Application of Susceptibility Imaging Method by Minimum-Structure Inversion to Underwater Target Detection
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摘要: 磁测技术对发现和确定水下目标十分有效，已被广泛用于探测水下沉船、水下掩埋管线、电缆和其他(电)磁性物体.然而，水下目标往往具有较强的剩磁与退磁特点，影响了定性解释结果和定量计算精度.为了更精准地确定和刻画水下目标的位置与形状，以磁异常模量数据为基础，采用最小结构模型进行磁化率成像反演；在模型试验中采用了L1范数和L2范数分别进行反演，并且对两者结果进行了对比分析，结果显示L1范数反演结果具有更为规则与清晰的边界，而L2范数反演结果则相对更为平滑.因此，对于常见水下目标磁测，L1范数反演更适合.以港珠澳大桥沉管隧道磁测数据为例，基于磁异常模量的最小结构模型反演了水下沉管的埋设状态，其平面位置、宽度和埋深具有较好的精准度.因此，本方法能够基于磁测数据更加精细地计算水下掩埋目标的位置和规模，具有较强的实际应用价值.Abstract: 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.
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图 1 正演磁异常(a)与下延35 m之后的磁异常(b)
Fig. 1. Synthetic magnetic anomaly (a) and its downward continuation by 35 m
图 2 下延35 m之后的磁异常模量
Fig. 2. Synthetic magnetic magnitude transform Ta after downward continuation of 35 m
图 3 单一长方体模量Ta异常反演结果
Fig. 3. Recovered susceptibility model from the synthetic magnetic magnitude transform Ta caused by the single cuboid
图 4 港珠澳大桥岛隧工程位置示意
Fig. 4. Location sketch map of the island-tunnel project for the Hong Kong-Zhuhai-Macao Bridge
图 5 磁力测线位置分布(a)与磁异常(b)
Fig. 5. Distribution of the magnetic survey lines (a) and the magnetic anomaly map (b)
图 6 磁异常模量Ta及反演剖面位置
Fig. 6. Magnetic magnitude transform Ta and the horizontal locations of the profiles for inversion
图 7 两条测线对应观测ΔT磁异常及磁异常模量反演得到的磁化率成像结果
Fig. 7. Profiles of the ΔT magnetic anomaly and the cross-sections of the inversed susceptibility
表 1 单一长方体模型参数
Table 1. Parameters of single cuboid model
(长，宽，高)(m) 中心埋深(m) 磁化倾角(°) 磁化偏角(°) 磁化率(SI) (40, 8, 10) 50 40 25 1.2 
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表 2 反演过程相关设置参数及估算的管线几何参数
Table 2. Parameters used in the inversion and the estimated geometric parameters of the pipelines
剖面 反演参数 结果几何参数 坐标范围(m, m) 网格数(个, 个) 网格间距(m, m) 位置-深度(m, m) 高度-宽度(m, m) AA′ (0, 660) (166, 50) (4, 2) (303, 32) (12, 25) BB′ (0, 660) (166, 50) (4, 2) (304, 33) (10, 26)
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表 3 反演结果与实际情况对比
Table 3. Comparison between the inversed results and the actual values
数据来源 AA′ BB′ 中心点位置 中心点埋深 高度 宽度 中心点位置 中心点埋深 高度 宽度 反演结果 303 32 20 28 304 35 19 29 实际情况 302 34 8 22.5 306 34 8 22.5 注：数据单位，m.
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