A new perspective to model subsurface stratigraphy in alluvial hydrogeological basins, introducing geological hierarchy and relative chronology
Section snippets
Rationale and aims of the work
Modelling the alluvial stratigraphy of sedimentary basins is important for many applications, like management of groundwater and shallow geothermal resources, or geohazard evaluation. The stratigraphic architecture of alluvial basins derives from multiple geological processes, acting at different spatial and temporal scales. The result is a nested, hierarchical arrangement of the sedimentary bodies and their internal heterogeneities: highest-hierarchy and large stratigraphic units are built-up
General workflow
The proposed workflow is illustrated in Fig. 2. It involves three main steps: the geological interpretation of the data-set in terms of hierarchy and relative chronology rules; the coding of these data using a standardized framework; the computation of the set of interfaces honouring the observations and the presented rules.
As for most geological modelling studies, field and subsurface data represent the starting point. The dataset is organized using a GIS, which permits to manage scale,
Raw data and introduction of the illustrative example
The dataset used to illustrate the proposed workflow derives from the stratigraphic study of the Quaternary fill of the Po Foreland Basin (Cavalli, 2012; Bersezio et al., 2016, Fig. 3). The selected area belongs to the glacio-fluvial and alluvial terraced landscape at the northern alpine margin of the Po Basin. Its stratigraphic architecture is the result of the progressive southward entrenchment and filling of alluvial valleys into the Pliocene-Lower Pleistocene marine sediments, followed by
Interpreting the cross-sections
The subsurface stratigraphic data are analysed with a rigorous GIS data-management method to preserve internal consistency during subsequent elaborations. It involves the following steps:
- (1)
1-D analysis (Fig. 4-A). The sharp “coarse-grained above fine-grained” sediment contacts are picked within each borehole, thus subdividing the stratigraphic logs into a sequence of lithotextural intervals, bounded by abrupt textural contacts (compare to the criteria adopted in Fig. 1). The sediment packages in
Computational steps
In the previous steps, the geological interpretation and the raw data have been coded to obtain a table, containing the coordinates of the contact points and their corresponding attributes (Fig. 4-B). In addition to the input table, HIEGEO reads a parameter file (JSON format) that contains information about the discretization grid, like the minimum and maximum x and z coordinates, and the size of the cells (Δx and Δz). A discretization grid is required to allow the export of the interpreted
Summary and conclusions
The proposed workflow integrates hard geometrical constraints (spatial coordinates of the contact points) and shows how hierarchical rules can be employed in addition to relative chronology to delineate stratigraphic entities in a geological model. This was illustrated on the alluvial stratigraphy of the Po Plain sedimentary basin (Italy). The Python™ code HIEGEO demonstrates, by using a simple illustrative case, how the rules derived from the geological knowledge can be formalized, yielding
Authorship statement
CZ and RB conceived the presented idea, provided the geological data-frame and wrote the manuscript. AC developed the PythonTM script, performed computations and wrote the section related to the modelling procedure. PR aided in interpreting the results and worked on the manuscript. RB and PR supervised the findings of this work.
Computer code availability
The Python™ module HIEGEO can be downloaded and checked at the following link: https://bitbucket.org/alecomunian/hiegeo. Developer: Alessandro Comunian. Dipartimento di Scienze della Terra, Università degli Studi di Milano, via Cicognara 7, 20129 I-Milano, Italy. [email protected].
Funding
This work was supported by funds to RB [grant-RV_ATT_COM16RBERS_M] and to AC [grant-PSR2018_ACOMUNIAN].
Declaration of competing interest
No potential conflict of interest was reported by the authors.
Acknowledgments
The final version of the manuscript benefits from the thoughtful suggestions of F. Wellmann and two anonymous Referees.
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