The term "field work" is pretty generic; it means a lot of different things depending on the context. Let me cite a few examples (not an exhaustive list at all):
1) For a construction engineer, field work might
mean being out on the construction site, looking at problems on the spot and suggesting fixes;
2) For a machinery engineer, field work might
mean taking vibration measurements on
operating machinery in an industrial environment such as a power plant or a steel mill;
3) For a radio/TV engineer, field work might
mean resolving an issue with an antenna;4) For an electrical power engineer, field work might mean investigating breaker and/or
transformer problems in a power plant or
distribution center;
5) For a drilling engineer, field work might mean figuring out why a drill is not advancing and working out a solution;6) For a controls engineer, field work might mean tuning a control system to the desired dynamic
response on actual operating machinery.Etc, etc. In one sense, "field work" is the opposite of "office work," meaning that it is usually done
on the job site rather than in an office
environment.