A GPS-time transmitter was prototyped in 2018 and ruggedized to allow, without any modification, synchronization of different types of seismic recorders in GPS-time denied underground mining environment. The system, with a maximum drift on the order of only a portion of microsecond per day, can transmit simulated false or real GPS-time signal without cabling from the surface. This provides opportunities for geophysical surveys in deep exploration and mining tunnels where access to such a signal is cost ineffective via surface cabling or even sometimes impractical. In this work, we present a background to how such a system was realized and then showcase two surveys, one from the Neves-Corvo mine at 650 m depth (Portugal) and another one from the South Deep gold mine (South Africa) at approximately 3000 m where the system was employed to wide range of recorders including nodal, cabled and DAS for improved targeting and higher-resolution imaging possibilities.