A high-precision magnetic data processing method for ancient tomb detection

Abstract

The present invention provides a high-precision magnetic survey data processing method for ancient tomb detection. The method comprises the following steps: Step 1: measuring magnetic anomalies at various points on the surface; Step 2: interpolating microtopography data within and around the magnetic measurement area onto regular grid nodes; Step 3: weighting the measured magnetic susceptibility of various soil types to obtain a magnetic susceptibility value at each grid node; Step 4: forward modeling the magnetic anomaly caused by the microtopography around the ancient tomb at the magnetic measurement point; Step 5: eliminating the microtopography magnetic anomaly from the measured magnetic anomaly, i.e., correcting the microtopography magnetic influence, and performing polarization processing on the corrected magnetic anomaly; and Step 6: calculating the vertical first-order derivative of the polarization magnetic anomaly or using a potential field separation technique to obtain a residual polarization magnetic anomaly, which serves as key data for interpreting the structure of the ancient tomb. The present invention eliminates the influence of microtopography and utilizes a series of techniques to process the magnetic survey data, thereby enhancing the magnetic data's ability to detect ancient tombs.

Description

Technical Field

[0001] The present invention relates to the field of ancient tomb detection and processing methods; in particular, to a high-precision magnetic measurement data processing method for ancient tomb detection. Background Art

[0002] Archaeology plays a crucial role in historical research and is a vital cultural endeavor. Traditional archaeological techniques, such as the "Luoyang shovel" drilling method, often damage cultural relics. From the perspective of cultural relic protection, archaeological surveys should prioritize techniques that avoid damaging cultural deposits at sites (Lin Jinxin et al., 2014). Furthermore, the traditional "Luoyang shovel" drilling method has a limited detection depth (Sun Pingping et al., 2019). Compared to the traditional "Luoyang shovel" drilling method, geophysical methods are not only nondestructive but also fast, relatively inexpensive, and capable of detecting large depths. These methods are ideal for preliminary archaeological surveys (Yuan Bingqiang et al., 2005; Wang Chuanlei et al., 2013; Lin Jinxin et al., 2014; Yuan Bingqiang et al., 2015), and their application in archaeological surveys is increasing (Lin Jinxin et al., 2014). Among geophysical methods, magnetic exploration is a key method, having been used as early as 1958 to detect ancient tombs and delineate the locations of ancient iron smelting furnaces and brick kilns (Qu Zan, 1993). On the one hand, many archaeological targets have magnetic differences from the surrounding media. For example, ancient kiln sites often have strong magnetism, which provides a physical basis for the effectiveness of magnetic detection. On the other hand, magnetic detection has the advantages of high efficiency and low cost, and therefore plays an important role in archaeological investigation and protection (Yang Tianmin et al., 2013; Lin Jinxin et al., 2014).

[0003] Magnetic archaeological surveys primarily target underground ancient tombs, ancient architectural ruins, ancient kiln sites, and ancient smelting sites (Zhang Yinsheng, 1999). Soil, rocks, and clay artifacts that have been burned by fire generally exhibit strong magnetism. Rammed earth, foundations, and tomb bricks exhibit distinctly higher magnetic properties compared to untouched soil. Some funerary objects in ancient tombs may also exhibit significant magnetism. This often results in the observation of significant high magnetic anomalies in these tombs, making magnetic methods a crucial tool for the exploration of ancient tombs, such as imperial mausoleums. Lin Jinxin et al. (2014) conducted magnetic exploration on the accompanying tombs of the Western Xia royal mausoleum. Based on several magnetic anomalies with densely distributed contour lines, combined with other geophysical anomalies, they inferred the burial mounds, burial chambers, and passageways of the underground accompanying tombs. Yuan Bingqiang et al. (2015) conducted magnetic exploration in the Qin Shi Huang Mausoleum area and concluded that the accompanying tombs in this area were generally unburned. The burial pits had been disturbed, with weak magnetic anomalies of 10–20 nT above them. High-precision magnetic surveys could be used to locate the accompanying tombs. Sun Pingping et al. (2019) conducted a series of nondestructive surveys at Maoling using a combination of geophysical methods, including drone aerial surveys, ground surveys, high-density electrical methods, geological radar, high-precision magnetic surveys, and microgravity. They used profile fitting of magnetic survey results with reference to physical property data of the Qin Shi Huang Mausoleum to preliminarily determine the extent of the underground palace and the fine rammed earth structure within the burial mound. Shi Chaoyang et al. (2021) also conducted a comprehensive geophysical survey of the tomb of Concubine Wei in the Tang Zhaoling Mausoleum. Using 2.5D inversion fitting of high-precision magnetic survey profiles, they identified the engineering geological structure of the backfill of Concubine Wei's tomb and roughly delineated the planimetric extent of the underground palace.

