Find and flag dead traces


     Some prestack processes such as DMO require extra traces (pads)
so that the seismic energy can be spread out spatially.  Not all
of the pad traces will have energy and should not be included in
things like process stack (which is a true average of all the traces
in the gather being stacked).

   My test of the dead trace picker is:

sioseis << eof
procs syn despike prout end
syn
    ntrcs 1 secs 1 tva .25 1500 .2 .55 1500 -.001 x 100 xinc 25 end
end
despike
   thres 1.E-20 1.E+20 
   sets 0. 1.
   kill yes
   lprint 15
   end
end
prout
   fno 0 lno 999999 ftr 0 ltr 9999 end
end
end
eof


     thres 1.E-20 1.E+20 means to look for amplitudes less than 1.E-20 
or greater than 1.E+20 are "spikes".