A method for determining the water distribution characteristics of the vadose zone based on soil and rock parameters

Abstract

A method for determining the distribution characteristics of vadose zone water based on soil and rock parameters is proposed. This method employs a high-density electrical resistivity tomography (EDT) system using both the Wenner α and Wenner β sensors for identification and measurement. Numerical analysis is then used to establish a resistivity forward model of the measured profile, based on a thorough understanding of the soil and rock characteristics of the work area and the conductivity information of the water body. The forward model typically considers various scenarios, including but not limited to situations where pores and microfractures are completely, partially, or not filled by water. Through numerical analysis and interpretation of the measured resistivity profile and the resistivity profile obtained from the forward model, the distribution characteristics of vadose zone water are ultimately delineated. This method is efficient and environmentally friendly, causing no pollution to the underground environment. It can quickly determine the range and depth of vadose zone water, providing a solid foundation for subsequent ecological restoration and construction, as well as for the investigation and prediction of environmentally polluted sites.

Description

Technical Field

[0001] This invention belongs to the field of groundwater resource exploration, and specifically relates to a method for determining the distribution characteristics of vadose zone water based on rock and soil parameters. Background Technology

[0002] Vadose zone water refers to groundwater buried in the vadose zone. Its main characteristics are climate control, pronounced seasonality, significant variability, abundant water during the rainy season, and scarce or even dried-up water during the dry season. It serves as a crucial conduit connecting surface water and groundwater. It is a primary source of water for surface vegetation and a significant medium for the horizontal and vertical diffusion and transport of surface pollutants. Therefore, determining the extent and depth of vadose zone water is of great significance for ecological restoration and construction, as well as for the investigation and prediction of contaminated sites.

[0003] Traditional methods for exploring vadose zone water typically involve on-site investigations combined with drilling, and long-term dynamic monitoring to determine the extent and depth of vadose zone water. This approach is labor-intensive, time-consuming, and lacks systematic consistency in the selected sites, making it difficult to accurately determine the occurrence patterns of vadose zone water. Furthermore, the extensive drilling work carries the risk of damaging surface vegetation and original strata; moreover, it can pollute the subsurface environment in areas already experiencing surface pollution. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for determining the water distribution characteristics of the vadose zone based on soil and rock parameters.

[0005] The technical solution adopted in this invention is: a method for determining the distribution characteristics of water in the vadose zone based on soil and rock parameters. The key technical points include: drilling in a defined target area; collecting groundwater samples and soil and rock samples from the strata belonging to the vadose zone; conducting soil and rock analysis and water conductivity analysis to establish a forward model of resistivity profile; and generating a resistivity profile map of the forward model through calculation and inversion; laying out high-density electrical resistivity measurement experimental profile lines in the working area; using data collected by the high-density electrical resistivity α device (reflecting the horizontal distribution characteristics of water in the vadose zone) and data collected by the β device (reflecting the vertical distribution characteristics of water in the vadose zone) to invert and obtain a measured resistivity profile map; and comparing and numerically analyzing the resistivity profile map of the forward model with the measured resistivity profile map to delineate the spatial distribution characteristics of water in the vadose zone.

[0006] In the above scheme, geotechnical analysis and water conductivity analysis are carried out, including obtaining the main parameters of the soil and rock mass where the vadose zone is located, which are composed of resistivity and porosity, including the conductivity of groundwater and phreatic water, and the direction of recharge and migration.

[0007] In the above scheme, the measurement direction of the test profile of the high-density electrical resistivity tomography should be consistent with the direction of the geotechnical parameter acquisition exploration line. Based on the geological conditions, the acquisition parameters, including electrode distance, power supply time, and power supply voltage, are initially selected. After the test profile measurement line is completed, it is inverted and compared with the known strata and groundwater layers. Finally, the acquisition parameters, including electrode distance, power supply time, and power supply voltage, are determined.

[0008] In the above scheme, high-density electrical resistivity tomography experimental profile lines are laid out in the working area, using a "rice" shaped wiring or grid wiring method, and small electrode spacing is selected.

