Reconstructing subsurface sandbody connectivity from temporal evolution of surface networks

Abstract

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

Differential Bank Migration Limits the Lifespan and Width of Braided Channel Threads

“…The experiment featured a distributary braided channel with significant variability in thread width (Figure 1c). Overhead photographs were collected for each minute of the experiment, and converted to maps of surface‐water presence (Figure 1d) using a principal component analysis (Chadwick, Steel, et al., 2022; Steel et al., 2022). Estimating the location of the wet‐dry interface from plan imagery is inherently subjective (Baki & Gan, 2012; Gupta et al., 2013); to account for this ambiguity, we applied our analysis to four separate, plausible realizations of the wet‐dry interface based on threshold values of the surface‐water principal component (see contour lines in Figure 1d) (Chadwick, Steel, et al., 2022).…”

Modeling the Dynamic Response of River Deltas to Sea‐Level Rise Acceleration

“…The correspondence of the surface reworking timescale to our modified "instantaneous" compensation timescale is of particular interest, as it represents a consistent relationship between surface dynamics and subsurface architecture when SLR conditions are approximately steady in time (Figure 8c). This connection between surface dynamics and subsurface structure is not new to this study; previous work (e.g., Barefoot et al, 2021;Esposito et al, 2018;Li et al, 2016;Steel et al, 2022;Straub & Esposito, 2013;Straub et al, 2015;Yu et al, 2017) found relationships between channel mobility, depositional behavior, and stratigraphy. Here, we are able to isolate the effect of SLR and show a consistent correlation between the two, wherein the vertical construction of the subsurface happens at a consistent rate which is proportional to the visitation of the surface by channels under approximately steady rates of SLR.…”

Sensitivity of Subsurface Permeability in Coastal Deltas to Their Morphodynamic and Geomorphic Characteristics

“…A high permeability body is defined as any cluster of model cells that have k ij greater than the 75th percentile of the k ij within the model. The connectivity of the high permeability bodies (Co) is calculated by dividing the area of the largest high permeability body (Ahp) by the sum of the area of all high permeability bodies (Steel et al., 2022): $$$\text{Co}=\frac{\text{Ahp}}{\sum \text{Ahp}}.$$$ …”

“…Although variations in the specific discharge of the three deltas types is small, delta type is important when considering barriers to groundwater flow. Steel et al (2022) state that the presence of low permeability barriers may be more important to groundwater flow through the delta than the permeable pathways; this conclusion was made by physically modeling the formation of a delta within a basin that is void of waves and tides. The permeability maps in this study confirm that fluvial deltas have an accumulation of low permeability material along the shoreline that blocks some of the permeable pathways from extending from the delta apex to the shoreline.…”

“…Steel et al. (2022) also used a physical delta model to characterize the connectivity of sand bodies within the delta and suggested that groundwater flow in a delta may be controlled by the lower permeability sediments rather than the connective sand bodies. The formation of channels within a delta, as well as the deposition of both high and low permeability sediments in a deltaic environment is largely controlled by the morphodynamic influences within the receiving deltaic basin.…”