The CMS-based resistivity.net website is down due to Typo3/PHP version and security reasons. As it has been outdated since years we do no strive to rescue. Instead, we will generate a Sphinx page documenting the BERT API and some additional information.
In the meanwhile I like to draw your attention to the main project pages of the pyGIMLi project (hosted on github) and of the BERT project page on gitlab where all information on BERT is available. Note that you will need a gitlab account that can be activated for BERT by email@example.com or firstname.lastname@example.org.
BERT is an expert system for ERT inversion, please have a look at the Tutorial, if you are a Windows user, particularly on Appendix A for working on a command shell.
If you are looking for fool-proof programs with a GUI (perfectly suited for students or beginners) on Windows you might try DC2dInvRes and DC3dInvRes. Note that development ceased long time ago, nevertheless they can be of great help, particularly DC2dInvRes, for 3D problems I strongly recommend using BERT instead of DC3dInvRes. Download and install theses programs and call Generate License Code and send the code along with your affiliation to thomas@resistivity. There have been other softwares like Ra2dTomo, TT2dTomo, TreeBERT (aka DC2dTree) that are outdated but possibly available upon request.