Study on Electrical Characteristics of Magmatic Rock Intrusions in Metamorphic Rock Formations— Taking the Mupi Tunnel of Jiuzhaigou-Mianyang Highway as an Example

The stratum lithology and geological structure of the highway tunnel in the mountainous areas of western China are 7 complex, and the engineering geological conditions are complicated. When the highway tunnel passes through different 8 lithological strata, its structural design and construction are completely different. Therefore, the design and construction of 9 the supporting tunnel are used in the tunnel survey. The identification of the contact boundary between magmatic rock and 10 metamorphic rock and the grade of surrounding rock is very important. The data processing, forward numerical simulation, 11 1D, 2D, and 3D inversion of the G8513 Jiuzhaigou-Mianyang Expressway Mupi Tunnel survey by magnetotelluric method 12 reveals the ground level along the design line of the tunnel to the depth of 50m depths within the design elevation of the 13 tunnel. Electrical characteristics, focusing on the identification of the contact boundary between magmatic rock and 14 metamorphic rock. This study provides the electrical characteristics of the magmatic rock and metamorphic rock contact 15 boundary of the Mupi Tunnel. It is speculated that the boundary is revealed by the tunnel construction excavation, which 16 verifies the correctness of the geophysical inversion model and provides a more detailed design basis for the tunnel design. It 17 is believed that taking the Mupi tunnel survey as an example, this research can provide detailed geophysical basis for the 18 identification of magmatic rock and metamorphic rock in highway tunnels, as well as the geological survey and design of 19 highway tunnels. a more efficient and accurate means of identifying the lithology boundary of intrusive magmatic rocks in 543 metamorphic rocks for highway tunnel design.


Introduction 22
Western China is a mountainous region, with complex topographical conditions and geological structures, and poor 23 engineering geological conditions. In order to meet the design requirements, the route inevitably needs to be designed to pass 24 through deep-buried super-long tunnels in the hilly area. Tunnel engineering occupies a large proportion of the highway 25 construction in the western mountainous areas 29 . 26 If the highway tunnel must pass through different lithological strata, if the structure and foundation form are not properly 27 selected, it is easy to cause collapse and structural instability during construction and later highway operation. Therefore, in 28 the preliminary survey and design process, the stratum lithology should be fully identified and provided More accurate 29 information to avoid subsequent problems. 30 Geophysical prospecting is one of the important methods used in various stages of tunnel engineering geological survey 7-31 22 . Engineering geological surveying, drilling, geophysical prospecting and many other methods are used to comprehensively 32 evaluate engineering geology in order to provide as detailed basic information as possible for the design and construction of 33 highway tunnels. Magnetotelluric method is widely used in highway tunnel surveys because of its large survey depth and high 34 efficiency 1-7 . 35 In the past few years, Yu N i an has carried out the application and research of magnetotelluric sounding in the engineering 36 geological survey of railway tunnels 15 . Zhao Guoze 22 , Yang Wencai 23 and others 24-25 have studied the magnetotelluric method 37 to detect the deep electric structure of the crust and the interpretation of magnetotelluric data under complex structural 38 conditions. Booker and Parker has written and published many papers in various aspects such as processing, inversion, 39 interpretation, respectively studying the methods and applications of magnetotelluric from acquisition to geological 40 interpretation, and has obtained many research results 13,18,20 . 41 We launched the investigation and application research of magnetotelluric sounding method in highway tunnels. The 42 application research of the electrical characteristics of magmatic rock and metamorphic rock contact boundary in the 43 investigation of highway tunnels. 44 Taking the Mupi Tunnel of Jiuzhaigou-Mianyang Highway as an example, the study of the electrical characteristics of the 45 magmatic rock intrusions in the metamorphic rock formations of highway tunnels by magnetotelluric method is carried out, 46 revealing the various layers within a depth of 50m within the design elevation of the Mupi tunnel's electrical characteristics 47 of the structure, focusing on the forward numerical simulation, inversion calculation and geological interpretation of the 48 electrical characteristics of the tunnel located in the metamorphic rock containing magmatic rock intrusions, and the technical 49 route of the verification results revealed by the tunnel construction excavation to solve the scientific problems and engineering 50 technical solutions of the validity and reliability of the application of the magnetotelluric method to the survey of magmatic 51 rock and metamorphic rock formations in highway tunnels. 52

