HCX area is a typical mountainous-hilly landform with an altitude of 180 m - 760 m. In the middle part, there is a long and narrow mountain range in the direction of NE and SW, with an altitude of 400 m - 760 m; in the both limbs, there are hills, with an altitude of 180 m - 450 m and relatively small fluctuation.

Quartz sandstone of T₃x and sandstone and mudstone of J₂s and J₁ are mainly exposed on the surface. More than 88% of the strata in the area are Jurassic strata, and the excitation and receiving conditions are relatively good.

There are a few surface control points in the 3D area, the results from the surface of the adjacent area show that: The surface structure of this area is two-layer structure, and the thickness of low velocity layer is between 2 m and 8 m; the velocity of the high velocity layer is affected by the lithology of the surface. The velocity of the high velocity layer in Jurassic strata is generally between 2200 m/s and 3000 m/s; the velocity of high velocity layer in Triassic strata is about 3000 m/s, as shown in Figure 1.

The East high point of HCX structure is an asymmetric anticline with steep southeast flank and gentle northwest flank, and the southeast flank is cut by C② fault. The general structure shape of the shallow layer (above T₃x¹) is basically consistent with the surface structure, with simple structure shape and undeveloped faults. The anticline structure in the middle layer (T₁j²₂ − P₁l) is divided into several fault blocks by faults. The time window of the target layer is in the range of 500 ms - 1500 ms.

Four large faults are developed in the East high point of HCX structure, which are all reverse faults. They are mostly developed in Permian and Triassic vertically and distributed in the both limbs of the structure. The strike is nearly NE, which is consistent with the axial direction of the structure, and most of them disappear with the dip of both ends of the structure.

T₁j is the main target layer of HCX project. Its buried depth is relatively shallow, ranging from 940 m to 2900 m, and the high part of the main structure is 940 m to 1600 m.

The synthetic well records show that the reflection characteristics of the standard layer in the target layer are obvious, which is the basis for obtaining high-quality seismic data. In order to predict T₁j²₂ thin reservoir (6 m - 22 m) according to geological requirements, the frequency bandwidth of seismic data in the target layer should be more than 10 Hz - 60 Hz.

Twice 2D seismic exploration, twice 3D gas reservoirs and twice 3D reef exploration have been completed in HCX area and adjacent areas. The parameters of the four times 3D geometry are obviously different. The bin of the 3D gas storage project is 20 m × 20 m, the line spacing is not more than 240 m, and the maximum offset is about 4000 m.

Based on the comprehensive analysis of geological tasks, working area conditions and actual data, it is concluded that this 3D exploration is faced with two key and difficulties. The first is to improve the seismic reflection energy, signal-to-noise ratio and longitudinal resolution of T₁j to ensure high-fidelity data to meet the needs of fracture prediction and reservoir prediction. The second is to ensure the high precision imaging of T₁j, which lays a foundation for the prediction of high quality reservoir and small fault fractures.

According to the key and difficulties, the project has formulated targeted technical ideas. First, fine surface structure investigation, system excitation parameter test, and supporting high sensitivity single point geophone reception, in order to improve the signal-to-noise ratio of single shot data. Second, to improve the imaging quality of shallow and steeply dipping interface, small bin and small line spacing are adopted. Thirdly, the geometry with high density and high folds is adopted to ensure the imaging effect of complex structures. Fourthly, wide-azimuth geometry acquisition and processing is used to characterize the anisotropy of geological bodies to meet the needs of reservoir prediction (Han, J. W., Zhang, W., Zhang, H., Xiao, H., & Geng, C., 2020).

According to the regional drilling situation, four points in different parts of the structure were selected to build geophysical models, and then the acquisition parameters were demonstrated. On the premise of satisfying the requirement of longitudinal resolution, the parameters such as the maximum protection frequency, bin size, folds, the maximum offset, the receiving line distance and the offset aperture are demonstrated successively (Zhang, J. H., 2017). In the parameter demonstration, various factors should be fully considered (Zhang, W. C., 2017). For example, the selection of the maximum offset considers the buried depth of the target layer, the dynamic correction tensile distortion, the accuracy of velocity analysis, the stability of reflection coefficient, the convergence of diffraction energy, and the AVO effect (Guo, W., 2017). In addition, it is better to combine the illumination analysis of the model (Li, W. W., 2008) and the analysis of the actual data of the adjacent areas.

Figure 2 is the illumination analysis results of observation systems with different offsets based on the 2D geological model established by the depth profile of old data. As can be seen from the figure, after the offset of 4000 m, the growth trend of illumination energy of TT₁f slows down.

Figure 3 and Figure 4 are comparative analysis diagrams of illumination curves at different structural positions and different offsets. After the offset reaches 4000 m, the increase range of illumination energy curves of TT₁j²₂ and TT₁f is small. Therefore, the maximum offset is recommended at about 4000 m.