The low frequency noise is very bad for ground-based gravitational-wave interferometers. For example, seismic noise from ground motion, such as waves hitting th

The low frequency noise is very bad for ground-based gravitational-wave interferometers. For example, seismic noise from ground motion, such as waves hitting the west coast for LIGO Hanford, or logging operations for LIGO Livingston; also the control noise is really bad at low frequencies. Generally most noises apart from the quantum noise scale inversely to the frequency. The ground-based detectors are most sensitive at 100Hz, which is around the frequency of the binary black hole inspiral. Supermassive black holes are more like millihertz. We do not have any concrete way yet to measure the gravitational waves from supermassive black holes, but there are some ideas. For example we can look at the time dilation of frequencies of nearby pulsars (https://en.wikipedia.org/wiki/Pulsar https://en.wikipedia.org/wiki/Pulsar) as a kind of gravitational wave interferometer in space, however to get a signal-to-noise ratio greater than one many years (decades) of data is needed.