Double BSRs of the Northern Hikurangi Margin: A Full Waveform Inversion

Abstract

Gas hydrates are an ice-like mixture of water and gas occurring as cage assemblies of water molecules surrounding gas molecules, and typically occur in the shallow subsurface of continental margins or permafrost regions. Hydrate layers are typically identiable in seismic section by the occurrence of a bottom-simulating reflector (BSR). BSRs are seismic reflections that mimic the topography of the seafloor and denote the phase boundary of gas and hydrates, known as the base of the gas hydrate stability zone (BGHSZ). The position of the phase boundary relative to the seafloor is known to change in environments when presented with a shift in the local temperature or pressure conditions, leading to subsequent dissociation or reformation of hydrates. The time scale for which hydrate dissociation occurs is relatively unknown, and has implications in constraining the role of hydrate as a "carbon capacitor" that may store and release large amounts of carbon through glacial cycles. Gas hydrate dissociation and its time scale may also affect slope stability and play a role in climate change. A high-resolution 2-D seismic survey conducted on the northern Hikurangi Margin shows the presence of two BSRs, indicating a recent shift in the BGHSZ. The deeper BSR, BSR-2, has previously been established as a paleo-BSR, marking the paleo-level of the BGHSZ. Gas hydrates at this level were thought to have undergone dissociation following tectonic uplift leading to the formation of the shallower BSR-1 at the new BGHSZ. Recent research suggests that hydrate dissociation may occur at the timescale of glacial periods, due to the endothermic nature of hydrate dissociation combined with slow dissipation of pore water freshening and increased pore pressure, which inhibits further dissociation in semi-permeable sediments. Assuming that the depressurization signal associated with tectonic uplift and the subsequent dissociation of hydrates in the paleo-BSR started at the end of sea level rise approximately 8000 years ago, this sets a clear time frame for the investigation of hydrate dissociation in the Hikurangi Margin. The application of a full waveform inversion (FWI) on seismic data yields a high-resolution velocity model, which is useful for extracting quantitative information on properties of the subsurface. This is achieved through iterative reduction of mist between synthetic and real data, resulting in a high resolution velocity profile best representing the behaviour of the subsurface. In this context, an FWI provides valuable information on the nature of the deeper BSR-2. This application explores two scenarios: Either the presence of a low velocity layer at BSR-2 indicating free gas trapped in pore space without any remaining hydrate, or the presence of a high-velocity hydrate layer overlying a low-velocity free gas layer at BSR-2 inferring the presence of hydrates. Results from an FWI place local high velocity layers above both BSRs, suggesting the presence of hydrates at both the current and paleo BGHSZ. The latter implying ongoing dissociation at the paleo-BSR long after depressurization from local uplift. Hydrate dissociation is frequently cited as a background feedback mechanism in models forecasting climate change, as hydrates re-adjust to changes in the BGHSZ and potentially release carbon. However, these results suggest that hydrate dissociation occurs at glacial timescales, and that hydrates may hold on to their carbon content for a while.