Construction method of loess tableland gradual change type layered surface layer model

Abstract

The invention discloses a construction method of a loess tableland gradual change type layered surface layer model, and belongs to the field of geological exploration, and the method comprises the following steps: firstly, obtaining loess layer thickness and physical property parameter vertical function of a loess layer survey point, and speed, density, quality factor and other data of a high-speed layer; dividing the loess layer into horizontal thin layers according to the obtained data, establishing a horizontal layered geometric model according to the horizontal thin layers and the high-speed layer, and determining the speed, density and quality factor of each horizontal thin layer of the geometric model according to the obtained vertical function and the speed, density and quality factor of the high-speed layer; the model constructed by the method can reflect the overall characteristics of the loess tableland, and the model can be used for carrying out simulation research on the loess tableland.

Description

Technical Field

[0001] The invention belongs to the field of geological exploration, and in particular relates to a method for constructing a loess plateau gradual layered surface model. Background Art

[0002] The thick loess surface layer of the loess plateau is a special geological body. Structurally, it has a low degree of compaction, loose structure, low degree of particle cementation, and is prone to plastic deformation under the action of external forces. The physical properties such as porosity, density, velocity, and quality factor show a continuous and gradual change in the vertical direction with the change of compaction degree. From the perspective of topography, under the influence of long-term weathering and water scouring and cutting, gullies are crisscrossed, forming complex landforms such as ditches, beams, plateaus, hills, and slopes with different shapes, and the lateral inconsistency is extremely strong. From the perspective of seismic response characteristics, it has poor elasticity, low velocity, and strong anisotropy. The high-speed pressure wave excited by the earthquake source has extremely poor coupling with the low-speed loess medium, and the conversion rate of elastic seismic waves is extremely low. The wave field of seismic waves propagating in the loess layer is extremely complex, and interference waves are developed. In addition, it also undergoes strong absorption attenuation and shielding effects, resulting in weak energy, narrow bandwidth, and low signal-to-noise ratio in the loess plateau seismic records. In addition, the serious static correction problems caused by the complex surface make the overall quality of the records extremely poor. Based on these characteristics, it is very difficult to directly use actual seismic data to study the geological structure characteristics and seismic response laws of loess layers. By establishing a suitable loess geological model and using the model forward modeling method to conduct research, we can effectively avoid the above difficulties and obtain valuable and regular insights.

[0003] At present, there are two main methods of geological modeling: physical simulation and numerical simulation. One is physical simulation, which refers to the method of using various modeling materials to simulate the actual geological body to establish a miniature physical model. This method needs to be completed in a professional laboratory using professional equipment and materials, which is costly and has a long modeling cycle. In addition, the materials used in the modeling and the seismic sources used for seismic excitation are quite different from the actual situation. Using physical models to conduct forward modeling research on the actual geological body will bring large errors. At present, the existing related patent CN114429728A realizes the low-speed and high-attenuation characteristics of the simulated loess layer, but cannot simulate the gradual change of the physical properties of the loess layer. In addition, once the physical model is established, the physical property parameters of the model are determined and cannot be changed and debugged, which will bring great inconvenience in the forward modeling research. One is numerical simulation, which refers to the method of using professional modeling software to simulate the actual geological body to establish a numerical model. This method is simple and easy to operate, has low cost, and is the most commonly used modeling method. For special geological bodies such as loess on the surface of the loess plateau with gradual physical properties in the vertical direction, a continuously changing medium model is often established. This model has a high degree of simulation of the gradual change characteristics of the physical properties of the loess layer, but it can only realize the establishment of some simple continuous change models, such as linear gradient. It is often difficult to achieve complex changes, and the gradual assignment of physical property parameters is not easy to achieve. Once the model is built, the physical property parameters cannot be flexibly adjusted.

[0004] Existing research on loess layer modeling has stopped at the establishment of numerical models such as loess thickness and quality factor Q field. These can only reflect a single characteristic of the loess layer and cannot simulate the internal structure of the loess layer and the gradual characteristics of various physical properties.

[0005] In summary, the existing numerical model of loess layer reflects a single characteristic, and it is impossible to simulate and study the overall characteristics of the loess layer using this model. Summary of the invention

[0006] The purpose of the present invention is to provide a method for constructing a gradually changing layered surface model of a loess plateau, so as to solve the technical problem that the numerical model of the loess layer in the prior art is a model reflecting a single characteristic and the overall characteristics of the loess layer cannot be simulated and studied using the model.