[0004] Although magnetic exploration has played an important role in archaeological work such as ancient tomb detection and has achieved significant results, some natural environmental influences, such as microtopography, can significantly interfere with magnetic measurement results. Artificially excavated mounds at archaeological excavation sites can cause strong magnetic anomalies, significantly interfering with the identification of underground burial chambers (Yan Guilin et al., 1993). Wang Chuanlei et al. (2013), while surveying smelting sites in the Tonglushan Ancient Copper Mine Reserve, also found that several major field ridges with elevation differences of 0.2 to 2 meters within the terraced fields within the survey area were clearly reflected on the magnetic anomaly contour map, demonstrating that topographic changes significantly influence magnetic measurement results. Although the influence of microtopography on measurement results has been discovered in previous archaeological magnetic surveys, due to technical limitations, this magnetic influence has not been corrected or eliminated; only a limited analysis of these anomalies has been performed during the interpretation of magnetic data. Furthermore, it is generally believed that because the targets of archaeological magnetic exploration are shallowly buried, there is a good correspondence between the archaeological detection targets and magnetic anomalies. Therefore, magnetic data are not processed and are directly analyzed and interpreted in conjunction with known data (Wang Chuanlei et al., 2013). However, magnetic anomalies are a comprehensive reflection of the magnetic inhomogeneities of all underground geological bodies. Archaeological detection targets are often small in scale, and when they are buried at a greater depth, the magnetic anomalies they cause are very weak. Therefore, it is necessary to use specific processing techniques to process magnetic data to better interpret the archaeological detection targets.

[0005] Based on the above problems, the present invention proposes a high-precision magnetic data processing method for ancient tomb detection. Compared with the existing magnetic data processing method, it corrects the influence of magnetic anomalies of microtopography and performs polarization and vertical derivative calculations. Therefore, this technology is more practical, less affected by interference, and has stronger detection capabilities for ancient tombs. Summary of the Invention

[0006] The present invention aims to provide a high-precision magnetic data processing method for ancient tomb detection. The prior art has two shortcomings: first, microtopography (such as steep terrain, pits and mounds at archaeological excavation sites) significantly interferes with magnetic measurement results. Existing methods do not address the elimination of such anomalies, but rather attempt to distinguish these interfering anomalies during interpretation, which is detrimental to the precise detection of archaeological targets. Second, archaeological targets are often small in size, and when buried at a greater depth, the resulting magnetic anomalies are very weak, making direct interpretation of the actual measured magnetic data extremely difficult. Therefore, specific techniques must be employed to process the magnetic data, highlighting the magnetic anomaly characteristics of the detected target and enabling better interpretation of the archaeological target. The present invention provides a high-precision magnetic data processing method for ancient tomb detection. Compared to existing magnetic data processing methods, this method corrects for the influence of microtopography magnetic anomalies and separates and extracts polarization and local magnetic anomaly signals. Consequently, this method is more practical, less susceptible to interference, and possesses enhanced ancient tomb detection capabilities.

[0007] The present invention is achieved through the following technical solutions:

[0008] The present invention relates to a high-precision magnetic measurement data processing method for ancient tomb detection, comprising the following steps:

[0009] Step 1: Use UAV lidar to measure the microtopography of the magnetic exploration area and its periphery, and interpolate the microtopography data measured in the entire area to regular grid nodes to obtain the x, y, and z coordinates of each grid node.

[0010] Step 2: Based on the actual magnetic susceptibility of various soils (including rammed earth, flower soil, and raw soil) exposed at the excavation site around the ancient tomb and the distribution range and thickness of various soils obtained by on-site detection, the following formula (1) is used for weighted calculation to obtain the magnetic susceptibility distribution with the same microtopographic grid size as the first step, that is, the magnetic susceptibility value at each grid node is obtained:

[0011]

[0012] In the formula is the weighted magnetic susceptibility value at the (x,y)th grid node; κ i (x,y) is the actual magnetic susceptibility value of the i-th layer of soil at the (x,y)-th grid node, h i (x, y) is the thickness of the soil layer (which can be estimated from the actual cross section); L is the number of soil layers.

