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)