Calculating a fault throw. At least some embodiments are methods of determining an underground surface or horizon including: identifying an occluded zone residing between a first and second faults, the occluded zone not penetrated by an actual borehole, and the first and second faults intersect an expected location of the surface; calculating a fault throw for the first fault; and calculating the underground surface using the fault throw. Calculating the fault throw may include: calculating a first pseudo depth at a first end of the first fault, the calculating the first pseudo depth using at least one actual depth value that resides across the first fault from the first end; calculating a second pseudo depth at a second end of the first fault, the second end distinct from the first end; and determining the fault throw using the first and second pseudo depths.