Mining Contract: Roof Resistivity Monitoring System (RRMS)

Contract Status & Impact

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

Over 1,500 roof falls occur each year in U.S. coal mines in spite of over 100 million roof bolts being installed each year. While design methods such as the Coal Mine Roof Rating (CMRR) and Analysis of Roof Bolt Systems (ARBS) have proven to be effective, they are usually generally applied and do not account for the localized geologic variability, lensing, and changes of horizontal stress, nor for the time-variant changes in stress caused by retreat mining. The lateral variability and vertical anisotropic nature of sedimentary lithology make the classical approach of assessing stress levels or rock quality based on point measurements or one-time characterization a poor and expensive method of predicting localized phenomena.

Electrical resistivity methods inject a small current into the earth and the resulting voltage is measured. In the case of a Roof Resistivity Monitor System (RRMS), as the micro-fractures develop, natural conductivity is disrupted and resistivity increases. The rate of resistivity change is therefore a direct correlation with fracture development. Existing roof bolts can be used as contact points for electrical current (I) injection and as voltage (V) measurement points. By using different I/V probe location geometries, effective area and in-depth measurements can be created to generate a 3-D map of the resistivity properties of the rock (called electrical time-lapse tomography). Whereas in exploration or mapping applications the arrays are moved to create a picture of rock variations, in the RRMS application the probes are stationary. In its simplest form, RRMS requires only four bolts to act as probes. Despite these attributes, the RRMS system needs to be updated to accommodate the greater potential provided by modern electronics, needs to exploit a real-time alarm system to warn miners that change is occurring, and needs to be hardened and simplified for routine use.

Under this contract, C-2 Innovations, Inc., is developing time-based matrix resistivity systems, proving their viability under typical working conditions at an active coal mine, gathering data to demonstrate sensitivity to changes in roof stability, and assessing the ease of installation, design, and testing of an alarm system. It is anticipated that the contractor will apply for an experimental permit and test a prototype system in 2017. The ultimate goal is to develop a practical system that will provide miners with a real-time localized means of monitoring roof stability.