Page 278 - Advanced Mine Ventilation

Premining Degasification of Coal Seams 255

Figure 15.6 The downhole drill monitor.

also be known so that the deflection device can be properly oriented. Also, the Mine
1
Safety and Health Administration (MSHA) requires that the azimuth of degasification
boreholes be plotted on mine maps to prevent inadvertent mining through such holes.
Borehole survey instruments incorporate sensors for the azimuth, pitch, and roll and
a coal thickness indicator. The latter indicates the thickness of the coal between the
borehole and the floor or the roof, depending on the orientation of the surveying
tool. Fig. 15.6 shows the basic components of the survey instrument system, namely
the DDM and the readout unit.
The DDM system consists of a downhole survey probe and a portable data collec-
tion and display unit situated outside the borehole.
The downhole survey probe is a battery-powered microprocessor-controlled data
acquisition system contained in a 12 ft. long coppereberyllium tube. It is located
just behind the downhole motor. The DDM remains downhole until the target depth
is reached or until a battery change is needed. A triaxial magnetometer is used to mea-
sure the magnetic azimuth. Three accelerometers are used to measure pitch and roll of
the drill bit. A solid-state gamma detector is used to monitor small amounts of natural
gamma radiation emitted from the overlying and underlying shale deposits.
An approximation of roof and floor coal thickness can be made from the observed
gamma ray count and the known half-depth value for gamma rays in coal. A built-in
computer program controls collection and transmission of data to the collection and

1
Mine Safety and Health Administration (MSHA) is the US certification agency for all mine equipment.