Thank you for your suggestion Sébastien.
In fact I found the solution to my problem, after some investigations in the fortran code.
The sel2vtk code I used is the version 0.5.5 available online
here.
The option BIG-ENDIAN was one of the problems, that I simply solved in changing the function which opens the SLF file in :
open(fu_s,file=trim(teldat),form='UNFORMATTED',status='old',iostat=f_error,CONVERT="BIG_ENDIAN")
But the other problem is that sel2vtk was not reading the date, which is represented by a succession of integers (year, month, day, hour...) contained in my SLF file (in my Telemac CAS file, I defined the starting date of the simulation).
I added a step to read these data because otherwise the program was affecting theses integers to the next variables (mesh attributes: number of elements, points...).
Best regards
Luc
PS : It did not investigate the vtk file structure/format but does someone tried to condense these files? Because the VTK files generated with sel2vtk are approx. 10 times much larger than the initial SLF file... This is caused by a lot of redundancies between the files generated (and also the use of an ASCII format).