Fig. 2.8 Isogram of vertical first derivative after the Bouguer anomaly in Meng Xing-Jihei area extends for 20km (unit: 10-8s-2).
2.3.2. 1 working method and quality evaluation of magnetotelluric sounding
(1) Work scale and technical parameters
The survey work is generally carried out according to the scale of 1:110,000, and1magnetotelluric survey points are arranged every 8km on average. In some non-key areas with relatively simple structures, the distance between points becomes larger, about 15km. In view of the fact that the electrical characteristics of the depth of 10km are mainly revealed in the transition zone between basin and mountain, the longest period of collecting medium and high frequency data in the field is about 300s, and the general collection and recording time is 2h, which is called shallow point. However, in order to better understand the deep structural characteristics in the area, in practical work, 1 deep points with recording time longer than 10h are generally arranged every two or three shallow points.
Fig. 2.9 Isogram of vertical first derivative after the Bouguer anomaly in Meng Xing-Jihei area extends for 50km (unit: 10-8s-2).
(2) Instrument and performance
In the field work, V5-2000 magnetotelluric sounder produced by Phoenix Geophysical Company of Canada is used. Before field work, field experiments were carried out on the instruments 1490 and 1545, including instrument calibration and field measurement. Figure 2- 10a, b and c are the calibration results of instrument host 1545 and magnetic probe 16 14 and 16 15, which fully meet the requirements of field work.
(3) Consistency check
In the field, it is particularly important to use two instruments to work at different measuring points on the same survey line at the same time to ensure the consistency of the instruments. Therefore, two instruments are used to compare and measure at four measuring points at different times. The results show that the results measured by different instruments at the same measuring point are basically the same. Fig. 2. 1 1a is the result measured by 1545 instrument at the same point, and fig. 2. 1 1b is the result measured by 1490 instrument at B338 point.
Figure 2. 12 shows the consistency test of two instruments at the same measuring point. In the figure, the lower river line is the measured value of instrument 1490, and the solid line is the measured value of instrument 1545. Figure 2. 13 shows the consistency test of two instruments with different polarization modes. After calculation, the mean square relative error of TE mode apparent resistivity is 4.95%, and that of TM mode apparent resistivity is 3.70%, both of which are less than 5% required by the specification.
Figure 2. 10 instrument calibration curve
Figure 2. 1 1 instrument measurement results
Fig. 2. Consistency curves of121490 and 1545 test points.
Figure 2. Instrument consistency of13 and 1490# under different polarization modes.
(4) Instrument inspection after field measurement.
After field measurement, calibrate the instrument again to check its stability. The calibration results show that the calibration results before and after field work are completely consistent, and the calibration results of the two instruments are also consistent. Figure 2. 14a, b and c show the calibration curves of the instrument host 1490 and two magnetic probes16161.
(5) Handling of mountain interference
The mountainous area in the working area is thick but soft due to fallen leaves and residual soil, and it is difficult to bury probes due to forest coverage. At the same time, due to the shaking of trees, the interference is very serious According to the operation requirements of the instrument, the instrument cannot be placed under a tree to avoid interference caused by the vibration of the wind shaking the trees. This interference mainly affects the sounding data in the middle and low frequencies (Figure 2. 15). The reason for this conclusion is that the estimated apparent resistivity data is less than the real apparent resistivity due to the noise of the magnetic field, and the high-frequency and low-frequency data of the apparent resistivity curve in the figure are seriously reduced, which is its concrete performance. In the process of work, the probe is buried deeply, but the effect is not ideal. Therefore, in order to overcome the interference of tree vibration, advanced technologies such as remote reference and cross reference are adopted, and the data of the current point is calculated by using the magnetic field reference with little interference from the reference point.
Figure 2. 14 instrument calibration curve
Fig. 2. 15 magnetotelluric sounding data of disturbed measuring points in forest area
(6) Quality evaluation of observation data
According to Technical Specification for Magnetotelluric Sounding (DZ/T0 172- 1997), the quality of all observation data is evaluated. Quality evaluation is mainly based on apparent resistivity and reference phase. See Table 2.4 for quality evaluation of survey lines crossing boundary faults of major basins.
