Salt lead time as a function of depth

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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, 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 be the path length in the salt. The lead time is the difference in traveltime for a salt path length of . Thus,

Figure 11.1a.  Gulf Coast interval velocity.
Table 11.1a. Calculation of lead time .
(km) (km/s) (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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