Marine VSP Quality Factor Estimation Using Dual Sensor Ghost Removal
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Solution Overview
Problem
Existing methods for estimating seismic attenuation quality factors (Q) in marine Vertical Seismic Profiles (VSPs) face challenges due to poor quality of initial traces caused by multiple casing strings and interference from source ghosting, leading to inaccurate Q estimation for shallow subsurface regions.
Innovation Solution
Deployment of a dual sensor system comprising a hydrophone and a geophone at the sea floor, using a matching filter to eliminate source ghosting and generate a reference trace for Q estimation, which provides a more accurate and time-depth compensated attenuation value.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single sensor (hydrophone or geophone) is used for Q estimation, then the measurement setup is simple, but the accuracy of Q estimation deteriorates due to source ghosting and poor trace quality
Solution Approach 1:
The patent combines both hydrophone and geophone sensors into a dual-sensor system. The hydrophone records the source ghost-free signal while the geophone provides the vertical component, and their combined data enables accurate Q estimation by eliminating source ghosting effects that plague single-sensor approaches.
Solution Approach 2:
The patent uses the hydrophone as an intermediary to obtain a ghost-free reference signal. By recording the seismic wavefield without source ghosting interference, the hydrophone provides a clean reference that mediates the Q estimation process, allowing accurate attenuation measurement when combined with geophone data.
2Reliability
If a reference trace from deeper subsurface is used, then the trace quality is improved, but the representativeness for shallow subsurface Q estimation deteriorates
Solution Approach 1:
The patent segments the Q estimation process into shallow and deep components. By using the dual sensor to capture shallow subsurface signals directly at the surface, it obtains high-quality shallow Q values that would otherwise be contaminated. The deeper traces provide additional quality but are treated as separate from the shallow Q estimation, avoiding the information loss problem.
3Ease of manufacture
If multiple casing strings are present in the borehole, then the well construction is practical, but the trace quality for Q estimation deteriorates
Solution Approach 1:
The patent uses the surface dual sensor as an intermediary measurement system that bypasses the borehole environment entirely. By recording seismic signals at the surface before they enter the problematic borehole section with multiple casing strings, it obtains high-quality reference traces that are completely free from casing-related contamination, enabling accurate Q estimation despite the impractical well construction conditions.
Data Source
AI summary
A seismic attenuation quality factor Q is determined for seismic signals at intervals of subsurface formations between a seismic source at a marine level surface and one or more receivers of a well. Hydrophone and geophone data are obtained. A reference trace is generated from the hydrophone and geophone data. Vertical seismic profile (VSP) traces are received. First break picking of the VSP traces is performed. VSP data representing particle motion measured by a receiver of the well are generated. The reference trace is injected into the VSP data. A ratio of spectral amplitudes of a direct arrival event of the VSP data and the reference trace is determined. From the ratio, a quality factor Q is generated representing a time and depth compensated attenuation value of seismic signals between the seismic source at the marine level surface and the first receiver.