Table 2.4 Quality Evaluation of Main Profile Survey Lines
Interpretation results of MT profile processing across main fault zone in 2.3.2.2
(1) MT sounding profile of zhalantun-Lin Dian.
This section starts from Zhalantun, Inner Mongolia in the west and ends in Lin Dian, Heilongjiang in the east, with a total length of about 260km (Figure 2. 16). The crossing area is the transition area between Daxing 'anling and Songliao Basin, where Nenjiang-Kailu fault passes in geology and the maximum gradient abrupt zone of Daxing 'anling gravity gradient zone passes in geophysics. Gravity and MT profile show that the basement of Songliao basin, which is roughly bounded by Qiqihar, is 2 ~ 5 km deep, and it has obvious "sandwich" electrical structure with two low resistances in the upper and lower parts and one high resistance in the middle. The western part is not the traditional western slope area of Songliao Basin, but there is a deep fault depression. The electrical structure at the depth of 10km is completely consistent with the low-resistivity layer in the lower part of the eastern Songliao Basin, indicating that the eastern and western areas of Qiqihar have similar basement characteristics. According to the regional geological data, nearly 100 sets of basic rocks and ultrabasic rocks are exposed in the northern part of the western margin of Songliao Basin. Recently, drilling near Baicheng in the south of Jilin Oilfield (Tao 5 well) found that strong deformed rocks, including rootless ultrabasic rocks and marble blocks, were found in the interval of 530 ~ 550 meters. The basic-ultrabasic rock belt is consistent with the Nenjiang-Kailu fault zone and the beaded strong magnetic anomaly belt on the eastern edge of Daxing 'anling, and it is also the abrupt change zone of lithospheric thickness and Moho surface on the east and west sides of Meng Xing-Jihei area. Especially in recent years, a Carboniferous magmatic arc with northeast distribution has been established in Daxinganling area, and its genetic type has the characteristics of continuous evolution from subduction to collision. Therefore, it can be basically determined that there is a concealed paleosubduction zone under the coverage area of the western margin of Songliao Basin, and the low resistivity anomaly inclined to the west may be an accretionary complex in the paleosubduction zone. As a structural weak zone, this ancient subduction zone not only obviously restricted the formation and evolution of Songliao basin, but also obviously controlled the formation and evolution of lithospheric structure in this area.
Fig. 2. 16 Interpretation map of electrical profile processing in the western margin of Songliao basin (see fig. 2. 1XB5 for profile location).
(2) MT sounding profile of Danqinghe-Daotai Bridge
This section is located in the east of the study area, with a total length of 64km. This profile passes through Fangzheng basin of Jiamusi-Yitong fault zone. The results of magnetotelluric sounding (Figure 2. 17) show that Fangzheng Basin has a "sandwich" electrical structure similar to Songliao Basin. Both sides of the upper low resistivity layer are controlled by normal faults, and the lower low resistivity layer is controlled by thrust faults. This feature is basically consistent with the whole Jiamusi-Yitong and Yilan-Lan Shu fault zones. The drilling results in Daqing Oilfield have confirmed that the upper low resistivity layer is Paleogene and the lower high resistivity interlayer is Lower Cretaceous, but the geological significance of the lower low resistivity layer is still unclear. According to the analysis of electrical structure, Jiamusi-Yilan fault developed thrust structure before Paleogene fault depression, and Paleogene fault depression developed from early thrust structure.
Fig. 2. 17 electrical section processing and interpretation diagram of Danqing River-Daotai Bridge (see fig. 2. 1DB4 for location).
(3) Baoqing-Dangbizhen magnetotelluric sounding profile.
