[libria]

D of earth = 12756km, and assuming a sphere (my physics prof would be proud), I'm computing that 730km covers a ~6.6 degree arc. Also assuming a 100m underground tunnel, the distance would be 21m shorter than its above-ground measurement. Someone needs to recheck my math, though.

[sprovoost]

According Arethuza's comment below, at least one of the labs is 1400 meters below ground. Also you need to calculate the straight-line distance between the two points, not the distance over the surface. Although for calculating the difference that's probably a good enough approximation. So if both stations are 1400 deep and your calculation is correct, the difference should be 14x21=300 meters. Which is a lot more than the 15m effect they found, which suggests they thought about this.

[lutorm]

They claim the geodesic measurements are done to within 0.6m, and the majority of that uncertainty is introduced by the need to triangulate down the tunnel to the experiment.