As of version 1.02 SegyMAT is intended to work with Octave version >2.1.64

If you have an older octave version installed you may need to :

1) change line (app 399) in ReadSegy
from : if (outtrace==1),
to   : if (outtrace==-1),

2) comment line (app 214) in ReadSegyFast :
from : Data=ibm2num(uint32(Data));
to   : % Data=ibm2num(uint32(Data));

These two changes have been successfully applied to octave version 2.1.57.

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* A note on speed
SegyMAT in Octave runs MUCH slower (about a factor 5) than in Matlab when reading in SEG-Y header values. If you just need to read the trace data try using :
data=ReadSegy(filename), which makes use of ReadSegyFast. 
In any case (Matlab or octave) this is much faster, and specifically, Matlab and Octave performs equally well.