The Wireless Signal Networks (WSNs) use the signal strength variation of wireless nodes in the host medium as the main indicator of events or physical property changes in the medium. With the concept and application of the Underground Wireless Signal Networks (UWSNs) where the host medium is a soil, the real-time global subsurface monitoring of wireless underground sensor networks can be achieved, whereas traditional wireless sensor networks provide point sensing and monitoring. In this paper, we present two representative experiments of subsurface monitoring such as leakage monitoring and earthquake monitoring using wireless sensor nodes and UWSNs. However, the deployment of underground wireless signal networks has more constraint than aboveground deployment, because the radio signal attenuation is much higher than above ground communication. To realize UWSNs for practical subsurface monitoring applications, it is necessary to extend the underground communication radius. In the paper, we provide the estimations of extending underground communication radius of underground wireless nodes using the underground radio propagation model [1]. Based on the estimations, we introduce a concept of Low Frequency Wireless Signal Networks (LFWSNs) for subsurface monitoring applications as well as experimental platforms for the low frequency wireless signal networks.