The location of abandoned mine workings using thermal techniques

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Abstract

Between 1926 and 1956, laboratory experiments and underground observations demonstrated the relationship between barometric pressure decrease and the efflux gas in mines. This resulted in the establishment of early warning systems in some British mines when significant falls of atmospheric pressure were expected. In 1993, ‘frost circles’ were observed on the ground surface, above abandoned mine shafts. These were caused by warmer mine air escaping into the atmosphere, via the mine shafts. The temperature of the ground above, and in the vicinity of, abandoned mines was subsequently monitored using digital thermometers, a precision radiation thermometer and infrared thermal photography. This was followed by the use of a longer-term, ground temperature monitoring technique, using digital temperature recorders, which monitored the ground temperature every 2.5 minutes for a period of 14 days. The ground temperatures above the abandoned mine shafts were also found to be related to fluctuations in barometric pressure, agreeing with the earlier observations. During periods of high barometric pressure mine gases were prevented from escaping into the atmosphere and accumulated in the mine voids. Following a drop in barometric pressure the mine gases were capable of escaping into the atmosphere, where they underwent oxidation. This produced an increase in ground temperature in the order of a few degrees centigrade. The temperature differential may be sufficient to be detected by remote thermal imaging techniques, provided that they are undertaken in suitable climatic conditions. This type of remote, reconnaissance technique, is particularly applicable to the British Isles due to the abundance of abandoned mines workings and the regular fluctuations in climatic conditions.

Introduction

During underground coal mining operations, ventilation is essential to prevent gas accumulation; this is controlled by fans, screens, air locks and doors. Investigations and experiments have concluded that the reservoir of gas that exists in certain rock formations can expand and contract with changes in atmospheric pressure. The idea that the efflux of methane gas in mines was associated with changes in barometric pressure has long been known (Fig. 1) (Carter and Durst, 1955, Harries, 1926).

Experiments have been undertaken to forecast the emissions of methane with barometric pressure changes (Durst, 1956). These were carried out with some success and have been compared with actual observations (Fig. 2). Observations carried out over a 5 day period on a long-wall face clearly demonstrate the relationship between barometric pressure decrease and increase in methane at the edge of the goaf (Fig. 3).

Owing to gas emissions caused by a reduction in atmospheric pressure, early warning systems were established in some British mines. For instance, in December 1949, the Central Forecasting Office of the Meterological Office issued special reports when sharp falls of pressure were expected. These sharp falls were defined as one of 4–8 mb, in 3 h, Durst (1956). A very sharp fall was considered as one occurring when the pressure fell by >8 mb in 3 h. In one period, from March 1951 to April 1952 warnings were issued on 36 occasions and 50% of these were successful. This indicated the difficulty of accurate and precise forecasting.

During the winter of 1993, an airborne survey was being undertaken at the former Wet Earth Colliery site in the Lancashire Coalfield to observe mining-induced fault reactivation (Donnelly, 1994). This is a green-field site adjacent to the River Irwell, in a region of urban occupation. At the time of this survey a ground frost prevailed at the site. From the air, circular features were observed on the ground although there were no visible mine shafts.

These circular features, or ‘frost circles,’ corresponded to the approximate position of a mine shaft marked on a mine plan; it was suspected that the other features may also represent the site of abandoned mine shafts. A ground inspection immediately followed the airborne observations, but the circular features were not visible. It was hypothesised that the air in the mine shafts was slightly warmer than the surrounding rock mass and produced a sufficient temperature difference to change the crystallinity of the ground frost, but insufficient to melt it completely.

This paper documents the monitoring of ground temperatures above abandoned mine shafts during different barometric pressure conditions. This resulted in the provision of information on optimum conditions for the remote thermal detection of mine shafts.

Section snippets

Hazards presented by abandoned mine shafts

Abandoned mine shafts in the vicinity of a property or structure represent a potential hazard. It is essential, prior to development in mining regions, that the extent and depth of previous mining in the near-surface environment is determined accurately. However, this can be difficult to ascertain.

A range of geophysical and other site investigation techniques are available to detect mine shafts and have been used with variable success over the past few decades. (Jackson et al., 1987, McCann et

Direct measurements

At a site in the North Yorkshire coalfield, a Hanna, hand held, digital thermometer, with a 0.1° resolution, was linked to a 100 mm long soil probe and ground temperature measurements were undertaken over known mine shafts positions. The temperature of the centre of the shaft was taken first, then the ambient ground temperature was monitored at 1.0 m intervals to a distance of ca. 20 m, from the shaft. However, the ground temperature above the mine shaft was found to be lower than the ambient.

Mine shaft A

Three digital temperature recorders were programmed to record the temperature every 2.5 min, for a period of 14 days. One instrument was installed directly above the central position of a mine shaft, immediately beneath the vegetation cover, the second at the depth of 0.75 m, also in the central position of the mine shaft. The third was buried 30 m from the shaft, again below the vegetation, to record the ambient temperature of the ground. It was thought necessary to cover the instruments in an

Mine shaft B

The three temperature recorders were re-installed on a second mine shaft, under different barometric pressure conditions than the previous investigation at mine shaft A. Recent investigations monitoring radon gas emissions from mine shafts has revealed that gas often escapes from a shaft into the atmosphere along a preferential route, up the walls of the shaft. This results in a typical ‘rabbit ear’ distribution of the data (Donnelly, 1996). These observation suggested that the central portion

Remote thermal detection of abandoned mine shafts

Based on the findings of this paper, a research project has been commissioned by the Natural Environment Research Council (NERC) and the British Geological Survey (BGS). The remote detection of the thermal anomalies between abandoned mine shafts and their surroundings should be possible using a pre-dawn acquisition of the thermal band 11 on the NERC, ATM or the Daedalus 1230 Thermal Line Scanner. Previously, these have been used to detect temperature variations over landfill sites in the order

Conclusions

The results of these investigations have support experiments and observations made in the early and mid part of the 1900s. That is, fluctuations in barometric pressure can control the efflux of gas in mines. The mine gases may subsequently escape, via mine shafts, into the atmosphere.

Mine gasses which escapes from a mine shaft may cause a subtle increase in temperature of the ground surface. This has been observed in the form of ‘frost circles’ (where slightly warmer, mine air, rising up a mine

Acknowledgements

This paper is published with the permission of the Director of the British Geological Survey (NERC) and IMC Consulting Engineers Ltd. The views expressed are those of the authors and not necessarily those of the BGS (NERC) and IMC.

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