SEG-D is an uncommon format. From my experience, here is how you need to approach this problem. You can't easily jump straight from SEG-D to SU. I've had extensive experience with SU and the best methodology is to first take your current seismic data type (SEG-D) and convert to SEG-Y. Only after converting to SEG-Y format can you then jump to SU.
So, here is an outline and workflow of a solution:
1.) Download and use this tool for SEG-D to SEG-Y format: http://users.chariot.net.au/~witek/segd2segy.htm
2.) Now that you have all your SEG-D data converted to SEG-Y data, use the following BASH script to convert a single SEG-Y file to SU:
#!/bin/bash
echo Provide Input File: ;
read sdata;
segyread tape=${sdata} verbose=1 endian=0 segyclean > ${sdata}.su
echo "New File: ${sdata}.su";
rm -rf binary header;
echo Done.;
***Keep in mind that - based on the type of machine you have - endian may need to be set to 1.