7/29/2019 Basics of Seismic Velocities http://slidepdf.com/reader/full/basics-of-seismic-velocities 1/5 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 geophysical industry. This includes developments in both geophysical techniques and geophysical interpretation as applied to the understanding of the earth’s geology. However, we are also aware that our journal is passed around the office to nongeophysicists, and is 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 geophysical tutorial-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 find it 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 is an 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 from semiinfrequent to fairly frequent. -BRIAN RUSSELL Chairman, TLE Editorial Board Basics of seismic velocities Basics of seismic velocities By GEORG E B. AMERY Houston, Texas U nderstanding and interpreting seismic velocity data appear to be difficult for two reasons. First, complexity is introduced when 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 variations on 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 is usually a rather homogeneous rock unit, thin (inches) but of significant lateral extent, and with a rather constant interval velocity. In terms of reflection seismic data, the thinnest unit we might examine effectively is the sequence or sequence set (several hundreds of feet). The characteristic velocity of this larger unit is the interval-time-weighted average of the veloc- ities of all the lamina that compose the unit. This velocity is usually referred to as interval velocity ( V or V i ), although it actually is an average. On the short end of the spectrum, a sonic log with 2 ft spacing is probably measuring velocities at the lamina-set or bed level. Average velocity (V&& is the average of all the interval velocities 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