Application of Susceptibility Imaging Method by Minimum-Structure Inversion to Underwater Target Detection

Some defects still exist in the magnetic survey for the underwater targets especially in case of the objects with small sizes. In general, the interpretation is dominated by qualitative approach and the accuracy of the quantitative results is low, which is mainly caused by the remanent magnetization and the self-demagnetization in the underwater targets. These have great impact on the analysis and inversion of the magnetic anomaly data. To obtain a robust result, the minimum-structure inversion is applied to the magnetic magnitude transform data to image the location and shape of the underwater target. In the inversion, the L₁ norm and L₂ norm are utilized respectively to measure the complexity of the susceptibility structure. The corresponding results show that, to recover the cuboid model, the results by the L₁ norm has much clearer boundaries and that by the L₂ norm much smoother. Therefore, to detect the common underwater objects, the L₁ norm is more appropriate to be adopted in the inversion. Moreover, the susceptibility imaging method is applied to the field data in the island-tunnel project for the Hong Kong-Zhuhai-Macao Bridge. The practical results verify that the horizontal location, width and depth of the underwater covered pipelines can be accurately determined by the minimum-structure inversion of the magnetic magnitude transform. In short, both the synthetic and the field examples indicate that the susceptibility imaging method in this paper, can calculate the locations and sizes of the underwater covered objects accurately and thus the method is valuable to be applied to the field data.