Project Overview 53
2.1．Introduction to the geographic location of the study area 54 The entrance of the tunnel is located near the Mupi Tibetan Township Government of Pingwu County, Mianyang City,  55 Sichuan Province, China, as is shown in Fig. 1a. The exit is located in Jinfeng Village, Mupi Township, Pingwu County, on 56 the left bank of Huoxi River. The exit is close to Huaneng Yinping Hydropower Plant. There is no road connection between 57 the entrance and exit. The S205 road is about 100-300m away from the entrance of the tunnel, so the traffic is relatively 58 inconvenient. 59 The proposed Mupi highway tunnel is left and right separated. The left tunnel design is shown in Fig. 1b  The site is located on the east side of the Qinghai-Tibet Plateau, the Motianling mountain system, with steep mountains, 84 complex topography, dense vegetation on the surface, and tectonic denudation of mid-mountainous landforms with large 85 topography, as shown in Fig. 2a, c. 86 The tunnel is located on a wedge-shaped block bounded by the Wenxian arc-shaped structural belt in the north, the 87 Minjiang-Xueshan-Huyaguan fault zone in the west, and the Longmenshan fault zone in the southeast. The tectonic traces in 88 the block are mainly controlled by the three above-mentioned tectonic belts, but in the later period, affected by the arc structure 89 of Wenxian County, they all present a curved arc protruding southward. The Indosinian period was strongly squeezed by the 90 SN-trending tectonic movement, and a series of near-EW-trending folds and faults formed the basic structure of the area. 91 The faults and folds in the work area are relatively developed and large in scale, extending from dozens to nearly a hundred 92 kilometers. There are mainly large folds and fault structures such as the Daqiao syncline, the veneer inverted anticline, the 93 Baima arc structure belt, and the Nanping anticline. The tunnel is located near the core of the veneer inverted anticline. 94 According to the ground geological survey and the drilling revealed, the main lithology of the project area is gravel soil, 95 Mupishaizi intrusion (TJS), and Dashaba formation (AnZd). 96 Dashaba Formation (AnZd): The lithology is dominated by light gray to dark gray garnet metamorphic porphyritic mica 97 nachoschist, chlorite nachoschist, and biotite sericite schist, with a small amount of metamorphic sandstone and quartz veins, 98 and the thickness is greater than 540m. See Fig. 2b for details. 99 Mupishaizi intrusive (TJS): mainly medium to fine-grained biotite quartz diorite, gray to off-white, the mineral composition 100 is mainly quartz, followed by diorite and biotite, granular cryptocrystalline structure, massive structure, the rock is hard. Local 101 granodiorite or granite. See Fig. 2c, d, Fig. 9.

102
The groundwater in the tunnel site is mainly composed of loose accumulation layer pore water and bedrock fissure water.  In this geophysical exploration work, a survey line is arranged along the axis ZK124+670~ZK125+330 of the highway 136 tunnel survey line (Fig. 1b, 3). The survey line is 660m long and the measuring point distance is mainly 30m. The focus of the survey is to explore the engineering geological conditions from the ground to 50m below the tunnel 160 elevation, especially the contact boundary between magmatic rock and metamorphic rock, so as to provide reliable basis for 161 engineering geological survey and detailed basic information for tunnel design. 162

Mathematical Model of MT method for Highway Tunnel 163
Assuming that the magnetic field is the input signal and the electric field is the output signal, the correlation between the 164 magnetic field and the electric field signal is shown in the figure below (Fig. 4). Using the electric and magnetic signals from 165 the surface observations to analyze the signals in the relevant domains, the mathematic model of the magnetotelluric method 166 for the geological body of the proposed Mupi tunnel can be obtained.
Introduce the definition of the plane electromagnetic wave impedance and then it can be obtained (the detailed process can 183 be seen in Reference 7-12 ): 184 The above formula shows that the electrical resistivity of the tunnel can be calculated by measuring the orthogonal 186 horizontal component of the electromagnetic field on the surface. 187 H γ , taking the practical unit system, the above formula 188 can be written as: 189 The above formula 3-4 characterizes the relationship between the surface electromagnetic measurement value of the 191 highway tunnel under the condition of uniform isotropic medium and the apparent resistivity of the tunnel site. It is a 192 comprehensive feature of the surrounding rock of the tunnel, including the lithology, structure and groundwater of the 193 surrounding rock and other information of the tunnel. 194

Data acquisition and quality assurance 195
Before the formal work started, the regional geological data was collected, and the electrical parameters of the work area 196 were measured. The analysis shows that the apparent resistivity of the gravel soil s ρ is 50-600Ω.m, and the resistivity of the 197 complete schist and diorite is s ρ >3100 Ω.m. Have the prerequisites for highway tunnel survey.

