Seismic Velocity Profile Determination via Virtual Source Migration
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional seismic velocity profiling techniques, such as check shots and zero-offset vertical seismic profiles, face challenges in obtaining accurate profiles in complex overburden conditions due to distortion of first arrival waveforms and difficulty in measuring shear wave velocities.
Innovation Solution
The method involves creating a virtual seismic source at a downhole receiver location, using multiple seismic receivers to record and cross-correlate wave signals, which allows for the derivation of seismic velocity profiles that are insensitive to overburden complexity and enables the measurement of both P-wave and S-wave velocities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional check shot technique is used with surface source, then velocity profile can be obtained, but first arrival waveforms are distorted and difficult to pick in complex overburden
Solution Approach 1:
The patent inverts the conventional check shot configuration by placing receivers in the borehole and using surface sources, then applying migration techniques to back-propagate the recorded wavefields to determine velocities. This inversion approach transforms the distorted first arrivals into coherent migrated events that are easy to pick, directly resolving the detection difficulty while maintaining velocity accuracy
Solution Approach 2:
The patent introduces migration as an intermediary processing step between data acquisition and velocity determination. This migration process acts as a mediator that transforms the complex, distorted wavefields recorded in the borehole into simplified, focused images where first arrivals are clearly identifiable, thereby solving both the detection difficulty and accuracy issues
2Measurement precision
If source is placed in borehole to shorten travel path, then first arrival measurement is improved, but it is cumbersome to place and operate physical source downhole
Solution Approach 1:
Instead of placing the source downhole as conventionally done, the patent inverts the configuration by using surface sources with borehole receivers, then applying migration to achieve the same measurement objectives. This inversion eliminates the operational complexity of downhole source deployment while maintaining accurate first arrival measurement through computational back-propagation
Solution Approach 2:
The patent replaces the mechanical complexity of physically deploying and operating sources downhole with a computational approach. By using surface sources with borehole receivers and applying migration algorithms, the system achieves equivalent measurement results without the mechanical operational difficulties, substituting physical complexity with computational processing
3Quantity of substance
If conventional check shot is used, then P-wave velocity can be measured, but shear wave velocity measurement is difficult or impossible
Solution Approach 1:
The patent makes the borehole receiver system universal by enabling it to detect both P-waves and S-waves using the same surface source configuration. The migration process preserves mode information, allowing the single receiver system to perform multiple measurement functions (P-wave and S-wave velocity determination) that would traditionally require separate specialized equipment
Solution Approach 2:
The patent uses migration as an intermediary that preserves and enhances wave mode information. The migration process separates and focuses different wave types (P-waves and S-waves) into distinct migrated events, making both P-wave and S-wave velocities measurable from the same data set, thereby enabling dual velocity measurement capability
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 provides accurate and undistorted seismic velocity profiles, including shear wave velocities, even in complex overburden conditions, and is capable of delivering precise velocity estimates at great depths, as demonstrated by examples in the deepwater Gulf of Mexico.
Implementation Method 1
cross correlating the wave signal traces unm(t) and unk(t) to obtain uconvnmnk(t)
Data Source
AI summary
A seismic velocity profile in a region of interest in a subsurface formation is determined using at least the following steps.(a) Activating a seismic source at a location n, thereby exciting a wave in the subsurface formation.(b) Recording a wave signal trace unm(t) against time t, at a seismic receiver m.(c) Recording a wave signal trace unk(t) against time t at a seismic receiver k.(d) Cross correlating the wave signal traces unm(t) and unk(t) to obtain uconvnmnk(t).(e) Repeating these steps, for different locations n;(f) Summing uconvnmnk(t) over all locations n, to obtain a signal trace uvsmk(t) which corresponds to the signal received by the seismic receiver k from the virtual source at the position of seismic receiver m;(g) Deriving the seismic velocity based on the time of first arrival of the wave in uvsmk(t) and the predetermined distance between the seismic receiver m and the seismic receiver k.