Cross-borehole pulse radar has been employed to detect a deeply located empty tunnel.
Ji-Hyun Jung, Se-Yun Kim 2014, 'Enhancement of Cross-Borehole Pulse Radar Signature on a Partially Water-Filled Tunnel',International Journal of Antennas and Propagationhttp://dx.doi.org/10.1155/2014/396835. Retrieved from DOAJ CC BY 4.0 (https://creativecommons.org/licenses/by-sa/4.0/legalcode)