Scientists have long sought accurate monitoring of seismic activity to identify natural phenomena such as earthquakes, volcanic eruptions and the leakage of fluids stored deep underground. Time-lapse four-dimensional seismic monitoring surveys that employ an active seismic source can accurately map the subsurface, and comparing results from different surveys can show how fluids such as CO2 move in deep geological reservoirs. However, the expense of such surveys limits how often data can be gathered, meaning that subsequent analysis often has poor temporal resolution. An alternative that provides a continuous dataset is the passive monitoring of ambient seismic noise, but the accuracy of this approach depends on the ambient sources, which can change over time.