Method for determining the degree of karst development based on calculation of karst rate of geophysical profile

Abstract

This invention discloses a method for determining the degree of karst development based on karst ratio calculation from geophysical profiles. Using a high-density resistivity geophysical profile, detection is conducted, and data is rapidly acquired and processed by a system to obtain the electrical variations of the horizontal and vertical rock and soil within a 50-meter depth underground, ultimately forming a geophysical profile resistivity image. Utilizing the electrical differences between karst anomaly sections on the geophysical profile and the surrounding rock mass, the horizontal and vertical karst development ranges, cave ceiling depths, and cave floor depths on the geophysical profile are delineated. Finally, the karst ratios of the geophysical line, geophysical surface, and geophysical body are calculated, thus determining the degree of karst development in a karst site. Based on the concepts of line karst ratio, surface karst ratio, and body karst ratio, this invention clarifies the calculation method for the karst ratio of the geophysical line, geophysical surface, and geophysical body based on geophysical profiles, and proposes quantitative indicators for determining the degree of karst development in a site.

Description

Technical Field

[0001] This invention relates to the field of engineering geological exploration technology, specifically to a method for determining the degree of karst development based on the calculation of karst rate from geophysical profiles. Background Technology

[0002] Karst landforms are a general term for surface and underground features formed by the dissolution of soluble rocks by groundwater or surface water. The area of ​​soluble carbonate rocks in my country reaches 3.44 × 10⁶ km². 2 Karst regions account for more than one-third of China's land area, with carbonate rocks exposed covering approximately 910,000 km², widely distributed and covering a large area, mainly in Guangxi, Guizhou, Yunnan, and Guangdong. With the increasing number of engineering projects being constructed in karst areas, karst disasters caused by unfavorable karst geology, especially karst seepage and karst collapse, have become prominent problems in engineering construction. Karst development has a significant impact on foundation stability. The "Code for Design of Building Foundations" stipulates that Class A and Class B buildings should avoid areas with strong karst development. Assessing the degree of karst development at a site is of great guiding significance for building site selection and road alignment. Therefore, improving the quantitative evaluation of karst development in engineering areas can effectively guide the management of karst disasters in engineering areas, and has significant technical and economic value for water conservancy, transportation, and urban construction in karst regions.

[0003] Because the development of karst caves is concealed, the number of boreholes can be arranged in the site selection or feasibility study stage, resulting in less information, longer time consumption, and higher costs. The linear karst rate calculated based on the boreholes is not representative and cannot accurately and comprehensively reflect the degree of karst development in the site area.

[0004] Geophysical testing methods offer advantages such as non-destructive testing, high speed, high efficiency, reliable accuracy, strong representativeness, and low cost in karst exploration. They have been widely used in engineering surveys and quality inspections, achieving good detection results and economic benefits. Current literature and research mainly focus on the interpretation of geophysical results, with no research on the calculation of karst ratio based on geophysical profiles or on criteria for judging the degree of karst site development.

[0005] The "Code for Design of Building Foundations" (GB5007-2011) uses borehole encounter rate and linear karst rate as quantitative indicators to measure the intensity of karst development at a site. In addition to these two indicators for assessing the degree of karst site development, the concepts of surface karst rate and volumetric karst rate are also mentioned in production work. In actual production, the number of boreholes at an exploration site is limited, and since boreholes are often "one-hole views," it is often impossible to obtain the planar or three-dimensional dimensional information of the karst caves revealed by the boreholes. Therefore, calculating the surface karst rate or volumetric karst rate based on borehole data is quite difficult.

[0006] Therefore, current descriptions of surface karst ratio and volume karst ratio remain at the conceptual level, and corresponding standards and regulations do not provide specific calculation methods for surface karst ratio and volume karst ratio. Surface karst ratio and volume karst ratio are indicators that determine the degree of karst development of a site in two planes and three-dimensional space, respectively, and can more accurately reflect the intensity of karst development at a site. Summary of the Invention

[0007] The purpose of this invention is to improve the quantitative prediction and evaluation method of karst disasters in engineering areas, and to propose a method for determining the degree of karst development based on the calculation of karst rate from geophysical profiles, which can effectively guide the management of karst disasters in engineering areas.

