Investigation of a bottom-simulating reflector at the Hikurangi subduction margin by means of 3D full-waveform inversion

In this study we image and model the physical properties of a bottom-simulating reflector (BSR) at the Hikurangi subduction margin, New Zealand. We achieve this by developing a novel approach that combines 3D full-waveform inversion (FWI) velocity modelling with rock-physics modelling and amplitude-versus-offset (AVO) analysis.

In our research we focus on the Puke Ridge, a thrust ridge at the accretionary wedge of the northern Hikurangi subduction zone, offshore the North Island of New Zealand. Previous seismic images revealed a BSR along the ridge. A BSR in seismic data shows the base of the gas hydrate stability zone indicating gas hydrate overlaying gas in the subsurface, however, the concentrations of gas hydrate and gas cannot be directly quantified from the seismic image alone. We apply FWI to a 3D seismic multichannel streamer dataset (NZ3D) acquired in 2018. FWI produces physical property models of the subsurface with superior resolution compared to other inversion methods - with length scales comparable to that of the layer thicknesses recovered in nearby boreholes. We use FWI to generate a P-wave velocity model of the Puke Ridge and its BSR resolving the complex velocity structure and subsurface geometry. Additionally, by combining FWI results for different offset ranges of the data, we investigate the AVO behaviour of the BSR and draw conclusions about the elastic parameters in the subsurface.

Two IODP boreholes located in the vicinity of our study area provide information about the mineral composition and porosity which we use to perform rock-physics modelling. This allows us to calculate the densities as well as P- and S-wave velocities for different saturations of gas hydrate and gas in the subsurface. Correlating the results with those from the FWI will allow us to determine gas hydrate and gas saturation for the Puke Ridge BSR.