9781422285534

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.