Lincan Yan

Measurements and Modeling of Through-the-Earth Communications for Coal Mines

This paper presents modeling results from the National Institute for Occupational Safety and Health research into through-the-earth (TTE) communications technology for underground coal mines. Research focuses on the factors controlling the propagation and coupling of radio signals between transmit and receive antennas separated by earth or coal. Most TTE systems use single or multiturn loops of conductor for the transmit antenna. We compare the magnetic field distribution predicted from analytical formulas to the predictions of a method of moments computational electromagnetic (CEM) code. The predictions are compared in free space, in a homogeneous earth, and with the effect of the presence of the surface of the earth. The evaluations are done with the transmit loop buried in the earth and with the loop above the surface. The analytic results are shown to agree reasonably well with the more detailed CEM predictions for the situations considered, reducing the need for expensive and complicated CEM codes in analyzing simple TTE configurations. The predictive methods are applied to TTE measurements made in 94 different coal mines by the Bureau of Mines in the 1970s, and the implications for the apparent conductivity of the earth are discussed.

Measurements and modeling of through-the-earth communications for coal mines

This paper presents modeling results from the National Institute for Occupational Safety and Health research into through-the-earth (TTE) communications technology for underground coal mines. Research focuses on the factors controlling the propagation and coupling of radio signals between transmit and receive antennas separated by earth or coal. Most TTE systems use single or multiturn loops of conductor for the transmit antenna. We compare the magnetic field distribution predicted from analytical formulas to the predictions of a method of moments computational electromagnetic (CEM) code. The predictions are compared in free space, in a homogeneous earth, and with the effect of the presence of the surface of the earth. The evaluations are done with the transmit loop buried in the earth and with the loop above the surface. The analytic results are shown to agree reasonably well with the more detailed CEM predictions for the situations considered, reducing the need for expensive and complicated CEM codes in analyzing simple TTE configurations. The predictive methods are applied to TTE measurements made in 94 different coal mines by the Bureau of Mines in the 1970s, and the implications for the apparent conductivity of the earth are discussed.

Simulation and measurement of through-the-earth (TTE), extremely low-frequency signals using copper-clad, steel ground rods

The underground mining environment can greatly affect radio signal propagation. Understanding how the earth affects signal propagation is a key to evaluating communications systems used during a mine emergency. One type of communication system is through-the-earth, which can utilize extremely low frequencies (ELF). This paper presents the simulation and measurement results of recent National Institute for Occupational Safety and Health (NIOSH) research aimed at investigating current injection at ELF, and in particular, ground contact impedance. Measurements were taken at an outside surface testing location. The results obtained from modeling and measurement are characterized by electrode impedance, and the voltage received between two distant electrodes. This paper concludes with a discussion of design considerations found to affect low-frequency communication systems utilizing ground rods to inject a current into the earth.

Statistical analysis and modeling of VLF/ELF noise in coal mines for through-the-earth wireless communications

The through-the-earth (TTE) wireless communication system, in which the RF signal can directly penetrate the earth separating a transmitter and receiver, is likely to survive a mine disaster because it requires no cabling between the surface and underground. One of the biggest challenges for TTE communication is that the ambient electromagnetic (EM) noise can be significant and impose a limitation on the reception sensitivity. Both underground and surface RF noise characteristics were obtained and analyzed from experimental data collected at several coal mines. The results show the surface has a higher noise level than underground. Moreover, the parameters obtained in an empirical noise model can be used to predict a TTE systems performance at a specific mine site.

E-fields of electrode-based through-the-earth (TTE) communication

An electrode-based through-the-earth (TTE), or linear TTE, communication system sends its signal directly through the earth overburden of a mine by driving an AC current into the earth. The resultant current, present at the receiver, is detected as a voltage and communication is established. The electrode-based TTE system may achieve a considerably large transmission range >305 m (1000 ft). This kind of system may be implemented for emergency communication close to an area such as a refuge alternative or other strategic location to establish communication between underground miners and the surface. Since it does not rely on extensive infrastructure underground, this communication would provide an alternate path out of the mine. Given the attenuation characteristics of the earth-which is the major factor determining the performance of TTE systems-the receiver sensitivity, transmitter antenna length and orientation, working frequency, and transmitted current required for communication can be estimated and determined. In an effort to estimate the earth attenuation, the analytic solution for the electrical field distribution of an electrode-based TTE communication system in a homogenous half-space is derived. A prototype system was built and tested at a mine site. The test data is compared with the modeling results, and a close agreement is found between them.

