## 3 Geophysical Techniques and Geophysical Characteristics
### Geophysical Characteristics of the Study Area  
3.2.1. Regional Geophysical Characteristics of Changde City  
The 1:500,000 gravity measurement data of the region shows that the study area is located at the eastern and southwestern edges where the gravity is high, within the Mayang-Changde-Xiangtan-Hengyang arc. Bouguer gravity abnormal values are \((-50\)\(\sim\)\(-10)\times 10^{-5}\) m/s\({}^{2}\). The arc gravity height is consistent with the outline of the Mesozoic red bed basin, and has a large mass deficit relative to the basement, which mainly reflects the uplift of the metamorphic crystalline basement and Mohosurface [19; 20]. The aeromagnetic background field in the study area is \(\Delta\)T positive anomaly, and \(\Delta\)T abnormal values vary from 15 nT to 40 nT. Aeromagnetic \(\Delta\)T anomaly is generally small in the northwest and northeast. From north to south, the \(\Delta\)T anomaly gradient gets smaller and smaller. Aeromagnetic anomaly contour lines bulge from the middle to the northwest and northeast directions. The directions of aeromagnetic anomaly contour lines in the survey area are not in harmony with the strata and tectonic strike lines, indicating that the Quaternary strata exposed in the survey area have the characteristics of a low and slow magnetic field or they have no magnetic field. The study area is located in the Changde mantle-uplift district, which includes the whole Dongting Lake area. The Changde-Anren transition fault zone divides it into two local mantle-uplift districts, namely Yuanjiang and Taoyuan, with completely different strike directions. The study area is located between two secondary mantle-uplift districts. The Changde-Anren transition fault zone spreads in a northwest direction from the middle of the study area. The buried depth of the Moho surface is \(31\sim 32\) km, and the two mantle-uplift districts correspond to the Mesozoic Dongting basin. 3.2.2. Petrophysical Characteristics of the Study Area  
The four detection methods in this test involve two kinds of parameters, namely electrical parameter resistivity and wave velocity. The resistivity statistics of different formations in the region are shown in Table 1, and their electrical characteristics are as follows: Quaternary Holocene, Quaternary middle Pleistocene series Mawangdui formation, and Quaternary lower Pleistocene series Changde formation. All of these formations have relatively low resistivity characteristics. The average value is generally less than 300 \(\Omega\cdot\)m, and the resistivity is relatively higher when containing more gravel, while more silty clay content leads to lower resistivity. The average resistivity of silty shale in the Ordovician Baishuixi formation in this area is 1000.3 \(\Omega\cdot\)m, and it is buried deep in this area with less water content, showing moderate resistivity characteristics. The Dolomitic limestone of the Cambrian Tanxi formation has the highest resistivity within the range of detection depth in the study area. The average value is 2788.5 \(\Omega\cdot\)m. The relative resistivity of micritic argillaceous limestone in the Cambrian Winitang formation is lower, with an average value of 1757.3 \(\Omega\cdot\)m. The resistivity is lower than that of the Tanxi formation because of its Argillaceous components. However, the resistivity of the Cambrian Niuittang formation, with an average value of 798.2 \(\Omega\cdot\)m, is sharply lower than that of other Cambrian strata in the area due to the carbonaceous content. When there are fault structures in the concealed strata, Quaternary bedrock has a natural surface water infiltration channel of the Huxiang Basin, thus presenting locally low resistivity characteristics [21; 22; 23; 24; 25; 26]. Different strata have different physical properties. Resistivity parameter detection can be used to detect the buried pre-Quaternary bedrock in the area, which can be used to delineate geological anomalies such as structures.