We have been approached by multiple gas companies to perform as-built surveys of their pipelines. Unfortunately, most of the pipelines are already back-filled. I've used my Schonstedt in the past to detect steel lines. While this method is not exact, it was good enough for my purposes at the time (rough location on a boundary plat). I have little other experience in this regard. There are more specialized pipeline locating devices. Does anyone have experience/advice? Note: some of the pipelines are poly, with a tracer wire above the pipe. Some are steel.