Dependence of velocity-depth curves on geology

Problems in Exploration Seismology and their Solutions

Series	Geophysical References Series
Title	Problems in Exploration Seismology and their Solutions
Author	Lloyd P. Geldart and Robert E. Sheriff
Chapter	5
Pages	141 - 180
DOI	http://dx.doi.org/10.1190/1.9781560801733
ISBN	ISBN 9781560801153
Store	SEG Online Store

Problem 5.5a

Why do the velocity-depth curves for various areas shown in Figures 5.5a and 5.5b depart from each other? Incorporate your knowledge of the geology of the various areas in your answer.

Solution

Velocities are increased not only by compaction with depth of burial but also by cementation and other factors attendant upon age. They are also affected by lithology and other factors. The U.S. Gulf Coast and offshore Venezuela sediments are predominantly young siliciclastics that have never been buried deeper than they are now. Hence their velocities generally relate to the maximum porosities such as shown in Figure 5.1a. Similar reasoning applies to the shallower portions of the offshore U.S. East Coast and the Gulf of Alaska curves of Figure 5.5b. The high-velocity values in the upper part of the Gulf of Alaska curve probably indicate limestone or volcanics. The Texas Gulf Coast-1 curve penetrated Cretaceous rocks containing limestone at fairly shallow depths whereas the Gulf Coast-2 well did not encounter this section until a depth of 4.25 km. The Illinois Basin and Permian Basin wells contain much older, higher-velocity rocks including limestone.

Figure 5.5a.  Velocity-depth, U.S. Gulf Coast.

Problem 5.5b

Plot the shale and limestone values from Figure 5.5c for depths of 1000 and 2000 m on the velocity-depth Figure 5.5a. How do they compare?

Figure 5.5b.  Velocity-depth, selected wells.

Solution

The values read from Figure 5.5c are:

shale at 1000 m	2.2 km/s
shale at 2000 m	2.8
limestone at 1000	4.2
limestone at 2000	5.4

Figure 5.5c.  Velocity/depth relations (from Jankosky, 1970).

These points are plotted as triangles on Figure 5.5a for regions where the sections are sand-shale. The shale values fit nicely, but the limestone values are much too large, as one would expect. The limestone values are also plotted on Figure 5.5b where they fit in nicely with data for regions that are mostly carbonate.

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Velocities in limestone and sandstone	Effect of burial history on velocity
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Geometry of seismic waves	Characteristics of seismic events
Table of Contents (book)

Also in this chapter

• Maximum porosity versus depth
• Relation between lithology and seismic velocities
• Porosities, velocities, and densities of rocks
• Velocities in limestone and sandstone
• Dependence of velocity-depth curves on geology
• Effect of burial history on velocity
• Determining lithology from well-velocity surveys
• Reflectivity versus water saturation
• Effect of overpressure
• Effects of weathered layer (LVL) and permafrost
• Horizontal component of head waves
• Stacking velocity versus rms and average velocities
• Quick-look velocity analysis and effects of errors
• Well-velocity survey
• Interval velocities
• Finding velocity
• Effect of timing errors on stacking velocity, depth, and dip
• Estimating lithology from stacking velocity
• Velocity versus depth from sonobuoy data
• Influence of direction on velocity analyses
• Effect of time picks, NMO stretch, and datum choice on stacking velocity

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