Geop 397 - Velocity Analysis

Question 1

Why do you do velocity analysis?

Answer 1

to determine lithology velocities of the subsurface

Question 2

How do you get the lithological velocities?

Answer 2

well logs and seismic data

Question 3

Talk about velocities from well logs

Answer 3

• sonic log (p-wave)

- dipole sonic (s-wave)

Question 4

Talk about seismic velocities

Answer 4

from processing techniques

aka “processing velocities”

Question 5

Types of Velocity

Answer 5

• Lithological velocity
• average velocity
• RMS velocity

Question 6

What is lithological velocity?

Answer 6

velocity of each layer (aka Vint). V int is derived from sonic log and dipole sonic log

Question 7

How is Vint calculated?

Answer 7

Transit time is measured in microsec/m or microsec/ft.

Vint = 1/transit time

Question 8

How do you calculate Vint from Transit time ?

Answer 8

Vint = 1/Transit time.
eg.
-you’re given something in m/microsec
- multiply by 10 to the 6th so it’s metre per sec
The ans will be a 4 digit number…. m/s
in range of 1000-5000 m/s

Question 9

How do you calculate Vavg

note: Vavg is velocity over a set of layers

Answer 9

Vavg is computed over a range of lithological layers.

Vavg = ((Vint1 * deltaT1) +(Vint2* deltaT2)+…) / (deltaT1 +deltaT2)

Question 10

When are Vavg calculated?

Answer 10

• in processing

- useful to know avg velocity to a target

Question 11

How is Vrms calculated?

RMS = Root Mean Square

Answer 11

Vrms = Square Root (V1sq * deltaT1 +(V2sq*deltaT2)/(deltaT1+deltaT2…)

Question 12

What is velocity analysis used for?

Answer 12

velocity analysis is used to determine processing velocities

Question 13

What is the Processing Sequence with respect to velocity analysis

Answer 13

CDP gathers (w/o NMO)
Velocity Analysis (to get NMO velocities 1 Km * 1Km grid
NMO Correction, mute
Residual statics 
(apply statics)
THEN BACK UP TO CDP no NM), with statics

Question 14

why is Velocity Determination Required ?

Answer 14

Primary Velocity (Residual Statics, DMO, Stack, Migration)
Multiple attenuation (primary and or multiple velocity)

Question 15

Which is slower ? Multiple or Primary Velocity?

Answer 15

Multiple Velocity is slower than Primary Velocity
T2=(X2/V2)+ T2
Tp = 1sec (TWT of primary)
Tm= 2 sec (TWT of Multiple)