In the petroleum industry, the most common and effective method of obtaining a clear picture of the Earth's subsurface is seismic exploration. This technique is based on principles of Earth physics. Seismic exploration is the use of seismic energy to probe beneath the surface of the earth, usually as an aid in searching for economic deposits of oil, gas or minerals, but also for engineering, archeological and scientific studies. In exploration seismology, the seismic method is applied at or near the earth's surface to measure the elastic properties of the subsurface and to detect variations in those properties. Variations and discontinuities in subsurface elastic properties are usually indicative of changes in lithology or, rarely, pore fluids.