[0008] The technical solution of this invention: a method for determining the degree of karst development based on karst ratio calculation from geophysical profiles, comprising the following steps:

[0009] Step 1: Based on the information from the engineering exploration boreholes, identify the boreholes with karst phenomena and mark them as exploration boreholes;

[0010] Step 2: With the exploration well as the center, plan the geophysical exploration area, and carry out engineering geological and karst hydrogeological mapping in the exploration area to obtain comprehensive geological information in the site area. Select areas with good outcrops and no karst development, and use geophysical methods to obtain the general apparent resistivity variation range of each rock and soil body in the site area to assist in geophysical interpretation.

[0011] Step 3: Based on the karst information initially revealed by the borehole in Step 1, and taking into account the strike and dip geological information of the rock strata, a high-density resistivity profile is laid out. With the help of the high-density resistivity method test, the apparent resistivity distribution of the underground medium in each profile within a 50-meter range is obtained, and finally a geophysical profile resistivity image is formed.

[0012] Step 4: Since the karst caves and dissolution fissures on the geophysical profile show obvious electrical differences from the surrounding rock mass, based on the electrical differences, the geophysical profile in Step 3 is interpreted using the borehole information obtained in Step 1 and the general apparent resistivity of normal rock and soil obtained in Step 2. This yields basic information on the horizontal and vertical karst development range, the burial depth of the cave roof, and the burial depth of the cave floor on the geophysical profile, which is used to calculate the karst rate of the geophysical line, the karst rate of the geophysical surface, and the karst rate of the geophysical body.

[0013] Step 5: Based on Step 4, determine the degree of karst development at the site according to the karst rate of the geophysical line, the karst rate of the geophysical surface, and the karst rate of the geophysical body. The degree of karst development at the site is divided into strong development, moderate development, and slight development. When any one of the karst rates of the geophysical line, the geophysical surface, and the geophysical body meets the corresponding development criteria, the site can be determined to have the corresponding degree of karst development. The criteria for strong development are: karst rate of the geophysical line > 20%, karst rate of the geophysical surface > 4%, or karst rate of the geophysical body > 0.8%; the criteria for moderate development are: karst rate of the geophysical line 5%–20%, karst rate of the geophysical surface 0.25%–4%, or karst rate of the geophysical body 0.0125%–0.8%; the criteria for slight development are: karst rate of the geophysical line < 5%, karst rate of the geophysical surface < 0.25%, or karst rate of the geophysical body < 0.0125%.

[0014] In the aforementioned method for determining the degree of karst development based on the calculation of karst rate from geophysical profiles, step two involves integrating geological information including the surface karst strata and their occurrence, groundwater level, the main direction of karst development, and the general apparent resistivity variation range of each rock and soil body within the site area.

[0015] In the aforementioned method for determining the degree of karst development based on the calculation of karst ratio from geophysical profiles, the arrangement of the high-density resistivity method profile in step three is as follows: First, several survey lines are laid out along the main direction of karst development. In the main direction of karst development, the spacing between the survey lines is determined by the actual size of the engineering area, and the survey lines are laid out at equal intervals according to the accuracy requirements and the economic efficiency of the project. Second, perpendicular to the main direction of karst development, the arrangement of the survey lines is based on the resistivity anomaly areas in the main direction of karst development after the geophysical profile is interpreted, and the lines are densely arranged. Alternatively, the arrangement can be the same as the survey lines in the main direction of karst development, with equal intervals and a single arrangement.

[0016] In the aforementioned method for determining the degree of karst development based on the karst rate calculation of geophysical profiles, the determination methods for the karst rate of the geophysical line, the karst rate of the geophysical surface, and the karst rate of the geophysical body in step five are as follows: the karst rate of the geophysical line is obtained by measuring the ratio of the sum of the lengths of the vertical karst anomaly segments of the geophysical profile to the total vertical geophysical length; the karst rate of the geophysical surface is obtained by calculating the ratio of the sum of the areas of the karst anomaly segments of the geophysical profile to the total area of ​​the geophysical profile; and the karst rate of the geophysical body is obtained by calculating the ratio of the sum of the volumes of the karst anomaly segments of the geophysical profile to the volume of soluble rock in the geophysical profile.

