Transcript No Slide Title

Environmental and Exploration Geophysics II
Dipping layer reflection events and the
common midpoint gather
tom.h.wilson
[email protected]
Department of Geology and Geography
West Virginia University
Morgantown, WV
Tom Wilson, Department of Geology and Geography
The effective source
receiver geometry for the
records shown at right
and in the data you are
interpreting over Granny
Creek consists of source
and receiver sharing the
same surface location.
But - this is not the way
seismic data is usually
collected.
Tom Wilson, Department of Geology and Geography
NMO correction of the reflection events appearing in the shot
records across relatively horizontal strata yields a more accurate
Tom Wilson, Department of Geology and Geography
image of subsurface geology.
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
How do you find depth h, velocity V and dip ?
tapex
t0

tapex
t0
2h cos 
2h
V
V
 cos 
1 tapex 
  
cos 
 t0 
Tom Wilson, Department of Geology and Geography
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
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
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
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
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
• 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 next Monday. Chart is due next Wednesday.
•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