A method for determining soil physical crust thickness

Abstract

The present invention discloses a method for determining the thickness of a physical soil crust, comprising the following steps: S1, collecting an original crust soil sample with a certain thickness using a ring knife through an artificial simulated rainfall test or under natural conditions in the field; S2, scanning the crust soil sample using CT, and determining the first picture that can represent the crust soil sample, and selecting pictures of crust soil samples within a certain thickness range; S3, performing median filtering and image enhancement processing based on the selected pictures, and extracting a porosity vertical distribution curve; S4, based on the principle that the soil porosity in the crust area will be significantly lower than the soil porosity below, using a "CT scan + porosity threshold" crust thickness judgment method, and combining it with a vertical sliding method, determining the physical crust thickness. The present invention expands the application scope of determining soil physical crust using CT and porosity threshold methods.

Description

Technical Field

[0001] The invention relates to the technical field of soil physical structure, and in particular to a method for determining the thickness of soil physical crust. Background Art

[0002] Physical crust is a phenomenon of dense structure of soil surface, which is formed by rainfall hitting the soil surface, breaking and scattering of surface aggregates, filling of soil pores with fine particles, soil compaction, and deposition of suspended sediment. The physical crust of soil has a dense pore structure, which has a direct impact on water infiltration, and thus affects the hydrology, erosion and ecological processes of the slope.

[0003] Thickness is an important quantitative index of physical crusts. It can be used to indicate the degree of development of crusts and plays an important role in studying the hydrological and erosive effects of crusts with specific development degrees. However, there is no standard method for quantifying the thickness of physical crusts. Commonly used quantification methods include vernier caliper method, microscopic observation method and CT scanning method. The vernier caliper method directly measures the thickness of crusts using a vernier caliper or ruler, but this method is destructive and the measurement results are inaccurate due to the uneven development of the bottom of the crust. The microscopic observation method uses a microscope to observe the longitudinal sections of the crust, and subjectively compares or combines image processing software to semi-quantitatively extract the thickness of the crust. This method has the advantages of being intuitive and clear, but the preparation of crust sections is relatively complicated, and the thickness quantification process is somewhat subjective. The CT scanning method is a method developed in recent years. It uses CT scanning combined with porosity thresholds to extract the thickness of the crust. This method has the advantages of being objective and standardizable. Since Armenise et al. proposed it in 2018, Zhu et al. (2022) and Feng et al. (2024) have applied it in red soils in southern my country and yellow soils on the Loess Plateau, respectively, and achieved relatively satisfactory results. However, this judgment method has a premise, that is, the default physical crust thickness is less than 10mm. Many studies and practices have shown that the thickness of the structural crust of special soils or the sedimentary crust accumulated over a long period of time may be greater than 10mm. In this case, using this judgment method will lead to large errors. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a method for determining the thickness of soil physical crust, which can objectively quantify the thickness of soil crust and improve the measurement accuracy of the crust thickness.

[0005] Technical solution: The method for determining the thickness of soil physical crust of the present invention comprises the following steps:

[0006] S1, collect undisturbed crust soil samples with a certain thickness using a ring cutter through artificial simulated rainfall test or under natural conditions in the field;

[0007] S2, using CT to scan the crust soil sample and determine the first image that can represent the crust soil sample, and select images of crust soil samples within a certain thickness range;

[0008] S3, performing median filtering and image enhancement processing according to the selected image, and extracting the vertical distribution curve of porosity;

[0009] S4, based on the principle that the soil porosity in the crust area will be significantly lower than the porosity of the soil below, the "CT scan + porosity threshold" crust thickness judgment method is used, combined with the vertical sliding method to determine the physical crust thickness.

[0010] Furthermore, in step S1, a portable rainfall simulator is used to carry out a rainfall simulation test, and the rainfall intensity and rainfall time are set according to the local normal rainfall.

[0011] Furthermore, in step S1, the ring knife material has a greater penetrating ability for X-rays in CT scanning.

