Method for modeling the infiltration capacity of geological substrata

Abstract

The invention relates to a method for modeling the infiltration capacity of geological substrata comprising the following steps: - Observation (E1) of a terrain so as to obtain geological data determining the infiltration capacities of the terrain; - classification (E2) of said geological data which determine the infiltration capacities according to several hydraulic types of the substrata, said hydraulic types being a function of physical properties of the terrain; - mapping (E3) of the classified data; - association (E4) of the mapped data with geological objects of the terrain; - interpolation and extrapolation (E5) of the mapped data; and - establishment (E6) of a model of the hydraulic types of the terrain and its infiltration capacity. Figure for the abstract: Fig. 1

Description

Title of the invention: Method for modeling the infiltration capacity of geological substrata

[0001] The invention relates to the field of hydraulic behavior of geological substrata and geological science. The invention finds its application in particular in the prevention and management of flood risks in territories.

[0002] Geological substratum is understood to mean natural, solid or consolidated materials (soils, sediments or rocks), which constitute the natural support of topography, vegetation and surface artefacts (constructions, coverings, agriculture, etc.). The geological substratum considered may be visible at the outcrop and extend in depth up to several tens of metres.

[0003] Methods for preventing flood risks based on probabilistic approaches are known from the prior art. According to these approaches, hypotheses concerning the terrain are made and allow flood risks to be calculated.

[0004] Such solutions have limitations, as the results obtained are based on more or less realistic hypotheses and the uncertainty of which is not controlled. Flood risk predictions are therefore not sufficiently reliable.

[0005] The prevention methods of the prior art are based on probabilistic approaches and not on deterministic approaches because the latter would require precise and quantitative data on the infiltration capacity of geological substrata. Indeed, such data are continuously required on a site in order to obtain a complete model making it possible to predict the flood risks of said site. Such a solution would be costly in terms of time and money and technically impossible to obtain on a territorial scale.

[0006] The infiltration capacity of geological substrata represents, whatever the spatial scale, an intrinsic physical property of the materials, which is a function of their hydraulic conductivity (or permeability) in m / s, which controls the infiltration flow of water (m / s) in the substratum and makes it possible to establish a hydraulic balance in relation to rainfall, surface runoff and evapotranspiration. The invention aims to provide a method allowing the modeling of the infiltration capacities of geological substrata and thus the prevention of flood risks, in a simple and economical manner.

[0007] The invention proposes for this purpose a method for modeling the infiltration capacity of geological substrata comprising the following steps: - observation of a terrain so as to obtain geological data determining the infiltration capacities of the terrain; - classification of said geological data determining the infiltration capacities according to several hydraulic types of substrata, said hydraulic types being a function of physical properties of the terrain; - mapping of classified data; - association of mapped data with geological objects in the field; - interpolation and extrapolation of mapped data; and - establishment of a model of the hydraulic types of the terrain and its infiltration capacity.

[0008] The invention thus provides a naturalistic geological method based on the observation of terrains (rocky substrates, soils, etc.) which makes it possible to characterize and predict the hydraulic properties and the infiltration capacity of said terrains.

[0009] The method according to the invention makes it possible to easily and quickly map (without resorting to complex and expensive technologies) the hydraulic types on the surface and to construct a model of surface and volume properties (infiltration, runoff, etc.). These properties can be characterized and mapped at the same time on a local, regional or national scale.

[0010] Models of surface and volume properties can be obtained using digital tools such as a geographic information system (GIS), or digital meshers, etc. These models can be transferred into 3D-4D models of surface and underground flows or be used in flood risk prevention mapping and de-impermeabilization of urbanized surfaces, without resorting to 3D-4D models. Such mapping can be used for example in territorial planning.

[0011] This simple classification of the hydraulic types of substrata, based on a naturalistic approach, makes it possible to create a spatialized and realistic model of the infiltration capacities of substrata. This approach makes it possible to provide territorial managers with additional critical data for flood risk management.