198
This work uses the V8 multi-function electrical method produced by Phoenix, Canada, and collects and arranges two 199 orthogonal electromagnetic field information on the ground at a certain distance (this exploration is mainly 30m), that is, point 200 by point on the survey line. Measure the four-component Ex, Hy or Ey, Hx parameters of the electromagnetic field. This work 201 collects natural electromagnetic fields with a frequency between 10-10000 Hz, and the average acquisition time for each 202 measuring point is about 15 minutes, and the exploration depth is controlled within 700m. Due to insufficient collection time, 203 some data in the low frequency range is messy, and the overall goal is to balance efficiency and quality. 204 The V8 magnetotelluric data is preprocessed by the SSMT 2000 software provided by Canadian Phoenix Company. The 205 time series signal is fast Fourier transformed to obtain the magnetotelluric field self-and cross-power spectrum. The software 206 uses robust processing technology 20 to estimate the tensor impedance. 207 Data acquisition is strictly implemented in accordance with the relevant regulations of highway geophysical prospecting to 208 ensure the reliability of the original data, the tunnel construction also verified the reliability of the data. The typical data curve 209 is shown in Fig. 5.  In magnetotelluric field data collection, in addition to visual observation and recording of raw data such as time domain, 225 frequency domain, and power spectrum domain, some instruments can quickly obtain a 1-dimensional inversion map of a 226 single measurement point. If the instrument does not have this function, you can After the acquisition is completed, the data 227 is imported and processed by the computer, and the commonly used one-dimensional BOSTIC inversion, OCCAM inversion 14 , 228 etc. Fig. 6 shows a one-dimensional inversion diagram of the Mupi Tunnel ZK124+670 measuring point, which can be seen 229 directly. If it is assumed to be a three-layer geoelectric model, this data curve is a typical high-low-high H-shaped.

2-D inversion 240
The investigation's internal work use the magnetotelluric processing and interpretation system ( Other inversion methods are also similar processes. After selecting a suitable inversion model, the apparent resistivity 248 contour profile of the inversion model can be drawn, as shown in Fig. 7. The same process can also be used to obtain the 249 phase contour profile. After selecting a suitable inversion model and drawing the apparent resistivity contour profile of the inversion model, on 285 this basis, refer to the 1:200,000 regional geological data (Pingwu sheet) and the results of the previous geological survey to 286 complete the preliminary Mupi tunnel, the electrical properties profile is shown in Fig. 8a. 287 After discussing with geologists to determine the intrusion method of the magmatic rock intrusion 27 , it is clear that the 288 emplacement mechanism is mainly the passive emplacement rock wall expansion method, and then the electrical characteristic 289 profile is modified. And its electrical characteristics model geological interpretation, see Fig. 8b. 290 1. The ZK124+670～ZK125+030 section of the tunnel, the apparent resistivity value is ρ=1300～3100Ω.m, combined 291 with the geological data, it is inferred to be the electrical performance of the metamorphic rock mass. The rock mass in this 292 section is broken and weak with water. At ZK125+030, the resistivity gradient has the largest lateral change, which is 293 presumed to be the boundary between metamorphic rock and magmatic rock. Combined with the occurrence of metamorphic 294 rock exposed on the surface, the maximum gradient direction of the resistivity curve is inferred to be the contact direction of 295 magmatic rock and metamorphic rock. 2. The section of the tunnel body from ZK125+030 to the exit of the tunnel, the resistivity value ρ>3100Ω.m, combined 297 with geological data, it is inferred to be the electrical performance of the magmatic rock mass. The rock mass in this section 298 is relatively broken and weak. 299 3. At ZK125+030, the resistivity gradient has the largest lateral change, which is presumed to be the boundary between 300 metamorphic rock and magmatic rock. Combined with the occurrence of metamorphic rock exposed on the surface, the 301 direction of the maximum gradient of the resistivity curve cluster is inferred to be the contact relationship

Validation of electrical model 340
In order to complete the application research of magnetotelluric in the investigation of magmatic rock intrusions, with the 341 help of the later service of Mupi Tunnel, the actual excavation data of the tunnel was collected, and comparative analysis was 342 carried out, and the inversion and interpretation work was further iterated again. 343 The design (geological survey) and construction excavation have relatively few changes. The design (geological survey) 344 work is relatively conservative and the overall control is better. 345 The whole Mupi Tunnel has been completed in the early period of the Spring Festival in 2020, and it has been completed 346 and accepted. 347 Now give the lithological boundary identification verification of the tunnel construction excavation, and give the success 348 and shortcomings of the geophysical interpretation: 349 1. Identification of the boundary between metamorphic rock and magmatic rock (mupishaiziyan intrusion): 350 The   124+  340  370  400  430  460  490  520  550  580  610  640  670  700  730  760  790  820  850  880  910  940  970  125+  000  030  060  090  120  150  180  210  240  270  300  boundary between metamorphic rock and magmatic rock, which verified the correctness of the inversion method and electrical 354 interpretation. Excavation of the magmatic rock (mupishaiziyan intrusion) is shown in Fig. 9. However, the magmatic rock 355 intrusion interface has a large water gushing during tunnel construction, so the level of surrounding rock has not been changed, 356 and the overall design and geological survey are well controlled. 357 2. Insufficiency of geophysical exploration 358 Although the analysis and prediction of rock burst is not the focus of the original survey, but the problem of rock burst 359 during the construction of the Mupi tunnel is not mentioned in the original survey report, and it is still insufficient. The 360 analysis and prediction of rock burst 26   Based on the topographic and lithological characteristics of the Mupi Tunnel, this study abstracted a typical electrical model 391 for numerical simulation 8-12 , and generated a two-dimensional profile corresponding to the apparent resistivity and phase, as 392 shown in Fig. 10.

