Seismic Refraction SurveyingApplied SeismologyEarthquake SeismologyRecordings of distant or local earthquakes are usedto infer earth structure and faulting characteristics. Applied SeismologyA signal, similar to a sound pulse, is transmitted intothe Earth. The signal recorded at the surface can beused to infer subsurface properties. There are two main classes of survey Seismic Refraction the signal returns to the surface by refraction at subsurface interfaces, and is recorded at distances much greater thandepth of investigation. Seismic Reflection the seismic signal is reflected back to the surface at layer interfaces,and is recorded at distances less than depth of investigation. !istory of SeismologyE"ploration seismic methods developed from early work on earthquakes #$%& 'rish physicist, Robert (allett, makes firstuse of an artificial source in a seismic e"periment. #$$$ August Schmidt uses travel time vs. distance plots to determine subsurface seismic velocities. #$)) *.+. +nott e"plained refraction and reflection of seismic waves at plane boundaries. #)#, A. (ohorovicic identifies separate - and S waves on traveltime plots of distant earthquakes, and associates them with base ofthe crust, the (oho. #)#& Seismic refraction developed to locate artillery guns by measurement of recoil. #).# /Seismos0 company founded to use seismic refraction to map salt domes, often associated with hydrocarbon traps. #)., -ractical seismic reflection methods developed. 1ithin #, years, the dominant method of hydrocarbon e"ploration. Applications Seismic Refraction Rock competence for engineering applications 2epth to 3edrock *roundwater e"ploration 4orrection of lateral, near5surface, variations inseismic reflection surveys 4rustal structure and tectonics Seismic Reflection 2etection of subsurface cavities Shallow stratigraphy Site surveys for offshore installations !ydrocarbon e"ploration 4rustal structure and tectonics Stress and StrainA force applied to the surface of a solid body creates internal forces within the body Stress is the ratio of applied force 6 to the area across which it is acts. Strain is the deformation caused in the body, and is e"pressed as the ratio of change in length 7or volume8 to original length 7or volume8. Tria"ial StressStresses act along three orthogonal a"es, perpendicular to faces of solid, e.g. stretching a bar-ressure6orces act equally in all directions perpendicular to faces of body, e.g. pressure on a cube in waterStrain Associated with Seismic 1aves 'nside a uniform solid, two types of strain can propagate as wavesA"ial StressStresses act in one direction only, e.g. if sides of barfi"ed 4hange in volume of solid occurs. Associated with - wave propagation Shear StressStresses act parallel to face of solid, e.g. pushing along a table 9o change in volume. 6luids such as water and air cannot support shear stresses. Associated with S wave propagation. !ooke0s :aw!ooke0s :aw essentially states that stress is proportional to strain. At low to moderate strains !ooke0s :aw applies and a solid body is said to behave elastically, i.e. will return to original form when stress removed. At high strains the elastic limit is e"ceeded anda body deforms in a plastic or ductile manner itis unable to return to its original shape, being permanently strained, or damaged. At very high strains a solid will fracture, e.g. in earthquake faulting. 4onstant of proportionality is called the modulus, and is ratio of stress to strain, e.g. ;oung0s modulusin tria"ial strain. Seismic 3ody 1avesSeismic waves are pulses of strain energy that propagate in a solid. Two types of seismic wave cane"ist inside a uniform solidA8 - waves 7-rimary, 4ompressional, -ush5-ull8(otion of particles in the solid is in direction of wavepropagation. - waves have highest speed. s surface 76ree surface8, and two types of surface wave can e"istA8 Rayleigh waves -ropagate along the surface of Earth Amplitude decreases e"ponentially with depth. 9ear the surface the particle motion is retrograde elliptical. Rayleigh wave speed is slightly less than S wave ?).@