![]() Most objects exhibit diffuse reflection, with light being reflected in all directions. All the light travelling in one direction and reflecting from the mirror is reflected in one direction reflection from such objects is known as specular reflection. Reflected light obeys the law of reflection, that the angle of reflection equals the angle of incidence.įor objects such as mirrors, with surfaces so smooth that any hills or valleys on the surface are smaller than the wavelength of light, the law of reflection applies on a large scale. Objects can be seen by the light they emit, or, more often, by the light they reflect. In particular, we'll use rays and wave fronts to analyze how light interacts with mirrors and lenses. ![]() Rays and wave fronts can generally be used to represent light when the light is interacting with objects that are much larger than the wavelength of light, which is about 500 nm. If the source is a long way away, the wave fronts can be treated as parallel lines. For a source like the Sun, rays radiate out in all directions the wave fronts are spheres centered on the Sun. A wave front is the line (not necessarily straight) or surface connecting all the light that left a source at the same time. A ray is a thin beam of light that travels in a straight line. Most modelling uses P-waves, but S-waves are also modelled in some cases.Light is a very complex phenomenon, but in many situations its behavior can be understood with a simple model based on rays and wave fronts. A ray tracing algorithm is used to calculate the travel times and the model is adjusted iteratively to reduce the misfit between observed and modelled times. An initial model of variations in seismic velocity is set up, based on whatever knowledge is available from other sources. The main modelling approach used for WARR profiles is to match predicted travel times, based on the geology, with those observed in the data. The processing approach used in standard seismic reflection profiling is not appropriate for wide-angle data. For the top few kilometres of the crust, such as when investigating beneath a thick layer of basalt, a range of 10–20 km may be appropriate, while for the lower crust and mantle, offsets greater than 100 km are normally necessary. The offset range used depends on the depth of the target. The three components allow the recording of S-waves as well as the P-waves that single component instruments can record. The sound waves are normally recorded using 3-component seismometers, with ocean-bottom seismometers (OBS) used offshore. Exceptionally, the sound waves from nuclear explosions have been used to look at the structure of the upper mantle down to the base of the transition zone at 660 km depth. explosive charges set off in shallow boreholes or seismic vibrators onshore or air guns offshore. naturally occurring sources, such as earthquakes, or anthropogenic sources, such as quarry blasts, or "active", sometimes referred to as "controlled source", e.g. The source of the seismic waves may be either "passive", e.g. ![]() The acquisition setup depends on the type of seismic source being used and the target of the investigation. ![]() In comparison to the typical seismic reflection survey, which is restricted to relatively small incidence angles due to the limited offsets between source and receiver, wide-angle reflection and refraction (WARR) data are acquired with long offsets, allowing the recording of both refracted and wide-angle reflection arrivals. ![]()
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