Mathematical modeling and measurement of electric fields of electrode-based through-the-earth (TTE) communication

There are two types of through-the-earth (TTE) wireless communication in the mining industry: magnetic loop TTE and electrode-based (or linear) TTE. While the magnetic loop systems send signal through magnetic fields, the transmitter of an electrode-based TTE system sends signal directly through the mine overburden by driving an extremely low frequency (ELF) or ultralow frequency (ULF) AC current into the earth. The receiver at the other end (underground or surface) detects the resultant current and receives it as a voltage. A wireless communication link between surface and underground is then established. For electrode-based TTE communications, the signal is transmitted through the established electric field and is received as a voltage detected at the receiver. It is important to understand the electric field distribution within the mine overburden for the purpose of designing and improving the performance of the electrode-based TTE systems. In this paper, a complete explicit solution for all three electric field components for the electrode-based TTE communication was developed. An experiment was conducted using a prototype electrode-based TTE system developed by National Institute for Occupational Safety and Health. The mathematical model was then compared and validated with test data. A reasonable agreement was found between them.

Time Domain and Frequency Domain Deterministic Channel Modeling for Tunnel/Mining Environments

Understanding wireless channels in complex mining environments is critical for designing optimized wireless systems operated in these environments. In this paper, we propose two physics-based, deterministic ultra-wideband (UWB) channel models for characterizing wireless channels in mining/tunnel environments - one in the time domain and the other in the frequency domain. For the time domain model, a general Channel Impulse Response (CIR) is derived and the result is expressed in the classic UWB tapped delay line model. The derived time domain channel model takes into account major propagation controlling factors including tunnel or entry dimensions, frequency, polarization, electrical properties of the four tunnel walls, and transmitter and receiver locations. For the frequency domain model, a complex channel transfer function is derived analytically. Based on the proposed physics-based deterministic channel models, channel parameters such as delay spread, multipath component number, and angular spread are analyzed. It is found that, despite the presence of heavy multipath, both channel delay spread and angular spread for tunnel environments are relatively smaller compared to that of typical indoor environments. The results and findings in this paper have application in the design and deployment of wireless systems in underground mining environments.

Simulation and Measurement of Through-the-Earth, Extremely Low-Frequency Signals Using Copper-Clad Steel Ground Rods

The underground mining environment can greatly affect radio signal propagation. Understanding how the earth affects signal propagation is a key to evaluating communications systems used during a mine emergency. One type of communication system is through the earth (TTE) that can utilize extremely low frequencies (ELF). This paper presents the simulation and measurement results of recent National Institute for Occupational Safety and Health (NIOSH) research aimed at investigating current injection at ELF , and in particular ground contact impedance. Measurements were taken at a surface testing location, outside. The results obtained from modeling and measurement are characterized by electrode impedance and the voltage received between two distant electrodes. The paper concludes with a discussion of design considerations found to affect low-frequency communication systems utilizing ground rods to inject a current into the earth.

Antenna arrangement investigation for through-the-earth (TTE) communications in coal mines

The MINER Act of 2006 requires the installation of post-accident, two-way, communications and electronic tracking systems for all coal mines. A through-the-earth (TTE) wireless communication system sends its signal directly through the overburden of a mine but can have limitation relative to performance, reliability, and transmission range. The National Institute for Occupational Safety and Health (NIOSH) conducted experiments at a coal mine for different TX/RX antenna arrangements using a NOISH TTE prototype system. This system uses multi-turn, relative small TX loop antenna instead of single turn, relative large TX loop antenna. The objectives of the test are to evaluate the performance of the system, to evaluate the path loss and optimize working frequency for the mine, to characterize surface and underground electromagnetic noise, and to investigate the feasibility of horizontal TTE communication and its advantage over vertical TTE communications. In this paper, the performance of a magnetic loop TTE communication system was evaluated for various antenna arrangements. A fairly large communication range was achieved for horizontal TTE transmission. While vertical TTE communication between underground and the surface may be restricted by factors like deployment challenges of the surface TX antenna and short transmission ranges, horizontal TTE communication within the tunnel can reach relative large distances and can thus establish a more reliable communication. Moreover, the combination of vertical and horizontal TTE communication may provide a way to considerably increase communication range.

Loop coupling and field distribution in earth for horizontal positioning in VLF/ELF through-the-earth wireless mine communications

A through-the-earth (TTE) wireless communication system is the system most likely to survive after a mine disaster because its signal penetrates the earth directly and does not use wires connecting the surface and underground components. Typically, the transmit (Tx) and receive (Rx) antennas are vertically separated by the earth overburden in a coaxial arrangement. In this paper, we show that there are advantages to separating the Tx and Rx antennas horizontally (co-planar arrangement), hence, communicating within the mine itself, and offering the advantage of mobility for one of the antennas. In this paper, we present a two-layer model, with both Tx loop and Rx loop antennas buried at the same depth underground. The magnetic field distribution results at the Rx loop are obtained for various conditions, such as earth conductivity, conductivity contrast between two layers, and co-planar and co-axis for the same Tx and Rx separation distance, providing us with an understanding of the parameters that control the performance and a concise method to predict the performance of a co-planar TTE loop communication system.