This section is located in the east of Heilongjiang Province, passing through the northern edge of Xingkai Block from south to north, passing through Dunhua-Mishan Fault and the eastern edge of Boli Basin, and reaching Baoqing in the eastern edge of Jiamusi Block, with a total length of 130km (Figure 2. 18). The results of magnetotelluric sounding show that the whole Xingkai block has high resistivity characteristics, and the Dunhua-Mishan fault zone has a "sandwich" electrical structure similar to the Jiamusi-Yilan fault zone. Between the north of Dunhua-Mishan fault and Baoqing, Mesozoic and Upper Paleozoic are exposed on the surface, partly covered by Cenozoic basalt, and there is deep low resistivity anomaly at the junction of Paleozoic and Mesozoic in southern Baoqing. The electrical structural characteristics of the profile show that there is no stable high-resistivity block in the area north of Dunmi fault, and the accretionary complex belt between Wandashan terrane and Jiamusi terrane may pass along the profile.
Fig. 2. 18 Baoqing-Dang wall dielectric profile processing interpretation diagram (see fig. 2. 1DB2 for location).
The above two profiles reveal that Jiamusi-Yitong fault zone and Dunhua-Mishan fault zone are both composed of two main faults, both of which have a "sandwich" electrical structure with double low-resistivity layers in the vertical direction. The burial depth and thickness of the upper low-resistivity layer, the middle relatively high-resistivity interlayer and the lower low-resistivity layer are basically the same, and the lower low-resistivity layer is controlled by two thrust faults, while the upper low-resistivity layer is controlled by two opposite normal faults. This feature shows that Jiamusi-Yitong fault and Dunhua-Mishan fault have undergone at least two stages of evolution. According to the drilling data, it is confirmed that the upper low-resistivity layer is Paleogene and the middle high-resistivity interlayer is Lower Cretaceous, indicating that the early thrust fault should be active in the late Early Cretaceous or later. Different from the early understanding, the electrical profile does not show that the upper Paleogene fault depression has the characteristics of east fault and west overload. These two faults extend eastward and are both cut by the central Schott-Alin fault in Russia. According to the data of G.L.Kirillova(2003, 2005), the central Xihaote-Alin fault is a sinistral strike-slip fault, and the strike-slip structure occurred in the Late Cretaceous. This further proves that the thrust and strike-slip time of Jiamusi-Yilan fault and Dunhua-Mishan fault occurred before the late Cretaceous. Near Jixi basin on the north side of Dunhua-Mishan fault, the high-grade metamorphic rocks of Mashan Group in basement thrust northwest on the coal-bearing strata of Muling Formation in Early Cretaceous, which fully shows that there were strong sinistral strike-slip and thrust nappe events in this area from the end of Early Cretaceous to the beginning of Late Cretaceous, which became the basis of Paleogene extension deformation.
2.3.2.3 Huanan-Raohe magnetotelluric sounding profile.
Jiamusi Block and Wandashan Block are two important tectonic units in Meng Xing-Jihei Lithospheric Block, which play an important role in the structure and evolution of the lithosphere. In particular, the Wandashan terrane, as a part of the huge Xihaote-Alin Mesozoic accretion terrane, is of great significance for understanding the evolution of the ancient Pacific Ocean and the characteristics of the current lithospheric structure in this area. It can be said that this area is a symbolic and representative area to understand the lithospheric structure and dynamic evolution of the continental margin of Northeast Asia. Over the years, although there have been many studies on the nature and relationship between Jiamusi block and Wandashan block (Zhang Yixia et al.,1998; Wook Kim et al.,1994; Fang et al., 2002; Mao Ye et al.,1994; Zhang Xingzhou et al., 199 1,1992; Liu Jinglan et al.,1988; Liu Xianwen et al., 1994), but these studies are mostly based on surface geological data, lacking deep geophysical research and basis. Because the Manzhouli-Suifenhe geoscience section is located in the south, Jiamusi block and Wandashan terrane have not been exposed. Therefore, the study of these two tectonic units and their relationship has been in the stage of surface geological research for a long time, and there is a lack of understanding of their deep structural characteristics. In order to solve this problem, we carried out magnetotelluric profile detection in this area in 2002.
Location and tectonic background of (1)MT section
MT profile starts from Huanan (east longitude 130 38' 58 ",north latitude 46 1 1' 1") in the middle of Jiamusi massif in the west and ends near Wulin Cave (east longitude 133 39' 8) 50 kilometers south of Raohe County bordering China and Russia in the east.
