sù yuán
to investigate the origin of sth
to trace a river upstream back to its source
zhuī sù
lit. to go upstream
to trace sth back to
to date from
huí sù
to recall
to look back upon
sù
to go upstream
to trace the source
zhuī gēn sù yuán
to pursue sth back to its origins
to trace back to the source
to get to the bottom of sth
xún gēn sù yuán
see 追根溯源[zhui1 gen1 su4 yuan2]