reflection seismology Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismi ...

, 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 Hyperbolic is an adjective describing something that resembles or pertains to a hyperbola (a curve), to hyperbole (an overstatement or exaggeration), or to hyperbolic geometry. The following phenomena are described as ''hyperbolic'' because they ...

and it is the principal criterion that a

geophysicist Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' som ...

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

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 A geophone is a device that converts ground movement (velocity) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analyzed for structure of ...

sensor array A sensor array is a group of sensors, usually deployed in a certain geometry pattern, used for collecting and processing electromagnetic or acoustic signals. The advantage of using a sensor array over using a single sensor lies in the fact that an ...

s, 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. For a non-dipping reflector, this meant that the source and receiver station would have to move the same distance-in opposite directions-from the reflection (or mid-) point. One problem still remained. The reflections had different traveltimes on each pair of sources and receivers, so it would be necessary to correct for these differences (moveouts) prior to array formation." Coupled with the normal moveout correction, Mayne stated, "the method was primarily intended to attenuate systematic surface noise, and to average out near-surface aberrations in travel paths. It was soon realized, however, that it alone could also substantially attenuate the insidious multiple reflection."

References

{{Reflist Seismology