Setting up the velocity model

The Velocity Model tool in g-Space allows users to create different types of velocity models for seismic data interpretation. This chapter describes the available options, how to set up each model, and the parameters required for accurate velocity modeling.

1. Constant Model

The Constant Model generates a velocity model with a uniform velocity value throughout the entire geological section. The user needs to set the following parameters:

•Datum: The reference depth from which the model is calculated.

•Max Depth: The maximum depth for the velocity model.

•Depth Step: The vertical resolution or step size for calculating velocity.

•Velocity: The constant velocity value that will be applied throughout the entire depth range.

2. Constant Layered Model

The Constant Layered Model allows for the definition of velocity between multiple horizons with constant velocities for each layer. The user must:

•Select Horizons: Choose the horizons that will define the layers within the model.

•Set Velocity for Each Horizon: Specify the constant velocity for each layer between the horizons.

•Set Parameters:

oDatum: The reference depth.

oMax Depth: Maximum depth for the model.

oDepth Step: Vertical resolution for velocity calculation.

oNumber of Supersamples: Controls the number of intermediate velocity samples for improved precision in the model.

3. Markers + Horizons

In the Markers + Horizons model, the velocity is derived using both well markers and seismic horizons. This method interpolates velocities between well markers, adjusted by the seismic horizons, allowing for a more geologically driven velocity model. The user must:

•Select Markers: Choose the well markers to anchor the velocity model.

•Select Horizons: Add horizons to further refine the model’s vertical velocity structure.

•Adjust Parameters: Set additional parameters such as depth step and max depth for model accuracy.

4. Grid velocity by markers + horizons

5. Checkshot Extrapolations

6. Checkshots + Markers

The Checkshot Extrapolation option allows users to incorporate checkshot data for accurate time-depth conversion and velocity model creation. This method extends velocities from available checkshots through extrapolation to provide a smooth, reliable velocity field. The process includes:

•Selecting Checkshots: Import or select checkshot data from wells.

•Extrapolation Settings: Define the method of extrapolation for areas beyond available checkshot data.

7. Checkshots + Markers + Horizons

This option combines Checkshots, Markers, and Horizons to create a detailed velocity model. By integrating all three data types, the model reflects both well control (checkshots and markers) and seismic horizon information. The process involves:

•Selecting Checkshots: Import checkshot data.

•Selecting Markers: Choose well markers to define key depth points.

•Selecting Horizons: Add seismic horizons for further refinement.

•Setting Extrapolation Options: Define how velocity is interpolated or extrapolated between the data points.

8. From Vrms

The From Vrms option generates a velocity model based on RMS velocity data. This method is particularly useful when working with pre-stack seismic data where Vrms velocities have been calculated. The workflow includes:

•Importing Vrms Data: Load the RMS velocity information into the model.

•Defining Parameters: Set up the depth step, max depth, and interpolation method for converting Vrms to interval velocities.