This document discusses velocity analysis techniques for seismic data processing. It introduces normal moveout (NMO) correction, where the proper velocity must be chosen to horizontally align reflections. The velocity spectrum method is described, which applies hyperbolic transformations and weighted stacking to plot coherence across a range of velocities. This velocity spectrum helps identify the correct velocities to use for NMO correction and horizontally aligning reflections in common midpoint gathers. Deeper reflections generally have higher velocities due to increasing pressure and temperature with depth in the Earth.

1. Seismic Data Processing
Lecture 12
Velocity Analysis
Dr Amin Khalil
School of Physics, USM

2. 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.

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

5. We 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.

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

7. Another Method:

9. Velocity Spectrum Method

10. 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.

11. Example for velocity
spectrum method:
Starting from the synthetic CMP of the form

12. 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.

13. Output:

15. Factors controlling seismic Velocity

21. Seismic velocities of common rock types

22. Thank you