The effects of non-stationary noise on electromagnetic response estimates
The effects of non-stationary noise on electromagnetic response estimates
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DOI：
10.1046/j.1365-246x.1998.00661.x
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发表时间：
1998-11
影响因子：
2.8
通讯作者：
R. Banks
中科院分区：
文献类型：
--
作者：
R. Banks
摘要自然电磁时间序列中的噪声通常是非平稳的。具有高磁噪声水平的数据部分会使阻抗产生偏差，并产生不可靠的误差估计。包含电磁通道之间相干噪声的部分也会产生不适当的阻抗和误差。答案是计算尽可能短的数据段的响应值，即与所选带宽兼容并且需要过度确定阻抗和相干的最小二乘估计的数据段的响应值。只选择那些可靠的值，而衡量接地阻抗估计可靠性的最佳单一衡量标准是它们的时间不变性，这是通过测量和预测的电场之间的一致性来检验的。复数解调是用来探测20-6000 S周期内电磁场的时间结构的方法。对于超过300 S的周期，在阻抗值的时间序列中很容易识别出噪声段。相应的估计值严重偏离正常值，偏向低阻抗值，并与低相干性相关。相干性的阻抗曲线是特别有价值的诊断辅助工具。对于低于300时的S，阻抗偏置随着相干性的降低而系统性地增加，从而确定输入通道噪声是原因。通过选择具有高相干性的截面(相当于截面上的阻抗不变)，可以确定无偏阻抗和实际误差。高相干段之间阻抗值的分散是由于输入和输出通道之间相干的噪声，这意味着存在两个或多个系统，可以为其定义一致的响应。在地球响应和噪声响应明显不同的情况下，可以通过剔除不能为所有响应元素生成令人满意的值的部分来改进对前者的估计。
SUMMARY The noise in natural electromagnetic time series is typically non-stationary. Sections of data with high magnetic noise levels bias impedances and generate unreliable error estimates. Sections containing noise that is coherent between electric and magnetic channels also produce inappropriate impedances and errors. The answer is to compute response values for data sections which are as short as is feasible, i.e. which are compatible both with the chosen bandwidth and with the need to over-determine the least-squares estimation of the impedance and coherence. Only those values that are reliable are selected, and the best single measure of the reliability of Earth impedance estimates is their temporal invariance, which is tested by the coherence between the measured and predicted electric fields. Complex demodulation is the method used here to explore the temporal structure of electromagnetic fields in the period range 20‐6000 s. For periods above 300 s, noisy sections are readily identified in time series of impedance values. The corresponding estimates deviate strongly from the normal value, are biased towards low impedance values, and are associated with low coherences. Plots of the impedance against coherence are particularly valuable diagnostic aids. For periods below 300 s, impedance bias increases systematically as the coherence falls, identifying input channel noise as the cause. By selecting sections with high coherence (equivalent to the impedance being invariant over the section) unbiased impedances and realistic errors can be determined. The scatter in impedance values among high-coherence sections is due to noise that is coherent between input and output channels, implying the presence of two or more systems for which a consistent response can be defined. Where the Earth and noise responses are significantly diVerent, it may be possible to improve estimates of the former by rejecting sections that do not generate satisfactory values for all the response elements.