Iterate until it stabilizes around optimized values of V0 and K. If necessary enlarge the a priori uncertainty range associated to  updated V0 and K , to allow the plugin to scan a wider range of possibilities. Remember that Velocity function V0 + K*Z (V0 = constant velocity factor (instantaneous velocity at the surface), K = constant compaction factor,)  is non-linear function in the time to depth conversion formula. In case of a priori K values very different from the starting default k= 0.2 , it is advised to input a first k value consistent with existing calibrated sonic .