[0007] To achieve the above-mentioned purpose, the technical solution of a method for constructing a loess plateau gradient layered surface model provided by the present invention is:

[0008] A method for constructing a loess plateau gradient layered surface model comprises the following steps: 1) selecting survey points in a loess plateau study area, obtaining loess layer thickness data, a vertical function of loess layer velocity, a vertical function of loess layer density, a vertical function of loess layer quality factor, and the velocity, density and quality factor of a high-speed layer underlying the loess layer; 2) obtaining the velocity distribution of the loess layer according to the vertical function of the loess layer velocity, and dividing the loess layer into multiple layers according to the loess layer thickness and the loess layer velocity distribution; 3) obtaining the velocity distribution of the loess layer according to the vertical function of the loess layer velocity, and dividing the loess layer into multiple layers according to the loess layer thickness and the loess layer velocity distribution. The horizontal thin layers are of equal thickness; a horizontal layered geometric model is constructed using the divided horizontal thin layers and the high-speed layer underlying the loess layer; 3) the velocity, density and quality factor of each horizontal thin layer in the horizontal layered geometric model are determined by using the vertical function of the loess layer velocity, the vertical function of the loess layer density and the vertical function of the loess layer quality factor and assigned to each horizontal thin layer, and the high-speed layer is assigned according to the velocity, density and quality factor of the high-speed layer underlying the loess layer, thus completing the construction of the horizontal surface loess plateau gradient layered surface model.

[0009] The beneficial effect is: firstly, the thickness of the loess layer, the vertical function of the physical parameters, and the velocity, density, and quality factor of the high-speed layer at the loess layer survey point are obtained. Then, the loess layer is divided into horizontal thin layers according to the obtained data, and a horizontal layered geometric model is established according to the horizontal thin layers and the high-speed layer. Then, according to the obtained vertical function and the velocity, density, and quality factor of the high-speed layer, the velocity, density, and quality factor of each horizontal layer of the geometric model are determined to complete the construction of the horizontal surface loess plateau gradual layered surface model. By dividing the loess layer into horizontal thin layers, the gradual characteristics of the velocity, density, and quality factor of the loess layer can be reflected. The model constructed by this method can reflect the overall characteristics of the loess plateau, and the model can be used to simulate the loess plateau.

[0010] As a further improvement, it also includes obtaining the undulating surface trace line of the survey point, cutting the horizontal thin layer of the horizontal surface loess plateau gradual layered surface model according to the trace line, discarding the part above the surface trace line, and completing the construction of the undulating surface loess plateau gradual layered surface model.

[0011] The beneficial effect is that by using the surface trace lines to cut the model, various loess tableland terrain forms such as surface ditches, ridges, plateaus, and hills can be simulated, and the terrain characteristics of the loess tableland can be directly observed through the model.

[0012] As a further improvement, in step 1), when selecting the survey points in the loess plateau study area, the survey points are selected on the loess plateau with flat terrain.

[0013] The beneficial effect is that it is faster to establish models for flat plateaus than for terrains such as gullies, ridges or slopes, and the models established based on data obtained from flat plateaus can ensure the originality, stability and integrity of the loess layer structure.

[0014] As a further improvement, in step 2), when the loess layer is divided into multiple horizontal thin layers of equal thickness, the thickness of each horizontal thin layer is less than a set maximum thickness, and the velocity difference between adjacent horizontal thin layers is less than a set maximum inter-layer velocity difference.

[0015] The beneficial effect is that the gradual change of the loess plateau can be better displayed by setting the maximum thickness and the maximum inter-layer velocity difference.

[0016] As a further improvement, in step 3), when determining the velocity, density and quality factor of each horizontal thin layer of the horizontal layered geometric model, the velocity, density and quality factor of a point at the same relative position of each water layer thin layer is used as the velocity, density and quality factor of each horizontal thin layer.

[0017] The beneficial effect is that the attribute of a point at the same relative position in each horizontal thin layer is used as the corresponding attribute data of the horizontal thin layer, which not only simplifies the value-taking process of each attribute and facilitates operation, but also ensures that the distance between each adjacent value-taking point is equal to the thin layer thickness, ensuring the uniform distribution of the value-taking points.

