Transcript No Slide Title

Environmental and Exploration Geophysics II
Coincident Source receiver Concepts
Extracting Velocities from Seismic Data
tom.h.wilson
[email protected]
Department of Geology and Geography
West Virginia University
Morgantown, WV
Tom Wilson, Department of Geology and Geography
Common Midpoint (CMP) gather, also often
referred to as Common Depth Point (CDP)
Tom Wilson, Department of Geology and Geography
Common Midpoint Gather
Doug Smith’s seismic Data Processing site
http://www-geo.phys.ualberta.ca/~doug/G438/Assignments07/Lectures/Common_Midpoint.pdf
Tom Wilson, Department of Geology and Geography
Moveout and the moveout correction
t
Tom Wilson, Department of Geology and Geography
Redefine the reflection time equal to the 0-offset arrival
time (t0) plus the t (drop from t0 or “moveout”).
Tom Wilson, Department of Geology and Geography
Assume t2 is small relative to other terms and
can be ignored to approximate the moveout
t is the normal moveout correction
Tom Wilson, Department of Geology and Geography
Look at the reflection time distance
relationship in terms of t2 versus x2
Square both
sides of this
equation
Tom Wilson, Department of Geology and Geography
The hyperbola becomes a straight line
Tom Wilson, Department of Geology and Geography
In the t2-x2 form, the slope is 1/V2
Tom Wilson, Department of Geology and Geography
The moveout velocity
V is derived from the slope of the
reflection event as portrayed in
the t2-x2 plot. The derived
velocity is referred to as the
Normal Moveout Velocity, NMO
velocity, or, just VNMO.
Tom Wilson, Department of Geology and Geography
The VNMO is used as a correction velocity
2
x
 2  t2  2
V NMO
Tom Wilson, Department of Geology and Geography
If the velocity is accurately determined
the corrected time  equals t0
Fun with hyperbolas and ellipses
Tom Wilson, Department of Geology and Geography
If the correction velocity (VNMO) is too high then the
correction is too small and we still have a hyperbola
Tom Wilson, Department of Geology and Geography
2
If VNMO
 1
1 
 V then  2  2   1
 V VNMO 
2
And we have
Tom Wilson, Department of Geology and Geography
Velocity Analysis
Tom Wilson, Department of Geology and Geography
Discussion of problems 4.1
Read over and think about how you are
going to solve problems 4.5 and 4.8
Tom Wilson, Department of Geology and Geography
Dix Interval Velocities
The Dix interval velocity
assumes VNMO velocities
are equivalent to RMS
velocities. Starting with the
basic definition of the RMS
velocity we derive the
interval velocity Vn.
Vn2 
n
VRMS 
n
2
V
 i ti
i 1
n
 ti
i 1
2
2
VRMS
t

V
RMSn 1t0 n 1
n 0n
tn
Read over discussions of Dix Interval Velocities (pages 170 -181)
Tom Wilson, Department of Geology and Geography
Using Fault Polygons
General Steps
•Create a fault polygon set: usually identified by
horizon
•Digitize your fault polygon, double click and assign a
fault to it
•Digitize fault polygons for all faults intersecting that
surface and assign the appropriate fault to each
polygon
•Under Fault Polygon Management Specify “Fill Color
Based on Associated Fault Surface”
• Go to next horizon
Tom Wilson, Department of Geology and Geography
See revisions (on line) to Parts 3 and 4 of the
3D seismic Interpretation procedural notes
Tom Wilson, Department of Geology and Geography
Horizon > Polygon Smooth
Another useful tool
– smoothing can
help fill gaps in your
interpretations
Tom Wilson, Department of Geology and Geography
Some structural views and potential traps
should be emerging in your interpretations
Tom Wilson, Department of Geology and Geography
To Do List
•Work problem 4.1 (Bring questions in Monday).
4.1 is due next Wednesday.
•Digitize additional horizons
•Discuss divisions of labor with your team mates
•Read over discussions of Dix Interval Velocities
(pages 170- 181)
Tom Wilson, Department of Geology and Geography