Salt lead time as a function of depth
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| Series | Geophysical References Series |
|---|---|
| Title | Problems in Exploration Seismology and their Solutions |
| Author | Lloyd P. Geldart and Robert E. Sheriff |
| Chapter | 11 |
| Pages | 415 - 468 |
| DOI | http://dx.doi.org/10.1190/1.9781560801733 |
| ISBN | ISBN 9781560801153 |
| Store | SEG Online Store |
Problem 11.1a
The velocity of salt is nearly constant at 4.6 km/s. Calculate the amount of lead time per kilometer of salt diameter as a function of depth assuming the sediments have the Louisiana Gulf Coast velocity distribution shown in Figure 11.1a.
Background
Early seismic prospecting for salt domes involved locating geophones in different directions from the source at roughly the same distance from it. Rays that passed through salt arrived earlier than those that did not, the reduction in traveltime due to the high velocity in salt being the lead time.
Solution
The first two columns of Table 11.1a were obtained from the dashed curve in Figure 11.1a. The third column gives the lead time per kilometer of salt, that is, $ \Delta t=(1/V_{i}-1/4.6) $ s/km.
The lead time decreases rapidly with depth to the top of the dome because compaction causes the sediment velocity to increase.
Problem 11.1b
Early refraction work searching for salt domes in the Gulf Coast considered a significant “lead” to be 0.25 s. Assuming a range of 5.6 km and normal sediment velocity at salt-dome depth of 2.7 km/s, how much salt would this indicate?
Solution
Let $ x $ be the path length in the salt. The lead time is the difference in traveltime for a salt path length of $ x $. Thus,
$ {\begin{aligned}0.25=x\left(1/2.7-1/4.6\right);\quad x=1.6\ {\rm {km}}.\end{aligned}} $
| $ z $ (km) | $ V_{i} $ (km/s) | $ \Delta t $ (ms/km) |
|---|---|---|
| 0.25 | 1.70 | 371 |
| 0.50 | 1.92 | 303 |
| 0.75 | 2.11 | 257 |
| 1.00 | 2.30 | 217 |
| 1.25 | 2.46 | 189 |
| 1.50 | 2.63 | 163 |
| 1.75 | 2.80 | 140 |
| 2.00 | 2.93 | 124 |
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| Geologic interpretation of reflection data | 3D methods |
Also in this chapter
- Salt lead time as a function of depth
- Effect of assumptions on refraction interpretation
- Effect of a hidden layer
- Proof of the ABC refraction equation
- Adachi’s method
- Refraction interpretation by stripping
- Proof of a generalized reciprocal method relation
- Delay time
- Barry’s delay-time refraction interpretation method
- Parallelism of half-intercept and delay-time curves
- Wyrobek’s refraction interpretation method
- Properties of a coincident-time curve
- Interpretation by the plus-minus method
- Comparison of refraction interpretation methods
- Feasibility of mapping a horizon using head waves
- Refraction blind spot
- Interpreting marine refraction data
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