We use seismic ambient noise recorded by dense ocean bottom nodes (OBNs) in the Gorgon gas field, Western Australia, to compute time-lapse seafloor models of shear-wave velocity. The extracted hourly cross-correlation (CC) functions in the frequency band 0.1-1 Hz contain mainly Scholte waves with very high signal-to-noise ratio. We observe temporal velocity variations (dv/v) at the order of 0.1% with a peak velocity change of 0.8% averaged from all station pairs, from the conventional time-lapse analysis with the assumption of a spatially homogeneous dv/v. With a high-resolution reference (baseline) model from full waveform inversion of Scholte waves, we present an elastic wave equation based double-difference inversion (EW-DD) method, using arrival time differences between the reference and time-lapsed Scholte waves, for mapping temporally varying dv/v in the heterogeneous subsurface. The time-lapse velocity models reveal increasing/decreasing patterns of shear-wave velocity in agreement with those from the conventional analysis. The velocity variation exhibits a similar to 24-hr cycling pattern, which appears to be inversely correlated with the diurnal variations in sea level height, possibly associated with dilatant effects for porous, low-velocity shallow seafloor and rising pore pressure with higher sea level. This study demonstrates the feasibility of using dense passive seismic surveys and wave-equation time-lapse inversion for quantitative monitoring of subsurface property changes in the horizontal and depth domain. Unlike seismic waves generated by earthquakes or human-made sources, seismic ambient noise (passive seismic) is the ubiquitous background vibration of the solid Earth recorded by seismic sensors. We use seismic interferometry to extract Scholte waves at hourly intervals from ambient noise data collected by dense ocean bottom nodes. Scholte waves travel along the interface between the ocean and the seafloor. This workflow facilitates continuous passive monitoring of the seafloor, without using human-made seismic sources. Conventional passive monitoring techniques usually assume a spatially homogeneous relative velocity changes. With this assumption, the waveform differences on the extracted Scholte waves reveal temporal variations in the velocity of shear waves up to 0.8%. The velocity variation in this study exhibits a similar to 24-hr cycling pattern, which seems inversely correlated with diurnal variations in sea level height, possibly associated with dilatant effects for porous, low-velocity shallow seafloor and rising pore pressure with high sea level. Furthermore, we push the limits of passive monitoring with advanced wave-equation based inversion technique, allowing us to mapping the velocity change into detailed spatial distribution. Therefore we not only infer how velocity changes in time, but also provide insights on where the velocity changes occur in 3-D beneath the seabed. We present a method for space-time monitoring of subsurface velocity changes in the horizontal and depth domain using seismic ambient noise We compute time-lapse images of seafloor seismic velocity and observe diurnal shear-wave velocity changes up to 0.8% The wave-equation based method is suitable for 4-D subsurface passive monitoring using dense seismic arrays
