Stratigraphic Layers

The subsurface is built by stacking stratigraphic layers, each with its own geometry and set of hydrogeological properties. This representation directly feeds the simulation engine: the parameters of each layer determine the propagation speed, adsorption, and reactions within that part of the volume. IsoFind includes eight reference lithologies that cover the majority of situations encountered in hydrogeology and prospecting.

The Eight Reference Lithologies

Each preset lithology defines four typical values used by the simulation engine: effective porosity, hydraulic conductivity (K) in m/s, longitudinal dispersivity (αL) in meters, and the fraction of organic carbon (foc). These presets are defined in the engine's LAYER_PRESETS structure. They serve as a starting point and can be adjusted on a case-by-case basis when field data is available.

Lithology (key)	Porosity	K (m/s)	Dispersivity αL (m)	foc
Gravel (gravier)	0.30	1e-2	5.0	0.0005
Coarse Sand (sable_grossier)	0.28	1e-3	2.0	0.001
Fine Sand (sable_fin)	0.30	1e-4	1.0	0.002
Silt / Loam (limon)	0.35	1e-5	0.5	0.005
Silty Clay (limon_argileux)	0.40	1e-6	0.3	0.010
Clay (argile)	0.45	1e-8	0.1	0.020
Chalk / Limestone (craie_calcaire)	0.15	1e-5	1.0	0.001
Fractured Schist (schiste_fracture)	0.05	1e-6	2.0	0.002

The default values in LAYER_PRESETS are representative of a median context. Reality often presents significant deviations: well-sorted aeolian fine sand does not have the same permeability as silty fine sand from a valley floor. Replacing these values with site measurements immediately improves simulation fidelity. Existing presets can also be overridden via the template_overrides field in the simulation request.

Layer Geometry

The geometry of a layer is defined by two surfaces: its top (upper surface) and its base/bottom (lower surface). Each surface can be constant (horizontal plane), variable via polygon (surface defined by points), or interpolated from borehole logs. All three modes can be combined within a single project.

Geometry Mode	Required Data	Use Case
Horizontal Plane	A single depth	Initial draft, simple sites
Inclined Plane	Dip and azimuth, or 3 points	Regularly dipping layers
Surface by Points	List of x, y, z points	Geometry known at specific locations
Borehole Log Interpolation	drilling_logs tables	Dense borehole sites, high precision needed
DEM or External Surface Import	Raster file or mesh	Linking with previous geotechnical studies

Adding a Layer

Layers are added in the Subsurface tab of the 3D Visualization module. The order in the stack defines the stratigraphy: the first layer added is at the surface, and each subsequent layer is placed below it. Layers can be reordered using drag-and-drop.

3D Visualization > Subsurface > Add Layer > Select Lithology > Define Geometry > Adjust Properties > Validate

The layer appears in the scene with a default color associated with its lithology. This color can be modified for report renderings, though default colors follow standard geological mapping conventions.

Per-Layer Properties

Beyond the four main properties inherited from the chosen lithology, each layer accepts an extended set of parameters used by the simulation.

Property	Field	Role in Simulation	Unit
Effective Porosity	porosity	Calculating effective velocity from Darcy velocity	unitless
Hydraulic Conductivity K	K_ms	Darcy's Law, solving the flow field	m/s
Longitudinal Dispersivity αL	alpha_L_m	Plume spreading in the flow direction	m
Organic Carbon Fraction	foc	Adsorption of organic compounds, Kd = Koc × foc	fraction
Lithology Key	lithology_key	Reference to the preset used	string
Top Depth	depth_top_m	Upper boundary of the layer	m
Bottom Depth	depth_bottom_m	Lower boundary of the layer	m

Transverse dispersivity (αT) is not defined per layer; it is automatically calculated as a fraction of αL. Properties such as pH, Eh, and Fe/Mn associated with local geochemistry are not attached to the lithology but are interpolated via IDW from field samples. Redox equilibria are resolved in the Nexus bridge, not in the lithology preset.

Leaving all properties at their default values is possible for an initial exploratory simulation, but it underutilizes the module's potential. Final simulations used in a report should start from properties adjusted to site measurements. For targeted adjustments without creating entire layers, the template_overrides field in the simulation request allows redefinition of a parameter on an existing template.

Borehole Log Integration

If the project contains borehole logs entered in the Prospecting module or imported from a file, IsoFind can automatically build the stratigraphy by interpolating the positions of layer interfaces. The user retains control to correct inconsistent interpolations or to integrate geological knowledge not captured by the boreholes.

Subsurface > Build from Logs > Borehole Selection > Interpolation Method > Preview > Validate

Three interpolation methods are available: Delaunay Triangulation (fast, suitable for dense datasets), Ordinary Kriging (highest quality, slower), and Inverse-Distance Weighting (compromise). The result is previewed before final validation.

Tectonic Layers and Faults

For sites where the subsurface presents structural discontinuities (faults, thrust contacts, fractured zones), IsoFind supports discontinuity surfaces that cut through the layer stack. Each block defined by these discontinuities carries its own stratigraphy. This representation allows for correct simulation of cases where a fault acts as a drain or a hydraulic barrier.

For simple cases, it is more efficient to treat the area as an effective medium without explicitly representing faults. Explicit fault representation increases scene complexity and calculation time, requiring data that is not always available. Use it only when its absence would clearly skew the interpretation.

Exporting the Subsurface Model

The constructed model can be exported in several formats for external use: glTF for interoperable 3D visualization, VTK for scientific post-processing, and CSV per cell for integration into third-party tools. The export includes geometry, properties, and references to the original layers.