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摘要:
陆地可控源电磁法的观测资料可依据频段范围近似地划分为近区场、中间区场及远区场，但采用测量相互正交电、磁分量，并计算视电阻率的资料处理方式只适用于远区场数据.为更有效地利用陆地可控源电磁法不同区间场的观测资料，本文结合三维数值模拟技术并采用电场分量直接进行反演的策略，对不同区间电场的响应特征与探测效果进行了分析.数值模拟结果表明：近区电场的异常响应最明显，异常响应不随频率发生显著变化，但纵向分辨能力差；远区电场异常响应随频率发生显著变化，其探测深度取决于频率的高低；中间区场较为复杂，地表电场异常响应的等值线中心并不是位于异常体中心正上方，而是在沿场传播方向上向异常体与围岩的分界面处偏移，并且发现中间区场资料的加入会影响反演结果的准确性.综合合成数据和野外实测资料的反演结果，发现结合近区场和远区场资料而舍弃中间区场资料的反演效果更佳，这为陆地可控源电磁法资料的反演解释提供了一种有效途径.
Abstract:The land controlled-source electromagnetic (CSEM) method observes the electromagnetic field of wide frequency band excited by single source. According to the physical nature of the electromagnetic field in frequency domain,the observation of the land CSEM method can be approximately divided into three sections:the near-zone field,the middle-zone field and the far-zone field. The conventional land CSEM method such as CSAMT only uses the far-zone field data,which possibly fails to utilize the information contained in the near-zone and middle-zone. We have developed 3D forward and inversion tool for land CSEM method,which is based on the open source EM modular platform ModEM. Our algorithm solves the primary and secondary electrical field separately in the forward modeling,and directly fits Ex component in all of the three zones in the inversion scheme. After discussing the response of electric field Ex in different field zones,we investigate the best frequency combinations for 3D inversion. The 3D CSEM numerical simulation shows that in near-zone field the abnormal response of Ex has large amplitude and does not change with frequency significantly,and the inversion result of the synthetic data shows that the near-zone field has poor resolution in depth. The simulation also shows that the center of the surface Ex abnormal response contour in the middle-zone field is not located above the center of the anomaly body,but is offset from the transverse electrical interface of resistivity model,which results in the deviation of the inverted resistivity anomaly from the actual position in the synthetic inversion test. The far-zone field Ex abnormal response varies with frequency,which can be used to distinguish the electrical layers at different depths. By comparing and analyzing the inversion results of different frequency combinations of synthetic data and field data,we find that for land CSEM 3D inversion the best frequency choice is to combine the data in the near-zone field and far-zone field together,abandon the data in middle-zone field.
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Key words:
- Land CSEM /
- Field-zone /
- Frequency combination /
- 3D forward and inversion
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图 2
依据均匀半空间电场Ex响应特征划分近区场、中间区场以及远区场
Figure 2.
The near-zone field, middle-zone field and far-zone field are divided according to the characteristic of the half-space electric field Ex response
图 3
近区场(0.2 Hz)、中间区场(20 Hz)、远区场(1260 Hz)在地表测区ΔEx与Δψ分布图
Figure 3.
The relative changes of Ex amplitude ΔEx and phase Δψ of different frequencies at different zones for all observation points
图 4
测点A与测点B的ΔEx与Δψ随频率变化曲线
Figure 4.
The relative changes of Ex amplitude (ΔEx) and phase (Δψ) of 3D anomaly model to background layered model at observation points A and B
图 5
剖面x=0 m不同频率的电场矢量图
Figure 5.
Electric field vector of different frequencies at profile x=0 m
图 6
各单频率反演结果(x=0 m)
Figure 6.
Inversion results for the single-frequency at profile x=0 m
图 7
各单频率反演的模型响应数据与合成数据的Ex幅值比
Figure 7.
Amplitude ratio of the Ex between the response data of the inversion model and synthetic data for the single-frequency
图 8
组合不同场区数据的反演结果(剖面x=0 m)
Figure 8.
The inversion results for different frequency combinations at profile x=0 m
图 10
基于发射源T2采集各个测点视电阻率
Figure 10.
Apparent resistivity of each observation point based on the transmitter T2
图 12
源T1激发下各测点电场Ex幅值
Figure 12.
Ex amplitude of each observation point based on transmitter T1
图 13
参考电阻率模型合成数据反演结果(剖面z=35 m，发射源为T1)
Figure 13.
Synthetic inversion results of the reference model based on transmitter T1 (z=35 m)
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