[0012] The invention not only makes it possible to improve the modeling of the physical process (flow, water infiltration, etc.) but also to provide a simple map of the infiltrating zones and the non-infiltrating zones. Such a map can be used, for example, by decision-makers for land use planning. More generally, the method of the invention is easily applicable by geologists or geotechnicians working in design offices or for communities. The hydraulic types are applicable regardless of the scale of investigation of the terrain.

[0013] Particularly convenient preferred features of the modeling method according to the invention are presented below.

[0014] Physical properties include the nature of the terrain, permeability,

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[0022] porosity and infiltration capacity of the ground. Hydraulic types include: - a first hydraulic type corresponding to an impermeable substratum; - a second hydraulic type corresponding to a ground; - a third hydraulic type corresponding to a rocky substratum with matrix porosity; and - a fourth hydraulic type corresponding to a rocky substratum with fractures and / or open karsts. The second hydraulic type includes hydraulic subtypes depending on the infiltration capacity of the soils. Hydraulic subtypes of the second hydraulic type include: - a first hydraulic subtype corresponding to soils with zero to very low infiltration capacity; - a second hydraulic subtype corresponding to soils with very low to low infiltration capacity; and - a third hydraulic subtype corresponding to soils with a high infiltration capacity. The third hydraulic type includes hydraulic subtypes depending on the porosity and permeability of the bedrock. Hydraulic subtypes of the third hydraulic type include: - a first hydraulic subtype corresponding to rocky substrates with low porosity and low permeability; - a second hydraulic subtype corresponding to rocky substrates with medium porosity and medium permeability; and - a third hydraulic subtype corresponding to rocky substrates with high porosity and high permeability. The fourth hydraulic type includes hydraulic subtypes based on fractures and karst networks. Hydraulic subtypes of the fourth hydraulic type include: - a first hydraulic subtype corresponding to rocky substrates with little fracture and without karst network; - a second hydraulic subtype corresponding to rocky substrates with a high density of open fractures and without a karst network; and - a third hydraulic subtype corresponding to rocky substrates with or without fractures, and presenting an open karst network. According to another aspect, the invention also relates to a geographic information system comprising a computer program. The computer program comprises instructions adapted to the implementation of each of the steps of the modeling method having at least one of the preceding characteristics, when said program is executed on a computer.

[0023] The geographic information system may comprise - a geographic or hydraulic data acquisition module configured to acquire geological data from the field observation stage; - a database management module configured to manage one or more of said geological data; - a georeferencing module configured to place an image in a terrestrial reference frame; and - an exploitation and mapping module configured to process and analyze geological data.

[0024] According to another aspect, the invention also relates to a means of storing information, removable or not, partially or totally readable by a computer or a microprocessor comprising code instructions of a computer program for the execution of each of the steps of the modeling method having at least one of the preceding characteristics.

[0025] Other features and advantages of the invention will become apparent in the description below with reference to the appended drawings, given by way of non-limiting example: - [Fig. 1] represents a method of preventing hydraulic modeling according to an exemplary embodiment of the invention; - [Fig.2] represents a classification of hydraulic types according to an exemplary embodiment of the invention; - [Fig.3] represents a field of hydraulic properties of hydraulic types; - [Fig.4] is a schematic block diagram of a geographic information system according to an exemplary embodiment of the invention; and - [Fig.5] a schematic block diagram of an information processing device for implementing one or more embodiments of the invention.

[0026] [Fig.l] represents an exemplary embodiment of a hydraulic modeling method according to the invention.

[0027] The method comprises a first step of observation E1 of a terrain. This is a visual observation of the terrain. The observation can be carried out with or without technological devices. Observation without technological devices, called human observation, has the advantage of being inexpensive. Observation with technological devices, called technological observation, may involve the use aerial images, remote sensing tools, or any device for capturing images of the terrain. In one example, both human observation and technological observation are used.

[0028] The first step of observation El of the terrain makes it possible to obtain hydraulic data on the terrain. The hydraulic data provide intrinsic physical properties of the observed terrain. The physical properties are for example the nature of the terrain (soil, bedrock, etc.), its porosity, its permeability, its infiltration capacity, etc.

