Seismology—the study of elastic waves that travel on and beneath the surface of the earth—allows scientists to “see” underground. Geophysicists are scientists who study Earth's seismicity, gravity, magnetism, and other physical properties. At the KGS, they develop and use techniques and equipment to gather information about subsurface rocks, hazards, and such resources as oil, natural gas, and water.
Geophysicists use a technique called seismic reflection to identify underground rocks, resources, and structures without digging or drilling. They create a vibration at the earth's surface by pounding the ground or setting off explosives to generate seismic waves.
Methods used by geophysicists to generate waves to study the subsurface are known as active sources. (Earthquakes and regular cultural sources of seismicity, such as heavy traffic, are passive sources.) KGS scientists use several active sources, including a specially equipped seismic vehicle that drives a weight into the ground with a series of low-power impacts, a bungee-assisted weight drop, and a sledgehammer.
Surface waves and body waves are the two main categories of seismic waves. Surface waves, as the name suggests, travel on Earth's surface, and body waves travel beneath it. Because seismic waves behave differently when they encounter different kinds of rocks and structures, data collected during seismic studies provide an image of the subsurface.
Portable sensors called geophones are used to detect seismic waves generated by an active source. Researchers stick rows of geophones in the ground then create a seismic wave. They then use specialized software to create images from the recorded data to identify different types of rock layers, faults, and other geologic structures. Multiple geophones are deployed during each seismic study. Typically, scientists at the KGS have used 24 geophones for shallow investigations and as many as 480 geophones for investigations at depths greater than half a mile.
In Kansas, seismic surveys are used to
- map subsurface geologic structures that provide clues to the state’s geologic history and earthquake activity
- evaluate the risk of subsidence—sinking or settling of the earth's surface—for existing or future highways, buildings, and bridges
- map existing sinkholes in the Red Hills to help understand how and why they formed
- evaluate the risk for collapses into subsurface voids created by dissolution of salt around Hutchinson
- map coal beds and detect abandoned mines in eastern Kansas
- evaluate bedrock for fractures and voids at sites being considered for wind turbine installations and other projects
Besides using seismic reflection to study the subsurface, the KGS has a network of permanent seismic monitors throughout Kansas that detect and measure passive seismic waves generated by earthquakes. Seismometers at seven stations record ground movement, and the data collected can be analyzed to determine the direction, intensity, duration, depth, epicenter, and magnitude of an earthquake. The data also provide insight into whether and how rocks in the subsurface moved after an earthquake.
Seismology and Its Applications in Kansas, Kansas Geological Survey Public Information Circular 37