The processing of aeromagnetic data is first based on polarization processing, and then the target processing is carried out according to the requirements. The method and means are similar to gravity data processing. As can be seen from Figure 4-7, the high-value anomaly area of Δ T anomaly after polarization is consistent with the understanding of earthquake.
Figure 4-7 contour map of high-precision aeromagnetic pole measurement in Hefei Basin
(A) the fault structure of Hefei Basin
Magnetic anomaly characteristics of 1. fault structure
As we all know, it is an effective method to determine basement faults, especially large faults, by using aeromagnetic data. The reason is that the magnetism of geological bodies often changes after suffering from faults, which may lead to the decrease of magnetism and the formation of a narrow negative anomaly zone. Or magnetic substances invade along the fracture, forming a linear or beaded abnormal zone; Or the dislocation line of magnetic anomaly indicates that the geological bodies on both sides of the fault have moved relatively; In addition, the most common is the magnetic gradient zone, which is also one of the main manifestations of fracture. These characteristics objectively reflect the fault structure, which is closely related to our understanding of the development and evolution of regional structures and geological structures and even the formation of ore deposits. Therefore, it is of great practical significance to identify deep faults and regional large faults by using aeromagnetic data.
2. Methods and principles of delineating faults
According to the characteristics of magnetic anomalies and geological structural conditions in this area, the principles we have determined are as follows: (1) mainly delineating those fault structures with obvious anomalies and large scale; ② The faults determined by aeromagnetism are mainly basement faults, which may not be completely consistent with the distribution position of fault structures in sedimentary layers. In this case, we should make a comprehensive analysis and emphasize the leading role of aeromagnetic interpretation results in aeromagnetic interpretation data.
3. Main fault structures
According to the magnetic anomaly signs and delineation principles of the above faults, 76 fault structures have been delineated in this area, including three major first-class faults that control the boundary of the basin, namely, Tanlu fault zone, Jinzhai-Shucheng fault zone and Yingshang fault zone. Generally speaking, there are five faults in the secondary uplift and depression of the basin, including Feizhong fault (F4), Shushan fault (F5), Feixi-Hanbaidu fault (F6), Gaotangpu fault (F9) and Zhujiada fault (F 10). There are three major faults outside Hefei Basin, namely Xiaotian Yimei Fault (F8), Hu Lin Fault (F9) and Maotanchang Fault (F 1 1). The rest are common basal fractures.
4. Distribution law of faults and their functions in geological structures.
As mentioned above, according to the manifestations (characteristic lines) of faults in the magnetic field, there are 76 faults in the inner circle of the area, of which 1 1 is mainly characterized by long fracture extension and obvious characteristic lines. It is also clearly shown on the aeromagnetic upward continuation map, indicating that it has a certain cutting depth, and most of the other faults are general basement faults. The above 76 fault structures can be divided into four groups according to their distribution direction on the plane. One group is a NE-trending fault, that is, the Tan-Lu fault zone, and its strike-slip geological process is very obvious. In addition, there are a series of smaller fault structures such as Hu Lin fault and Gaotangpu fault, which have played a very important role in the evolution of geological structures. The other group is east-west and nearly east-west faults. The main fault is Fei Zhong fault, followed by Shushan fault, Feixi-Hanbaidu fault, Jinzhai-Shucheng fault and Maotanchang fault to the south. Originally, they may all be east-west fault structures, but in the process of interaction between the North China plate and the Yangtze plate, especially the strike-slip action of the Tan-Lu fault zone, they turned to a certain extent. In addition, there are few NW-trending faults, such as Zhujiada fault, and some smaller fault structures, mostly internal faults in the basin, which mainly control various blocks with different properties in the basin, especially in the central and northern parts of North China Platform. Followed by the north-south fault structure, this interpretation results delineated nearly ten north-south fault structures, all of which are small in scale, but also have cutting control effect on some fault blocks.
(2) regional magnetic anomaly characteristics and basement lithofacies structure
The spatial distribution of regional magnetic anomalies is basically a comprehensive reflection of the results of different protolith formation and tectonic evolution in different stages of basement crystalline rock series. Combined with the characteristics and distribution laws of magmatic activities in different periods, we can roughly understand the basement properties of the study area and delineate the zoning. The study area is generally divided into six different types of magnetic fields, and the differences of magnetic anomalies and morphological changes in the six magnetic fields reflect the differences and differences of basement lithofacies in this area.
According to the physical data, the magnetic materials in the survey area and its surroundings mainly include deep old metamorphic rocks (Archean and Middle and Lower Proterozoic), Mesozoic and Cenozoic volcanic rocks, partial sedimentary rocks and multi-stage intrusive rocks. The magnetic changes and spatial occurrence of these rocks have caused various magnetic anomalies, and the comprehensive superposition of various anomalies has formed the complex magnetic field in Hefei Basin.