[0013] Step 3: Use the measured microtopography as the top interface data and the plane where the lowest point of the microtopography is located as the bottom interface data to form a double-interface model. The space between the upper and lower interfaces is filled with the magnetic susceptibility values ​​established in the second step. Use this model to forward calculate the magnetic anomaly caused by the microtopography (mainly various types of soil) around the ancient tomb at the magnetic measurement point. During the calculation, the magnetic soil layer between the upper and lower interfaces is divided into N vertically juxtaposed prisms. The calculation formula (2) is as follows:

[0014]

[0015] Where ΔT topo (x oi ,y oi ,z oi ) is any magnetic measurement point (x oi ,y oi ,z oi ) (the probe height is added to the z coordinate to better fit the actual situation); nonlinear function f i (m j ) is used to calculate the magnetic anomaly caused by the j-th prism at the i-th point, and its expression (3) is:

[0016] f i (m j )=H ax cosIcosA′+H ay cosIsinA′+Z a sinI (3)

[0017] Where H ax 、H ay and Z a are the three components of the magnetic anomaly, I is the geomagnetic inclination, and A′ is the magnetic azimuth of the survey line. The calculation assumes that there is no residual magnetization effect and the magnetization direction is consistent with the local geomagnetic field.

[0018] Using the Poisson formula of gravity and magnetic potential field, we can get equations (4), (5), and (6)

[0019]

[0020]

[0021]

[0022] Where Mx, My, and Mz are the three components of magnetization intensity, and V is the gravitational potential, whose second-order derivative can be calculated by the following equations (7), (8), and (9);

[0023]

[0024]

[0025]

[0026] (x,y,z) are the coordinates of any calculation point, (ξ j ,η j ,ζ j ) is the coordinate of any point in the divided prism, ξ j1 ~ξ j2 ,η j1 ~η j2 ,ζ j1 ~ζ j2 are the coordinate ranges of the j-th prism in three directions, and we get Vxy, Vxz, and Vyz;

[0027]

[0028]

[0029]

[0030] Step 4: Eliminate the micro-topography magnetic anomaly from the measured magnetic anomaly, that is, correct the micro-topography magnetic influence, and perform polarization processing on the corrected magnetic anomaly to obtain the polarized magnetic anomaly.

[0031] Step 5: Calculate the vertical first-order derivative of the polarization magnetic anomaly or use the potential field separation technology to obtain the residual polarization magnetic anomaly as the key data for the interpretation of the ancient tomb structure.

[0032] The present invention has the following advantages:

[0033] (1) The method of the present invention addresses the problem of magnetic anomaly interference caused by micro-topography (such as steep slopes of terraced fields, earth mounds at archaeological excavation sites, etc.) when magnetic exploration is used for archaeological exploration. Based on the magnetic exploration area and the surrounding micro-topography measured by lidar and the magnetic susceptibility of various soils in the exploration area, the magnetic anomaly caused by the micro-topography at each magnetic measurement point is calculated and eliminated from the measured magnetic anomaly, so that the magnetic anomaly is less affected by the micro-topography.

[0034] (2) The method involved in the present invention uses polarization, vertical derivative and other technologies to process the magnetic anomaly after micro-topography correction. Compared with the existing magnetic measurement data processing method for ancient tomb detection, the result can more clearly reflect the location and even the internal structure of the ancient tomb, and has a stronger ability to detect ancient tombs.

[0035] (3) The present invention is simple, easy to operate and has high accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1It is a flowchart of the method involved in the present invention;

[0037] Figure 2 The tested ancient tomb model and micro-topographic map provided by the embodiment of the present invention;

[0038] Figure 3 It is the magnetic anomaly map caused by micro-topography at all measuring points;

[0039] Figure 4 It is the measured magnetic anomaly map generated by the model at all magnetic measurement points;

[0040] Figure 5 It is the magnetic anomaly map after microtopography correction;

[0041] Figure 6 It is the polarization magnetic anomaly map;

[0042] Figure 7 It is the vertical first-order derivative diagram of the calculated polar magnetic anomaly. DETAILED DESCRIPTION

[0043] The present invention will be described in detail below with reference to specific embodiments. It should be noted that the following embodiments are only for further explanation of the present invention, but the protection scope of the present invention is not limited to the following embodiments.

[0044] Example 1

[0045] This embodiment relates to a high-precision magnetic measurement data processing method for ancient tomb detection. Figure 1 As shown, the following steps are included:

[0046] In order to make those skilled in the art better understand the technical solutions involved in the present invention, this embodiment takes a certain ancient tomb model data as an example (see Figure 2 , where the white solid line is the magnetic measurement range; the black shadow area in the shape of a field is the rammed earth wall model, and the four compartments in the middle are the tomb chambers). The technical solution in this application is explained, including the following main steps:

[0047] (1) Using UAV LiDAR to measure the magnetic field in the survey area ( Figure 2 White solid line in the middle) and peripheral microtopography, gridded microtopography data ( Figure 2 ), using the plane at the lowest point of the microtopography (i.e., the plane at an altitude of 192 meters) as the bottom interface data to form a microtopography dual-interface model, which serves as the basis for calculating microtopography magnetic anomalies. The magnetic anomaly caused by the microtopography at the magnetic measurement point is calculated point by point (in actual work, the magnetometer probe needs to be at a certain height above the ground, so this height needs to be added).