[0017] The method for determining the volume of the karst anomaly segment in the above geophysical profile is as follows: by interpreting multiple geophysical profiles, the area of ​​the karst low resistivity anomaly segment in each geophysical profile is calculated, and the elevation of the karst boundary feature points is obtained. Then, by combining the karst boundary feature points of multiple geophysical profiles, the surfaces are connected into a whole, and the karst volume of the continuous low resistivity anomaly segment is calculated by spatial interpolation.

[0018] The method for determining the volume of soluble rock in the above geophysical profile is as follows: the volume of soluble rock between two geophysical profiles can be obtained by multiplying the average area between the two geophysical profiles by the distance between the geophysical profiles. The same method can be used to obtain the volume of soluble rock between other profiles. By summing them up, the volume of soluble rock in the geophysical profile can be obtained.

[0019] The beneficial effects of this invention are as follows: Compared with the prior art, although the "Code for Design of Building Foundations" (GB5007-2011) clearly gives the calculation formulas for borehole and void ratio: namely, borehole void ratio = (number of boreholes with voids / total number of boreholes) × 100%, and linear karst ratio = (sum of drilling footage with voids / total drilling footage) × 100%, surface karst ratio and volume karst ratio only remain at the conceptual level. That is, surface karst ratio: the percentage of karst spatial morphology area per unit area, and volume karst ratio: the percentage of void volume to the measured soluble rock volume; no clear method is given for determining how to obtain surface karst ratio and volume karst ratio. Therefore, it is impossible to apply surface karst ratio and volume karst ratio to the judgment of the degree of karst development in a site area.

[0020] Based on the concepts of surface karst ratio and volume karst ratio, this invention proposes a method for calculating surface karst ratio and volume karst ratio based on geophysical profiles. Specifically, the geophysical surface karst ratio = (sum of areas of geophysically interpreted karst anomaly segments / geophysical profile area) × 100%; the geophysical volume karst ratio = (sum of volumes of geophysically interpreted karst anomaly segments / soluble rock volume) × 100%. Furthermore, based on this, a quantitative criterion for determining the degree of karst development at a site is proposed based on the linear karst ratio, surface karst ratio, and volume karst ratio of the geophysical profile. Compared with the karst ratio calculated by boreholes, this method can more accurately reflect the strength of karst development at a site, improving the reliability of judging the degree of karst site development. Attached Figure Description

[0021] Figure 1 This is a flowchart of the process of the present invention;

[0022] Figure 2 This is a layout diagram of the geophysical profile lines;

[0023] Figure 3 This is a geophysical profile.

[0024] Figure 4 This is a three-dimensional schematic diagram of the geophysical profile. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings and embodiments, but this should not be construed as limiting the present invention.

[0026] Embodiments of the present invention: A method for determining the degree of karst development based on karst ratio calculation from geophysical profiles, such as... Figure 1As shown, it includes the following steps:

[0027] Step 1: Based on the information of the engineering exploration boreholes, identify the boreholes with karst phenomena and mark them as exploration boreholes.

[0028] Step 2: Centered on the exploration borehole, plan the geophysical exploration area and conduct engineering geological and karst hydrogeological mapping within the exploration area to obtain comprehensive geological information of the site area, such as lithology, stratum dip, dip angle, and structure. Select areas with good outcrops and no karst development, and use geophysical methods to obtain the general apparent resistivity variation range of each rock and soil body in the site area to assist in geophysical interpretation.

[0029] Step 3: Based on the karst information initially revealed by the boreholes in Step 1, and taking into account geological information such as the strike and dip of the rock strata, a high-density resistivity geophysical profile is laid out. Figure 2 Using high-density resistivity testing, the apparent resistivity distribution of the subsurface medium in each profile within a 50-meter depth was obtained, ultimately forming a geophysical profile resistivity image. Figure 3 ).