[0009] In the above scheme, the water accumulation in the vadose zone is simplified to a needle-like structure, that is, a shape approximately resembling an elongated rotating ellipsoid. Therefore, the resistivity calculation formula for the established resistivity profile forward model is:

[0010]

[0011]

[0012] In the formula, V is the volume percentage of water, ρ n Resistivity, ρ, perpendicular to the major axis t Let ρ1 be the resistivity along the long axis, ρ2 be the resistivity of the water body, and ρ3 be the resistivity of the rock and soil body. By rearranging the above two formulas, we can obtain the following: the resistivity perpendicular to the long axis is always greater than the resistivity along the long axis.

[0013] In the above scheme, the condition for establishing the resistivity profile forward model is that the various parameters of the soil and the distribution of groundwater are spatially isotropic and homogeneous.

[0014] In the above scheme, the comparative analysis of the resistivity profile of the forward model and the measured resistivity profile refers to identifying the morphological differences in the distribution of the low-resistivity region. Specifically, by comparing the resistivity profile data of the Wenner α device and the Wenner β device, the transverse low-resistivity body and the vertical low-resistivity body are identified. The distribution of the vertical low-resistivity body is related to the distribution of water in the vadose zone.

[0015] In the above scheme, numerical analysis of the resistivity profile of the forward model and the measured resistivity profile refers to determining the degree of fit between the theoretical calculated value and the measured resistivity of the forward model by calculating the fitting difference. When the fitting difference meets the set value, it means that the resistivity profile of the forward model reflects the measured resistivity.

[0016] The beneficial effects of this invention are as follows: A method for determining the distribution characteristics of vadose zone water based on soil and rock parameters is employed. This method utilizes both the Wenner α and Wenner β devices in a high-density electrical resistivity tomography (EDT) apparatus for identification and measurement, followed by further analysis using numerical analysis. To address the issues of multiple solutions and ambiguity in high-density EDT, and to compare the data with actual high-density EDT measurements, a forward model of resistivity for the measurement profile is established based on a thorough understanding of the soil and rock characteristics of the work area. This model uses soil and rock parameters and the conductivity information of the water body to establish a forward model. The establishment of the forward model typically considers various scenarios, including but not limited to situations where pores and microfractures are completely filled with water, partially filled with water, or not filled with water. Through numerical analysis and interpretation of the measured resistivity profile and the resistivity profile obtained from the forward model, the distribution characteristics of vadose zone water are ultimately delineated. This method is highly efficient and environmentally friendly, causing no pollution to the underground environment. It can quickly obtain the range and depth of vadose zone water occurrence, providing a solid foundation for subsequent ecological restoration and construction, and investigation and prediction of environmentally polluted sites. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of the observation arrangement of the Wenner α device in the high-density electrical resistivity tomography (EDT) device in an embodiment of the present invention.

[0019] Figure 2 This is a schematic diagram of the observation arrangement of the Wenner β device in the high-density electrical resistivity tomography (EDT) device in an embodiment of the present invention.

[0020] Figure 3 This is a flowchart of a method for determining the water distribution characteristics of the vadose zone based on soil and rock parameters in an embodiment of the present invention. Detailed Implementation

[0021] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the following description is provided in conjunction with the accompanying drawings. Figures 1-3 The present invention will be further described in detail below with reference to specific embodiments.

[0022] Example 1:

[0023] Because groundwater and other natural water bodies have low resistivity, typically less than 100 Ω·m, and the higher the salt content, the lower the resistivity. The amount of water (or humidity) in soil and rock masses has a significant impact on their resistivity. Generally, soil and rock masses with high water content have lower resistivity, while those with low water content or that are dry have higher resistivity. The water content of soil and rock masses is mainly determined by their porosity and local hydrogeological conditions. Strata containing vadose zones are usually located above the water table. Water in the atmosphere seeps into the strata through gravity, and water in the water table infiltrates into the strata through capillary action. During this infiltration process, some water is retained in the soil and rock masses due to their adsorption. Generally, the smaller the pore diameter, the stronger the water absorption. Archie's formula is typically used to express the relationship between soil and rock parameters such as humidity and porosity and the resistivity of the soil and rock masses. The forward model established in this embodiment to reflect underground resistivity is a multi-scale forward model based primarily on Archie's formula.