(2)MT field data collection and processing
On-site measurement adopts GDP32-Ⅱ multifunctional electric measuring instrument produced by Zongge Company of the United States, which has high degree of automation, full functions and can be processed in real time. The instrument mainly includes: two-component electric field receiver (with unpolarized electrode); Two-component magnetic field receiver; Electric field preamplifier; Computer system and power supply system for data acquisition and real-time processing. In addition, the instrument has a perfect self-checking system, which effectively ensures the quality of field data acquisition. The data acquisition system adopts cascade sampling method to sample, and the sixth and eighth harmonics are averaged by Fourier transform to obtain the amplitude and phase of electric field and magnetic field. The frequency range of GDP32-Ⅱ MT acquisition program is from 0.0007(6/8 192) to 8 192Hz, which is divided into four groups and displayed with 6th and 8th harmonics. Only three groups of low frequency, intermediate frequency and high frequency are used in the work. See Table 2.5 for three sets of frequency settings.
Low-frequency data are continuously sampled, filtered, sampled and Fourier transformed in real time. For the three frequency bands in the table, the signal group (or signal string; Burst) mode, and data processing is performed between these signal groups. The acceptance and rejection of data are determined according to the setting of correlation and dispersion limits. GDP32-Ⅱ instrument has FFT and robust processing function, which ensures the timely processing of field measurement data. SHRED and NSAVG processing programs provided by Zongge company are used for secondary processing in the laboratory, and then various interpreted apparent resistivity and other parameters are obtained through static correction. Fig. 2. 19 is the apparent resistivity curve representing three sections (Jiamusi block high resistivity area, Baoqing east low resistivity area and east end high resistivity area).
Table 2.5 GDP32-Ⅱ Sampling Frequency Settings
Figure 2. 19 Measured MT curve of Huanan-Raohe River in different sections
(3) Electrical structural characteristics of Huanan-Raohe MT section.
On the basis of processing the measured MT data and determining the apparent resistivity parameter curve model, one-dimensional conventional inversion and two-dimensional smooth inversion methods are used for one-dimensional and two-dimensional inversion interpretation. Figure 2.20 shows the one-dimensional inversion results, which are given in the form of histogram. Figure 2.2 1 is the result of two-dimensional inversion and is given in the form of cross section.
Figure 2.20 One-dimensional inversion model of Huanan-Raohe MT profile
Fig. 2.2 1 2D mountain inversion profile of Huanan-Raohe (see fig. 2- 1 profile ⑥ for location).
The Huanan-Raohe magnetotelluric sounding profile describes the detailed structure of the crust and asthenosphere between Jiamusi and Raohe. One-dimensional inversion results give the longitudinal electrical structure relationship. In the west of Baoqing, there is a continuous high conductivity layer with a depth of more than ten kilometers in the crust, and the top boundary of asthenosphere is between 90 ~ 100 kilometers. In the eastern part of Baoqing, there is a highly conductive layer in the crust that deepens from east to west, with a depth of 20 ~ 30 kilometers, which may be a structural feature of the early subduction of the oceanic crust to the mainland, and the top boundary of the asthenosphere is 75 kilometers deep. The results of two-dimensional inversion show that the electrical structure of the profile is obviously zonal in the horizontal direction, and it is divided into two distinct electrical structure areas, east and west, with the position of 07 measuring point east of Baoqing as the boundary. The whole area to the west of Baoqing is characterized by high resistivity, which reflects the compositional characteristics of Jiamusi massif dominated by metamorphic crystalline rock series. The main body in eastern Baoqing is characterized by low resistivity, which reflects the composition characteristics of Mesozoic accretionary complex. Based on this, it can be accurately determined that the boundary between Jiamusi block and Wandashan terrane is in this position, but only in the shallow part of the crust. With the increase of depth, this position deviates and inclines to the west, indicating