[0029] The method comprises a second step E2 of classification of the hydraulic data. The hydraulic data are classified according to several hydraulic types, for example four as detailed in the example below. The hydraulic types are a function of the physical properties.

[0030] In the example of Figures 2 and 3, the hydraulic types comprise four types, i.e. a first hydraulic type T1, a second hydraulic type T2, a third hydraulic type T3 and a fourth hydraulic type T4. The evolution of the infiltration rate F over time is represented on the one hand. On the other hand, the underground flow is represented with the downstream runoff flow rate Q. The downstream runoff flow rate Q is a function of the rainfall flow rate R (mm / h), the hydraulic conductivity K (m / s), the storage coefficient (m2), the infiltration rate F (mm / h) and the time t.

[0031] The first hydraulic type Tl corresponds to an impermeable substratum or impermeable surface. The impermeable substratum is smooth, without porous networks and without fractures, with zero infiltration capacity. The impermeable substratum can be natural or artificial.

[0032] As visible in [Fig.2], the first hydraulic type Tl groups together the environments in which there is no infiltration (infiltration rate F=0 mm / h). Tl type environments do not present any underground flow.

[0033] Hydraulic type Tl media also have the following characteristics: - maximum cumulative rainfall of the watershed; - strong slope effect; - erosion if the ground is not consolidated.

[0034] The second hydraulic type T2 corresponds to a soil. This hydraulic type includes all soils, regardless of their origin (natural or anthropogenic, industrial or agricultural). The second hydraulic type T2 includes porous media with micrometric pore spaces, in which interactions with water are controlled by capillary forces.

[0035] The second hydraulic type T2 groups together environments presenting a low in filtration. The infiltration rate decreases over time (transient process) as seen in [Fig.2]. The underground flow of T2 type environments is low.

[0036] Hydraulic type T2 media also have the following characteristics: - variable storage capacity; - cumulative variable watershed rainfall; - significant effect of initial saturation; - dominant capillary forces; - strong slope effect; - erosion if the ground is not stable (vegetation).

[0037] Depending on the context of the study, the second hydraulic type T2 can be subdivided into several subtypes. These subtypes can be adapted according to local needs and conditions.

[0038] According to a preferred example, the subtypes of the second hydraulic type T2 are defined from the least infiltrating to the most infiltrating.

[0039] The second hydraulic type T2 comprises for example a first hydraulic subtype T2a, a second hydraulic subtype T2b and a third hydraulic subtype T2c.

[0040] The first hydraulic subtype T2a corresponds to soils with zero to very low infiltration capacity. These are soils with a hydraulic conductivity K of less than 10 15 m / s. These are soils dominated by clays or materials with a grain size equivalent to clays.

[0041] The second hydraulic subtype T2b corresponds to soils with a very low to low infiltration capacity. These are soils with a hydraulic conductivity K between 10 15 m / s and 10 12 m / s. The infiltration capacity can change depending on the water saturation. Soils of the second hydraulic subtype T2b also have variable porosity. The second hydraulic subtype T2b corresponds to soils characterized by a mixture of clay and coarser materials (sand, gravel).

[0042] The third hydraulic subtype T2c corresponds to soils with a high relative infiltration capacity. These are soils with a hydraulic conductivity K greater than 10 12m / s. The soils of the third hydraulic subtype T2c also have a high porosity O. The porosity O is notably greater than 40%. The third hydraulic subtype T2c corresponds to “Ethical” soils poor in clay, dominated by coarse materials, gravel, pebbles, blocks, generally not very thick (< 1 m) and in contact with a bedrock.

[0043] The third hydraulic type T3 corresponds to rock substrates with matrix porosity. In particular, these are rock substrates without fractures and open karst cavities, presenting matrix porosity (between the crystal grains).