The regional magnetic field in Hefei basin and its surrounding areas is mainly characterized by high magnetic anomaly zones with area distribution, such as Huoqiu strong magnetic anomaly zone, Dabie mountain strong magnetic anomaly zone and Zhangbaling strong magnetic anomaly zone, which is mainly the reflection of Archean strong magnetic basement and large-scale deep intrusive rocks. The magnetic anomaly areas with positive and negative changes are mostly massive, which are basically caused by Archean medium-weak magnetic rocks; In addition, there are large areas of regional negative magnetic anomaly belts in the south and southeast edge of the survey area, which mainly reflect the weak magnetic basement dominated by Yuanguyu.
According to the above analysis and some differences in basement structure, it can be divided into the following basement lithofacies structural areas: ① Archaean-dominated strong magnetic bedrock distribution area; (2) The distribution area of moderately strong magnetic bedrock dominated by Archean; (3) the distribution area of weak magnetic bedrock in Archean (including some middle and lower Proterozoic); ④ The distribution area of weak magnetic bedrock dominated by Yuanguyu; ⑤ Proterozoic weak magnetic bedrock distribution area, rich in Mesozoic volcanic rocks; ⑥ Intrusive rocks and subvolcanic rocks in different periods.
(3) delineation of magnetic anomalies and distribution law of local structural anomalies
In recent years, with the improvement of magnetometer accuracy and positioning accuracy, the decrease of flying height and the increase of measurement scale, the reflection of local anomalies in a small area is more clear, making the interpretation of local structural anomalies one of the main geological tasks of high-precision large-scale structural aeromagnetic interpretation. Advanced aerial survey and ground survey systems were used in the high-precision aeromagnetic survey in Hefei Basin, and many local anomalies were found. More importantly, the classification and interpretation of the geological causes of these local anomalies can solve some geological structure details and provide important clues for oil and gas exploration.
1. Method of highlighting local anomalies
There are many methods to highlight local anomalies, and two conversion processing methods are mainly used in this area: residual anomaly extraction and polar first derivative calculation.
The extraction and processing of aeromagnetic δ T residual anomaly is effective, which not only effectively eliminates or suppresses the regional background field, but also achieves the purpose of highlighting local anomalies, especially for low-amplitude local anomalies. The vertical first derivative diagram of aeromagnetic δ T polarization is obvious and reliable, which plays an important role in delineating and explaining local structural anomalies.
2. Determine the principle of local anomalies
Considering the magnetic field characteristics, geological conditions and practical significance of oil and gas exploration in the survey area, the determination of local anomalies in this area is mainly delineated in detail in areas with oil and gas prospects. For the area with large distribution of volcanic rocks at the edge of the basin, it is of little significance to find oil and gas, so there is no detailed delineation of local anomalies.
3. Classification of local anomalies
Based on the abnormal morphological characteristics of aeromagnetic Δ T profile, firstly, considering the amplitude and gradient of anomalies, the anomalies can be roughly divided into: high amplitude, steep gradient peak-like and chaotic strong magnetic anomalies; Local elevation anomaly with moderate amplitude and steep slope; Weak magnetic anomaly with low amplitude and small gradient; High frequency weak anomaly with small amplitude but fast gradient change. Secondly, considering the abnormal shape and magnetic field background, local negative anomalies and isolated positive anomalies are separated respectively.
4. Geological causes of local anomalies
The complex structure and frequent magmatic activity in the survey area have caused various forms of local anomalies. According to the morphological characteristics and petrophysical data of the anomalies, and repeatedly comparing with the known geological conditions and related geophysical data, we have made geological classification and interpretation, and identified 27 bedrock uplift anomalies (Grade A), 4 basic volcanic rock anomalies (Grade B), 65,438 intermediate-acid intrusive rock anomalies (Grade C) and volcanic rocks.
5. Distribution law of local structural anomalies
(1) Most of the local structural anomalies are in the same direction as the adjacent faults: most of the local structural anomalies in the survey area are closely related to fault activity, mostly distributed along faults or parallel faults, especially controlled by NE and NW faults.
(2) The local structural anomalies are obviously controlled by the regional structure: the local anomalies delineated in the basin are mainly distributed in and around the depressions with thick caprock, and are obviously controlled by the regional structural strike, especially in the convex zone between the edge of the depression and the depression, which are mostly consistent with the direction of the regional structural line.
(3) The distribution of local structural anomalies in the north and south of the survey area is quite different: there are more anomalies reflecting bedrock uplift in the north, and structural anomalies reflecting shallow sediments and volcanic anomalies in the south are more developed than in the north.
(4) There are many local structural anomalies reflecting shallow structures: the local magnetism of sedimentary rocks causes many structural anomalies of sedimentary layers, and the anomalies caused by the multilayer distribution of volcanic rocks are also related to structures. These two types of anomalies account for more than 70% of the total number of anomalies, which undoubtedly provide rich geological information for shallow caprocks. With the continuous improvement of exploration degree, we should study them more carefully.
(5) There are many local structural anomalies: most of the local structural anomalies in the basin are distributed in bands, and there are many obvious structural anomalies.