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摘要:
大地电磁法一直以来基本都遵循场源为平面电磁波的假设，这种假设普遍适用于中高频的大地电磁研究，但在具有显著非平面波场特性的区域或开展长周期大地电磁研究时，则对场源平面波的理论基础带来了挑战，若不满足理论假设，将会导致深部探测的可靠性降低，勘探目标定位风险增高.本文通过选择线状、片状及非规则场源模型来模拟极光电集流、赤道环电流和横向波动的波数场源，开展了大地电磁场源效应的模拟研究，对比分析了非平面波模式和平面波模式的响应差别.模拟表明大地电磁场源效应强度与模型平均电阻率正相关，与频率和测点与源水平距离负相关，且与场源的高度、宽度和横向波长等参数均有关系，非平面波模式和平面波模式正演响应在一些情况下具有超过10%的差异.基于场源效应特征，归纳并提出了五种场源效应校正方法，一是计算极限模型来截断场源效应影响频段的频域截断法，二是对多个同步观测的邻近测点进行平均的测点平均法，三是延长测点观测时间的时间域延长法，四是剔除较强垂直磁场分量所对应场源效应较强段数据的时间域剔除法，五是直接考虑场源的反演，五种方法均能一定程度上降低大地电磁场源效应的影响.对于受场源效应影响的大地电磁数据，若直接采用平面波方式进行反演则可能在深部产生较大畸变，若在获知场源相关参数的基础上，可开展考虑场源的大地电磁反演来消除场源效应影响.
Abstract:Magnetotelluric generally follows the source for the plane wave assumption, which is generally applicable to the high frequency magnetotelluric study, but in the area with significant non-plane wave field characteristics or in long period magnetotelluric study, bring challenges to the theoretical basis of the plane wave source, if the theoretical hypothesis is not satisfied, the reliability of deep exploration will be reduced and the risk of exploration target positioning will be increased. This paper selects linear, sheet and irregular field sources model to simulate the auroral electric current, equatorial ring current and transverse wave number field sources, carries out simulation study of source effects of geodetic electromagnetic field, and analyzes the differences between non-plane wave mode and plane wave mode. The simulation has shown that the source effects strength of the electromagnetic field is positively correlated with the average resistivity of the model, negatively correlated with frequency and the horizontal distance between the measured point and the source, and is related to the height, width and transverse wavelength of the field source. Difference between the forward response of the non-plane wave mode and the plane wave mode is over 10% in many cases. Based on source effects characteristics, this paper summarized and put forward the five effect correction methods. First one is a computing model limit of truncation effect source spectrum frequency domain truncation method. The second one is the average method for multiple simultaneous observation of adjacent points. The third one is to prolong the time of the measuring point observation time domain extension method. The fourth one is to eliminate the stronger the magnetic field component perpendicular to the corresponding source effects strong piece of data of time domain method. The fifth one is directly consider the inversion of field source. All of 5 methods, to a certain extent, can reduce the effect of electromagnetic field source to the earth. For the magnetotelluric data affected by the field source effects, if the inversion is carried out directly in the way of plane wave, there may be great distortion in the deep part. If the relevant parameters of the field source are obtained, the magnetotelluric inversion considering the field source can be carried out to eliminate the influence of the field source effects.
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Key words:
- Magnetotelluric /
- Source effects /
- Non-plane wave /
- Source effects correction
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图 2
均匀半空间模型线电流源正演响应
Figure 2.
Response of line current source in uniform half-space model
图 5
稳定区域模型线源与平面波源正演响应及其相对误差
Figure 5.
Response and relative error between line source and plane wave source in stable region model
图 7
活动区域模型线源与平面波源正演响应及其相对误差
Figure 7.
Response and relative error between line source and plane wave source in active region model
图 10
波数场源100 Ωm模型正演响应
Figure 10.
Responses of wave number field source in 100 Ωm model
图 11
波数场源10000 Ωm模型正演响应
Figure 11.
Responses of wave number field source in 10000 Ωm model
图 12
线电流源正演结果测点平均校正前后对比
Figure 12.
Comparison of average calibration of measuring points before and after in linear current response
图 13
北京地磁台不同观测时长倾子幅值对比
Figure 13.
TipMag of different observation time at the Beijing Geomagnetic Observatory
图 14
线电流源-稳定区域模型Bz响应断面
Figure 14.
Bz responses of line current source in stable region model
图 15
北京地磁台2019年度磁场时间序列
Figure 15.
The magnetic field time series of Beijing Geomagnetic Platform in 2019
图 16
时间域剔除法校正北京地磁台倾子幅值
Figure 16.
The time domain elimination method corrects the tilting amplitude of the Beijing geomagnetic platform
图 17
地球平均电阻率模型反演响应和RMS收敛曲线
Figure 17.
Inversion responses and RMS of earth mean resistivity model
图 18
地球平均电阻率模型带源与不带源反演结果
Figure 18.
Inversion results of earth mean resistivity model with and without source
表 1
10000 Ωm均匀半空间模型极光电集流场源效应频段
Table 1.
The source effects spectrum of aurora electric collector field in 10000 Ωm model
测点纬度 测点与线源水平距离/km 线源与平面波源相对误差＞1%的频段/s 线源与平面波源相对误差＞10%的频段/s 视电阻率 相位 视电阻率 相位 0° 7338 ＞1050 ＞175 ＞4000 ＞1950 15° 5670 ＞610 ＞110 ＞2200 ＞1220 30° 4003 ＞270 ＞60 ＞1220 ＞690 45° 2335 ＞95 ＞27 ＞480 ＞380 60° 667 ＞44 ＞125 ＞1500 ＞630 75° 1000 ＞35 ＞22 ＞310 ＞1700 90° 2668 ＞120 ＞30 ＞610 ＞420 
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