[Pyramid #3/34] Alt Spaceships: Basement PF?

[DaltonS]

I'm trying to design a largely underground Mars base using the "Building" rules in Pyramid 3/34: Alternate GURPS Alternate Spaceships p.9-10, and would like to figure out the radiation protection factor (PF) for inhabited "Basement" systems (ie. those that are underground). There are two such systems (including the core) in the upper hull section, with both middle and lower hull sections completely buried. Spaceships 5 p.41 describes calculating PF for systems of a ship in space, but this assumes all hull sections are equally exposed and nothing outside is adding its protection. I would like to be able to calculate cumulative long term exposure based on the following:

Average exposure on the surface of Mars is about 1/4 rads per day at the zero datum altitude. <snip> Mars suits cut the dose in half (PF 2), and the habitats used by the first manned missions would be PF 20. Later permanent structures sheltered by bricks and dirt have a PF of 200 or more.

GURPS Mars, page 20

Any help would be greatly appreciated.

Dalton “GURPS is about realistic roleplaying, right?” Spence