[0018] As a further improvement, in step 1), the loess thickness is obtained by surface survey methods such as small refraction, micro-logging or bedrock surface survey.

[0019] As a further improvement, in step 1), the velocity of the loess layer and the velocity of the high-speed layer underlying the loess layer refer to the longitudinal wave velocity of seismic wave propagation, and the vertical function of the loess layer velocity and the velocity of the high-speed layer underlying the loess layer are obtained by surface investigation, rock sampling and detection, or seismic recording refraction wave velocity measurement.

[0020] As a further improvement, in step 1), the vertical function of the density of the loess layer and the density of the high-speed layer underlying the loess layer are obtained by vertical sampling and detection.

[0021] As a further improvement, in step 1), the vertical function of the quality factor of the loess layer and the quality factor of the high-speed layer underlying the loess layer are obtained by micro-logging survey, ultra-deep micro-logging survey, velocity empirical formula method, seismic record spectral ratio method or frequency shift method. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the process of constructing the loess plateau gradual layered surface model in the present invention;

[0023] Figure 2 It is a schematic diagram of a horizontal surface loess plateau gradually changing layered surface model of the loess plateau in the method for constructing the loess plateau gradually changing layered surface model in the present invention;

[0024] Figure 3The schematic diagram of the loess plateau gradient layered surface model on undulating terrain is a method for constructing the loess plateau gradient layered surface model in the present invention. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention, that is, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. The components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0026] Example of a method for constructing a loess plateau gradient layered surface model:

[0027] The construction method of the gradual layered surface model of the loess plateau is as follows: Figure 1 As shown, the method comprises the following steps:

[0028] 1) Select survey points in the loess plateau study area, obtain the loess layer thickness data, the vertical function of the loess layer velocity, the vertical function of the loess layer density, the vertical function of the loess layer quality factor, and the velocity, density and quality factor of the high-speed layer underlying the loess layer.

[0029] Among them, when selecting survey points in the loess study area, survey points were selected on the loess plateau with flat terrain.

[0030] The thickness of loess is obtained through surface surveys such as small refraction, micro-logging or bedrock surface surveys. The velocity of the loess layer and the velocity of the high-speed layer underlying the loess layer refer to the longitudinal wave velocity of seismic wave propagation. The vertical function of the loess layer velocity and the velocity of the high-speed layer underlying the loess layer are obtained through surface surveys, high-speed layer rock layer sampling and detection or seismic record refraction wave velocity measurement. The vertical function of the loess layer density and the density of the high-speed layer underlying the loess layer are obtained through vertical sampling and detection. The vertical function of the quality factor of the loess layer and the quality factor of the high-speed layer underlying the loess layer are obtained through micro-logging surveys, ultra-deep micro-logging surveys, velocity empirical formula method, seismic record spectral ratio method or frequency shift method. The above-mentioned data can also be directly obtained from previous geological survey results and related literature data.

[0031] Specifically, this application takes a loess plateau work area in the western part of the country as an example, and selects one survey point on a plateau with a large area and a flat surface within the work area.

[0032] Obtain the loess thickness data of the survey point. Based on the relatively flat distribution of the bedrock surface below the loess layer in the work area, the elevation of the exposed bedrock surface was surveyed and measured in the valleys around the plateau, the elevation of the underlying bedrock surface at the survey point was linearly interpolated, and the surface elevation of the survey point was measured on the spot. The difference between the surface elevation and the bedrock elevation was used to calculate the loess thickness of the survey point to be 155m, thus determining the loess thickness of the model to be built to be 155m.

[0033] Obtain the vertical function of the loess layer velocity and the velocity of the underlying high-speed layer of the loess layer. A 50m micro-logging survey was carried out at the survey point, and it was found that within the range of 0-50m, the loess layer velocity was roughly linearly distributed, which can be expressed as:

[0034] V=300+10d

[0035] Wherein, V is the velocity of the loess layer, unit: m / s; d is the burial depth of the loess layer, unit: m.

