I looked at simplifying the HAM1 ground feedforward a little, by adding a single vertical sensor that wasn't epoxied to the floor. Not suprising that this doesn't have quite as good coherence as the 2 epoxied vertical sensors that are currently being used for the ff. Attached plot shows the different coherences. Red coherence between the B_X (test sensor, no epoxy) and B_Z (epoxied sensor). Blue is the coherence between the test sensor and the HEPI Z L4Cs, green is the epoxied to HEPI Z L4Cs. Brown is coherence the summed epoxied sensors with the HEPI Z L4Cs.

Comparing the green and blue coherences, it seems that epoxying might not be totally necessary. The sum of the epoxied sensors is more coherent with HAM1 HEPI than any of the single sensors, compare brown to blue or green.