Seljeskog and Halverson

My grandfather, Seger (Segierd ot Sigurd) Seljeskog (Sebyerkog or Selgeskog) came to America pre-Ellis Island, abt 1889 with his mother, Malena Halverson Seljeskog. As I've noticed, there are so many variants of their last name... I was told that the court house that housed grandpa Seger's birth certificate (in Strand?) burnt down. I am having a bugger of a time finding his ancestors, probably because of the name fluctuation, and the same with Malena's. I do know that a couple of Seger's cousins, Theodore Seljeskog and Sigsbee (Doctor) Seljeskog (which I have a little bit of information on) also lived here in the US. Sigsbee in Minnesota and Theodore in California. Any information any of you can give me on these names will be greatly appreciated!