[0012] Further, in step S2, the crust sample is scanned using industrial nano CT to obtain a tomographic image sequence of the crust soil sample; using the software view, an image in which the area ratios of air and soil particles are approximately equal is selected as the first image representing the crust soil sample.

[0013] Furthermore, in step S3, the grayscale histogram is combined with the manual judgment method to determine the threshold for segmentation, thereby obtaining a binary image of the soil structure, and the porosity is extracted using visualization software to obtain a vertical distribution curve of the porosity of the crust soil sample.

[0014] Further, in step S4, the "CT scan + porosity threshold" crust thickness judgment method is as follows: assuming that the crust thickness is x mm, according to the extracted porosity vertical distribution curve, using the vertical sliding method, slide downward in sequence, and compare the porosity values ​​of each layer from 0 to x mm with the average porosity values ​​of the layers from x+a to h-10 mm, where a is the resolution of the CT scan of the crust sample, h is the height of the ring cutter, and h-10 means eliminating the influence of the 10 mm thickness at the bottom of the ring cutter;

[0015] The criteria for determining the crust thickness are as follows: when the porosity values ​​of each layer from 0 to x mm are not greater than the average porosity value of the x+a to h-10 mm layers, and the porosity value of the x+a mm layer is greater than the average porosity value of the x+2a to h-10 mm layers, the physical crust thickness of the soil sample is determined to be x mm.

[0016] Further, in step S4, the vertical sliding refers to taking the layer corresponding to the first tomographic image as the boundary according to the CT scan resolution, and determining the crust thickness according to the judgment standard of the crust thickness; if it cannot be judged, taking the layer corresponding to the second image as the boundary to judge the crust thickness; sliding down in sequence until sliding to the pth image, when the judgment condition is met, stopping sliding to determine the crust thickness.

[0017] Compared with the prior art, the present invention has the following significant effects:

[0018] 1. In the "CT scan + porosity threshold" crust thickness determination method of the present invention, the crust thickness is set to x mm, and a vertical sliding method is proposed to determine the crust thickness. In addition to being applicable to most soils with a crust thickness of less than 10 mm, it is also applicable to special soil structure crusts and long-term accumulated sediment crusts with a thickness greater than 10 mm, thus expanding the scope of application of this method for determining crust thickness;

[0019] 2. Based on the principle that the soil porosity in the crust area is significantly lower than that of the soil below, and combined with the vertical distribution curve of porosity, the subjectivity of the existing method in the quantification process can be reduced, and the accuracy and quantification efficiency of extracting crust thickness can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a flow chart of the present invention;

[0021] Figure 2 Schematic diagram of the method for determining the thickness of the physical crust based on "CT scan + porosity threshold" when the thickness of the physical crust is x mm; the red fluctuation curve in the figure is the porosity measurement value, the blue line is the average porosity of all soil slices in the lower layer, and the black line is the crust thickness. DETAILED DESCRIPTION

[0022] The present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0023] like Figure 1 As shown, the method for determining the thickness of soil physical crust of the present invention comprises the following steps:

[0024] Step 1, collecting an original crust soil sample with a certain thickness using a ring cutter through an artificial simulated rainfall test or under natural conditions in the field;

[0025] In this embodiment, a typical red soil bare sloping farmland was selected at the Red Soil Ecological Experimental Station of the Chinese Academy of Sciences in Yujiang District, Yingtan City, Jiangxi Province. Large clods of topsoil and plant roots were removed. After appropriate leveling, a 2m×2m runoff micro-area was enclosed to carry out artificial simulated rainfall experiments, and a crusted soil with a certain degree of development was prefabricated.

[0026] A portable rainfall simulator was used to carry out simulated rainfall tests. According to the local routine rainfall, the rainfall intensity was set to 30 mm / h. A PVC ring knife with a diameter and height of 50 mm was used to select representative points in the micro-area at 10 min and 60 min of rainfall, and one original soil sample of the crust was taken from each point.

[0027] In this embodiment, the ring knife refers to a ring knife made of PVC or other materials with greater penetration ability of X-rays in CT scanning, and the inner diameter d and height h of the ring knife are both 50 mm.

