Geology Reference
In-Depth Information
Fire-Depth Estimation
E stimating fire depth using remote sensing is and will remain a complex problem, as the intent is to quantify a
parameter that is not directly measurable by remote sensing. To model the subsurface condition based on what is
observed on the surface requires simplifications and assumptions of naturally occurring processes. Most fire-depth
estimation models are based on investigating the location of a surface thermal anomaly and its unique sectional profile.
The simplest and crudest method for fire-depth estimation is a geometric method (Saraf et al., 1995). According to
this method, if the location of the surface thermal anomaly associated with an underground fire, and the dip of the
proximally outcropping coal seam are known, then a simple geometric construction can help to give an approx-
imate estimate of fire depth (Figure 14.4.4).
The two inaccurate assumptions made in this model are that the proximally outcropping seam is the one on fire, and
the surface thermal anomaly is vertically above the underground heat source. Peng et al. (1997) expanded the
geometric model to include more geologic (stratigraphic and structural) information of the site under consideration.
In the absence of any other sophisticated equipment and data, this technique is still very useful to give a first-order
estimate of fire depth.
Other depth-estimation techniques are based on numerical methods that rely on the physical principles of heat-
energy transfer. Numerical models can be broadly divided into two categories: (1) conduction as the sole means
of heat transfer from the subsurface to the surface and (2) a combination of conduction and convection processes
for heat transfer. In both categories, the heat source is modeled as either a point source or a linear/cylindrical
source; and the overlying material as one constituting a semi-infinite medium. Most efforts to understand the
transfer of heat energy along the surface are supported by finite element analysis. Both types of models have their
limitations as discussed below.
Most conduction-based models (Mukherjee et al., 1991; Cassells, 1998; Prakash and Berthelote, 2007; Berthelote
et al., 2008) are based on the assumption that the heat transfer from the subsurface to the surface is linear in nature
and after a certain time, the system reaches a steady state. The shape of the surface thermal anomaly at this point
holds clues to the depth of the heat source. Shallow fires quickly increase the surface temperature, whereas deeper
fires cause the surface thermal anomaly to be widespread and very subtle (Figure 14.4.5). A profile of the surface
thermal anomaly from a shallow-heat source is therefore considerably different than from a deeper source.
Distance
Surface
thermal anomaly
X
Y
X
Y
Dip angle
Depth
Fire
Figure 14.4.4. Diagram illustrating a simple-geometric construction for estimating fire depth. The top portion of
the diagram shows a plan view and the bottom a cross-sectional view through the line
X-Y
. The distance between
the coal seam and surface thermal anomaly can be measured
or from remote sensing images. In addition, by
knowing the dip direction of the coal seam below the anomaly, the normal distance from the surface thermal
anomaly to the coal seam, is a quick approximation of fire depth. Adapted from Saraf et al. 1995.
in situ
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