[0024] For high-density electrical resistivity tomography (EDS), different measuring devices yield varying results for different anomalies. For water in the vadose zone, the main characteristic is low resistivity, and due to gravity and capillary action, the water distribution in its occurrence area is predominantly vertical. Among high-density EDS measuring devices, the Winner α device is less susceptible to interference, has excellent data acquisition, and accurate data inversion. It can effectively identify the distribution of groundwater beneath the vadose zone and the geoelectric characteristics of the strata. However, when detecting low resistivity beneath high-resistivity bodies, it can expand the distribution range of the low-resistivity body, affecting the analysis and identification of its distribution area. In contrast, the Winner β device in high-density EDS can better distinguish lateral resistivity variations and also reflects low-resistivity anomalies beneath high-resistivity bodies well. Therefore, in this implementation example, both the Winner α and Winner β devices are used for measurement. The collected data is then processed and analyzed to obtain measured resistivity profile data. Mathematical analysis is performed on this data, comparing it with the resistivity profile data obtained from the forward model, to ensure the accuracy and stability of the analysis of the vadose zone water occurrence area.

[0025] The method for determining the water distribution characteristics of the vadose zone based on soil and rock parameters used in this embodiment specifically includes the following steps:

[0026] Step 1: Understand and collect geological and hydrological data at the work site, and determine the soil and hydrological parameters that need to be collected.

[0027] First, the regional geological conditions of the work area are understood, specifically including the thickness of major strata, the development of geological structures, hydrogeological conditions, meteorological and hydrological conditions, and detailed information on Quaternary strata. The collected data is analyzed to preliminarily determine the main strata containing vadose zone water and their information. The collected data is then screened to determine other geotechnical and hydrological parameters that need to be collected through drilling or excavation. Furthermore, the scale and precision of sample collection are determined based on the workload. Generally, the direction of the sampling exploration line should be consistent with the terrain direction or the direction of groundwater flow. When the direction of groundwater flow cannot be determined, the sampling exploration line should be perpendicular to the direction of the main surface water system flow.

[0028] Step 2: Collect the necessary geotechnical and hydrological parameters. Geotechnical parameters include mechanical and physical parameters, while hydrological parameters mainly include information on precipitation, groundwater replenishment, and discharge.

[0029] According to Archie's formula, the specific parameters that need to be collected include the resistivity of the soil and rock, the resistivity of water filling the pores (groundwater resistivity), porosity (void volume ratio), and water saturation. It is also necessary to understand the internal stress and tectonic force direction of the soil and rock, as well as the resistivity of surface water, the groundwater recharge method, flow direction, and discharge method, to provide more technical support for model building.

[0030] Step 3: Combining parameter data and relying on Archie's formula and other technical formulas, establish a multi-scale forward model of resistivity; the forward model of resistivity profile in the vadose zone is mainly established using Archie's formula, the expression of which is:

[0031] ρ=aφ -m S -n ρ0

[0032] In the formula: ρ is the resistivity of the rock and soil, ρ0 is the resistivity of the water, φ is the porosity, S is the water saturation, n is the saturation index (usually varying between 1 and 2), m is the porosity index (usually around 1.3 for looser strata), and a is the proportionality coefficient (varying between 0.6 and 1.5).

[0033] For water in the vadose zone, its typical occurrence can be simplified to a needle-like structure, that is, a shape approximately resembling an elongated ellipsoid of revolution. Its resistivity can then be expressed by the following formula:

[0034]

[0035]

[0036] In the formula, V is the volume percentage of water, ρ n Resistivity, ρ, perpendicular to the major axis tLet ρ be the resistivity along the major axis, ρ1 be the resistivity of water, and ρ2 be the resistivity of soil and rock. Rearranging the above two formulas, we get: the resistivity perpendicular to the major axis is always greater than the resistivity along the major axis.

[0037] The model should also consider the relationship with temperature, especially for water in the vadose zone, which is mainly located above the water table and close to the surface, making it more significantly affected by temperature. When the temperature is below 0℃, the water in the vadose zone may freeze, causing a sharp increase in resistivity and forming high resistance, which will also have a significant impact on the final analysis results.

[0038] Step 4: Based on the on-site construction conditions and work objectives, arrange the high-density electrical resistivity tomography (EPT) test profiles. According to the measurement results of the test profiles, select appropriate measurement profile arrangement methods and measurement parameters.