[0028] Collecting original crusted soil samples with a certain thickness means placing the sampling ring on the soil surface with a certain crust layer, and slowly and evenly pressing the ring downwards to prevent the soil crust surface inside the ring from being uneven or cracked.

[0029] Step 2, scanning the crust soil sample using CT;

[0030] The crust soil samples are scanned using X-ray synchrotron radiation micro-CT or industrial CT to obtain a high-resolution tomographic image sequence of the soil samples. The images are batch processed using image processing software such as ImageJ to extract the porosity of the soil samples and obtain a vertical distribution curve of the porosity in the range of 0 to 50 mm.

[0031] In this embodiment, the crust sample was scanned by industrial nano CT (Nanotom S) with a scanning voltage of 100 kV and a current of 100 μA to obtain a tomographic image sequence of the crust soil sample with a tomographic image spacing (ie, resolution) of 0.025 mm.

[0032] Step 3, image processing;

[0033] Considering that the image of the upper part of the crust soil sample includes the space between the overlying plastic wrap, the ring knife cover and the topsoil, it is necessary to determine the first picture that can represent the crust soil sample. The image in which the black (air) and gray (soil particles) areas in the software view are roughly equal is used as the first image, and the corresponding soil depth is 0.025mm. At the same time, considering the edge effect at the bottom of the ring knife, the thickness of 10mm at the bottom is removed to eliminate the influence (in this embodiment, the height of the ring knife h = 50mm). Finally, the selected area of ​​interest is determined to be 0-40mm, that is, from the first crust image to the following 1600 images (i.e. 0-40mm).

[0034] Step 4, extracting the vertical distribution curve of porosity;

[0035] Median filtering and image enhancement were performed on 1600 images in Image J software. In order to eliminate the influence of soil sample variation at the ring knife wall and possible inclination of the crust surface, a circular region of interest with a diameter of 4 mm was cut out in the center of the crust soil sample, and the grayscale histogram combined with manual judgment method was used to determine the threshold for segmentation to obtain a binary image of the soil structure. The porosity was extracted using the software's built-in function, and the vertical distribution curves of the porosity of the 0-40 mm layer of the two crust soil samples were obtained.

[0036] Step 5: Based on the principle that the soil porosity in the crust area is significantly lower than the porosity of the soil below, the "CT scan + porosity threshold" crust thickness judgment method is used, combined with the vertical sliding method to determine the physical crust thickness.

[0037] The method of judging the thickness of crust by “CT scanning + porosity threshold” is as follows: collect the original soil sample of crust with a circular knife in the test area, obtain the slice of the whole soil sample after CT scanning, and use software to obtain the soil porosity value of different soil layer depths. Figure 2 As shown, the crust thickness of the soil sample to be tested is set to x mm, and a vertical distribution curve is constructed according to the extracted porosity. The porosity values ​​of each layer from 0 to x mm are slid downward in sequence by vertical sliding, and the porosity values ​​of the layers from x+a to h-10 mm are compared with the average porosity values ​​of the layers from x+a to h-10 mm. According to the principle that the soil porosity in the crust area is significantly lower than the porosity of the soil below, in this embodiment, the crust thickness judgment standard is: the porosity values ​​of each layer from 0 to x mm are not greater than the average porosity values ​​of the layers from x+a to h-10 mm, and the porosity value of the layer from x+a mm is greater than the average porosity values ​​of the layers from x+2a to h-10 mm, then the physical crust thickness of the soil sample is determined to be x mm, where a is the resolution of the CT scan of the crust sample, h is the height of the ring cutter, and h-10 means eliminating the influence of the 10 mm thickness at the bottom of the ring cutter.

[0038] Vertical sliding means taking the layer (0.025 mm) corresponding to the first tomographic image as the boundary according to the CT scanning resolution (such as 25 μm), and judging the crust thickness according to the judgment standard (the porosity value of each layer in 0-x mm is not greater than the average porosity value of the layers x+a-h-10 mm, and the porosity value of the x+a mm layer is greater than the average porosity value of the layers x+2a-h-10 mm). If it cannot be judged, the layer corresponding to the second image is taken as the boundary to judge the crust thickness, and slide down in sequence until it slides to the pth image. When the judgment standard is met, stop sliding and determine the crust thickness.

