(A) data preprocessing
After processing (Figure 4-8), the data collected in the field mainly include: apparent resistivity, phase, coherence, principal axis orientation, dip angle and other necessary parameters that can be used to explain and provide certain information for interpreters. However, due to observation errors, static effects caused by additional charges generated by surface inhomogeneity, rugged terrain and other interference factors, the processed magnetotelluric sounding data have abnormal jumps, curve shifts and curve distortions at individual frequency points and certain frequency bands. Therefore, preprocessing measures such as smoothing curves to eliminate serious sudden frequency hopping points, static correction and terrain correction to suppress static effect, random noise and terrain influence are adopted.
Figure 4-8 Processing Flowchart
(2) Recognition of polarization mode
According to the information of impedance trend and dip trend, combined with the corresponding data of adjacent measuring points and referring to the regional structural characteristics, the polarization mode can be distinguished. TE indicates that the electric field polarization direction is parallel to the structural strike, and TM indicates that the electric field polarization direction is perpendicular to the structural strike.
(3) One-dimensional and two-dimensional inversion
On the basis of qualitative and semi-quantitative interpretation, one-dimensional inversion is carried out by using measured magnetotelluric sounding data (including apparent resistivity and phase curve, etc.). ) into one-dimensional geoelectric profile parameters to provide the geoelectric model of the working area. If the geoelectric profile in the work area is a one-dimensional or nearly one-dimensional two-dimensional model, the one-dimensional inversion result can be taken as the final result. In the case of two-dimensional (or nearly two-dimensional, three-dimensional), two-dimensional inversion is needed. Compared with one-dimensional inversion, two-dimensional inversion can better solve the problems of electromagnetic field diffraction (returning diffracted waves, correctly reflecting electrical and structural characteristics) and three-dimensional (using combined average method) in low resistivity layers (Figure 4-9).
Figure 4-9 Profile of Continuous Medium Inversion of Electric Method 700 Line in Hefei Basin
(d) Integrated information modeling
Based on magnetotelluric data, combined with seismic, gravity and magnetic, drilling and logging results, an electric-geological model is established.
(5) Interpretation of the results
1. Profile feature
Electrical characteristics of (1) north-south section (700-line section): reflected in the section, its electrical structure is divided into three blocks by Shucheng fault and Fei Zhong fault. The southern block is Dabie block, which is roughly divided into shallow high-resistivity layers (Dabie Group and Foziling Group metamorphic rocks) and deep low-resistivity layers (Paleozoic North China Platform).
The northern block belongs to the North China Platform, which is divided into shallow low-resistivity caprocks (including Cenozoic and Middle Jurassic with extremely low resistivity) and deep high-resistivity basement (Huoqiu Group).
From Shucheng fault to Fei Zhong fault, the overall electrical property is low, and there is a relatively high-resistivity body with a local area greater than 300 Ω m, which may represent that Fang Hushan is a Feilai peak, and its electrical structure is relatively complex, but its basement is mainly low-resistivity.
(2) Electrical characteristics of the east-west profile: The 365-line profile shows that the east-west electrical characteristics of Hefei Basin have not changed much, and the main change zone is in the basin boundary faults (Tanlu fault and Wuji fault). The east-west electrical structure in the north of the basin is mainly divided into two parts, and the Huoqiu fault nose belt in the west is divided into shallow low-resistivity caprock and deep high-resistivity metamorphic basement of Huoqiu Group. The Cretaceous resistivity in Dongqiao syncline area is very low and there are many thick layers, the Jurassic resistivity is medium, and the basement is lower Paleozoic resistivity (Figure 4- 10).
Figure 4- 10 Inversion Profile of Electrical 2D Continuous Media in Hefei Basin
Line 340 reflects that the east-west electrical property in the south of the basin has little change, and the basement is mainly low resistivity (the low resistivity layer in the Upper Paleozoic is very obvious).
2. Fracture analysis
On the electrical profile, faults generally show as follows: sudden change of curve type, change of curve mode, obvious difference of electrical layer series or obvious dislocation of buried depth of electrical layer, obvious decrease of resistivity caused by broken stratum and water filling, and dense, steep or dislocation distortion of isolines on the resistivity isoline profile.
(1) Tanlu fault: The Tanlu fault is reflected as a ne-trending isoline intensive zone on the electrical plane, and the Chihe-Xishanyi fault (commonly known as Tanlu fault) controls the eastern boundary of Hefei Basin. On the profile (line 340), the shallow part (late stage) of the Tanlu fault is a west-dipping normal fault, and the deep part (early stage) may be a compressive reverse fault. The electrical data show that its main activity time is after Cretaceous, and it should be dominated by compression in the early stage, and it may be an extensional normal fault with steep fault plane in the later stage.
(2) Fei Zhong fault: the plane of Fei Zhong fault is nearly east-west, and the profile (line 700) is mainly characterized by early reverse fault, late normal fault and southward dip. The electrical properties of the basement on the north and south sides of the fault are very different. The basement on the west side of the north side of the fault is Archean metamorphic rock series (Huoqiu Group) with high resistivity. The basement in the east is dominated by Lower Paleozoic with relatively high resistivity. In addition, there are low resistivity clastic rocks in the Upper Paleozoic in Dingyuan Depression, Wushanmiao and the eastern area. The basement on the south side of Fei Zhong fault is dominated by low resistivity strata (upper Paleozoic clastic strata).
(3) Feixi Fault: The electrical plan shows that Feixi Fault is a nearly east-west distribution electrical change zone. According to the electrical (stratigraphic) structure on both sides of the fault, it is characterized by multiple thrusts and normal faults, and its profile is inclined to the south.
