Magnetic anomaly characteristics

(1) magnetic anomaly? Relationship between t and polarization magnetic anomaly

Magnetic anomaly is the manifestation of underground magnetic inhomogeneity, and high and low anomalies reflect the magnetic changes of different underground geological bodies. Generally speaking, most sedimentary rocks are nonmagnetic or weakly magnetic, and there is no magnetic interface in sedimentary rock series. However, the crystalline basement and various igneous rocks in sedimentary basins are generally very magnetic, and the fluctuation of basement and the change of lithology may cause magnetic anomalies.

At low latitudes, due to the influence of oblique magnetization, the position of the main body of magnetic anomaly and the magnetic body deviates vertically, which brings great difficulties to explain the magnetic anomaly and correctly understand the position of the magnetic body. In order to make the plane position of magnetic body vertically correspond to the extreme position of magnetic anomaly, magnetic anomaly is usually polarized and converted into vertical magnetic anomaly with vertical magnetization, so that the center position of magnetic anomaly corresponds to the center position of geological body, and the position of geological body can be accurately determined through the interpretation of polarized magnetic anomaly.

Through the comparative analysis of aeromagnetic anomalies (Figure 2-3) and polarized magnetic anomalies (Figure 2-8) in the northern South China Sea, the necessity of polarization of magnetic anomalies in the South China Sea and the effect of polarization technology can be well explained.

1. Xisha trough basin

Xisha trough basin is located in the north of17 N, between11~114 E. The western end of the basin is Bao Dao sag in Qiongdongnan basin, the middle is Changchang sag, and the east is the deep water part of Xisha trough. In magnetic anomalies? On the T-plane, Xisha trough basin is a high magnetic anomaly area near east-west, while on the polarized magnetic anomaly map, it is a low magnetic anomaly area near east-west, which corresponds well with the known depression position.

2. Other regions

In Huizhou sag of Pearl River Mouth Basin, the low-value center of magnetic anomaly is biased towards Xi Hui low uplift in the south, while the low-value center of polarized magnetic anomaly corresponds to the center of Huizhou sag. The magnetic field in Zhongjiannan basin is not obvious, but the polarization magnetic anomaly is obvious, and the overall trend is the same as that in Zhongjiannan basin, which is northeast. The magnetic anomalies in Baiyun sag, Chaoshan sag and southwest basin of Taiwan Province are obviously higher, and the magnetic anomalies after polarization are obviously lower.

Through the above comparative analysis examples, it can be seen that the magnetic anomaly after polarization treatment can reflect the location of geological bodies more accurately, thus indicating that the interpretation of magnetic anomalies in the South China Sea must be based on magnetic anomalies? Practice has proved that the frequency domain variable magnetization direction adaptive filtering polarization technology adopted in this project is effective. Therefore, in the description of magnetic anomaly characteristics, we mainly rely on the polarized magnetic anomaly map.

(2) Zoning characteristics of polarized magnetic anomalies

The magnetic field characteristics in the study area are complex. According to the amplitude of magnetic anomaly, the trend of abnormal isoline and the change characteristics of abnormal gradient, it is divided into the following areas (Figure 2-8):

1. Guangdong, Fujian and other land areas have relatively low magnetic fields.

This area mainly includes Guangdong, Fujian and other land areas, and the overall performance is the background of negative magnetic anomalies. Local abnormal development, complex trend, seems to be dominated by NNE-NNE direction, followed by NW direction. Several high-value anomalies basically correspond to topographic highs and are related to the development of igneous rocks.

Figure 2-8 Aeromagnetic Magnetic Anomalies in the Northern South China Sea and Its Adjacent Land Areas

I— northern high-amplitude positive and negative magnetic anomaly areas: I 1— western low-amplitude magnetic variable area, I 2— northeastern high-amplitude magnetic variable area, I 3— central and eastern high-amplitude magnetic variable area, and I 4— central and southern high-amplitude magnetic variable area; Ⅱ-complex magnetic anomaly area in southwest China; ⅲ-banded magnetic anomaly area in the central basin; Ⅳ —— Magnetic anomaly zone with high and low variation in the east.

2. North high-amplitude positive and negative magnetic anomaly area (area I)

This area is located between the northern shelf and continental slope of the South China Sea, starting from Qiongdongnan Basin in the north and reaching Taiwan Province Strait Basin and Southwest Taiwan Province Basin in the west. The characteristics of magnetic anomalies are generally in an alternating pattern, which can be divided into two major anomaly areas, namely, the western region (Ⅰ1) and the eastern region (Ⅰ 2-Ⅰ 4). The western region, including Zhu San sag, Shenhu uplift and Qiongdongnan basin, has a negative magnetic anomaly background, and the anomaly amplitude in most areas is between-150 ~ 0nt, with small amplitude and small gradient change, so it is called the western variable magnetic low region. The eastern region includes Dongsha Uplift, Panyu Low Uplift, Baiyun Depression, Southwest Taiwan Basin and Gabi Basin. Magnetic anomalies are distributed in alternating bands in northeast China, which can be further divided into three zones: Northeast High Magnetic Anomaly Zone (Ⅰ 2), which mainly refers to Dongsha Uplift and Panyu Low Uplift. It is a positive magnetic anomaly background, with an amplitude of 0 ~ 0 ~ 150 nt and a maximum of 3050 nt. The high-amplitude magnetic low zone (Ⅰ 3) in central and eastern China mainly refers to the area from Baiyun Depression to Penghu Beigang, with negative magnetic anomaly background, and the amplitude is mostly between-150 ~-50nt. The variable magnetic high area (Ⅰ 4) in the south-central part mainly refers to the Gabi basin and the south of the southwest platform basin, and it is an area with alternating positive and negative anomalies. The positive anomaly amplitude is mostly between 20 and 70 nt, and the negative anomaly amplitude is mostly between-100 and-20-70 nt, and the anomaly gradient changes little.

3. China Southwest Complex Magnetic Anomaly Area (Area II)

This area is located in the south of north latitude18 and west of east longitude116, mainly including Xisha and zhongsha islands. The background of magnetic anomalies in this area is complex. Generally speaking, the magnitude of magnetic anomalies in the west (Xisha Islands) is smaller than that in the east (dongsha islands), and the local magnetic anomalies are irregular in shape, with different sizes, and the anomaly gradient changes greatly and the trend is chaotic.

4. banded magnetic anomaly area in the central basin (area III)

This area is located in an ocean basin with a water depth of over 3000 meters. The magnetic anomalies are mainly distributed in the east-west direction with alternating positive and negative stripes, which have the characteristics of typical ocean basin "magnetic stripe". The northwest of this area is a bimodal basin, the northeast is Bijianan basin, and the east is Luzon Trough. The phenomenon and trend of magnetic anomalies in every region have changed. Shuangfengnan basin is a high-amplitude negative magnetic anomaly area, and the magnetic anomaly trend of Bijianan basin and the west side of Luzong trough changes from near east-west to northeast.

5. East high-low variable magnetic anomaly zone (Ⅳ area)

This belt includes Manila Trench, Luzon Trough, Luzon Island and Taiwan Province Island. The trend of magnetic anomalies is mainly near north-south direction, followed by north-north direction. The trenches and troughs in the west and north are dominated by negative magnetic anomalies, and there are large-scale high-value positive anomalies in the southeast, which roughly correspond to Luzon Island.