflections are termed 'deghosting.'
Data Source
AI summary
A method according to one aspect for deghosting seismic data includes determining wave heights in an area wherein marine seismic data have been acquired by actuating a seismic energy source in a body of water and detecting seismic energy at each of a plurality of spaced apart seismic receivers deployed in the water. A reflectivity distribution function of the water surface is determined using the determined wave heights. A ghost function is determined from the reflectivity distribution function. The ghost function is applied to the marine seismic data to deghost the marine seismic data. In other embodiments, the reflectivity distribution function may be used to invert notional source signatures from near field seismic energy or modeling/attenuation free-surface multiples for both land and marine seismic data.