In depth migration, the process of generating a velocity model involves estimating the subsurface velocities of the Earth to improve the accuracy of seismic imaging. It is a crucial step in seismic data processing for mapping subsurface structures accurately. Here's a general overview of the steps involved in generating a velocity model for depth migration:
Acquire seismic data: Seismic data is collected by deploying seismic sensors (geophones or accelerometers) on the Earth's surface or by using borehole sensors. Energy sources such as seismic vibrators or explosive charges are used to generate controlled seismic waves.
Preprocessing: The acquired seismic data is subjected to various preprocessing steps to remove noise, correct for instrument response, and perform quality control checks. This ensures that the data is suitable for further analysis.
Velocity analysis: Velocity analysis is performed to estimate the subsurface velocities at different depths. This process involves picking and analyzing seismic wave arrivals in the recorded data to determine the travel times and velocities of the subsurface reflections. Common techniques include semblance analysis, velocity spectrum analysis, and moveout analysis.
Velocity modeling: Based on the velocity analysis results, a velocity model is constructed. A velocity model represents the subsurface velocities at each point in the survey area as a function of depth. Different methods are used to interpolate and extrapolate the velocities between and beyond the analyzed data points to create a continuous velocity model.
Velocity model building: The velocity model is refined and improved by incorporating additional information, such as well log data, seismic horizons, and geologic interpretations. Advanced techniques, such as tomography and inversion, may be employed to iteratively update and optimize the velocity model based on the observed seismic data.
Depth migration: Once a reliable velocity model is established, depth migration algorithms are applied to the seismic data. Depth migration techniques, such as Kirchhoff migration or reverse-time migration, use the velocity model to accurately position the seismic reflections in their correct subsurface locations. This process improves the resolution and fidelity of the subsurface images, providing a more accurate representation of the Earth's structure.
The process of generating a velocity model in depth migration is an iterative one, often involving multiple iterations of velocity analysis, model building, and migration to refine the subsurface image and improve the accuracy of the velocity model.