The notice may be served personally upon the owner, or may be given by the mailing of the notice by registered mail to the owner to his or her last known address and by publishing the notice three times in a legal newspaper published in the county in which the cemetery is located. In the event that the whereabouts of the owner is unknown, then the notice may be given by publishing the notice three times in a legal newspaper as required by this section. The cemetery authority may file an affidavit in the proceeding to the effect that the owner is unknown and that the cemetery exercised diligence in attempting to locate the unknown parties. The affidavit shall be conclusive to that effect.