Seismic Depth Imaging

Depth imaging converts seismic reflection data recorded in time, into a depth domain that represents the true positions of subsurface features.

The depth processing accounts for lateral variations in seismic wave velocities through different rock layers, which improve the accuracy of subsurface images compared to time-domain seismic methods.

Depth Imaging Workflow

Initial Velocity Model Building

An initial velocity model of the subsurface is constructed using available geological information and well data. This model estimates how fast seismic waves travel through different rock layers. In conventional depth imaging RMS velocity is used to create initial interval velocity model. But in advance depth imaging you can combine sonic velocity with seismic velocity to create initial depth model for subsequent tomographic iterations.

The RMS velocity is often measured over a certain time interval (usually in milliseconds) and represents an average velocity of seismic waves over that interval. Where as, the depth intervals are typically used to describe velocities in terms of depth below the surface in meters per second.

Depth Migration

The seismic data is migrated using the initial velocity model to convert reflection times into accurate depth positions.

Velocity Refinement Using Seismic Tomography

The general steps involved in updating velocity models using seismic tomography are:

Initial Velocity Model Building

• Start with an initial velocity model, which can be a simple homogeneous model or a more complex one based on prior studies.

Ray Tracing

• Calculate the paths (rays) that seismic waves travel through the Earth using the initial velocity model.
• Determine the travel times for these rays from the source to the receivers.

Inversion

• Compare observed travel times with those predicted by the initial model.
• Use mathematical inversion techniques to adjust the velocity model so that the predicted travel times better match the observed data.
• Common inversion methods include least squares, conjugate gradient, and damped least squares.

Iteration to update velocity

• Update the velocity model with the new information obtained from the inversion process.
• Repeat the ray tracing and inversion steps with the updated model to further refine the velocity.
• Continue iterating until changes between successive models become minimal or converge to a satisfactory level.

Validation

• Validate the final velocity model by comparing it with independent datasets, such as sonic velocity logs, Overlay velocity on seismic section.
• Ensure the model is consistent with known geological discontinuity and tectonic features such as faults.

The seismic tomography is computationally intensive and required specialized software and high-performance computing resources.

Welltie Tomography

Input Data

• Horizon grids, Well Markers, Interval Velocity and Time Gathers.

Steps for Welltie tomography

1) Create horizon model

2) QC for mistie value at well locations by creating RC line, and note down the mistie value at each well location.

3) Find out average mistie value for all well locations (e.g. 24.34m).

4) Create marker map using mathematical operations (horizon model – average mistie value for all locations).

5) Generate mistie map using mathematical operations (horizon map-marker map).

6) Create pencil : using model maps, mistie maps, interval velocity

7) Perform tomography : build matrix, keeping ray shooting angle 30 degree for shallow and 20 degree deeper, update only velocity and delta keeping epsilon zero.

8) Solve matrix

9) Migrate horizon model from time to depth using updated velocity and parameters.

10) Create Stack with mute

11) Run anisotropic migration using updated velocity and anisotropic parameters epsilon and delta.

12) Compare initially migrated horizon model with horizon model migrated after welltie tomography.

13) Check mistie value again at each well location.

Anisotropy

Isotropic Medium

A material is isotropic if its properties are the same in all directions. The seismic waves travel with the same velocity regardless of the direction of propagation in isotropic medium. For example, an idealised, homogeneous Earth model where seismic wave velocity is constant in all directions and locations.

Anisotropy Medium

A material is anisotropic if its properties vary depending on the direction. In an anisotropic medium the seismic waves travel with different velocities depending on their direction of propagation.

An anisotropic models account for directional variations in seismic velocity, and it provides a more accurate representation of the Earth’s subsurface.

Learn More About Seismic

Click 👉 Seismic Data Acquisition

Click 👉 Seismic Survey Geometry

Click 👉 Seismic Data Processing

Click 👉 Seismic Depth Imaging

Click 👉 Seismic Data Interpretations