science

Normal moveout

Normal moveout is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Normal moveout rather than just read about it. In short: 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.

Key takeaways

Normal moveout belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Normal moveout to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Normal moveout from memory before moving on to harder problems.

Reference excerpt

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 + x 2 v 2 {\displaystyle t^{2}=t_{0}^{2}+{\frac {x^{2}}{v^{2}}}}

where x = offset; v = velocity of the medium above the reflecting interface; t 0 {\displaystyle 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. 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

Illustrations

Worked examples

Example 1 — a first encounter with Normal moveout

Start with the simplest possible case. Write down what Normal moveout claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Normal moveout before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Normal moveout ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Normal moveout

In research: Normal moveout appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Normal moveout in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Normal moveout is common in secondary-school and first-year university syllabi. It links to neighbouring topics Seismology, so understanding it makes those chapters shorter.
In everyday life: Look for Normal moveout outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Normal moveout in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Normal moveout means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Normal moveout out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Normal moveout in simple terms?

Why does Normal moveout matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Normal moveout?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Normal moveout.