"Field expedition" is more descriptive then just "expedition". You don't have to use the more descriptive form but it sounds more natural and clear when you do. An expedition is a journey or voyage undertaken by a group of people with a particular purpose. There are different types of expeditions, like "military expedition", "religious expedition" or "corporate expedition". Generally speaking, scientists do their work in a laboratory but sometimes they have to travel outside the laboratory and they usually call this a "field expedition". However, if a scientist says "I'm going on an expedition" nobody will assume she is going on a "military expedition", but to avoid misunderstanding it's probably better to be more descriptive.