Effects of weathered layer (LVL) and permafrost

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Problem 5.10a

Assume that raypaths have angles of approach of , , , , and in the subweathering where the velocity is 2400 m/s. For a weathered layer 10 m thick with velocity 500 m/s, how do travel-times through the weathered layer compare with that for a vertically traveling ray? What are the horizontal components of the raypaths in the LVL?

Solution

Referring to Figure 5.10a,

Figure 5.9a.  Effect of overpressure on sonic and resistivity logs for an offshore Gulf of Mexico well (from MacGregor, 1965).
Table 5.10a. Traveltimes and for raypaths in the LVL and permafrost.
Incident In low-velocity layer In permafrost
(m) (ms) (m) (ms)
0.0 20.0 0 27.8
0.4 20.0 27 28.8
0.7 20.1 60 32.4
1.1 20.1 113 42.0
1.4 20.1 363 104.6
1.4 20.1
1.8 20.3

Substituting the values of , we get the results in Table 5.10a. The traveltimes in the LVL vary by only 0.5% over most of the range of , and, even for , change by only 1.5%.

Problem 5.10b

For permafrost 100 m thick with a velocity of 3600 m/s, answer the questions in part (a).

Solution

We repeat the calculations of part (a) changing to 3.60 km/s and layer thickness to 100 m. The results are also shown in Table 5.10a. Because rays now have large horizontal components, the changes in and are considerable. This large ray bending makes corrections for permafrost very difficult. If , upcoming waves are totally reflected.

Figure 5.10a.  Raypath bending at LV base.

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