The development of an integrated through-the-earth-and-air (TTEA) communication system for enhanced real-time tracking and post-accident rescue in underground mines

Abstract

Underground mines remain one of the riskiest and most hazardous environments where people work. This can be attributed to the unacceptably high rates of accidents which often result in injuries and fatalities. However, miners are not always killed during the initial event of an accident. In some accidents, miners can survive the initial event, but remain trapped or lost underground in unknown locations for an extended period of time. If not located as quickly as possible, the trapped miners can succumb to their injuries and the lack of supply of life-sustaining essentials such as fresh air, water, food, and medication. The persistent occurrence of trapped miner-related fatalities highlights a critical gap in the existing tracking, locator, and two-way communication systems. For instance, through-the-air (TTA) communication systems enable real-time tracking, provide the last known locations of miners, and can support full-duplex two-way voice and text communication. However, these systems are highly susceptible to damage and failure post-accident due to their reliance on extensive infrastructure to operate. Conversely, through-the-earth (TTE) communication systems are less susceptible to damage because they transmit signals directly through rock without the need for any major infrastructure. However, the TTE systems are less effective compared to the TTA system, as they depend on the trapped miners to reach an antenna station to relay their locations. These contrasting limitations between the TTE and TTA systems hinder the effectiveness and survivability of each system when operated in isolation. To bridge this gap, this study proposes the integration of the TTE and TTA systems into a unified hybrid through-the-Earth-and-Air (TTEA) communication system. The TTEA system merges the higher data rate applications of the TTA system, and the survivability aspect of the TTE system for enhanced real-time tracking and sustainable connectivity post-accident. To develop the TTEA system, the TTE system was identified as the performance bottleneck in terms of data rates. This is due to the complex challenges of transmitting electromagnetic waves through conductive rock media for significant distances. As such, a large portion of this study focused on modelling signal transmission through rock for the design and optimisation of an electrically small antenna. The performance of a TTE system can be measured by the achievable penetration depth and data rates. Several antenna design models and optimisation techniques were explored. The various antenna design and optimisation configurations were modelled by varying operating frequencies, loop material, shapes, sizes, number of turns, transmitter power and current, and transmitter-receiver misalignment angles. The antenna design configurations were further optimised for Quality-factor (Q-factor) tuning and self-resonance matching by modelling a Q-aware (Q-factor consideration or bandwidth-limited) condition. The antenna optimisation techniques were applied to maximise magnetic flux coupling and field strength, minimise resistive losses, and match the resonance frequency to the selected carrier frequencies. For the bandwidth restrained TTE link, the data rates target was set at 300 bps for digital locations and tracking data only, at 1,000 bps for digital locations tracking data and text messaging, and at 2,400 bps for digital locations tracking data, two-way voice, and text messaging. The results showed that higher frequencies can achieve higher data throughput, but a limited range due to shorter skin depth. Conversely, lower frequencies can reach significant depth due to their larger skin depth, but limited data rates. This indicates that an optimal balance is critical for the various antenna design configurations for enhanced performance and practical deployment. Simulation results for an electrically small circular loop made of copper, with a 10 – 15 m radius, 10 – 20 turns, a 10 A drive current with a 100 W power budget can reach up to 1,000 m at 300 bps at 1 kHz or lower, about 600 m at 1,000 bps between 1 and 3 kHz, and up to 400 m at 2,400 bps at 3 kHz and higher. For the TTA link, wireless communication protocols including Wi-Fi 6, Zigbee, UWB, LoRa, Sigfox, and NB-IoT were selected and evaluated for their compatibility and integrability with the TTE system. The received signal strength indication (RSSI) thresholds and data rate capacities of the different protocols were simulated. To test the performance and reliability these protocols, simulations were conducted in a typical underground mine 100 m long straight drift, as well as different geometries. A multi-layered channel and signal propagation model approach was applied in an increasing level of complexity with the free space path loss (FSPL) used as a baseline reference point from the transmitter. A full model was developed through a systemic incorporation of several models, including the log-distance attenuation, log-normal shadowing, Rician fading, Rayleigh fading, wall material and roughness attenuation, antenna alignment and polarisation, and inter-symbol interference (ISI) losses. The full model was applied to assess signal fading effects such as path loss, poor line-of-sight (LoS), non-LoS (NLoS) conditions, material, and roughness reflections, refractions, and scattering, and multipath effects. The simulation results showed that UWB, Wi-Fi, and Zigbee are more suitable for high data rate applications, but localised area network. Sigfox could achieve full coverage in the underground openings, but failed to meet the minimum data rates required for the TTE channel. LoRa and NB-IoT achieved full coverage in the underground openings and their data rates are well aligned and matched the requirements of the TTE system channel. This allows for a less complex and an efficient modulation or signal processing scheme. Although not well tested underground yet, LoRa and NB-IoT proved to be the potential solutions for the TTA component of the integrated TTEA communication system. From the modelling and simulation results of the TTE and TTA systems, the feasibility of integrating the TTE and TTA systems into a unified system was evaluated. This was done by matching or aligning the data rates between the TTE and TTA systems. This was an integral part of developing the integrated TTEA communication system and ensuring an efficient and seamless switch between the TTE and TTA channels. A higher mismatch would require a more sophisticated signal modulation or processing scheme to be used without distorting the transmitted data. An amplify-and-forward (AF) and decode-and-forward (DF) relay schemes were considered and compared for the integration. The AF model is based on a continuous cascade, where both the signal and accumulated noise from the TTE link are amplified by the TTA link. This resulted in a higher data rates and significant noise accumulation. Conversely, the DF model simulated a realistic two-hop configuration, where the received TTE signal is demodulated, decoded, and re-encoded before retransmission over the TTA link. The DF approach offered a more reliable and stable integration for the TTE and TTA systems, with slightly lower data throughput than the AF relay. Across all TTA protocols, the DF relay was limited by the lower capacity TTE hop. A hybrid AF-DF relay was also considered, but showed similar results to the DF relay due to the TTE link limitations. Therefore, an adaptive DF dual-mode gateway was developed to facilitate the rerouting of transmission between the TTE and TTA links. The TTEA system prioritises transmission via the TTA link, and only reroutes via the TTE link if the TTA system is damaged and unavailable post-accident. The TTEA communication system showed the capability to transmit digital tracking data at 300 bps up to 1,000 m depth, digital tracking data and two-way text at 1,000 bps up to 600 m, and digital tracking data, two-way text and voice at 2,400 bps up to 400 m. This study proved the feasibility of a proof-of-concept of the proposed integrated TTEA communication system. A framework for an optimal deployment of the proposed TTEA system was developed. This was based on multi-input, multi-output (MIMO) and relay waveguide techniques. These techniques were incorporated to enhance transmission range and maintain sufficient data rates. The framework goes further to consider factors such as the mine layout, depth, rock properties, and inter-level spacing parameters. This study proposes that a prototype of the integrated TTEA communication proof-of-concept is developed and tested underground. It is anticipated that the integrated TTEA communication system will not be limited to tracking miners in real-time and locating them post-accident only, but will form the mine-wide scalable backbone infrastructure for underground mines. This can support various Internet of Things (IoT)-based applications to modernise underground mines and advance the state of underground communication systems and emergency mine rescue preparedness.