[0044] The third hydraulic type T3 includes all types of rocks, sedimentary (limestone-sandstone, etc.), metamorphic (schist-gneiss, etc.) and magmatic (basalt, granite, etc.). Whatever the origin of the rock, its classification in the third hydraulic type T3 indicates that said rock does not have fractures and open karst cavities. This classification also indicates that said rock is characterized by a matrix porous network (micrometric to centimetric). The third hydraulic type T3 thus includes environments with a low to moderate infiltration capacity. The infiltration rate decreases over time as visible in [Fig.2]. The interactions between rock and water in the third hydraulic type T3 are weakly controlled by capillary forces. The flow of water in a substratum of the third hydraulic type T3 is of the “Darcy” type. The underground flow is variable.The presence of the porous network can be controlled by sedimentary, diagenetic, metamorphic or magmatic processes.

[0045] T3 hydraulic type media also have the following characteristics: - variable storage capacity; - cumulative variable watershed rainfall; - significant effect of initial saturation and piezometric level; - negligible capillary forces; - strong slope effect; - erosion if the ground is not consolidated.

[0046] Depending on the local context of the study and the nature of the rocks, the third hydraulic type T3 can be subdivided into several subtypes. These subtypes can be adapted according to needs.

[0047] According to a preferred example, the subtypes of the third hydraulic type T3 are defined from the least porous and least permeable, to the most porous and most permeable.

[0048] The third hydraulic type T3 comprises for example a first hydraulic subtype T3a, a second hydraulic subtype T3b and a third hydraulic subtype T3c.

[0049] The first hydraulic subtype T3a corresponds to rocky substrates having low porosity O and low permeability K. The porosity O is notably less than 10%. The permeability K is less than 10 12m / s.

[0050] The second hydraulic subtype T3b corresponds to rock substrates with an average porosity O and an average permeability K. The porosity O is notably between 10% and 20%. The permeability K is between 10 12m / s and 109 m / s.

[0051] The third hydraulic subtype T3c corresponds to rocky substrates with high porosity O and high permeability K. Porosity O is notably greater than 20%. Permeability K is greater than 109m / s.

[0052] In a limestone rock for example: - the first hydraulic subtype T3a corresponds to a limestone rock with low porosity (< 10%) and low permeability (< 1 mD), resulting from the high proportion of intergranular calcitic cement or carbonate mud (> 30%); - the second hydraulic subtype T3b is characterized by medium porosities and permeabilities (resp. 10-20% and 1-100 mD) and a medium proportion of cement or mud (10-30%); - the third hydraulic subtype T3c corresponds to high porosities and permeabilities (resp. > 20% and > 100 mD). In this case, the infiltration capacity increases from the first subtype T3a to the third subtype T3c depending on the decrease in the proportion of intergranular calcitic cements.

[0053] The fourth hydraulic type T4 corresponds to rock substrates with fractures and / or open karst networks. The fourth hydraulic type T4 includes fractured and / or karstified rocks, regardless of the nature of said rocks and their matrix porosity (low or high). The rock substrates of the fourth hydraulic type T4 have variable hydraulic permeabilities or conductivities that can reach several Darcies (> 1 D or > 105 m / s).

[0054] The fourth hydraulic type T4 groups the rocky substrata with strong infiltration, as visible in [Fig.2]. The fourth hydraulic type T4 is the hydraulic type with the greatest infiltration potential and hydraulic conductivity (K > 10-5 m / s) on a large spatial scale ( > km2 ). The underground flow is strong.

[0055] T4 hydraulic type media also have the following characteristics: - high storage capacity - formation of aquifers; - probable resurgences; - significant effect of the piezometric level.

[0056] Depending on the local context of the study, the fourth hydraulic type T4 can be subdivided into several subtypes. These subtypes can be adapted according to local needs and conditions.

[0057] According to a preferred example, the subtypes of the fourth hydraulic type T4 are defined as a function of the density of the open fracture network, which increases by one subtype to another, and the presence of an open karst network.

[0058] The fourth hydraulic type T4 comprises for example a first hydraulic subtype T4a, a second hydraulic subtype T4b and a third hydraulic subtype T4c.

