Page 430 - Acquisition and Processing of Marine Seismic Data

CHAPTER

9

Velocity Analysis

OUTLINE

9.1 Types of Seismic Velocity 424 9.3 Velocity Analysis in Practice 436
9.1.1 Interval Velocity (V int ) 424 9.3.1 Supergather 436
9.1.2 Average Velocity (V ave ) 424 9.3.2 Effect of Reflection Depth 440
9.1.3 Instantaneous Velocity (V ins ) 424 9.3.3 Effect of Streamer Length 441
9.1.4 Root-Mean-Square Velocity 9.3.4 Effect of Fold Number 443
(V RMS ) 424 9.3.5 Effect of Multiple Reflections 443
9.1.5 NMO or Stacking Velocity 9.3.6 Effect of Time Gate 444
(V NMO ) 425 9.3.7 Effect of Velocity Increment 446
9.1.6 Dix Interval Velocity (V DIX ) 425 9.3.8 Effect of Semblance Sample Rate 446
9.3.9 Effect of Noise 447
9.2 Velocity Determination From Seismic
9.3.10 Effect of Muting 449
Data 426
9.2.1 Constant Velocity Scan 427 9.4 QC in Velocity Analysis 453
9.2.2 Constant Velocity Stack 428
9.2.3 Velocity Spectra 428

Seismic reflections are generated at interfaces velocity of the medium, and recorded reflections
characterized by impedance contrasts separat- are assumed to originate from the interfaces that
ing rock units with different velocity and separate two layers with different velocities.
density values. However, in seismic data proces- Velocity information is the main parameter to
sing, the density term is generally omitted since define the depth, dip, and the lateral locations
the variations in density are generally quite of the reflectors in the seismic data. It is also cru-
small, as compared to the variations in rock cial information for some of the processing
velocity. Therefore, it is supposed that the main methods, such as normal moveout (NMO) cor-
argument of the seismic method is seismic wave rection and migration. In this section, the factors

Acquisition and Processing of Marine Seismic Data 421 # 2018 Elsevier Inc. All rights reserved.
https://doi.org/10.1016/B978-0-12-811490-2.00009-8