[0048] (2) Based on the actual magnetic susceptibility of various soils exposed in the study area and the distribution range and estimated thickness of various soils obtained by field detection, the weighted calculation formula (1) is used to obtain the magnetic susceptibility distribution with the same size as the microtopography grid. Based on this, the magnetic anomaly caused by the microtopography at all measuring points is calculated and interpolated to the regular grid nodes ( Figure 3 ).contrast Figure 2 From the perspective of micro-topography, Figure 3 The results show that the magnetic anomaly changes caused by micro-topography are very obvious, especially at the two northwest-facing steep terrain slopes, which shows obvious magnetic anomaly changes, indicating that the calculation process is correct.

[0049]

[0050] In the formula is the weighted magnetic susceptibility value at the (x,y)th grid node; κ i (x,y) is the actual magnetic susceptibility value of the i-th layer of soil at the (x,y)-th grid node, h i (x, y) is the thickness of the soil layer (which can be estimated from the actual cross section); L is the number of soil layers.

[0051] (3) Figure 4 To measure the magnetic anomaly, interpolate it to the grid nodes of the same size as the microtopography magnetic anomaly, subtract the microtopography magnetic anomaly, and obtain the microtopography corrected magnetic anomaly ( Figure 5 ).contrast Figure 4 It can be seen that due to the influence of microtopography, Figure 4 The measured magnetic anomaly cannot directly reflect the location and morphological characteristics of the ancient tomb. The influence of terrain on the magnetic anomaly after microtopography correction has been significantly improved, and the anomaly caused by the two north-northwest steep terrain steps has been well eliminated. During the calculation, the magnetic soil layer between the upper and lower interfaces is divided into N vertically juxtaposed prisms. The calculation formula (2) is as follows:

[0052]

[0053] Where ΔT topo (x oi ,y oi ,z oi ) is any magnetic measurement point (x oi ,y oi ,z oi ) (the probe height is added to the z coordinate to better fit the actual situation); nonlinear function f i (m j ) is used to calculate the magnetic anomaly caused by the j-th prism at the i-th point, and its expression (3) is:

[0054] f i (m j )=H ax cosIcosA′+H ay cosIsinA′+Z a sinI (3)

[0055] Where H ax 、H ay and Z a are the three components of the magnetic anomaly, I is the geomagnetic inclination, and A′ is the magnetic azimuth of the survey line. The calculation assumes that there is no residual magnetization effect and the magnetization direction is consistent with the local geomagnetic field.

[0056] Using the Poisson formula of gravity and magnetic potential field, we can get equations (4), (5), and (6)

[0057]

[0058]

[0059]

[0060] Where Mx, My, and Mz are the three components of magnetization intensity, and V is the gravitational potential, whose second-order derivative can be calculated by the following equations (7), (8), and (9);

[0061]

[0062]

[0063]

[0064] (x,y,z) are the coordinates of any calculation point, (ξ j ,η j ,ζ j ) is the coordinate of any point in the divided prism, ξ j1 ~ξ j2 ,η j1 ~η j2 ,ζ j1 ~ζ j2 are the coordinate ranges of the j-th prism in three directions, and we get Vxy, Vxz, and Vyz;

[0065]

[0066]

[0067]

[0068] (4) Affected by oblique magnetization, Figure 5The corrected anomaly main position and shape still do not correspond to the location of the ancient tomb. The corrected magnetic anomaly needs to be polarized. When polarizing, the local magnetic inclination and declination are used to obtain the polarized magnetic anomaly ( Figure 6 ),and Figure 5 Compared with the unpolarized magnetic anomaly shown, the high magnetic anomaly moves northward and the magnetic anomaly shape is more symmetrical, which is consistent with the magnetic anomaly characteristics of the ancient tomb.