[0039] In this embodiment, because the total height h of the sampling ring cutter is 50 mm, in order to avoid the edge effect caused by the bottom disturbance, the bottom 10 mm range is removed, so the threshold calculation layer is set to x+a~40 mm, that is, the porosity mean is x+a~40 mm.

[0040] The following is the thickness extraction of two crust soil samples with different development degrees:

[0041] (I) Crust 1 (rainfall 10 minutes)

[0042] Preliminary processing. Based on the vertical distribution of porosity of the crust soil sample obtained after 10 minutes of rainfall, the porosity data was preprocessed. The coefficient of variation was calculated to be 67.7%. In order to reduce the abnormal fluctuation of the data, the porosity data was averaged with 5 values ​​as a window to obtain the available porosity value of the crust soil sample.

[0043] Judgment process. When the boundary layer is 0.025 mm, i.e. the first image: the porosity of the first image (0.025 mm deep) of the crust soil sample is 2.694%, and the average porosity of the 2nd to 1600th images (0.050 to 40 mm) is 6.126%; the porosity of the second image (0.05 mm) is 2.592%, and the average porosity of the 3rd to 1600th images (0.075 to 40 mm) is 6.129%. Although the porosity of the first image is not greater than the average porosity of the 2nd to 1600th images, it does not meet the requirement that the porosity of the second image is greater than the average porosity of the 3rd to 1600th images. Therefore, the boundary layer slides down to the second image, i.e., 0.050 mm. The average porosity corresponding to the 3rd to 1600th images (0.075-40 mm) is 6.129%, the porosity of the 3rd image (0.075 mm) is 2.715%, and the average porosity of the 4th to 1600th images (0.1-40 mm) is 6.131%, which satisfies that the porosity of the 1st and 2nd images is not greater than the average porosity of the 3rd to 1600th images, but does not satisfy that the porosity of the 3rd image is greater than the average porosity of the 4th to 1600th images (0.1-40 mm). Therefore, the boundary layer continues to slide down to the third image... until it slides to the 96th image (2.400mm), the porosity is 6.222%, the average porosity of the 97th to 1600th images (2.425-40mm) is 6.317%, the porosity of the 97th image (2.425mm) is 6.514%, and the average porosity of the 98th to 1600th images (2.450mm-40mm) is 6.317%, which satisfies the porosity of the 1st to 96th images is not greater than the average porosity of the 97th to 1600th images, and the porosity of the 97th image is greater than the porosity of the 98th to 1600th images. Therefore, according to the judgment standard, the crust thickness of the soil sample is the corresponding layer of the 96th image, that is, 2.400mm.

[0044] (ii) Crust 2 (60 minutes of rainfall)

[0045] Preliminary processing. Based on the vertical distribution of porosity of the crust soil sample obtained after 30 minutes of rainfall, the porosity data was preprocessed. The coefficient of variation was calculated to be 63.4%. In order to reduce the abnormal fluctuation of the data, the porosity data was averaged with 5 values ​​as a window to obtain the available porosity value of the crust soil sample.

