Seismic reflections. Seismic waves will be reflected at “discontinuities” in elastic properties...
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Transcript of Seismic reflections. Seismic waves will be reflected at “discontinuities” in elastic properties...
Seismic reflections
Seismic reflections• Seismic waves will be reflected at “discontinuities” in elastic
properties• A new ray emerges, heading back to the surface • Energy is “partitioned” – some is reflected, some is transmitted,
according to
(where Z=vρ is the “seismic impedance”)
• Note that the value of R can be negative (implies a polarity reversal)• These relationships are only valid for normal incidence (i.e., zero
offset) - no mode conversion in this model
Reflection traveltimes• Reflection arrival times will increase with distance (offset)• Change in arrival time is known as “moveout”• Moveout for a simple, two-layer model is easy to predict:
Leads to a hyperbolic traveltime equation:
“Normal moveout”
Reflection traveltimes“Normal moveout”
Or:
Approximate, parabolic form:
“Normal moveout”
Reflection traveltimes
Note that reflections and refractions co-exist (we will see both events on the seismic data record)
Reflection traveltimes
Note that reflections and refractions co-exist (we will see both events on the seismic data record)
to
Reflection traveltimes
Shot gathers
Global arrival times from major earthquakes show the same pattern
Shot gathers
Display of seismic data
Shot gathers
Shot gathers
Example of shot gathers from City Park, Kingston (2002)
Reflection traveltimes
What happens in multi-layered media?
Answer: we can still use the same equation, but v must represent the “Root mean square” velocity of the layers:
t
“Normal moveout”
Reflection traveltimes
In the case of multiple layers, the moveout is governed by the vrms for all the layers above a given reflection.
The weighting in the equation for vrms is the (vertical) time spent by the ray in each layer.
Note that the zero offset time for a given layer is still called to
As to increases, the moveout for each successive reflection gets smaller
Reflection seismograms
Each reflector in the subsurface gives rise to a reflection in the data.
The reflections consist of the downgoing wavelet, “convolved” with the reflection coefficient series.
(Recall: reflection coefficients depend on the impedance contrasts)
Reflection seismograms
Each reflection will follow a moveout trajectory according to the moveout equation
Reflection traveltimes
The equations predict the “Normal moveout” for a given reflection, which depends only on zero offset time, and on vrms.
By measuring the moveout on the data, we can therefore estimate vrms from the surface down to each, successive reflector
If we know vrms to successive reflectors, we can use the “Dix” equation to extract the interval velocity for the intervening layer
Next Lecture: Shot gathers
A collection of shot gathers from around the world (from Yilmaz, 2001)