| Controlled source electromagnetic methods(CSEM)such as controlled source audio magnetotelluric method(CSAMT),long offset distance transient electromagnetic method(LOTEM),wide field electromagnetic method(WFEM)and time frequency electromagnetic method(TFEM)have been widely used in hydrology,engineering geology and exploration of oil,gas and mineral resources,mainly due to the controllable source,known signal type and high signal-to-noise ratio.With increasingly human activities,strong noise interference has become a key factor affecting the exploration effect,especially in mining areas and cities.Surveying different electromagnetic components with different anti-interference ability and work efficiency.A single-component observation is preferred to improve the efficiency of exploration,and in combination with noise removal methods can improve the signal-to-noise ratio of the data,making it a cost-effective observation method.However,if we measure a single component,the recorded data may suffer from obvious source effect compared with the traditional multicomponent observation,due to the single-component apparent resistivity formula is mostly based on the assumption of uniform earth.When the geological settings are complex,source overprint effect and topography may cause inaccurate resistivity values,even in the "far-filed".How to capture the characteristics of strong noise and remove its influence on recorded data,and remove the source effect of single-component observation is the key problem to be solved.Therefore,in this paper we proposed a controlled source frequency domain electromagnetic method based on multiple reference channels,considering two perspectives:noisy reference channel(NRC)denoising and corrected reference channel(CRC)correcting source effect.1)We proposed the exploration and denoising methods based on NRC in this paper.From the basic principle of the CSEM and the characteristics of its observation device,taking the controlled electrical source E-Ex mode as an example,we proposed the NRCs Eβ and Hβcorresponding to Ex and Hy,which only record the "pure" noise without source information.The data of NRC and normal channel are recorded at the same location and time,but from different angles.Their noise components are highly homologous.Then,with Eβ/Ex<5%as the threshold,we analyzed the applicability of NRC through amounts of model tests,and illustrated its field operability.In the field test,we obtained simultaneously the normal signal and the"pure" noise signal.Then,the time-frequency characteristics of the noise contained in the normal data were analyzed based on the NRC.Based on the periodic characteristics of the transmitting signal,we constructed the joint signal-noise sparse matrix of the normal data by using Fourier transform and Legendre basis,and obtained the optimal denoising result by using Haar wavelet and ADF test iteratively.After that,we quantified the time-frequency characteristics of the noise by using the short-time Fourier transform,and proposed the robust reconstruction method for normal data,which successfully suppress the influence of impulse and other noise.Overall,we constructed the noise removal process based on the NRC to improve the signal-to-noise ratio of the data and realize the effective observation in the strong interference area.2)We proposed the single-component source effect(SSE)observation and correction method based on the CRC.First,we investigated the generation mechanism of SSE through model tests,and considered the SSE as a whole without subdivision,and then proposed the observation method of the CRC.Then,we proposed layout principle applicable to the CRC by considering the outline,terrain characteristics and noise distribution of the survey area.Based on the characteristics of single-component apparent resistivity and Cagniard resistivity formula,we proposed a single-component apparent resistivity equivalent current calculation method,and proposed a multi-point joint correction method for the SSE using the natural neighborhood method and the subsurface distribution of the CRC.The correction method is applied to improve the accuracy and resolution of single-component exploration methods.3)Field test studies were conducted in Qihe-Yucheng iron mine in Dezhou,the Zhangqiu Bucun coal mine in Jinan,and a station of Jinan Metro.The detection results by applying the proposed method matched well with the actual geological outline,and verify the effectiveness and applicability of the method proposed. |
