NMO Correction - West Virginia University

Report
Environmental and Exploration Geophysics II
Moveout and Coincident Source-Receiver Concepts &
3D Seismic Interpretation
tom.h.wilson
[email protected]
Department of Geology and Geography
West Virginia University
Morgantown, WV
Tom Wilson, Department of Geology and Geography
Just a reminder: pages 149 to 164 in Chapter 4 were on
your reading list. Continue reading Chapter 4.
We will be addressing some issues in a different
sequence than in the text.
1) Review TNMO relation on page 160
2) Understand the t2-x2 transformation (p165-167)
3) We will come back to a discussion of determining
velocities, thicknesses .. (p 170 – 180). Review for
now.
4) You will be expected to understand how to apply
relationships associated with the dipping interface
problem (p 192- 199). You will encounter additional
discussion of moveout in these pages.
Tom Wilson, Department of Geology and Geography
5) Read about multiple reflections and diffractions
(p206-217).
6) Familiarize yourself with the common depth point
concept (p 225-229).
7) Correcting for normal moveout (p232- 241).
Tom Wilson, Department of Geology and Geography
Objectives for the day
• Normal moveout (NMO) and its elimination (NMO
Correction)
• What do dipping layer reflections look like in the shot
record?
• Quantitative relationships for the dipping layer reflection
• The problem posed by dipping layers
• Common midpoint sorting & CMP gathers
• Transformation of the dipping layer reflection in the CMP
gather.
Tom Wilson, Department of Geology and Geography
Here is some shot data collected in Marshall Co. WV
Enhanced display
We’d like to turn
this into geology.
Why do the
amplitudes drop
off below 200ms?
Tom Wilson, Department of Geology and Geography
How do we get here?
How do we get from the shot data to the data
you’ve been interpreting in the Gulf – or
Tom Wilson, Department of Geology and Geography
this data from the North Sea ….
Tom Wilson, Department of Geology and Geography
or Appalachians
The short story
The effective source
receiver geometry for the
records shown at right
across the east margin of
the Rome Trough is
corrected so that the
source and receivers
share the same surface
location.
Note that critical
refractions point to
individual source points.
Tom Wilson, Department of Geology and Geography
Flatten in time
At this point it is apparent that something has to be
done to flatten out the hyperbolas to make them
look more like continuous geologic horizons
Tom Wilson, Department of Geology and Geography
Note that the reflection point coverage spans half the
distance between the source and receiver
Off-end
Split spread
The split spread provides symmetrical
coverage about the source
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 normal moveout velocity - VNMO
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
hyperbolas or 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 an ellipse
Tom Wilson, Department of Geology and Geography
Roll-along split-spread shooting geometry
Tom Wilson, Department of Geology and Geography
NMO corrected reflections
NMO correction of the reflection events appearing in
the shot records across relatively horizontal strata
yields a more accurate image of subsurface geology.
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Dipping Layer Reflection Event has Offset Apex.
How do you find depth h, velocity V and dip ?
tapex
t0

2h cos
tapex
t0
2h
V
V
 cos 
1 tapex 
  
cos 
 t0 
Tom Wilson, Department of Geology and Geography
Features of the reflection from a dipping
interface as observed in the shot record.
xapex  2h sin 
h
xapex
2 sin 
If you could not see the direct arrival
then you could solve for V using either
expressions for t0 or tapex.
Tom Wilson, Department of Geology and Geography
The NMO correction is symmetrical about the zero offset or
source point. The dipping layer reflection event is not.
Tom Wilson, Department of Geology and Geography
Reflection points
Tom Wilson, Department of Geology and Geography
Following the distribution of common reflection points
Tom Wilson, Department of Geology and Geography
Different source receiver combinations provide
information from the same reflection point
This is referred to as a stacking chart. The significance
of the name will become apparent later on.
Tom Wilson, Department of Geology and Geography
For next week at this time construct a
stacking chart for a symmetrical split spread
consisting of 12 geophones arranged 6 on
each side of the source.
Bring questions to class on Tuesday
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Stacking Chart
Split Spread Shooting Geometry (12 phones)
-6 -5 -4 -3 -2 -1
shot 1
Reflection
Points 02
4
6
8
10
Tom Wilson, Department of Geology and Geography
1
2
3
4
5
6
Time-distance relationship for reflections in a CMP
gather are identical to those in the shot record.
Tom Wilson, Department of Geology and Geography
Definition - CMP Gather: A collection of traces
sharing a common midpoint.
Tom Wilson, Department of Geology and Geography
Raypaths in the CMP Gather don’t necessarily
provide information from the same reflection point!
Tom Wilson, Department of Geology and Geography
But reflection events in a CMP gather have
a special property
Tom Wilson, Department of Geology and Geography
Even when the layer dips they remain hyperbolic
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
And they are symmetrical
Tom Wilson, Department of Geology and Geography
Symmetrical hyperbola are easy to NMO correct!
The effect of the moveout correction on the traces in the common midpoint
(CMP) gather is to create a composite normal incidence trace that effectively
shares a coincident source and receiver at the midpoint shared by all the traces
in the gather. We’ll discuss CMP data in more detail in a couple lectures.
Tom Wilson, Department of Geology and Geography
• Construct a stacking chart for a symmetrical split spread consisting of 12
geophones arranged 6 on each side of the source (see handout).
Bring questions to class This Wednesday. The chart is due next Monday.
• Complete your reading of Chapter 4. Dipping layer reflection events are
covered on pages 183-186, with additional discussion on pages 186-196.
The idea of common depth point sorting is discussed on pages 225 -229.
We’ve talked tangentially about resolution (217-219) and velocity analysis
(233-238). We will be talking about stacking of CDP gathers (238- 241)
and migration (241-244). Discussions of migration will come later but it is
helpful to be aware of the issues early on.
• Look over problems 4.1, 4.4 and 4.8.
Tom Wilson, Department of Geology and Geography

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