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20 Coal
are sent down into the ground where they are reflected or redirected by various rock
layers, in different ways, back to the surface. The echoed or returning waves are picked
up by devices called seismometers, which measure their strength, time delay, and
other features. The seismic survey is one of several methods that reveal the physical
features of what is underground.
There are other surface methods to locate coal. Devices called gravimeters
measure tiny changes in Earth’s gravitational pull that are caused by different kinds
of rocks. Magnetometers sense similar changes in the Earth’s natural magnetic field.
Chemical sensors can pick up faint traces of methane seeping up through the rocks
from deep coal.
Test Boreholes
If signs look good, the exploration company may drill some
boreholes
. This is done
in a similar way as that used to search for petroleum oil and natural gas (see other
titles in this series). The boreholes are spaced out to find the depth and extent of the
coalfield. Now and then, devices may be lowered into the borehole. These include
“sniffers” for gases such as methane, counters to detect any natural rays or radiation
from the rock, and seismic vibration producers to send out shock waves.
A seismograph
records vibrations
from below
ground.