| In recent years,with the increasing level of urbanization in China,the demand for the development and utilization of urban underground space is increasing.In the process of urban underground space development,problems such as groundwater loss,karst cavity and unknown underground geological conditions are often faced,resulting in frequent secondary disasters in the process of urban construction,seriously affecting the social economy and restricting the sustainable and healthy development of the city.Controlled source electromagnetic exploration has great exploration potential in urban environment because of its low cost,small destructiveness and no pollution.However,in the complex electromagnetic environment,the controlled source electromagnetic signal is greatly affected by noise.Especially in the urban environment with the most concentrated human activities,the intensity,frequency band and distribution range of noise are far beyond the traditional working environment.It is urgent to suppress noise through technical methods to obtain high signal-tonoise ratio exploration data.For urban environmental noise,using a single method cannot achieve noise suppression or removal,and using a variety of methods for processing cannot avoid the problem of evaluation and fusion of different methods.The existing evaluation methods and standards are not uniform,and the results cannot be directly compared.To this end,this paper establishes a unified standard data quality evaluation method.Specifically,the upper bound of noise is estimated by the improved spectral envelope algorithm based on Hilbert transform to evaluate the degree of noise influence,and verified by simulation and actual data.The practical application results show that the evaluation method proposed in this paper has good consistency and stability for the evaluation of data quality and denoising results.Based on the unified quantitative evaluation standard,this paper analyzes the urban environmental background noise from three aspects:spatial distribution,spectral characteristics and time-frequency characteristics.Focusing on the power frequency noise and the background noise corresponding to the target exploration frequency,the spatial distribution characteristics of background noise in urban environment are systematically analyzed by integrating the information of surface buildings and human activities.Through time domain,frequency domain and time spectrum analysis,the types and characteristics of background noise in urban environment are systematically divided,and the corresponding relationship between background noise in urban environment and human activity time is constructed.Aiming at the controllable source data in the urban environment,this paper uses a combination of multiple methods to accurately denoise.In this paper,an improved least squares inversion denoising method based on mixed basis is proposed.A low-frequency offset noise removal method based on step recognition is proposed.A data extraction method based on time-frequency analysis and a phase delay correction method based on zeropadding interpolation are constructed.Along with robust statistical method,formed a complete set of controlled-source electromagnetic denoising methods for urban complex environments.Finally,combined with the above research contents,this paper forms a complete set of controllable source electromagnetic data processing flow and writes the corresponding visual data processing software.Through simulation experiments and practical engineering applications,it is shown that the method proposed in this paper can effectively denoise the controllable source exploration data in the urban strong interference environment,so that the controlled-source electromagnetic method can obtain high-quality exploration data in the urban environment and obtain good exploration results. |