[0059] The first hydraulic subtype T4a corresponds to rocks with little fracture (n < 10 / m2) without a karst network.

[0060] The second hydraulic subtype T4b corresponds to rocks with a high density of open fractures (n = 10 / m2) without a karst network.

[0061] The third hydraulic subtype T4c corresponds to karstified rocks presenting an open karstic network with or without fractures.

[0062] [Fig.3] represents the field of physical properties of the different hydraulic types Tl, T2, T3, T4. The physical properties here are the hydraulic conductivity K (m / s), the permeability k (m2 or mD) and the porosity cp (%).

[0063] Thanks to this classification according to different hydraulic types, at the end of step E2, the substrata are characterized according to their infiltration capacity and the hydraulic types.

[0064] The method of spatial modeling of infiltration capacities then comprises a step E3 of mapping the hydraulic types. This involves mapping the observed hydraulic types using a digital medium.

[0065] The observed hydraulic types are associated with geological objects in the field during an association step E4 or correlation. The correlation step E4 consists of making the link between the geological objects and the hydraulic types; the association step can be carried out locally but also on a larger scale (regional scale for example).

[0066] The E4 association is carried out by means of geological rules, if any, or randomly. The geological rules are established for each studied terrain (or zone) or case study from observations of the specific hydraulic types and geological data.

[0067] At the end of the association step E4, we obtain 2D-3D concepts linking stratigraphic formations and hydraulic types T1 T2, T3, T4, or between geological structures and hydraulic types T1 T2, T3, T4. We obtain concepts linking a massif, a rock, or a stratum, to a hydraulic type.

[0068] The mapping process then comprises a step of interpolation and extrapolation E5 of the mapped data using the geological rules established in the previous step. During this step, the map of the terrain studied is meshed. The interpolation of the data makes it possible to move from point to digital meshes. For example, the meshes are square surface meshes of 250 x 250 meters.

[0069] As visible in [Fig.3], the method according to the invention allows, on the basis of the ob observations of the terrain carried out, to place in an area, any geological object, in a field of permeability, conductivity, porosity which corresponds to the hydraulic type. The process allows in an interesting and advantageous way, to create a link between a hydraulic / typological classification and physical parameters.

[0070] The modeling method finally comprises a step E6 of establishing a model of the hydraulic properties of the terrain. This is a model of flow and infiltration of the terrain. The model obtained is in particular a surface or volume model.

[0071] The method according to the invention is implemented by computer. Steps E2 to E6 are carried out using digital tools. Steps E2 to E6 are for example carried out using a geographic information system (GIS) 1.

[0072] The geographic information system 1, shown schematically in [Fig.4], comprises a module 10 for acquiring geographic or hydraulic data. The acquisition module 10 is configured to acquire data, in particular data from the observation of the terrain carried out in step EL

[0073] The geographic system 1 also comprises a database management module 11. The management module 11 is configured to create and manage one or more geographic databases. The management module 11 is in particular configured to manage the geological data acquired by the acquisition module 10.

[0074] The geographic information system 1 further comprises a georeferencing module 12. The georeferencing module 12 is configured to place an image in a terrestrial reference frame. The geographic information system 1 finally comprises an exploitation and mapping module 13. The exploitation and mapping module 13 is configured to process and analyze the geographic data. The exploitation and mapping module 13 makes it possible in particular to carry out steps E3 to E6.

[0075] The geographic information system is for example QGIS, which is a free cross-platform GIS software published under GPL license. Other GIS systems are possible.

[0076] The geographic information system comprises a computer program. The computer program comprises instructions adapted to the implementation of each of the steps of the method E1 to E6. [Fig. 5] represents a schematic block diagram of an information processing device 2 for the implementation of one or more embodiments of the invention. The information processing device 2 comprises a RAM memory 20, a processing unit 21, equipped for example with a processor, and controlled by the computer program stored in a ROM memory 22.

