SUADDNOISE - add noise to traces					

 suaddnoise <stdin >stdout  sn=20  noise=gauss  seed=from_clock	

 Required parameters:							
 	if any of f=f1,f2,... and amp=a1,a2,... are specified by the user
	and if dt is not set in header, then dt is mandatory		

 Optional parameters:							
 	sn=20			signal to noise ratio			
 	noise=gauss		noise probability distribution		
 				=flat for uniform; default Gaussian	
 	seed=from_clock		random number seed (integer)		
	f=f1,f2,...		array of filter frequencies (as in sufilter)
	amps=a1,a2,...		array of filter amplitudes		
 	dt= (from header)	time sampling interval (sec)		
	verbose=0		=1 for echoing useful information	

 	tmpdir=	 if non-empty, use the value as a directory path	
		 prefix for storing temporary files; else if the	
	         the CWP_TMPDIR environment variable is set use		
	         its value for the path; else use tmpfile()		

 Notes:								
 Output = Signal +  scale * Noise					

 scale = (1/sn) * (absmax_signal/sqrt(2))/sqrt(energy_per_sample)	

 If the signal is already band-limited, f=f1,f2,... and amps=a1,a2,...	
 can be used, as in sufilter, to bandlimit the noise traces to match	
 the signal band prior to computing the scale defined above.		

 Examples of noise bandlimiting:					
 low freqency:    suaddnoise < data f=40,50 amps=1,0 | ...		
 high freqency:   suaddnoise < data f=40,50 amps=0,1 | ...		
 near monochromatic: suaddnoise < data f=30,40,50 amps=0,1,0 | ...	
 with a notch:    suaddnoise < data f=30,40,50 amps=1,0,1 | ...	
 bandlimited:     suaddnoise < data f=20,30,40,50 amps=0,1,1,0 | ...	


 Credits:
	CWP: Jack Cohen, Brian Sumner, Ken Larner
		John Stockwell (fixed filtered noise option)

 Notes:
	At S/N = 2, the strongest reflector is well delineated, so to
	see something 1/nth as strong as this dominant reflector
	requires S/N = 2*n.

 Trace header field accessed: ns