[0030] Step 4: Since the karst caves and dissolution fissures on the geophysical profile show obvious electrical differences from the surrounding rock mass, based on these electrical differences, the geophysical profile from Step 3 is interpreted using the borehole information obtained in Step 1 and the general apparent resistivity of normal rock and soil obtained in Step 2. This allows for the acquisition of basic information such as the horizontal and vertical karst development range, the burial depth of the cave roof, and the burial depth of the cave floor on the geophysical profile. This information is then used to calculate the karst rate of the geophysical line, the karst rate of the geophysical surface, and the karst rate of the geophysical body.

[0031] Step 5: Based on Step 4, determine the degree of karst development at the site according to the karst rate of the geophysical line, the karst rate of the geophysical surface, and the karst rate of the geophysical body. The judgment method is shown in the table below.

[0032] Linear karst rate (%) Karst surface ratio (%) Karst ratio (%) Strong development >20% >4% >0.8% moderate development 5%～20% 0.25%～4% 0.0125%～0.8% Microdevelopment <5% <0.25% <0.0125%

[0033] Taking strong development as an example: when any one of the karst rates of the geophysical line, geophysical surface, and geophysical body reaches the standard, the karst development level of the site can be identified as strong development. The judgment methods for moderate and slight development are the same.

[0034] In step two, the comprehensive geological information includes the surface karst strata and their occurrence, groundwater level, karst depth, main direction of karst development, and the apparent resistivity range of general rock and soil masses. The main direction of karst development provides a basis for the layout of the high-density resistivity method observation system and for interpreting underground anomaly zones.

[0035] In step three, the high-density resistivity method profile is arranged as follows: several survey lines are laid out along the main direction of karst development. The spacing between the survey lines along the main direction of karst development is based on the actual size of the engineering area and is arranged equidistantly according to the accuracy requirements and the economic efficiency of the project. Perpendicular to the main direction of karst development, the survey lines are arranged in a denser manner according to the resistivity anomaly areas along the main direction of karst development after geophysical profile interpretation. Alternatively, the survey lines can be arranged in the same way as those along the main direction of karst development, with equal spacing and arranged all at once.

[0036] The methods for determining the karst ratio of geophysical lines, geophysical surfaces, and geophysical bodies in step four are as follows: the karst ratio of geophysical lines is obtained by measuring the ratio of the sum of the lengths of the vertical karst low-resistivity anomaly segments of the geophysical profile to the total vertical geophysical length; the karst ratio of geophysical surfaces is obtained by calculating the ratio of the sum of the areas of the karst low-resistivity anomaly segments of the geophysical profile to the total area of ​​the geophysical profile; and the karst ratio of geophysical bodies is obtained by calculating the ratio of the sum of the volumes of the karst low-resistivity anomaly segments of the geophysical profile to the volume of soluble rock in the geophysical profile.

[0037] Determination of the volume of the karst low resistivity anomaly section in the above geophysical profile ( Figure 4 The method is as follows: by interpreting multiple geophysical profiles, the area of ​​the karst low resistivity anomaly segment in each geophysical profile is calculated, and the elevation of the karst boundary feature points is obtained. Then, by combining the karst boundary feature points of multiple geophysical profiles, the surfaces are connected into a whole, and the karst volume of the continuous low resistivity anomaly segment is calculated by spatial interpolation.

[0038] The method for determining the volume of soluble rock in the above geophysical profile is as follows: the volume of soluble rock between two geophysical profiles can be obtained by multiplying the average area between the two geophysical profiles by the distance between the geophysical profiles. The same method can be used to obtain the volume of soluble rock between other profiles. By summing them up, the volume of soluble rock in the geophysical profile can be obtained.