[0077] The method according to the invention allows, thanks to simple field observations, to improve flow and infiltration modeling. This allows decision-makers to be provided with a simple map of infiltrating and non-infiltrating areas, for land use planning or the possible de-impermeabilization of urban systems.

[0078] Maps can be quickly and efficiently obtained for surveyed areas of several hundred km2.

Claims

Claims

1. Method for modeling the infiltration capacity of geological substrata comprising the following steps: - Observation (El) of a terrain so as to obtain geological data determining the infiltration capacities of the terrain; - classification (E2) of said geological data determining the infiltration capacities according to several hydraulic types (Tl, T2, T3, T4) of the substrata, said hydraulic types being a function of physical properties of the terrain; - mapping (E3) of the classified data; - association (E4) of the mapped data with geological objects of the terrain; - interpolation and extrapolation (E5) of the mapped data; and - establishment (E6) of a model of the hydraulic types of the terrain and its infiltration capacity.

2. The method of claim 1, wherein the physical properties include the nature of the terrain, the permeability, the porosity and the infiltration capacity of the terrain.

3. Method according to claim 1 or claim 2, wherein the hydraulic types (T1, T2, T3, T4) comprise: - a first hydraulic type (T1) corresponding to an impermeable substratum; - a second hydraulic type (T2) corresponding to a soil; - a third hydraulic type (T3) corresponding to a rock substratum with matrix porosity; and - a fourth hydraulic type (T4) corresponding to a rock substratum with fractures and / or open karsts.

4. Method according to claim 3, in which the second hydraulic type (T2) comprises hydraulic sub-types (T2a, T2b, T2c) depending on the infiltration capacity of the soils.

5. A method according to claim 4, wherein the hydraulic subtypes (T2a, T2b, T2c) of the second hydraulic type (T2) comprise: - a first hydraulic subtype (T2a) corresponding to soils with zero to very low infiltration capacity; - a second hydraulic subtype (T2b) corresponding to soils with very low to low infiltration capacity; and - a third hydraulic subtype (T2c) corresponding to soils with high infiltration capacity.

6. Method according to one of claims 3 to 5, in which the third hydraulic type (T3) comprises hydraulic sub-types (T3a, T3b, T3c) depending on the porosity and permeability of the rock substrates.

7. The method of claim 6, wherein the hydraulic subtypes (T3a, T3b, T3c) of the third hydraulic type (T3) comprise: - a first hydraulic subtype (T3a) corresponding to bedrocks having low porosity and low permeability; - a second hydraulic subtype (T3b) corresponding to bedrocks having medium porosity and medium permeability; and - a third hydraulic subtype (T3c) corresponding to bedrocks having high porosity and high permeability.

8. Method according to one of claims 3 to 7, in which the fourth hydraulic type (T4) comprises hydraulic sub-types (T4a, T4b, T4c) depending on fractures and karst network.

9. Method according to claim 8, in which the hydraulic subtypes (T4a, T4b, T4c) of the fourth hydraulic type (T4) comprise: - a first hydraulic subtype (T4a) corresponding to rock substrates with little fracture and without karst network; - a second hydraulic subtype (T4b) corresponding to rock substrates with a high density of open fractures and without karst network; and - a third hydraulic subtype (T4c) corresponding to bedrock with or without fractures, and presenting an open karst network.

10. Geographic information system comprising a computer program, the computer program comprising instructions adapted to the implementation of each of the steps of the method according to one of claims 1 to 9 when said program is executed on a computer.

11. Geographic information system according to claim 10 comprising: - a geographic or hydraulic data acquisition module (10) configured to acquire the geological data from the terrain observation step (El); - a database management module (11) configured to manage one or said geological data; - a georeferencing module (12) configured to place an image in a terrestrial reference system; and - an exploitation and mapping module (13) configured to process and analyze the geological data.

12. Information storage means, removable or not, partially or totally readable by a computer or a microprocessor comprising code instructions of a computer program for the execution of each of the steps of the method according to any one of claims 1 to 9.

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