Instructions for reading and displaying record sections from the Vidal
reflection/refraction profile.

S.R. 2008

DATA FORMAT:

The data are in SEG-Y format, in files called NNNN.sgy. Note that in some
instances, there is more than one shot in a sgy file.

SHOT DESCRIPTION:

Information about the various files, shots and geophones that recorded each
shots are provided in file reflog_vidal08.xls.

TO READ DATA:

Determine the shot you want to display (column labelled FFID in
reflog_vidal08.xls), and the segy file in which it was saved. For example, shot
1008 is saved in file 1005.sgy.

Then use the command ReadSegy, as follows:

[Data,SegyTraceHeaders,SegyHeader]=ReadSegy('1005.sgy');

The record section corresponding to shot 1008 is obtained by extracting the correct traces (see column labelled "Traces" in reflog_vidal08.xls):

whole traces: Data(:,181:240)
first 0.25s:  Data(1:500,181:204)

TO DISPLAY DATA:

Use command refsection as follows (for shots 1006 and 1008):

refsection(Data(1:500,61:120)',0,.5/1000,-1,'k',[],30,200,0)
refsection(Data(1:500,181:240)',0,.5/1000,-1,'k',[],30,200,0)

Type "help refsection" for usage. It is possible to control the scaling of the
traces and the color of the plot, to plot the traces as a function of exact
offset, to bandpass filter the data, and to time-shift the entire section.

It is also possible to plot several sections on the same figure by using the
command refsection several times in conjunction with the command "hold on".

Traces can be picked using the command "[xx,yy]=ginput" and then these picks
can be plotted using the commands "hold on; plot(xx,yy)"

The power spectrum of the traces can be calculated by using the powspec.m function (type "help powspec" for usage): 

[spec]=powspec(Data(1:500,181)',.5/1000);

DATA ANALYSIS:

1- Determine the exact geophone elevations and offsets from GPS measurements
2- Identify shot sections that can be appended to form updip
and downdip profiles along the entire line
3- When several redundant shots were fired from the same location, stack the
redundant shot sections (this may require some time shifting of the whole
sections if the shot was manually triggered)
4- Append shot sections using tshift option in "refsection.m" and by 
matching/correlating the overlapping segments of each profile
5- Perform refraction analysis for horizontally layered medium and for
dipping layered medium (as shown in class notes) - note that there are
time shifts due to changes in elevation between geophones (relative to
a datum). First perform the analysis without elevations correction,
and then apply an elevation correction.