EXTENDED MONITORING OF COAL MINE FIRES WITH THERMOCHRONS

Temperature monitoring is critical for successful exploration, design and assessment of coal mine and coal refuse fire abatement projects.  Boreholes are commonly used to explore subsurface fires by sampling gases and measuring temperatures from the burning horizon.  Relative changes in temperatures over time in the boring are direct indicators of fire activity.  Routine monitoring of borings is time consuming and often entails expensive sophisticated equipment.  Accurate temperature trends, whether cooling or heating, require many consistent readings over long periods.  Infrequent monitoring may miss changes caused by venting, weather and other environmental factors. The use of a novel thermochron device termed an ‘I-button’ can inexpensively and efficiently record temperatures in moderately warm holes over long periods.

The temperature logger I-button is a microprocessor about the size of a large watch battery, referred as a thermochron.  The I-button can be deployed to measure temperatures up to 125°C remotely down a boring, in a fire vent or shallow excavation. The device can be retrieved and downloaded in the field using a laptop computer.  The device has 8kb of memory, which allows storage of 328 days of data with a temperature recorded every 4 hours.  The sample rate can be varied from 1 second to 273 hours.  The data produces a text file that can be easily read and graphed in a spreadsheet program or reviewed immediately in the vendor-supplied software.

We have deployed thermochrons at four sites using slightly different techniques.  These sites vary from small fires, monitoring a few holes to a large fire where over fifty holes are being monitored for a long term.  At two small fire sites the devices were used to monitor the success of the abatement.  The monitoring showed a gradual reduction in temperatures. 

On a large mine fire site numerous thermochrons were installed on the cold side of a fire barrier to measure the integrity of the cutoff.  Another smaller mine fire site has 10 thermochrons installed to measure fire activity prior to abatement.  Some of the thermochron locations undisturbed by the excavation activities will be retained at this site to evaluate abatement success over a period of a year or more.