I've been working on implementing blended inertial isolation on the HAM1 HEPI. This requires designing higher UGF loops, which I've been struggling with. I have Arnauds commissioning scripts, which I used as a starting point, but it was really hard to get as high a UGF as he did. When I dug into his data I realized that his plant was very different from mine. The first attached pdf compares his cartesian super-sensor transfer functions and mine (LLO is green, LHO is red, dof order is X, Y, RZ, HP, Z, RX, RY, VP). The only known difference between the two sites is the grouting on the HEPI piers. To test this, I looked at HAM4 and 5 here. I chose these chambers (4&5) because they are oriented the same direction, their payloads are similar (enough) and HAM5 is not grouted, while HAM4 is. Second attached pdf shows what i found (blue is HAM4 (grouted), orange is HAM5 (no grout), dof order is the same). Clearly grouting makes a difference. On all DOFs except HP HAM4's modes are higher by ~20-40%.

I don't think this makes a huge difference for HAMs 2-6 right now, but for HAM1 it will be difficult to make higher gain loops without grouting. It's not clear that better isolation at HAM1 will get us much right now, though Jeff took some initial measurements a few days ago, in alog 20664.