Methods and seismic shot generation and data collection systems configured to determine a monotonically increased velocity v*(z) from a monotonically increased velocity model by requiring the monotonically increased velocity v*(z) to be nearest to a refraction velocity v(z) determined for an estimated depth z and to be characterized by a positive slope such that the refraction velocity v(z) increases with depth, and to generate a subsurface image based on the estimated depth z and the determined monotonically increased velocity v*(z).