Geology Reference
In-Depth Information
becomes prone to the danger of the overlying material collapsing. If a fire starts at a previously mined area that has
coal pillars left as support structures for the overlying material, the pillars may burn causing a bigger void and
subsidence of the overlying material. Coal fires can therefore contribute to the problem of land subsidence in a
coal-mining area.
The dangers of land subsidence are many. Infrastructures above and near the subsidence area can get damaged, the
surface can become inhabitable from massive cracking, coal reserves can get blocked for further exploration,
subsidence can open new pathways for oxygen to get into the subsurface and thus flare-up old fires or start new
ones, and land subsidence may injure or kill mine workers during mining operations.
Traditional and advanced remote sensing techniques can be used to detect and regularly monitor existing
subsidence areas, as well as those areas that are likely to be affected in the future. The traditional techniques are
based on visual interpretation of aerial photos where areas of subsidence are identified based on photo-interpreta-
tion elements such as tone, texture, shape, size, pattern, association, and user knowledge of the field area.
Subsidence areas appear as small sinkholes or larger troughs with massive cracks associated with the trough edges.
Digital remote sensing methods for land-subsidence detection and monitoring involve the use of InSAR. For a
good overview of the principles, techniques, and applications of InSAR, readers are referred to the paper by Gens
and van Genderen (1996) and the topic by Ketelaar (2009). A good list of references for subsidence mapping using
InSAR is also available at http://www.gi.alaska.edu/~rgens/teaching/literature/ insar_subsidence.html. The use of
InSAR techniques for identifying potential coal-fire related subsidence was first described by Prakash et al. (2001).
A report entitled
(Genderen et al., 2000) also discusses this topic. More recently, Voigt
et al. (2004) used InSAR techniques in a coalfield in China to map subsidence.
Coal Fire Interferometry
Greenhouse-Gas Emissions
E
stimating the greenhouse-gas emissions from an individual fire and projecting global estimates is one of the
key research questions that is now being actively pursued by coal-fire researchers. Research lead by the US
Geological Survey (Kolker et al., 2009) and other institutions carried out a field-based emission estimate at the
Mulga fire in Alabama followed by a field-based and airborne-temperature and flux estimate of coal fires in the
Powder River basin of Wyoming. Their results showed that an approximate (not definite) correlation exists
between land-surface temperatures of areas affected by coal-fire and the emanating CO
2
flux. Similar efforts were
made for emission estimates in select coalfields in China (Voigt et al., 2004), Australia (Carras et al., 2009), and
India (Prakash et al., 2010).
Gangopadhyay et al. (2005) attempted to use imaging spectroscopy to quantify CO
2
emissions based on spectral
absorption by CO
2
molecules in the infrared regions. Though the theoretical concept that this research was based
on is powerful, the authors could not convincingly quantify the increase in spectral absorption due to increased
CO
2
emissions.
Extrapolating emission estimates from the observations from one fire to another fire in its proximity, or even for the
same fire at different times, is challenging due to the spatiotemporal dynamics of burning. Therefore, extrapolating
the limited results that are available for greenhouse-gas emissions from individual fires, to project a global estimate
has inherent flaws and dangers. This explains why the greenhouse-gas-emission estimates for coal fires in China,
which reportedly range from 10Mt/yr to 200Mt/year (Rosema et al., 1999; Kuenzer et al., 2007b), have such large
variability.
