Introduction to Geophysics Ali Oncel Department of Earth Sciences KFUPM Thin Layer Effect Dipping Layer Refractions Introduction to Geophysics-KFUPM

Review: Refraction Inversion Introduction to Geophysics-KFUPM

Some pitfall should be considered- Low Velocity layer (V 2 <V 1 <V 3 ): We have assumed that our layers have successively higher and higher velocity. What happens if we have a velocity inversion - let’s say V 2 is less than V 1 and V 3 ? Thin layer (V 3 >V 2 >V 1 but h 2 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 2 nd layer is too thin. Problem Situations Introduction to Geophysics-KFUPM

Fig of Lillie leads to an over-estimation of the depth to the underlying Velocity Inversion-Hidden Layers No head waves are generated at a boundary where velocity decreases undetectable “hidden” layer Only one critical refraction appears in the time-distance plot

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. Introduction to Geophysics-KFUPM Velocity Inversion-Hidden Layers

Thin layers may be undetectable, if rays traveling to deeper levels arrive first due the V 2 -V 3 velocity contrast and the thinness of the V 2 -layer. Hidden Zones: Thin Layer Problem Introduction to Geophysics-KFUPM

Hidden Zones: Thin Layer Problem

The refraction travel times plotted below were computed for the model at right h 1 =10 feet V 1 =4000f/s h 2 =30feet V 2 =8000f/s V 3 =15000f/s Hidden Zones: Thin Layer Problem Introduction to Geophysics-KFUPM

h 1 =10 feet V 1 =4000f/s h 2 =20feet V 2 =8000f/s V 3 =15000f/s Hidden Zones: Thin Layer Problem Introduction to Geophysics-KFUPM

h 1 =10 feet V 1 =4000f/s h 2 =10feet V 2 =8000f/s V 3 =15000f/s The record appears to have only one refraction with time-intercept = seconds. What depth would be calculated for that refractor? Hidden Zones: Thin Layer Problem Introduction to Geophysics-KFUPM

h 1 =10 feet V 1 =4000f/s h 2 =10feet V 2 =8000f/s V 3 =15000f/s In this case we estimate the depth to the 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. Hidden Zones: Thin Layer Problem Introduction to Geophysics-KFUPM

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

t 1d ≠ t 1u T AB = T BA Introduction to Geophysics-KFUPM

For next time read over pages in Chapter 4 Introduction to Geophysics-KFUPM