393
The model is close to the actual undulating topography and the intrusive distribution of metamorphic rock and magmatic 394 rock. As shown in Fig. 10a, the apparent resistivity of metamorphic rock schist is 1000Ω.m, the apparent resistivity of 395 magmatic rock diorite is 4500Ω.m, and the study area is 5km long. It has a height of 3 km and a total of 100 measuring points. 396 The measuring point distance is 50m. The calculated frequency points are 41, which are evenly distributed with equal 397 logarithmic intervals between 10-10K Hz. 398 Use self-compiled electromagnetic separation highway tunnel magnetotelluric exploration software (MT-T-Highway 399 V1.0.0) to complete the forward numerical simulation calculation. 400 Fig. 10b is the apparent resistivity value, the abscissa is the frequency (unit is Hz), the ordinate is the apparent resistivity 401 value of the mode (the unit is Ω.m); Fig. 10c is the impedance phase diagram, and the abscissa is the frequency (unit is Hz), 402 and the ordinate is the impedance phase value of the mode (unit is °). 403 The forward numerical simulation calculation has a certain reference and guidance for the actual processing of the 404 distribution characteristics of the magnetotelluric field caused by lithology, and then makes more reasonable judgments and 405 geophysical inference interpretations that are closer to objective geological bodies. 406 For example, taking the dip angle of the lithological interface as an example, the phase map of the TE mode is closer to 407 the geological model than the apparent resistivity map of the TE mode. Therefore, phase inversion can be added to the 408 subsequent data processing and inversion maps, and the lithology interface can be inferred after comprehensive analysis. (in Fig. 11). The observed and calculated responses fit will. 444 Fig. 11 shows representative depth sections. At depths of 1,2 and 3 km (Fig. 11a, b,   In the future tunnel inversion, 3-D inversion can be tried 28 , and the tunnel surrounding rock grade can be comprehensively 520 estimated through the multi-dimensional inversion results such as 1-D, 2-D, and 3-D combined with the geological results. 521 6. conclusion 522 1. Through the data processing, inversion and interpretation of the magnetotelluric survey of the Mupi Tunnel of Jiuzhaigou-523 Mianyang Highway on the G8513 Line, the electrical characteristics of each layer from the ground along the design line of 524 the tunnel to the depth of 50m within the design elevation of the tunnel are revealed. Discussed the identification of the contact 525 boundary between magmatic rock and metamorphic rock. 526 2. This study provides evidence of magnetotelluric sounding of the contact boundary between magmatic rock and 527 metamorphic rock of the Mupi Tunnel, which is revealed by the excavation of the tunnel, which verifies the correctness of 528 the geophysical inversion model, and verifies the geophysical data acquisition, data preprocessing, and inversion. The 529 correctness of the performance method. Among them, the selection criteria of geophysical inversion methods and parameters 530 are not unique. This preliminary explanation is obtained using two-dimensional Occam inversion this time, and it is not ruled 531 out that other methods cannot obtain this model. 532 3. This study combines the geological results to give the tunnel electrical geological interpretation section, and the actual 533 excavation of the tunnel verifies the correctness of the established electrical geological model. It can provide experience and 534 lessons for the design and construction of tunnels with similar stratum lithology in the future. 535 4. This study also discussed the use of the finite element method to complete the magnetotelluric forward calculation based 536 on the veneer tunnel model, and has an overall understanding of the electromagnetic field distribution characteristics under 537 similar geological conditions. 538 5. Although the current general engineering survey processing software is mostly two-dimensional inversion, this research 539 also discusses the inversion model that tries to use the three-dimensional finite difference method to explain the MT inversion 540 of highway tunnels under complex geological conditions in the future. A meaningful attempt. The results of this research can 541 continue to be carried out in depth, and do a good job of complementing scientific research, survey and tunnel construction, 542 in order to provide a more efficient and accurate means of identifying the lithology boundary of intrusive magmatic rocks in 543 metamorphic rocks for highway tunnel design. 544           employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.