I investigated the asymmetry in the IP-L and IP-T resonance frequencies by measuring the transfer functions at the individual LVDT-actuator units H1, H2 and H3 separately. As can be seen in the plots attached, the lowest resonance frequencies in each case are:

• H1: 82 mHz,
• H2: 62 mHz,
• H3: 62 mHz.
• Frequency resolution: 5 mHz.
• Temperature: 23.0°C (O3GK T = 23 °C).

The resonant frequency in the IP_H1 transfer function is 20 mHz higher than in the H2 and H3 measurements! This suggests that we may be able to reduce the asymmetry by putting ballast mass on the IP just above the H1 LDVT-actuation unit, which is nearby one of the IP legs.