Seismic refraction – Multiple layers with velocity increasing with depth
The results for two layers can be generalized for an N-layer model and written as a series. See the text book for details. Consider the case where there is a uniform increase in velocity with depth. What will be the form of the ray paths and travel time curves?
Low-velocity layers
● for a head wave to propagate, an increase in velocity from one layer to the next is needed. If a decrease in velocity occurs, there will be no head wave and refraction will fail to detect the layer.
Example : If a 3-layer model has a LVZ in the second layer, then interpretation in terms of 2-layers will give a wrong answer e.g. soil-peat-bedrock. The peat is a low velocity layer.
Hidden layers
Even if a layer has an increase in velocity, then it possible for the head wave on the upper surface to never be the first arrival. Again if a 3-layer velocity model has a hidden second layer, then interpretation in terms of 2-layers will give a wrong answer.
● for a head wave to propagate, an increase in velocity from one layer to the next is needed. If a decrease in velocity occurs, there will be no head wave and refraction will fail to detect the layer.
Example : If a 3-layer model has a LVZ in the second layer, then interpretation in terms of 2-layers will give a wrong answer e.g. soil-peat-bedrock. The peat is a low velocity layer.
Hidden layers
Even if a layer has an increase in velocity, then it possible for the head wave on the upper surface to never be the first arrival. Again if a 3-layer velocity model has a hidden second layer, then interpretation in terms of 2-layers will give a wrong answer.
No comments:
Post a Comment