Are there any good open source (Linux) alternatives of
SeisImager which includes Pickwin and Plotrefa and file formats are .sg2?
Same question for
Actually, there are plenty of alternative DC Inversion software;
I hate using res2dinv; unfortunately it's relatively easy to get into. I would have much preferred ZondRes2D for it's user experience.
Look up DC2DInvRes (Resistivity.net), and BERT by Thomas Gunther. You may need to contact Thomas for Bert codes, however it's all done in Python and easily read.
Here is another one, but in matlab. (http://www.degruyter.com/view/j/acgeo.2015.64.issue-2/acgeo-2015-0071/acgeo-2015-0071.xml)
Here in another, again in matlab. https://github.com/cageo/Karaoulis-2013
I can address the Res2DInv. The answer unfortunately is no. The only freely available Resistivity Inversion software is R2 or R3 from Lancaster University. http://www.es.lancs.ac.uk/people/amb/Freeware/R2/R2.htm. This software is only free for students and projects that don't have a commercial component. The author of the software charges about as much as RES2DINV for commercial applications. Last I checked all of these programs are precompiled for Windows. You may be able to use a windows virtual machine in linux.
Check out Simpeg as well. It is a python library for Geophysical Modeling built by UBC. It is comprehensive and has routines for resistivity, em, mag, etc. http://simpeg.xyz/
Seismic Unix or Madagascar are big packages that might have what you need to do the same sorts of operations as SeisImager. There is also Opendtect for interpretation and visualization. Ask @kwinkunks for more information on the seismic end of things. He has a more comprehensive list of open source geoscience software.