I have a client that would like to use their older Zephyr model 2 antenna (on a rod) with their newer Geo7X in order to be able to reach into shallow trenches for top of pipe readings. Is this possible? I've never seen a compatibility chart on this.
Yes, you can use both Zephyr model 2 and 3 (rover editions) with the Geo 7X. Zephyr model 3 support requires OS v6.7.18 (released October 2017), as detailed in this release note. Details on downloading this OS (if not already installed) are in the release note.
In addition to the suggestions from Gareth, the latest Geo7X Data Sheet does make mention of support for Zephyr 2 and 3 in the Footnotes - lower left of page 2.
4 Requires Zephyr 2 or 3 antenna and CM option.5 Stated accuracy is only with Tornado or Zephyr 2 or 3 antennas.