Unfortunately, there are formats which are called ``SEGY'' but which are not true to the SEG's standards for SEGY. One common variation is to honor most of the SEGY convention, but have the traces be in an IEEE format.
Such data would be read via:
% segyread tape=/dev/rmt0 verbose=1 endian=1 conv=0 | segyclean > data.suwhere the ``conv=0'' tells the program not to attempt the IBM to float conversion.