that the boundary between shallow and deep structural units is not the same. The vertical electrical structures on both sides of the boundary further confirm this point. Fig. 2.2 1 shows that although Jiamusi block shows stable high-resistivity structural characteristics as a whole, there is a stable low-resistivity layer at the depth of 9 ~ 17 km, which shows that Jiamusi block is not a continuous high-resistivity block from the surface to the deep, that is, the horizontal high-speed body over 9km is rootless. Similarly, the Wandashan terrane also shows obvious differences in electrical structure in the shallow and deep parts. The outstanding performance is that there is a horizontal low resistance layer between 6 and 9 km, a layered high resistance layer above the low resistance layer, and the low resistance layer is the main body, including several high resistance blocks. The low resistivity anomalies between high resistivity blocks are almost vertical, extending from the near surface to the bottom of the lithosphere. Generally speaking, the low resistivity anomaly shows that the thickness of lithosphere is about 60 ~ 65km, which is consistent with the asthenosphere uplift characteristics along Dunhua-Mishan fault in the south about 60km (Wook Kim et al., 1994) and in the north about 60km. This seems to indicate that there is not only a boundary between Jiamusi massif and Wandashan terrane, but also an important boundary structural belt on the lithospheric scale in the eastern part of Jiamusi massif, south along Dunhua-Mishan fault and to the eastern edge of Breya massif in Russia. It should be pointed out that the resistivity of several measuring points near Baoqing is obviously lower than that in the western and eastern sections, and the inversion results of the measured data in the maximum observation period of the instrument show that the maximum depth of the measured data can not reach the lithospheric bottom boundary. This may be related to low resistivity, strong electromagnetic field absorption and shallow electromagnetic field penetration depth.
(4) Geological interpretation of Huanan-Raohe magnetotelluric sounding results and its tectonic significance.
In the previous study of deep geological structure, the scope of Jiamusi block and the location and nature of its eastern boundary were analyzed, but the specific location was only inferred from some superficial phenomena, lacking geophysical evidence of deep structure. The composition and structure of the so-called Wandashan ophiolite have not been studied. Huanan-Raohe magnetotelluric sounding profile has a clear understanding of the above problems. Judging from the electrical structure revealed by the 1-D and 2-D inversion results, the whole section is roughly divided into two parts, east and west, with the basin coverage area east of Baoqing as the boundary. There are obvious differences in electrical structure between them, which indicates that there is a fault structure on the lithosphere scale. The shallow high-resistivity layer in Jiamusi massif has no roots, and the low-resistivity layer with a thickness of 10km below it may be a detached structure in the shell, but it is not excluded that it may be a buried sedimentary rock layer. Wandashan terrane is a horizontal high-resistivity layer above a horizontal low-resistivity layer, and there are two high-resistivity blocks under it, separated by a nearly vertical low-resistivity zone. This structure shows that the Wandashan ophiolite is a thrust sheet structure with a thickness of 5 ~ 7 km. The high-resistivity body sandwiched in the low-resistivity body under the rock sheet may be an accretion block related to subduction or an early cracking block on the eastern edge of Jiamusi block. The vertical low resistance zone from the near surface to the asthenosphere may be strike-slip structure in the late Mesozoic and become the channel of basalt eruption in the Cenozoic.
Re-processing and interpretation of magnetotelluric sounding data in 2.3.2.4
According to the research task of the project, the existing deep sounding magnetotelluric sounding data in this area (Table 2.6) are collected systematically, and all the collected profile data are inverted by using the internationally advanced two-dimensional continuous automatic inversion technology. One-dimensional inversion is carried out on some sections that do not give the inversion depth of lithospheric bottom boundary, and the lithospheric bottom boundary is inferred and determined.