[0036] The near-surface velocity model of the first-arrival inversion of earthquake records near the survey point shows that the loess layer has a relatively stable linear change law from shallow to deep. Therefore, the above loess velocity vertical function is applicable to the entire loess layer from 0 to 155 m. The first-arrival wave velocity of the earthquake record near the survey point reflects the velocity of the underlying high-speed layer of the loess layer. Through measurement, it is determined that the velocity of the underlying high-speed layer of the loess layer is Vg = 2500 m / s.

[0037] Obtain the vertical function of the loess layer density and the density of the high-speed layer underlying the loess layer. Through the lithology sampling measurement of the survey point drilling, the change of the loess layer density with depth can be expressed as:

[0038] ρ=1.5867+0.002667d

[0039] Where ρ is the density of the loess layer, unit: g / cm 3 ; d is the buried depth of loess layer, unit: m.

[0040] Since the density of the high-speed layer is relatively stable, the high-speed layer density ρg=2.3g / cm 3 .

[0041] Obtain the vertical function of the quality factor of the loess layer and the quality factor of the high-speed layer underlying the loess layer. The formation quality factor reflects the absorption characteristics of the formation to seismic waves, and is generally exponentially positively correlated with the longitudinal wave velocity of the formation. Here, the relationship between the formation quality factor Q and the longitudinal wave velocity V in the "Road to Accurate Exploration - Analysis of High-Resolution Seismic Exploration System Engineering" by author Li Qingzhong is directly used as the vertical function of the quality factor:

[0042] Q≈14V 2.2

[0043] Wherein, V is the formation velocity, unit: km / s.

[0044] 2) The velocity distribution of the loess layer is obtained according to the vertical function of the loess layer velocity, and the loess layer is divided into multiple horizontal thin layers of equal thickness according to the thickness of the loess layer and the velocity distribution of the loess layer; the horizontal layered geometric model is constructed using the divided horizontal thin layers and the underlying high-speed layer of the loess layer. When the loess layer is divided into multiple horizontal thin layers of equal thickness, the thickness of each horizontal thin layer is less than the set maximum thickness, and the velocity difference between adjacent horizontal thin layers is less than the set maximum inter-layer velocity difference.

[0045] The multiple horizontal thin layers of equal thickness include the number of horizontal thin layers and thickness data. Specifically, according to the data obtained in step 1), the horizontal layered geometric model is determined to have a total of 31 horizontal thin layers, and the thickness of each horizontal thin layer is 5 meters.

[0046] 3) The velocity, density and quality factor of each horizontal thin layer in the horizontal layered geometric model are determined by using the vertical function of the loess layer velocity, the vertical function of the loess layer density and the vertical function of the loess layer quality factor, and values ​​are assigned to each horizontal thin layer. The high-speed layer is assigned values ​​according to the velocity, density and quality factor of the high-speed layer underlying the loess layer, thus completing the construction of the horizontal surface loess plateau gradient layered surface model.

[0047] Specifically, Figure 2 As shown, according to the vertical velocity function of the loess layer and the high-speed layer velocity, each layer of the horizontal layered geometric model is assigned a velocity. The velocity of each layer is obtained by substituting the bottom boundary depth of each layer into the vertical velocity function of the loess layer. V1, V2, V3...V31 are 350m / s, 400m / s, 450m / s...1850m / s respectively, and the high-speed layer velocity Vg=2500m / s.

[0048] According to the vertical function of loess layer density and the high-speed layer density, each layer of the horizontal layered geometric model is assigned a density. The density of each layer is obtained by substituting the bottom boundary depth of each layer into the vertical function of loess layer density. ρ1, ρ2, ρ3…ρ31 are 1.6 g / cm respectively. 3 , 1.61g / cm 3 、1.63g / cm 3 ……2.0g / cm 3 , high speed layer density ρg=2.3g / cm 3 .

[0049] According to the vertical function of the loess quality factor Q and the quality factor of the high-speed layer, a quality factor is assigned to each layer of the horizontal layered geometric model. The quality factor of each layer is obtained by bringing the velocity of each layer into the vertical function of the formation quality factor. Q1, Q2, Q3...Q31 are 1.39, 1.86, 2.42...54.19 respectively, and the quality factor of the high-speed layer Qg=105.10, completing the construction of the loess plateau gradient layered surface model with a horizontal surface.