[0039] Typically, the measurement direction of the test profile in high-density electrical resistivity tomography (EDT) should be consistent with the direction of the geotechnical parameter acquisition exploration line. Based on the geological conditions, preliminary selections are made for acquisition parameters such as electrode spacing, power supply time, power supply voltage, and current. After the test profile measurement is completed, it is inverted and compared with known strata and unconfined layers to finally determine suitable acquisition parameters such as electrode spacing, power supply time, and power supply voltage.

[0040] Step 5: Use the Winner α device and Winner β device in the high-density electrical resistivity tomography apparatus to perform measurements and record the surface undulations;

[0041] Appropriate parameters should be selected for high-density electrical resistivity tomography (EDT) measurements of data from the Wenner α and Wenner β devices. The electrode spacing should be as small as possible, such as 1m or 2m, and the power supply time should be extended as much as possible. Simultaneously, the influence of surface undulations on the measurement results should be considered; the surface elevation of each electrode should be numbered and collected for subsequent apparent resistivity profile inversion.

[0042] Step 6: Use inversion software to invert the apparent resistivity profiles collected by the two devices to obtain resistivity profile diagrams of the Wenner α device and the Wenner β device, respectively.

[0043] The collected data undergoes preliminary screening to remove distorted data points. Inversion methods include gradient method, least squares method, and simplex method. Among these, the least squares method is the most effective and commonly used. Finally, measured resistivity profiles of the high-density electrical resistivity α-meter and β-meter Wenner devices are obtained.

[0044] Step 7: Compare and analyze the measured resistivity profiles of the high-density electrical resistivity α-device and the β-device to identify the differences in the morphology of their low-resistivity regions.

[0045] Because the Winner α device has a better ability to identify vertical resistivity differences, while the Winner β device has a better ability to identify lateral resistivity differences, the lateral and vertical low-resistivity volumes were identified by comparing the resistivity profile data of the two devices. The distribution of the vertical low-resistivity volumes showed a strong correlation with the distribution of water in the vadose zone.

[0046] Step 8: Compare and analyze the measured resistivity profiles of the high-density electrical resistivity spectroscopy Wenner α and β devices with the resistivity profiles of the multi-scale forward model established using soil and groundwater parameters to determine the specific distribution range of water in the vadose zone. Evaluate the goodness of fit between the theoretical calculations and measured resistivity of the forward model, primarily by calculating the fit difference. The squared deviation between the theoretical calculations and the measured apparent resistivity is typically used as the metric, i.e.:

[0047]

[0048] In the formula: The objective function is E. k These are measured data; f k (x) represents the theoretical calculated value; k represents the order of the discrete points; m represents the number of sampling points; and x represents the theoretical model parameters.

[0049] when: When the time is right, the fit is optimal.

[0050] If the fit difference cannot meet the actual situation, the parameters in the forward model parameters should be appropriately modified or other scale model parameters should be selected, and the fit difference should be judged again until the fit difference reaches the accuracy.

[0051] From the surface downwards, the vadose zone is divided into the soil water zone, the transition zone, and the capillary water zone. Above the water table, supporting capillary water rises capillarily, forming the supporting capillary water zone. The lower part of this zone tends to be saturated, forming the saturated capillary water zone, with the capillary water content gradually decreasing upwards. The transition zone lies between the soil water zone and the capillary water zone. When the water table is shallow, the transition zone disappears, and the capillary water zone connects with the soil water zone. Therefore, the water in the vadose zone is directly related to the porosity and microfracture development of the rock and soil strata to which the vadose zone belongs. When the pores and microfractures in the rock and soil strata are filled with water, their resistivity is controlled by the electrical conductivity of the filling water, creating an electrical difference compared to rock and soil strata where the pores and microfractures are not filled with water. This embodiment utilizes this electrical difference to identify the main distribution characteristics of the water in the vadose zone. Normally, when the pores and micro-fractures of rock and soil strata are filled with water, they will show low resistivity characteristics. However, this characteristic is difficult to be effectively identified in a single high-density electrical resistivity device. Considering that the formation of soil water bands, transition zones and capillary water bands is mostly related to gravity flow and hairline phenomena, their direction is perpendicular to the surface.

[0052] Example 2:

[0053] This invention also provides a data analysis system for determining the distribution area of ​​vadose zone water based on soil parameters and high-density electrical resistivity tomography, including:

[0054] A data analysis system for soil and water parameters is used to establish a forward model of resistivity for the observed profiles.