Table 2.6 Statistical Table of Past Magnetotelluric Data in Meng Xing-Jihei Area
Basic principle of inversion of two-dimensional smoothing model for (1)MT data
Smoothing model inversion is an effective and robust inversion method to convert magnetotelluric sounding data into resistivity-depth model. k， 1995； Achia. , 199 1; StaffaP。 Me, sir. K, 199 1), for simple one-dimensional inversion, the electrical parameters of layered earth model-layer resistivity and thickness-are usually determined by the apparent resistivity and phase observed at each observation point, so that the observed data can be converted into a resistivity-depth function. In smooth model inversion, the number of layers of geoelectric model is determined by the number of observation frequency points. The thickness of each layer is determined by the penetration depth of electromagnetic wave with corresponding frequency, which remains unchanged in the inversion process, while the initial value of resistivity of each layer is determined by apparent resistivity. In the iterative inversion process, the layer resistivity is continuously corrected until the calculated magnetotelluric response is as close as possible to the observed data, and the resistivity model maintains a certain smoothness requirement. The smoothness of the inversion model requires little change in interlayer resistivity, which leads to a smooth change in the vertical direction of the model.
The lateral change of resistivity can be realized by two-dimensional inversion. In order to carry out two-dimensional inversion, it is necessary to calculate the apparent resistivity and impedance phase of a given section. In this paper, two-dimensional finite element method is used for forward simulation. For undulating terrain, the finite element mesh is divided along the terrain.
When doing two-dimensional inversion along the survey line, the number of transverse grids of the inversion model is determined by the number of observation points, and there is a column of grids under each observation point, and its thickness is determined by the one-dimensional observation frequency. In this way, the resistivity grid of two-dimensional inversion geodetic model can be obtained from the number of measuring points and the observation frequency of each measuring point. A column of resistivity distribution under each measuring point is consistent with the distribution of the electrical layer of each measuring point, and the resistivity value is located at the midpoint of the electrical layer. In two-dimensional inversion, the initial model resistivity (background resistivity) can be obtained from the inversion results of one-dimensional smooth model or the observed apparent resistivity by some averaging method. If there is prior information such as logging data, these special information can be added to the background model to reflect the electrical characteristics of geological structure. In this way, the resistivity distribution of the grid is equivalent to the resistivity model profile, and for a complete survey line, the corresponding resistivity distribution pseudo profile can be made by the resistivity grid.
In the inversion process, the grid resistivity of the model profile is iteratively adjusted until the apparent resistivity and impedance phase calculated by the model are as close to the observed data as possible, and the model meets certain constraints, including the background model constraint that limits the difference between the inversion model resistivity and the background resistivity containing known prior geological information, and the model smoothing constraint that limits the spatial variation of the model resistivity. Therefore, inversion of apparent resistivity and impedance phase into a geoelectric model with smooth resistivity change is an important means to effectively indicate the information contained in magnetotelluric sounding data. The smoothing model inversion method does not need the prior information of model parameters, and the model limitation can make the inversion model contain as much known geological information as possible.
To sum up, the automatic inversion method of two-dimensional smooth model has the following advantages:
1) Select one or both of TM mode and TE mode for inversion, and make full use of observation data to obtain more information about underground electrical distribution;
2) Using the observed apparent resistivity and impedance phase to carry out two-dimensional inversion simulation at the same time can make full use of the geological information contained in the observed data and reduce the non-uniqueness of inversion, and the inversion result is more reliable than only using apparent resistivity inversion;
3) In the two-dimensional finite element forward simulation, the influence of terrain fluctuation is considered, which avoids the static correction of conventional magnetotelluric sounding and makes the calculation result closer to the actual observation;
4) The whole inversion process is fully automated, and human intervention is not needed except for constraining the initial model, so the processing result is more objective.
(2) Re-processing of magnetotelluric sounding data in Songnan-Liaobei area.
1) 2D inversion of Zhaluteqi-Changtu profile. This section starts from Zhalute Banner in Inner Mongolia and ends in Changtu, Liaoning, with a total length of 330km and a measuring point of 69 MT The results of two-dimensional inversion show that the basin area is smaller and the depth is shallower, and the basin edge characteristics are obvious. The inversion result is shown in Figure 2.22.
2) 2D and 1D inversion of Keyouzhongqi-Liaoyuan profile. This section is located in the south of Songliao Basin, starting from Keyou Zhongqi, Inner Mongolia and ending in Liaoyuan City, Jilin Province, with a total length of 330km and a measuring point of 78 MT The two-dimensional inversion results clearly show the regional electrical pattern, the basin scope is obviously widened and the depth is obviously increased. See Figure 2.23 for two-dimensional inversion results and Figure 2.24 for one-dimensional inversion results.
