GG 450L: Geophysical Methods Lab
LAB 7:
Intro to Reflections
Due Tuesday 13 April
This lab will demonstrate some concepts of normal move-out (NMO) and reflection coefficients.
We will make use of some nice Matlab code written by Larry Braile of Purdue University.
First copy the following files from the homepage to your Z drive:
seis1ddata.m
seis1d.m
ricker.m
signal.m
txrefl.m
rfltim.m
1) Reflection travel times
a) Run “txrefl.m”
You will see two plots: a plot of interval velocities vs depth for 5 layers; a plot of reflection
arrivals vs distance. You will need to hit “enter” 5 times to see the travel time plot for each
reflecting horizon.
Notice that as the depth of the horizon gets deeper, the travel time also increases (of course!),
and the shape of the x-t curve flattens – why?
Make a copy of the x-t plot
b) Change the first number of line 15 of txrefl.m from 0.5 to 2.0 and rerun. Make a copy of this
x-t plot and compare it to the plot in a). How are they different?
2) a)Run “seis1d.m”. Again you will see a plot of interval velocity vs. depth, but the second plot
has two components: two-way travel time vs reflection coefficient and 5 seismic traces that are
the convolution of a “Ricker” (symmetric) wavelet with the reflection coefficient. The starting
wavelet is 10 Hz. Print the seismogram.
b) Open the file “seis1ddata.m” in Wordpad; change the line near the bottom “freq = 8” to “freq
= 2” and save the file; rerun; Print the seismogram and describe what is different from a).
c) Now add multiples: change the line near the bottom from “nmult=0” to “nmult=1” , then
change “freq = 2” back to “freq = 8” and save the file; rerun; Print the seismogram and compare
to a)
d) Change “freq” back to 2, save, rerun; Print the seismogram and compare it to c). Would you
be able to identify the three real reflections and the 4 mutliples below 1 sec two-way time?
