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Description:
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The near -surface of the Earth often has rapidly varying velocity changes that distort the
seismic images to a greater or lesser degree . Depending on the complexity of the velocity variations , a variety of methods have been developed to remove these distortions so that the deeper structures can be more properly imaged .
If the near -surface is characterized by layers , where the layers below the top layer have velocity variations that slowly vary laterally , an efficient and effective approach is the Refraction Delay -Time method (RDT ) . In this technique first -arrival picks are used to infer layer information in the shallow sub -surface . If the layer assumption fails and the velocity still generally increases with depth , then the First -Arrival Traveltime Tomography (FAT ) technique may produce a better solution using the same or more extensively picked traveltime data . However , if one or more of the layers decreases in velocity with depth (the “hidden layer” problem ) , the interpretation of the traveltimes can be difficult . In such a situation Early -Arrival Waveform (EAT ) Tomography may be able to handle the complexity at a greater computation cost .
These three techniques were applied to a 2D onshore example from Peru and a 2D Ocean Bottom Cable (OBC ) dataset from Malaysia . In both cases the more sophisticated techniques of FAT and EAT produced superior results than the RDT technique . In addition , the EAT method gave further improvement to the land example . |