Mass volume between two topographies. Filling hollow solid

Hi everyone, I am trying to calculate volume of the mass(filling) between two topographies. There are two topographies as shown in the pictures. One lies exactly on another only with changes in some points resulting in the shape of 1st topography (2nd topography is original). However, with my script, I am only getting a hollow shaped model family as shown in the picture. Which is not exactly what I want, I want the hollow to be completely filled by mass to get the correct filling volume). Any idea on how to solve this problem?