Introduction to Geophysics

1. Introduction to Geophysics Introduction to Geophysics-KFUPM Thin Layer Effect Dipping Layer Refractions Ali Oncel oncel@kfupm.edu.sa Department of Earth Sciences KFUPM

2. Review: Refraction Inversion Introduction to Geophysics-KFUPM XCross Knowns - Refraction Time intercept V1 Reflection Time intercept V2 Xcrit

3. Problem Situations Some pitfall should be considered- Low Velocity layer (V2<V1<V3): We have assumed that our layers have successively higher and higher velocity. What happens if we have a velocity inversion - let’s say V2 is less than V1 and V3? Thin layer (V3>V2>V1 but h2 very small): Another assumption we have made here is that the refraction from the top of the third layer, for example, will actually show itself, and not get buried somewhere beneath the earlier refraction and reflections. This can happen if the 2nd layer is too thin. Introduction to Geophysics-KFUPM

4. Fig. 4.10 of Lillie VelocityInversion-HiddenLayers • No head waves are generated at a boundary where velocity decreases undetectable “hidden” layer • Only one critical refraction appears in the time-distance plot Introduction to Geophysics-KFUPM leads to an over-estimation of the depth to the underlying

5. VelocityInversion-HiddenLayers Introduction to Geophysics-KFUPM The presence of the velocity inversion delays the refraction from interface 2 and leads us to overestimate its depth. In addition, we have entirely missed the presence of the second layer. We overestimate thickness because we incorrectly assume that the refraction event traveled down to the refractor with a single velocity of 15000fps.

6. Hidden Zones: Thin Layer Problem Thin layers may be undetectable, if rays traveling to deeper levels arrive first due the V2-V3 velocity contrast and the thinness of the V2-layer. Introduction to Geophysics-KFUPM

7. Hidden Zones: Thin Layer Problem Introduction to Geophysics-KFUPM

8. h1=10 feet V1=4000f/s h2=30feet V2 =8000f/s V3=15000f/s Hidden Zones: Thin Layer Problem The refraction travel times plotted below were computed for the model at right Introduction to Geophysics-KFUPM

9. Hidden Zones: Thin Layer Problem h1=10 feet V1=4000f/s Introduction to Geophysics-KFUPM h2=20feet V2 =8000f/s V3=15000f/s

10. Hidden Zones: Thin Layer Problem The record appears to have only one refraction with time-intercept = 0.0069 seconds. What depth would be calculated for that refractor? h1=10 feet V1=4000f/s h2=10feet V2 =8000f/s Introduction to Geophysics-KFUPM V3=15000f/s

11. Hidden Zones: Thin Layer Problem h1=10 feet V1=4000f/s h2=10feet V2 =8000f/s V3=15000f/s Introduction to Geophysics-KFUPM In this case we estimate the depth to the 15000 f/s refractor to be approximately 14.4 feet. We underestimate the depth because the seismic wave did not spend its time traveling only at the 4000 f/s velocity. It sped through the second layer at 8000 f/s thus reducing the time intercept and thus our estimate of h. As was the case for velocity inversion, we have again missed an entire layer.

12. Fig 4.13 of Lillie Single Dipping Refractor Surface Introduction to Geophysics-KFUPM

13. Introduction to Geophysics-KFUPM t1d≠ t1u TAB = TBA

14. Reading Assignment Introduction to Geophysics-KFUPM For next time read over pages 73-99 in Chapter 4