[0050] 4) If Figure 3 As shown, in order to further enable the model to reflect the topographic characteristics of the loess plateau, the undulating surface trace line of the survey point is obtained, and the horizontal thin layer of the horizontal surface loess plateau gradual layered surface model is cut according to the trace line, and the part above the surface trace line is discarded to complete the construction of the undulating surface loess plateau gradual layered surface model. The model can simulate various loess plateau terrains such as ditches, beams, plateaus, and hills. The model constructed in this application can reflect the overall characteristics of the loess plateau, and the model can be used to conduct an overall simulation study of the loess plateau.

[0051] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention is described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions recorded in the aforementioned embodiments without creative work, or replace some of the technical features therein with equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for constructing a loess plateau gradient layered surface model, characterized in that: The method comprises the following steps: 1) Select survey points in the loess plateau study area, obtain the loess thickness data, the vertical function of the loess velocity, the vertical function of the loess density, the vertical function of the loess quality factor, and the velocity, density and quality factor of the high-speed layer underlying the loess layer; 2) The velocity distribution of the loess layer is obtained according to the vertical function of the loess layer velocity, and the loess layer is divided into multiple horizontal thin layers of equal thickness according to the thickness of the loess layer and the velocity distribution of the loess layer; a horizontal layered geometric model is constructed using the divided horizontal thin layers and the underlying high-speed layer of the loess layer; 3) The velocity, density and quality factor of each horizontal thin layer in the horizontal layered geometric model are determined by using the vertical function of the loess layer velocity, the vertical function of the loess layer density and the vertical function of the loess layer quality factor, and values ​​are assigned to each horizontal thin layer. The high-speed layer is assigned values ​​according to the velocity, density and quality factor of the high-speed layer underlying the loess layer, thus completing the construction of the horizontal surface loess plateau gradient layered surface model.

2. The method for constructing a loess plateau gradient layered surface model according to claim 1, characterized in that: It also includes obtaining the undulating surface trace line of the survey point, cutting the horizontal thin layer of the horizontal surface loess plateau gradual layered surface model according to the trace line, discarding the part above the surface trace line, and completing the construction of the undulating surface loess plateau gradual layered surface model.

3. The method for constructing a loess plateau gradient layered surface model according to claim 1, characterized in that: In step 1), when selecting survey points in the loess plateau study area, survey points were selected on the loess plateau with flat terrain.

4. The method for constructing a loess plateau gradient layered surface model according to claim 1, characterized in that: In step 2), when the loess layer is divided into multiple horizontal thin layers of equal thickness, the thickness of each horizontal thin layer is less than the set maximum thickness, and the velocity difference between adjacent horizontal thin layers is less than the set maximum inter-layer velocity difference.

5. The method for constructing a loess plateau gradient layered surface model according to claim 1, characterized in that: In step 3), when determining the velocity, density and quality factor of each horizontal thin layer of the horizontal layered geometric model, the velocity, density and quality factor of a point at the same relative position of each horizontal thin layer are used as the velocity, density and quality factor of each horizontal thin layer.

6. The method for constructing a loess plateau gradient layered surface model according to any one of claims 1 to 5, characterized in that: In step 1), the thickness of the loess layer is obtained by surface survey methods such as small refraction, micro-well logging or bedrock surface survey.

7. The method for constructing a loess plateau gradient layered surface model according to any one of claims 1 to 5, characterized in that: In step 1), the loess layer velocity and the velocity of the underlying high-speed layer of the loess layer refer to the longitudinal wave velocity of seismic wave propagation. The vertical function of the loess layer velocity and the velocity of the underlying high-speed layer of the loess layer are obtained by surface investigation, rock sampling and detection, or seismic record refraction wave velocity measurement.

8. The method for constructing a loess plateau gradient layered surface model according to any one of claims 1 to 5, characterized in that: In step 1), the vertical function of the density of the loess layer and the density of the high-speed layer underlying the loess layer are obtained by vertical sampling and detection.

9. The method for constructing a loess plateau gradient layered surface model according to any one of claims 1 to 5, characterized in that: In step 1), the vertical function of the quality factor of the loess layer and the quality factor of the high-speed layer underlying the loess layer are obtained by micro-logging survey, ultra-deep micro-logging survey, velocity empirical formula method, seismic record spectral ratio method or frequency shift method.

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