Seismic Data ProcessingLecture 12

Velocity Analysis

Dr Amin KhalilSchool of Physics, USM

Introduction:

In NMO correction we realized that we must input velocity (RMS velocity) in order to correct for normal moveout. The choice of proper velocity is crucial as if the velocity is either too large or too small the traces will not align.

GoalIn NMO correction our goal is that all reflections must horizontally aligned.

methodWe apply CMP velocity pannel, from this pannel we select the velocity (rms) that result in horizontal align of each reflector.

We use iterative scheme to obtain the best velocity value for each reflector.

Deeper reflector usually have higher velocity.Why?!!!

Another Method:

Velocity Spectrum Method

Description of velocity spectrum method:

Velocity spectrum is hyperbolic transformation that can be applied through the folowing steps:

1- correct the CMP gather for arbitrary stacking velocity.2- Apply weighted stacking to measure coherency. Stacking will with one trace.3-repeat for next velocity.4-For a complete set of velocities we plot the results. This is called velocity spectrum.

Example for velocity spectrum method:

Starting from the synthetic CMP of the form

Example continued

For this CMP we apply the semblance for velocities ranging from 1000 m/s to 3000 m/s with velocity step of 50 m/s

The semblance is defined using the relation

Which is often simplified to:

For fewer sample points (2p) of the time window or gate.

Output:

Factors controlling seismic Velocity

Seismic velocities of common rock types

Thank you