[0055] The data acquisition system is achieved through the Winner α device and Winner β device in the high-density electrical resistivity measurement device;

[0056] The data analysis system is used to invert the resistivity profile and data of the data collected by the high-density electrical resistivity measurement device Wenner α and Wenner β respectively, and to perform comparative analysis of the measurement results of the two devices and numerical analysis of the resistivity profile data with the resistivity forward model.

[0057] The vadose zone water occurrence area analysis system determines the distribution area of ​​vadose zone water based on numerical analysis results.

[0058] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for determining the distribution characteristics of water in the vadose zone based on soil and rock parameters, characterized in that, The process includes drilling in the designated target area, collecting ground water samples and soil and rock samples from the vadose zone strata, conducting soil and rock analysis and water conductivity analysis to establish a forward model of resistivity profiles, and generating resistivity profile maps of the forward model through calculation and inversion; laying out high-density electrical resistivity measurement experimental profile lines in the work area, and using data collected by the high-density electrical resistivity alpha device (WENNA) mainly reflecting the horizontal distribution characteristics of water in the vadose zone and data collected by the WENNA beta device mainly reflecting the vertical distribution characteristics of water in the vadose zone to obtain measured resistivity profile maps; comparing and analyzing the resistivity profile maps of the forward model and the measured resistivity profile maps to delineate the spatial distribution characteristics of water in the vadose zone. The aforementioned comparative analysis of the resistivity profile of the forward model and the measured resistivity profile refers to identifying the morphological differences in the distribution of the low-resistivity region. Specifically, by comparing the resistivity profile data of the Wenner α device and the Wenner β device, the transverse low-resistivity body and the vertical low-resistivity body are identified, and the distribution of the vertical low-resistivity body is related to the distribution of water in the vadose zone.

2. The method for determining the water distribution characteristics of the vadose zone based on soil and rock parameters as described in claim 1, characterized in that, Geotechnical analysis and water conductivity analysis are conducted, including obtaining the main parameters of the soil and rock mass in the vadose zone, which are composed of resistivity and porosity, as well as parameters such as the conductivity of groundwater and phreatic water and the direction of recharge and migration.

3. The method for determining the water distribution characteristics of the vadose zone based on soil and rock parameters as described in claim 1, characterized in that... The measurement direction of the test profile of the high-density electrical resistivity tomography should be consistent with the direction of the geotechnical parameter acquisition exploration line. Based on the geological conditions, parameters including electrode distance, power supply time, power supply voltage and current acquisition are initially selected. After the test profile measurement line is completed, it is inverted and compared with the known strata and groundwater layers. Finally, the acquisition parameters including electrode distance, power supply time and power supply voltage are determined.

4. The method for determining the water distribution characteristics of the vadose zone based on soil and rock parameters as described in claim 3, characterized in that... In the work area, high-density electrical resistivity tomography experimental profile lines are laid out using a "rice" shaped wiring or grid wiring method, and small electrode spacing is selected.

5. The method for determining the water distribution characteristics of the vadose zone based on soil and rock parameters as described in claim 1, characterized in that, Simplifying the water-bearing structure in the vadose zone to a needle-like structure, that is, a shape approximately resembling an elongated ellipsoid of revolution, the resistivity calculation formula for the established resistivity profile forward model is: ； ； In the formula, V is the volume percentage of water. Resistivity perpendicular to the major axis Resistivity along the major axis For the resistivity of water bodies, Let be the resistivity of the soil and rock mass; rearranging the above two formulas, we have: the resistivity perpendicular to the major axis is always greater than the resistivity along the major axis.

6. The method for determining the water distribution characteristics of the vadose zone based on soil and rock parameters as described in claim 5, characterized in that, The resistivity profile forward model is established under the assumption that the parameters of the soil and rock and the distribution of groundwater are spatially isotropic and homogeneous.

7. The method for determining the water distribution characteristics of the vadose zone based on soil and rock parameters as described in claim 1, characterized in that, Numerical analysis of the resistivity profile of the forward model and the measured resistivity profile refers to determining the degree of fit between the theoretical calculation value and the measured resistivity of the forward model by calculating the fitting difference. When the fitting difference meets the set parameters, it indicates that the resistivity profile of the forward model reflects the actual geoelectric cross-sectional characteristics.

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