fieldworker vs investigator

fieldworker and investigator both are nouns.

Word	Noun	Adjective	Verb	Adverb
fieldworker	Yes	No	No	No
investigator	Yes	No	No	No

As nouns, investigator is a hypernym of fieldworker; that is, investigator is a word with a broader meaning than fieldworker:

Other hypernyms of fieldworker include research worker, researcher.

fieldworker (noun)	investigator (noun)
a researcher who works in the field	someone who investigates
	a scientist who devotes himself to doing research
	a police officer who investigates crimes