Claims

1. A method for determining the degree of karst development based on karst ratio calculation from geophysical profiles, characterized in that: The following steps are involved: Step 1: Based on the information from the engineering exploration boreholes, identify the boreholes with karst phenomena and mark them as exploration boreholes; Step 2: With the exploration well as the center, plan the geophysical exploration area, and carry out engineering geological and karst hydrogeological mapping in the exploration area to obtain comprehensive geological information in the site area. Select areas with good outcrops and no karst development, and use geophysical methods to obtain the general apparent resistivity variation range of each rock and soil body in the site area to assist in geophysical interpretation. Step 3: Based on the karst information initially revealed by the borehole in Step 1, and taking into account the strike and dip geological information of the rock strata, a high-density resistivity profile is laid out. With the help of the high-density resistivity method test, the apparent resistivity distribution of the underground medium in each profile within a 50-meter range is obtained, and finally a geophysical profile resistivity image is formed. Step 4: Since the karst caves and dissolution fissures on the geophysical profile show obvious electrical differences from the surrounding rock mass, based on the electrical differences, the geophysical profile in Step 3 is interpreted using the borehole information obtained in Step 1 and the general apparent resistivity of normal rock and soil obtained in Step 2. This yields basic information on the horizontal and vertical karst development range, the burial depth of the cave roof, and the burial depth of the cave floor on the geophysical profile, which is used to calculate the karst rate of the geophysical line, the karst rate of the geophysical surface, and the karst rate of the geophysical body. Step 5: Based on Step 4, determine the degree of karst development at the site according to the karst rate of the geophysical line, the karst rate of the geophysical surface, and the karst rate of the geophysical body. The degree of karst development at the site is divided into strong development, moderate development, and slight development. When any one of the karst rates of the geophysical line, the geophysical surface, and the geophysical body meets the corresponding development criteria, the site can be determined to have the corresponding degree of karst development. The criteria for strong development are: karst rate of the geophysical line > 20%, karst rate of the geophysical surface > 4%, or karst rate of the geophysical body > 0.8%; the criteria for moderate development are: karst rate of the geophysical line 5%–20%, karst rate of the geophysical surface 0.25%–4%, or karst rate of the geophysical body 0.0125%–0.8%; the criteria for slight development are: karst rate of the geophysical line < 5%, karst rate of the geophysical surface < 0.25%, or karst rate of the geophysical body < 0.0125%. In step three, the high-density resistivity method profile is arranged as follows: First, several survey lines are laid out along the main direction of karst development. The spacing between the survey lines along the main direction of karst development is determined by the actual size of the engineering area, and the survey lines are laid out at equal intervals according to the accuracy requirements and the economic efficiency of the project. Second, perpendicular to the main direction of karst development, the survey lines are laid out in a denser manner based on the resistivity anomaly areas along the main direction of karst development after geophysical profile interpretation. Alternatively, the survey lines can be laid out at equal intervals in the same way as the survey lines along the main direction of karst development. The methods for determining the karst ratio of the geophysical line, the karst ratio of the geophysical surface, and the karst ratio of the geophysical body in step five are as follows: The karst ratio of the geophysical line is obtained by measuring the ratio of the sum of the lengths of the vertical karst anomaly segments of the geophysical profile to the total vertical geophysical length; the karst ratio of the geophysical surface is obtained by calculating the ratio of the sum of the areas of the karst anomaly segments of the geophysical profile to the total area of ​​the geophysical profile; the karst ratio of the geophysical body is obtained by calculating the ratio of the sum of the volumes of the karst anomaly segments of the geophysical profile to the volume of soluble rock in the geophysical profile. The method for determining the volume of karst anomaly segments in geophysical profiles is as follows: by interpreting multiple geophysical profiles, the area of ​​the karst low resistivity anomaly segment in each geophysical profile is calculated, and the elevation of the karst boundary feature points is obtained. Then, by combining the karst boundary feature points of multiple geophysical profiles, the surfaces are connected into a whole, and the karst volume of the continuous low resistivity anomaly segment is calculated by spatial interpolation. The method for determining the volume of soluble rock in a geophysical profile is as follows: by multiplying the average area between two geophysical profiles by the distance between the geophysical profiles, the volume of soluble rock between the two geophysical profiles can be obtained. Using the same method, the volume of soluble rock between other profiles can be obtained. By summing them up, the volume of soluble rock in the geophysical profile can be obtained.

2. The method for determining the degree of karst development based on the karst ratio calculation of geophysical profiles according to claim 1, characterized in that: In step two, the comprehensive geological information includes the surface karst development strata and their occurrence, groundwater level, the main direction of karst development, and the general apparent resistivity variation range of each rock and soil body in the site area.

Images