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摘要
In the range 1- to 5 kHz, so called the audio magnetotelluric (AMT) 'dead band', signal power of the natural electromagnetic field has low values. It leads to the distortion of the impedance data and may cause misinterpretation. It is difficult to correct the distortion data with time series or power spectrum data processing methods. With a large number of experimental data, the Rhoplus method can be used to obtain a reasonable result. @@@ A series of AMT observation experiments were accomplished using the Phoenix MTU-5A instruments. The AMT data was recorded in different seasons and different days, some data was recorded both in day and night. On the basis of these data, firstly the distortion characteristics of AMT 'dead band' data were analyzed by using signal power, signal coherence, apparent resistivity, phase and phase tensor. Secondly the Rhoplus technique is applied to correct the distortion data. Its applicable conditions, processing scheme and key detailed techniques are presented. Finally, a large amount of data, including six sites and a test line, are provided to demonstrate the effectiveness of the method. @@@ The experiments show that the natural electromagnetic signal strength is rather low and the coherences of the orthogonal electromagnetic field components are low in the AMT 'dead band', which lead to the distortion of the impedance apparent res'istivity, phase and phase tensor data. The frequency range of AMT 'dead band' distortion can be wide upto 10 k similar to 100 Hz of which 5 k similar to 1 kHz is most obvious generally. The effect of the distortion is closely related to the observation time and more serious in autumn and winter than summer, and more serious in the daytime than nighttime. The processing results show that when the main noise in the AMT 'dead band' is incoherent noise, the distorted data can be picked out automatically by using the coherence threshold value, and the Rhoplus method can obtain a good result that is consistent with the undistorted reference data which is observed in the night time or summer season. When the main noise in the AMT 'dead band' is coherent noise, the distorted data can be picked out manually by the interactive processing, the Rhoplus method can obtain a good result that is consistent with the undistorted reference data too. For the case of distorted data has no manifestation distinction with the undistorted data, the Rhoplus method can identify the distortion and correct it by using the high quality data in the frequency band over or under the AMT 'dead band'. For the case of distorted data has a wide distortion band and poor quality, the Rhoplus method can be effective too by using the limited high quality data at some frequencies. @@@ AMT impedance apparent resistivity, phase and phase tensor data in the so called AMT 'dead band' may distort When observations are conducted in the daytime of autumn or winter season. The distorted data can be corrected effectively by the Rhoplus method, the combined data selection strategy of the coherence threshold value and interactive processing is fast and objective, the result curves are smooth and corrected data can be basically in line with undistorted observation data.
