[Therion] the volume of a cave

hi, it just happened that i was asked to estimate the volume of a "cave" (actually a small mine). therion dxf export is a triangulated surface of the cave wall. if the surface were oriented it would be extremely easy to compute the volume. (the surface of a real cave is always orientable: no klein cave). all recent versions of therion produce nice-looking dxf. i tried 5.3.5 and the volume seemed reasonable. then i tried 5.3.11 and i got an unreasonable value (there is also a commented option for new 3d, and the result was even more unreasonable). so i checked the dxf files. it seems that none of them is oriented. is it possible to make therion produce an oriented surface? thanks. marco