Well honestly digging is best but in my experience heat tape works fine on long distances I've used heat tape on 50 ft runs and that's up here in Canada where it drops below -30 some days. Run the heat tape on the bottom of the pipe and secure it every few ft with electrical tape and insulate it. Dont squeeze the insulation super tight or itll lose the r value. I like pex for the reason that if it freezes it won't crack or split like copper. Pex has a lot more give than copper. Even better yet if you use wirsbo the elasticity of the pipe allows it to expand and contract freely as you need. Not to say that pex can freeze and thaw unlimited amounts but usually when copper freezes it'll crack. I've never gone to a frozen water line call with copper and be able to re use it. But that's extreme cold like -30 or -40. If it does freeze you can use either a heat gun or something of the sort.
If you don't learn something new everyday at work, it's not even worth going.