Reconstructing subsurface sandbody connectivity from temporal evolution of surface networks

Characterization of groundwater aquifers and hydrocarbon reservoirs requires an understanding of the distribution and connectivity of subsurface sandbodies. In deltaic environments, distributary channel networks serve as the primary conduits for water and sediment. Once these networks are buried and translated into the subsurface, the coarse-grained channel fills serve as primary conduits for subsurface fluids such as water, oil or gas. The temporal evolution of channels on the surface therefore plays a first-order role in the 3D permeability and connectivity of subsurface networks. Land surface imagery is more broadly available than topographic or subsurface data, and time-series imagery of river networks can hold useful information for constraining the shallow subsurface. However, these reconstructions require an understanding of the degree to which channel bathymetry and river kinematics affect connectivity of subsurface sandbodies. Here, we present a novel method for building synthetic cross sections using overhead images of an experimental delta. We use principal components analysis to extract river networks from surface imagery, then couple this with an inverse-CDF method to estimate channel bathymetry, to generate a time-series of synthetic delta topography. This synthetic topography is then transformed, accounting for deposition and subsidence, to produce synthetic stratigraphy that differentiates coarse-grained channel fill from overbank and offshore deposition. We find that large-scale subsurface architecture is relatively insensitive to details of channel bathymetry, but instead is primarily controlled by channel location and kinematics. We analyse the connectivity of sand bodies and the geometries of barriers to flow and find that periods of rapid sea-level rise have more variability in sand body connectivity. We also find that barrier width decreases downstream during all sea-level phases. Our method generates synthetic stratigraphy that closely resembles the large-scale architecture and 2-dimensional connectivity of the real stratigraphy built during the experiment it was based on. We anticipate that it will be broadly applicable to other experimental and field-scale scenarios.