basics of seismic velocities

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GEOPHYSICAL TUTORIAL SERIES

A s the name of our publication suggests, THE LEADING EDGE has a mandate to seek out and publish articles that discuss the latest developments in the geophysicalindustry. This includes developments in both geophysical techniques and geophysical

interpretation as applied to the understanding of the earth’s geology. However, weare also aware that our journal is passed around the office to nongeophysicists, andis also read by many geophysicists who may not be familiar with certain techniques.For that reason, it is important to sometimes go back and look at the basics. From

time to time, an article is submitted that falls into the category of a geophysicaltutorial-that is, it explains a fundamental idea in a clear and concise fashion that can

be understood by all readers. In fact, readers who know the subject matter often findit a worthwhile exercise to reread this material. They are often surprised at some new

pearl of wisdom that they are able to glean from the article.The following article, “Basics of seismic velocities” by George Amery, is a good

example. It is a clear review of seismic velocity concepts and explains the differences between such things as interval, average, NMO, and rms velocity. The paper also looks

at the effects on velocity of horizontal and dipping layers.Have a look at this article. If you think your already know this material, pass it on

to a colleague (maybe a geologist or reservoir engineer). And, even better, if there isan area of geophysics that you feel needs such a tutorial, sit down and try to write it.We’d love to hear from you. Maybe we could convert the status of this series fromsemiinfrequent to fairly frequent.

-BRIAN RUSSELLChairman, TLE Editorial Board

Basics of seismic velocitiesBasics of seismic velocities

By GEORGE B. AMERY Houston, Texas

Understanding and interpreting seismic velocity data appear to be difficult for two reasons. First, complexity is introducedwhen we describe seismic velocities as interval velocity ( V or V i ), average velocity (I$&, normal moveout velocity ( VWO)and root-mean-square velocity (l&J. The second cause for

difficulty is due to the effect of physical-geologic variationson seismic velocity and our general lack of understanding of those effects.

Velocity of compressional wave propagation is a funda-mental physical property of rocks, a property that is deter-mined by its density and elastic moduli. Stratigraphers de-scribe rock layers, starting with the thinnest, as lamina, lam-ina sets, beds, bed sets, sequences, sequence sets. A lamina isusually a rather homogeneous rock unit, thin (inches) but of

significant lateral extent, and with a rather constant intervalvelocity. In terms of reflection seismic data, the thinnest unitwe might examine effectively is the sequence or sequence set(several hundreds of feet). The characteristic velocity of thislarger unit is the interval-time-weighted average of the veloc-ities of all the lamina that compose the unit. This velocity isusually referred to as interval velocity ( V or V i ), although itactually is an average. On the short end of the spectrum, asonic log with 2 ft spacing is probably measuring velocitiesat the lamina-set or bed level.

Average velocity (V&& is the average of all the intervalvelocities from the surface to the depth of a particular horizon. Normal moveout velocity (VmO) is the parameter that de-scribes the shape of a seismic reflection recorded at several

NOVEMBER 1993 THE LEADING EDGE



Figure 7. Time-depth plot generated from seismic veloci-ties and reflection times.

from seismic data are accurate to within a few percent and areuseful for time-depth determination (V’,) and migration(I/i ). However, any variation from the model may causesevere problems. Things such as dipping beds, and rapidlateral velocity changes within a layer can affect seismicvelocity calculations. The most severe problem relates toinadequate static corrections applied to correct for variationsdue to weathering and elevation changes at the surface. Theeffect of these problems is to add or subtract a constant time(Ml) from the reflection time. The effect on calculated VW0

increases greatly for the deeper reflections. Such errors mayoften be compensated for by spatial smoothing.to have a few well-velocity surveys in the area.

It also helps

T ime-depth plots from seismic data. The procedure for applying the interval velocities determined from seismic datais illustrated in Figure 7. Interval velocities (V,) calculatedfrom the Dix equation are used to construct a time-depth plotsimilar to those obtained from well-velocity surveys. Asshown in Figure 7, starting at the surface, interval thicknesses between adjacent reflectors are computed and added together to produce the depth (Z) of each reflector. That depth is plotted versus reflection time to produce the time-depth plot.Most velocity analysis programs automatically produce these

plots.If these time-depth plots are to be used for reflection

migration or time-to-depth conversion, they will probably

need to be smoothed or averaged. Adjacent velocity profilegathers will often result in variations that are impossiblegeologically. Converting these profiles to a useful velocityfield will require some form of spatial smoothing. An excel-lent way to smooth takes the form of an Iso-Depth plot, whichis overlain on the seismic section. Then, smoothing may be

performed in a geologically reasonable manner. It helps to beable to tie these smoothed time-depth plots to local well-ve-locity surveys. The resulting time-depth data may then beused effectively for seismic reflection migration and time-to-depth conversion, which should result in geologically reason-able seismic data. IE

NOVEMBER 1993 THE LEADING EDG

basics of seismic velocities

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