Lesson plan
Normal moveout
In reflection seismology, normal moveout (NMO) describes the effect that the distance between a seismic source and a receiver (the offset) has on the arrival time of a reflection in the form of an increase of time with offset. The relationship between arrival time and offset is hyperbolic and it is the principal criterion that a geophysicist uses to decide whether an event is a reflection or not. It is distinguished from dip moveout (DMO), the systematic change in arrival time due to a dipping layer. The normal moveout depends on complex combination of factors including the velocity above the reflector, offset, dip of the reflector and the source receiver azimuth in relation to the dip of the reflector. For a flat, horizontal reflector, the traveltime equation is: t^2 = t_0^2 + \frac{x^2}{v^2} where x = offset; v = velocity of the medium above the reflecting interface; t_0 = travel time at zero offset, when the source and receiver are in the same place. According to W. Harry Mayne, inventor of the Common Point Reflection Method in 1950, in order to avoid the "smearing" of recorded seismic data caused by the use of geophone sensor arrays, I needed a very long array to attenuate the noise, yet each point of the array needed to represent the same reflection point of the subsurface.…
Article summary
In reflection seismology, normal moveout (NMO) describes the effect that the distance between a seismic source and a receiver (the offset) has on the arrival time of a reflection in the form of an increase of time with offset. The relationship between arrival time and offset is hyperbolic and it is the principal criterion that a geophysicist uses to decide whether an event is a reflection or not. It is distinguished from dip moveout (DMO), the systematic change in arrival time due to a dipping layer. The normal moveout depends on complex combination of factors including the velocity above the reflector, offset, dip of the reflector and the source receiver azimuth in relation to the dip of the reflector. For a flat, horizontal reflector, the traveltime equation is: t^2 = t_0^2 + \frac{x^2}{v^2} where x = offset; v = velocity of the medium above the reflecting interface; t_0 = travel time at zero offset, when the source and receiver are in the same place. According to W. Harry Mayne, inventor of the Common Point Reflection Method in 1950, in order to avoid the "smearing" of recorded seismic data caused by the use of geophone sensor arrays, I needed a very long array to attenuate the noise, yet each point of the array needed to represent the same reflection point of the subsurface.…
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Discussion questions
- Summarise the article in your own words. Which single claim would you verify first, and why?
- Which claim in this article is most likely to be revised as the sources update? How would you check its current status?
- If two sources disagreed about one of these facts, what would you need in order to trust one over the other?
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