"shock" waves from an event (explosive, tectonic, impact, etc.) that travel through the earth and reflect off of subsurface structures (rocks) in the same manner as sound echos and water waves and ripples
George E. King Engineering, 5000 Oilfield Terms: A Glossary of Petroleum Engineering Terms, Abbreviations and Acronyms, US 2010. http://gekengineering.com/Downloads/Free_Downloads /Glossary_of_Petroleum_Engineering_Terms_25_August _2010.pdf, (Oil - Amer) {593}