Fig. 2.22 MT 2D inversion profile of zharute banner-Changtu.
Figure 2.23 2D Inversion Profile of Keyouzhongqi-Liaoyuanshan
Fig. 2.24 One-dimensional magnetotelluric inversion of Youzhongqi-Liaoyuan in Keke.
3) 2D inversion and 1D inversion of Wafangdian-Yingchengzi profile. This section is located in the south of the middle of Songliao basin, with a total length of 330km and a measuring point of 78 MT The two-dimensional inversion results are similar to "Keyouzhongqi-Liaoyuan Profile", but the basin is larger and deeper. Some small basins and depressions on the profile are also clearly reflected, and the inversion results are shown in Figure 2.25. According to the one-dimensional and two-dimensional inversion results of MT and the discontinuity of electric layer, it is judged that there are many lithospheric faults and basin-controlling faults, such as Xilamulun fault, Yilan-Yitong fault, Changchun-Siping fault and Nenjiang-Kailu fault. Except Songliao basin, there are intermittent high conductivity layers in the crust of 8 ~ 48km in this area, and the depth of asthenosphere is between 58 ~ 126km. The general feature is that the asthenosphere uplift area corresponds to the Mesozoic-Cenozoic depression area. The asthenosphere changes greatly at deep faults, indicating that some lithospheric faults also correspond to the uplift of the asthenosphere (Figures 2.24 and 2.26).
4) 2D inversion of Kezuohouqi-Gan 'an profile. This section is NNE, which is basically perpendicular to the first three directions. This profile starts from Kezuohouqi (Wengsi) and ends at Gan 'an in the middle of Songliao Basin. The total length of the profile is 290km, and the measuring point is 65 MT The two-dimensional inversion results clearly reflect that the basin margin gradually deepens northward. See Figure 2.27 for the inversion results.
(3) 2D and 1D inversion of magnetotelluric sounding data of Manzhouli-Suifenhe geoscience section.
The comprehensive research results of Manzhouli-Suifenhe geoscience section have been introduced earlier. In this paper, the advanced inversion software is used to carry out two-dimensional re-inversion on the data of 30 MT measuring points in geoscience transect research, and all the measuring points in geoscience transect domain 1300km length range are completed at one time. The inversion results clearly describe the electrical structural characteristics of the whole section. The characteristics of one-dimensional interpretation model and two-dimensional inversion profile are shown in Figure 2.28 and Figure 2.29 respectively. The main electrical structural features are summarized as follows:
1) according to the electrical differences, the profile domain is divided into seven electrical blocks, and the regional electrical changes of the two-dimensional inversion results of the whole profile are consistent with the blocks divided by one-dimensional interpretation and basically consistent with the geological structure zoning.
2) In the profile domain, except Songliao Basin, where the overall resistivity is low, it is impossible to determine whether there is a high conductivity layer in the crust, there are irregular high conductivity layers in the crust in other areas, with a depth of 20 ~ 38km and a thickness of 2 ~ 3km, and the resistivity is generally10 ~ 50 Ω m.. There are two highly conductive layers in the crust east of Dunhua-Mishan fault zone.
3) Songliao basin has a low resistivity layer with a thickness of at least 40km, and the resistivity is 3 ~ 8 Ω m. ..
4) The depth of the mantle high conductivity layer in the profile area varies between 60 ~ 1 18 km, which is basically mirror-image symmetry with the topographic relief. The thickness of the lithosphere near Manzhouli at the western end of this section is118 km; The thickness of the lithosphere in Hailaer basin, Bahrain basin and Songliao basin is about 60 kilometers; At the eastern end of this section, it is about 90 kilometers.
(4) 2D inversion of magnetotelluric sounding data in Tianchi volcanic area of Changbai Mountain.
