In a multilayer earth model, if the thickness of a layer is smaller than a theoretical limit, its refracted signals would not be recorded as first arrivals and the layer will be hidden. This event is known as blind zone. Also in a multilayer earth model if velocity of a layer is smaller than the surrounding layers (below and above layers), no refracted signal of their interfaces would appear as first arrivals. This event is called velocity inversion and the layer that has this property is said hidden layer. Due to these conditions, some errors would occure in calculation of depth and velocity of under laying layer in seismic surveys.
In this study it was shown that lateral arrivals such as post critical reflections are useful for identifying blind zone and its parameters. Results of using this method on synthetic data are presented. It was also shown that if there is any undulation on the surface of refractor, it could be used for identifying hidden layer and calculating its parameter by determination of the horizontal spacing between effects of this undulations on travel-time curve