Mining Contract: Two-Way, Through-the-Earth Emergency Communication System for Trapped Miners and the Surface

Contract Status & Impact

This contract is complete. To receive a copy of the final report, send a request to mining@cdc.gov.

The objective of this contract was to design and build a two-way TTE communication prototype based on Stolar’s expertise in radio geophysics and to field test it at several underground mining operations.

Prototype Development

Stolar created a TTE modem technology which uses low-frequency transceivers to communicate via text messaging for two-way communications. Stolar’s system accomplished this using relatively low-power instrumentation and simple, portable hardware.

At the heart of the TTE system is the transceiver electronics, also described as low-frequency modems. Two different prototype systems were built: the primary software-definable digital transceiver (SDDT) system, and a secondary field-programmable gated array (FPGA) system. The FPGA system does not compete directly with the SDDT, but explores the feasibility of a wideband frequency shift keying (FSK)-type modem for future implementation in a faster digital signal processing platform. The FPGA prototype was systematically field tested in parallel with the SDDT modem during this project to evaluate the strengths and weaknesses of each design. For each system, three transceiver units were built: two surface units (plastic housings), and one underground unit (X/P housing).

The antennas used for this project were rigid air-core loops of either 4-ft or 6-ft diameter. They can be used as either vertical or horizontal magnetic dipoles (VMD or HMD), depending on their orientation and the companion transceiver’s point of reference.

• VMD mode is optimal for low-noise environments where TTE signal sensitivity will be high and the depth of penetration will be in the upper 40% of the maximum capabilities of the system.
• HMD mode is optimal for higher-noise environments since less noise from horizontal power lines can couple to the vertical antenna. However, HMD mode has less signal sensitivity and will reduce the depth of penetration to less than 70% of the maximum capabilities.

The primary screen menu on the transceiver’s LCD display lets the user enter custom text messages or turn on a location beacon. Another screen menu lets the user select from a variety of predefined messages.

• The text messages use a user-selected frequency of either 2 kHz or 4 kHz. The default lower frequency is commonly used for depths of 500 feet and deeper. The higher frequency has less penetrating power in the earth (higher attenuation rate) and is typically reserved for short-range applications where background noise at 2-kHz may be higher than normal.
• In beacon mode, the modem is used as a continuous transmission source for detection and location by the surface gradiometer (Delta Tracker).

Two types of power packs were developed, a high power (24V) and low power (15V). The low-power pack is lithium-ion based and has a capacity of 50 hours in a small, lightweight enclosure. The high-power pack is lead-acid based and can run a transceiver nearly 30 hours. The packs can be linked to augment lifespan. Capacity estimates for a single pack assume a 90/10 duty cycle at maximum transmission loop current.

Field Testing

Stolar field tested the prototype at several hard rock and coal mining sites between May and October, 2010. Preliminary testing in the western US proved that Stolar’s TTE technology can provide communication through a variety of rock types with depths ranging from 700 to 1,300 feet, depending on local rock types and their physical and electrical properties. The most recent demonstration achieved two-way text messaging in the deepest part of a commercial coal mine in southwestern Pennsylvania (nearly 800 feet) with ample signal strength in reserve; the maximum range at this mine was projected at nearly 1,100 feet.

To help with field testing and data collection and to assist with modem setup at different sites, a diagnostic program was created. This program links wirelessly to the individual transceivers for real-time signal analysis including signal strength, decoding sensitivity and threshold, and background noise.