According to the current definition of active volcano, Tianchi volcano is a volcano with potential eruption danger. 1From July to August, 1995, the Seismological Bureau of China conducted MT detection at Tianchi volcano 15 in Changbai Mountain. Among them, the two-dimensional inversion results in the northeast show that there is a magma sac system at a depth of 20 ~ 25 km. Magmatic cysts may have roots, and the downward extension depth is worthy of further study (Liu Ruoxin et al., 1999). The research results of Tang Ji et al. (1997) also show that there are geological bodies with low resistivity at the depth of about 12km in Tianchi of Changbai Mountain and its eastern area, and the resistivity is several tens of ohm meters, which may be magma sacs in the crust (Tang Ji et al., 200 1). The one-dimensional inversion results also show that the depth of asthenosphere near the crater is obviously shallow. On a section several kilometers long, the depth of asthenosphere changes greatly, forming a sudden change of asthenosphere, which is a feature of a * * * volcanic area. Liu Ruoxin and others (1992, 1995, 1996) have pointed out that Tianchi volcano is a volcano with potential eruption danger. Whether there is an active magma system deep in the dormant active volcano is an important condition for estimating the future eruption risk (Liu Ruoxin et al., 1999). In this study, the two-dimensional inversion results of MT profiles in different directions of Tianchi volcano were collected. Figure 2.30 is the inversion result in the north-south direction (Tang Ji et al., 1997), and figure 2.3 1 is the inversion result in the north-north direction (Liu Ruoxin et al., 1995). From the inversion results in two different directions, it can be seen that there are low-resistivity bodies at a depth of about 20 kilometers below the n5 measuring point in the NE profile, and there are also low-resistivity bodies at a corresponding depth below the N07-N08 measuring point in the N-S profile, which is a reliable basis for the existence of magma sacs in deep volcanic areas.
Figure 2.25 2D Inversion Profile of Wafangdian-Yingchengzi
Figure 2.26 One-dimensional inversion of Wafangdian-Yingchengzishan
Fig. 2.27 MT 2D inversion profile of zuohouqi in jiananke.
Figure 2.28 One-dimensional MT interpretation model of Manzhouli-Suifenhe geoscience section
Fig. 2.29 Two-dimensional Inversion Section of Mansui Geoscience Section MT
Figure 2.30 North-South MT 2D Inversion Profile of Tianchi Lake in Changbai Mountain
Fig. 2.3 1 MT 2D inversion profile of northeast Tianchi in Changbai mountain
Fig. 2.32 NW-trending MT 2D inversion profile of Jingpohu volcanic area.
Fig. 2.33 NE-trending MT 2D inversion profile of Jingbohu volcanic area
Fig. 2.34 One-dimensional MT inversion results of line 2 in the northwest of Jingbo Lake.
Fig. 2.35 One-dimensional inversion results of MT in the northeast of Jingbo Lake
(5) 2D inversion of magnetotelluric data in Jingpohu volcanic area.
Jingbo Lake is located in Ning 'an County, Heilongjiang Province, on the northwest side of Dunhua-Mishan fault zone. There are 13 craters in the forest about 50 kilometers northwest of Jingbo Lake, which are named Holocene volcanic groups. In order to understand the deep structure of volcanic area and whether there are magma sacs in the deep, it is of great significance to study the prediction of volcanic eruption. In 2000, College of Earth Exploration Science and Technology of Jilin University conducted magnetotelluric sounding at 30 points in this area (Zhu et al., 200 1), and carried out two long sounding profiles in the northwest and northeast (Zhu et al., 200 1). Figures 2.32 and 2.33 show the two-dimensional inversion results of NW and NE respectively. The results of two-dimensional inversion show that there is indeed a magma sac in the deep part of the volcanic area (Zhu et al., 200 1), especially in the northwest profile, there is a low-resistivity body connecting from the upper part to the deep part near the crater, and the low-resistivity body has the characteristics of narrow upper part and wide lower part; The one-dimensional inversion results of some measuring points also show that the upper interface depth of asthenosphere in Jingbohu volcanic area is 70 ~ 100 km (Figure 2.34), and the asthenosphere depth in the crater and on both sides of the crater is obviously different, especially the asthenosphere becomes shallower towards the crater (Figure 2.35).
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