The Effect of Thin High Velocity Layers on Seismic Refraction Data: An Example from Mahanadi Basin, India

For the interpretation of seismic refraction profiles, an internal structure with continu ously increasing layer velocity with depth may be an inappropriate model to fit the data. Moreover, velocity inversions, wherein high velocity layers (HVL) alternate with low velocity layers (LVL), are common features in the real earth. In models where the relative thickness of the LVL is large compared to the HVL, the refraction travel time shows a shift. The minimum thickness of the LVL required for such a shift depends on its layer velocity. These shifts can be modeled to determine the thicknesses of the HVL and LVL. An example of this phenomenon is visible in the alluvium-covered Cuttack depression of the on-shore Mahanadi basin, where travel-time shifts can be correlated with thin volcanic layers in the well data. Reinterpretations of the seismic refraction data, after considering the existence of volcanic layers and using forward modeling and a 2-D ray-tracing technique, show the extent of these layers within the depression.