A computer-implemented method comprising: determining a depth of an underground surface using a plurality of actual depth values from actual boreholes (104,108,110,112), the determining comprising: identifying an occluded zone residing between a first (130,900,1002) and second (132,1008,1010) fault, the occluded zone not penetrated by an actual borehole (104,108,110,112), and the first (130,900,1002) and second (132,1008,1010) faults intersect an expected location of the underground surface; calculating a fault throw for the first fault (130,900,1002) by: calculating a first pseudo depth at a first end of the first fault (130,900,1002) residing within the underground surface, the calculating the first pseudo depth comprises using at least one actual depth value that resides across the first fault (130,900,1002) from the first end; calculating a second pseudo depth at a second end of the first fault (130,900,1002) residing within the underground surface, the second end distinct from the first end, the calculating the second pseudo depth comprises using at least one actual depth value that resides across the first fault (130,900,1002) from the second end; and determining the fault throw using the first and second pseudo depths; calculating the depth of the underground surface residing within the occluded zone, using the fault throw for the first fault (130,900,1002) and the actual depth values from actual boreholes (104,108,110,112). identifying an occluded zone residing between a first (130,900,1002) and second (132,1008,1010) fault, the occluded zone not penetrated by an actual borehole (104,108,110,112), and the first (130,900,1002) and second (132,1008,1010) faults intersect an expected location of the underground surface; calculating a fault throw for the first fault (130,900,1002) by: calculating a first pseudo depth at a first end of the first fault (130,900,1002) residing within the underground surface, the calculating the first pseudo depth comprises using at least one actual depth value that resides across the first fault (130,900,1002) from the first end; calculating a second pseudo depth at a second end of the first fault (130,900,1002) residing within the underground surface, the second end distinct from the first end, the calculating the second pseudo depth comprises using at least one actual depth value that resides across the first fault (130,900,1002) from the second end; and determining the fault throw using the first and second pseudo depths; calculating a first pseudo depth at a first end of the first fault (130,900,1002) residing within the underground surface, the calculating the first pseudo depth comprises using at least one actual depth value that resides across the first fault (130,900,1002) from the first end; calculating a second pseudo depth at a second end of the first fault (130,900,1002) residing within the underground surface, the second end distinct from the first end, the calculating the second pseudo depth comprises using at least one actual depth value that resides across the first fault (130,900,1002) from the second end; and determining the fault throw using the first and second pseudo depths; calculating the depth of the underground surface residing within the occluded zone, using the fault throw for the first fault (130,900,1002) and the actual depth values from actual boreholes (104,108,110,112).