Feixi fault is also the main electrical boundary of Hefei basin. The basement structure on the north and south sides of the fault is different. The south side of Carboniferous-Permian clastic rocks in Upper Paleozoic is dominated by low resistivity, and the north side is high resistivity.
(4) Shushan fault: The plane reflection of Shushan fault is complex, and its east-west distribution is similar to that of Feixi fault.
(5) Shucheng fault and Mozitan-Xiaotian fault: Shucheng fault (Jinzhai-West Tang Chi fault) and Mozitan-Xiaotian fault. Generally, it is distributed in the east-west direction, thrust in the early stage, and the fault plane is inclined to the south. In the later stage, the fault was normal and the fault plane tilted to the north. As can be seen from the north-south section, Shucheng fault is a very obvious electrical interface, which is supposed to be the combination zone between Dabie Mountain block and North China block. The Dabie block in the south rises above the North China block in the north.
The Mozitan-Xiaotian fault thrusts from south to north in the early stage and is a normal fault in the later stage, forming the Xiaotian Basin. The Lower Cretaceous strata dominated by pyroclastic sediments were deposited.
(6) Shouxian-Dingyuan fault: Shouxian-Dingyuan fault is the northern boundary of Hefei basin, and the fault plane is inclined to the south, which is an overthrust fault. Mesozoic and Cenozoic, with normal faults, formed a thick Mesozoic and Cenozoic sedimentary cover in the south of Hefei Basin.
(7) Infinite fault: Infinite fault is the eastward dip of the fault plane at the western boundary of Hefei Basin, with Shilichangshan basement exposed on the west side and Yingshang syncline belt on the east side.
3. Strata distribution
(1) Cenozoic: The Cenozoic resistivity in Hefei Basin is generally low, and the Quaternary is very thin; The Neogene in the west of the basin is 100 ~ 200 m thick, and the Yingshang syncline zone is thick: the Cenozoic is dominated by Paleogene deposits, mainly deposited in Yingshang sag, Shucheng syncline zone, Dingyuan sag and Feidong sub-sag. Cenozoic in Fei Zhong fault zone is generally distributed in the east-west belt on the south side of the fault, and the sediments near the fault are thick, that is, thick in the north and thin in the south (the fault in the north and the overload in the south).
(2) Mesozoic: The Cretaceous strata of Mesozoic are the thickest in Da Qiao syncline, reaching more than 3,000 meters. There are thin Cretaceous strata deposits and pyroclastic rocks in the south of the basin. Moreover, the distribution range of Cretaceous in the south is controlled by faults, and the north side of faults is generally undeveloped or eroded. The lower Cretaceous Heishidu Formation and Maotanchang Formation are mainly pyroclastic rocks in Xiaotian Basin in the southern section of Line 650.
The Mesozoic Jurassic is relatively thick, with a thickness of 2000~3000 m, except that the Huoqiu fault nose zone is slightly larger than 1000 m, which is widely distributed in almost the whole basin. The resistivity of Jurassic strata is moderate (80 ~100 Ω m).
(3) Paleozoic basement: After many movements in Indosinian, Yanshan and Himalayan periods, the basement structure of Hefei Basin was complex and the buried depth of the top surface of the basement changed greatly. The basement of Hefei basin is roughly bounded by Fei Zhong fault, which is divided into two parts:
The basement resistivity in the northern area is generally high. The basement of Huoqiu fault nose belt is dominated by Archaean Huoqiu Group, with resistivity greater than1000ω m, and the Da Qiao syncline belt is dominated by Lower Paleozoic with high resistivity. For example, the Upper Paleozoic strata with relatively low resistivity are distributed in the area from Wushanmiao to Yinxianji on the north side of Dingyuan sag and Fei Zhong fault, and the development thickness on the north-south profile is not large.
The basement resistivity in the south of the basin is generally low, which should be dominated by upper Paleozoic strata, and there are overthrust faults in lower Paleozoic or Foziling Group in some areas. The upper Paleozoic sedimentary basin may be covered under the nappe, which is of great significance for oil and gas exploration.
4. Tectonic movement
The tectonic movement of Hefei Basin is quite complicated, and the electrical data reflect Indosinian movement, Yanshan movement and Himalayan movement, which makes Hefei Basin develop a structural system with nearly east-west direction and obvious NNE reflection.
Before Indosinian, it was speculated that Dabie block and Huoqiu fault nose belt were in marine sedimentary environment, and the Dabie ophiolite suite developed in front of Dabie Mountain on the south side of the basin also provided proof for this. During Indosinian period, with the Dabie orogeny, a large nappe from south to north was formed, such as the overthrust nappe on the north side of Feixi fault, which made the Foziling Group nappe on the upper Paleozoic stratum. In the early Jurassic, Hefei basin was in a extensional environment, and Feixi fault reversed, showing a extensional normal fault, and a thick Jurassic stratum was deposited in the basin. During the Yanshanian period, Hefei basin was once again in a north-south compressive environment, and Feixi fault was thrust again. The Jurassic strata near the fault are uplifted, denuded and thinned (such as the relatively thin high-resistivity layers on both sides of Feixi fault in the north and south sections). After Yanshanian, Feixi fault returned to normal, and thick Paleogene strata were deposited on the south side near the fault. Due to this reverse movement of north-south compression and extension, a structural system of ridges and depressions alternating with each other in the east-west distribution of Hefei Basin has been formed.
The NE-trending structure dominated by the Tanlu fault is also well developed in Hefei Basin. The electrical data show that its main activity time is after Cretaceous, and it should be dominated by compression in the early stage (the north-south extension environment of Paleogene basin may be related to the nearly east-west compression reflected by Tanlu fault), and it may be an extension normal fault in the later stage.