[0046] Judgment process. When the boundary layer is 0.025 mm, i.e. the first image: the porosity of the first image (0.025 mm deep) of the crust soil sample is 0.319%, and the average porosity of the 2nd to 1600th images (0.050 to 40 mm) is 3.600%; the porosity of the second image (0.05 mm) is 0.406%, and the average porosity of the 3rd to 1600th images (0.075 to 40 mm) is 3.602%. Although the porosity of the first image is not greater than the average porosity of the 2nd to 1600th images, it does not meet the requirement that the porosity of the second image is greater than the average porosity of the 3rd to 1600th images. Therefore, the boundary layer slides down to the second image, i.e., 0.050 mm. The average porosity corresponding to the 3rd to 1600th images (0.075-40 mm) is 3.602%, the porosity of the 3rd image (0.075 mm) is 0.492%, and the average porosity of the 4th to 1600th images (0.1-40 mm) is 3.603%, which satisfies that the porosity of the 1st and 2nd images is not greater than the average porosity of the 3rd to 1600th images, but does not satisfy that the porosity of the 3rd image is greater than the average porosity of the 4th to 1600th images (0.1-40 mm). Therefore, the boundary layer continues to slide down to the third image... until it slides to the 411th image (10.275mm), the porosity is 4.423%, the average porosity of the 412th to 1600th images (10.300-40mm) is 4.442%, the porosity of the 412th image (10.300mm) is 4.566%, and the average porosity of the 413th to 1600th images (10.325mm-40mm) is 4.442%, which satisfies the porosity of the 1st to 411th images is not greater than the average porosity of the 412th to 1600th images, and the porosity of the 412th image is greater than the porosity of the 413th to 1600th images. Therefore, according to the judgment standard, the crust thickness of the soil sample is the corresponding layer of the 411th image, that is, 10.275mm.

[0047] The present invention uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only used to help understand the method and core ideas of the present invention. At the same time, for those skilled in the art, according to the ideas of the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for determining the thickness of soil physical crust, characterized in that: The steps include: S1, collect undisturbed crust soil samples with a certain thickness using a ring cutter through artificial simulated rainfall test or under natural conditions in the field; S2, using CT to scan the crust soil sample and determine the first image that can represent the crust soil sample, and select images of crust soil samples within a certain thickness range; S3, performing median filtering and image enhancement processing according to the selected image, and extracting the vertical distribution curve of porosity; S4, based on the principle that the soil porosity in the crust area is significantly lower than that in the underlying soil, The "CT scan + porosity threshold" crust thickness determination method is combined with the vertical sliding method to determine the physical crust thickness; The "CT scan + porosity threshold" crust thickness judgment method is as follows: assuming the crust thickness is x mm, based on the extracted porosity vertical distribution curve, using the vertical sliding method, slide downward in sequence, and compare the porosity values ​​of each layer from 0 to x mm with the average porosity of the layers from x+a to h-10 mm, where a is the resolution of the CT scan of the crust sample, h is the ring cutter height, and h-10 means eliminating the influence of the 10 mm thickness edge effect at the bottom of the ring cutter; The judgment standard of crust thickness is as follows: when the porosity values ​​of each layer from 0 to x mm are not greater than the average porosity value of the layers from x+a to h-10 mm, and the porosity value of the layer from x+a mm is greater than the average porosity value of the layers from x+2a to h-10 mm, the physical crust thickness of the soil sample is determined to be x mm; The vertical sliding means taking the layer corresponding to the first tomographic image as the boundary according to the CT scanning resolution, and determining the crust thickness according to the crust thickness judgment standard; if it cannot be judged, taking the layer corresponding to the second image as the boundary to judge the crust thickness; sliding down in sequence until sliding to the pth image, when the judgment condition is met, stopping sliding and determining the crust thickness.

2. The method for determining the thickness of soil physical crust according to claim 1, characterized in that: In step S1, a portable rainfall simulator is used to carry out a simulated rainfall test, and the rainfall intensity and rainfall time are set according to the local normal rainfall.

3. The method for determining the thickness of soil physical crust according to claim 1, characterized in that: In step S1, the material of the ring knife should have a large penetrating ability against X-rays in CT scanning.

4. The method for determining the thickness of soil physical crust according to claim 1, characterized in that: In step S2, the crust sample is scanned by industrial nano-CT to obtain a tomographic image sequence of the crust soil sample; and the image in which the area ratio of air and soil particles is roughly equal is selected as the first image representing the crust soil sample by using the software view.

5. The method for determining the thickness of soil physical crust according to claim 1, characterized in that: In step S3, the grayscale histogram is combined with the manual judgment method to determine the threshold for segmentation, and a binary image of the soil structure is obtained. The porosity is extracted using visualization software to obtain the vertical distribution curve of the porosity of the crust soil sample.

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