Bourg vs de Burgh

Bourg

name

  • A topographical surname from Anglo-Norman for someone who lived in a fortified place. 

  • A surname from French. 

de Burgh

name

  • A topographical surname from Anglo-Norman for someone who lived in a fortified place. 

How often have the words Bourg and de Burgh occurred in a corpus of books? (source: Google Ngram Viewer )