Time-distance curves for various situations

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	4
Pages	79 - 140
DOI	http://dx.doi.org/10.1190/1.9781560801733
ISBN	ISBN 9781560801153
Store	SEG Online Store

Problem 4.22

The situations shown in Figure 4.22a involve two rock types, the shaded one having the higher velocity [laterally varying according to the shading density in (i) and (j)]. Two dip cases are shown in (c). This figure is adapted from Barton (1929), one of the first publications in English on the seismic method. Sketch time-distance curves in the spaces above 0 [part (a) shows what is expected].

Solution

The time-distance curves are shown in Figure 4.22b. Traveltimes in the updip direction are earlier than those in the downdip direction. Parts (e) and (h) also involve a wide-angle reflection (see problem 6.13), which is not shown because it would be offscale at the top. The headwaves are tangent to diffractions that occur directly over the diffracting points. Curvature of the head-wave curves indicates changes in refractor velocity.

Figure 4.22a.  Various situations for drawing time-distance curves.

Figure 4.22b.  Time-distance curves for Figure 4.22a. Dir indicates a direct wave; $R$, a reflection; $H$, a head-wave; $D$, a diffraction.

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Partitioning at an interface	Seismic velocity
Table of Contents (book)

Also in this chapter

• Accuracy of normal-moveout calculations
• Dip, cross-dip, and angle of approach
• Relationship for a dipping bed
• Reflector dip in terms of traveltimes squared
• Second approximation for dip moveout
• Calculation of reflector depths and dips
• Plotting raypaths for primary and multiple reflections
• Effect of migration on plotted reflector locations
• Resolution of cross-dip
• Cross-dip
• Variation of reflection point with offset
• Functional fits for velocity-depth data
• Relation between average and rms velocities
• Vertical depth calculations using velocity functions
• Depth and dip calculations using velocity functions
• Weathering corrections and dip/depth calculations
• Using a velocity function linear with depth
• Head waves (refractions) and effect of hidden layer
• Interpretation of sonobuoy data
• Diving waves
• Linear increase in velocity above a refractor
• Time-distance curves for various situations
• Locating the bottom of a borehole
• Two-layer refraction problem

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