[0069] (5) Despite Figure 6 The shown magnetic anomaly of the pole can reflect the overall morphological characteristics of the ancient tomb, but the details of the tomb are not clear enough, especially the location of the tomb chamber and rammed earth wall. The vertical first-order derivative of the magnetic anomaly of the pole is calculated, as Figure 7 As shown, Figure 6 Compared to the tantalum magnetic anomaly, the vertical first-order derivative of the tantalum magnetic anomaly provides greater detail, showing low magnetic anomalies above the four burial chambers and high magnetic anomalies near the field-shaped rammed earth walls, consistent with the characteristics of an ancient tomb. This indicates that calculating the vertical first-order derivative is beneficial for interpreting the internal structure of the tomb and even the location of funerary objects.

[0070] At present, the commonly used methods for magnetic exploration of ancient tombs are based on measured magnetic anomalies (i.e. Figure 4 ) is explained, but in areas with relatively large microtopographic interference as shown in this example, the magnetic anomaly cannot reflect the structural characteristics of the ancient tomb. After using the present invention to correct the microtopographic magnetic anomaly and subsequently calculate the polarization and vertical derivatives, the results can more clearly reflect the location and even the internal structure of the ancient tomb. It can be seen that the magnetic data processing invention of the present invention has obvious advantages in ancient tomb detection.

[0071] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various variations or modifications within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A high-precision magnetic measurement data processing method for ancient tomb detection, characterized in that: The following steps are involved: Step 1: Measure magnetic anomalies at various points on the surface; Step 2: Use UAV lidar to measure the microtopography of the magnetic survey area and its surroundings, and interpolate the microtopography data measured in the entire area to regular grid nodes to obtain the x, y, and z coordinates of each grid node; Step 3: Based on the actual magnetic susceptibility of various soils exposed at the excavation site around the ancient tomb and the distribution range and thickness of various soils obtained through on-site detection, a weighted calculation is used to obtain the magnetic susceptibility distribution of the same microtopographic grid size as in Step 2, that is, the magnetic susceptibility value at each grid node; Step 4: Use the measured microtopography as the top interface data and the plane where the lowest point of the microtopography is located as the bottom interface data to form a dual-interface model. Fill the space between the upper and lower interfaces with the magnetic susceptibility values ​​established in the second step. Use this model to forward calculate the magnetic anomaly caused by the microtopography around the ancient tomb at the magnetic measurement point. During the calculation, the magnetic soil layer between the upper and lower interfaces is divided into N vertically juxtaposed prisms. Step 5: Eliminate the micro-topography magnetic anomaly from the measured magnetic anomaly, i.e., correct the micro-topography magnetic influence, and perform polarization processing on the corrected magnetic anomaly to obtain the polarized magnetic anomaly; Step 6: Calculate the vertical first-order derivative of the polarization magnetic anomaly or use the potential field separation technology to obtain the residual polarization magnetic anomaly as the key data for the interpretation of the ancient tomb structure.

2. The high-precision magnetic measurement data processing method for ancient tomb detection according to claim 1, characterized in that: In step 3, the weighted calculation formula is as follows: In the formula is the weighted magnetic susceptibility value at the (x,y)th grid node; κ i (x,y) is the actual magnetic susceptibility value of the i-th layer of soil at the (x,y)-th grid node, h i (x,y) is the thickness of the soil layer; L is the number of soil layers.

3. The high-precision magnetic measurement data processing method for ancient tomb detection according to claim 1, characterized in that: In step 4, the magnetic soil layer between the upper and lower interfaces is divided into N vertically juxtaposed prisms. The specific calculation formula is as follows: Where ΔT topo (x oi ,y oi ,z oi ) is any magnetic measurement point (x oi ,y oi ,z oi ) calculated on the magnetic anomaly, Nonlinear function f i (m j ) is used to calculate the magnetic anomaly caused by the j-th prism at the i-th point, and its expression is formula (3): f i (m j )=H ax cosIcosA′+H ay cosIsinA′+Z a sinI (3) Where H ax 、H ay and Z a are the three components of magnetic anomaly, I is the geomagnetic inclination, and A′ is the magnetic azimuth of the survey line; the calculation conditions are: assuming that there is no residual magnetization effect and the magnetization direction is consistent with the local geomagnetic field direction; Using the Poisson formula of gravity and magnetic potential field, we can get equations (4), (5), and (6): Where Mx, My, and Mz are the three components of magnetization intensity, and V is the gravitational potential, whose second-order derivative can be calculated by the following equations (7), (8), and (9): (x, y, z) are the coordinates of any calculation point, (ξ, η, ζ) are the coordinates of any point in the divided prism, ξ j1 ~ξ j2 ,η j1 ~η j2 ,ζ j1 ~ζ j2 are the coordinate ranges of the j-th prism in three directions, and we get Vxy, Vxz, and Vyz.

Images