Application Of Multi Station Remote Reference Magnetotelluric Sounding In Deep Structure Research Of Jizhong Depression

Jizhong depression is located in the economically developed Beijing,Tianjin and Hebei region,which is an extremely important energy prospect area in northern China.It can conduct deep geological investigation to find out the energy distribution,which can serve the national economic construction and solve the strategic scientific and technological problems called by the state for the deep march into the earth.In order to understand the deep geological information,we must obtain the deep electrical parameters first.The magnetotelluric sounding method based on the electromagnetic field propagation theory can observe the electrical parameters from shallow to deep,which is an effective method to study the electrical structure in the deep part of the earth,and has been widely used in recent years.But the field source of magnetotelluric method is the natural alternating electromagnetic field,which is weak,and is easily disturbed by electromagnetic noise sources such as urban stray current,high-speed railway and high voltage substation,especially the low-frequency signal which can reflect the deep geological information,and the interference is more serious.Although we can choose to avoid the interference source for a certain distance to reduce the interference,it is difficult to avoid various interference sources effectively in the urban intensive and industrial developed Jizhong depression.Therefore,it is urgent to analyze the signal characteristics of interference sources systematically,study more effective noise suppression methods,improve the quality of magnetotelluric data,and lay a foundation for high-precision geophysical inversion interpretation.Based on the geological survey project "investigation and evaluation of deep carbonate thermal reservoir in Jizhong depression" undertaken by the Institute of geophysical and geochemical exploration,Chinese Academy of Geological Sciences,in order to effectively formulate the technical index of magnetotelluric sounding,the project team carried out the technical test of magnetotelluric sounding anti-interference method.In this paper,the noise characteristics and de-noising methods of urban stray current interference source which has the most serious impact on magnetotelluric sounding are analyzed.Firstly,the relationship between interference source and influence distance is analyzed theoretically.Then,six magnetotelluric sounding points are arranged at different distances of Beijing south central axis,and the analysis is carried out according to the collected data.One is to analyze the characteristics of noise waveform,the correlation with urban power consumption and the range of signal radiation from the perspective of time domain;Secondly,from the perspective of frequency domain,the interference mode and influence range of noise signal on apparent resistivity and phase curve are analyzed,and the change trend of noise signal energy with distance is analyzed according to the spectrum density curve.According to the noise interference characteristics,far reference method is selected to denoise.In order to improve the conventional far reference method,11 remote reference stations are set up under the consideration of distance,structural unit and orientation.The data of different distance measuring points are processed by 11 remote reference stations respectively.The processing effect is evaluated.The advantages and disadvantages of far reference in strong interference area and the suggestions for far reference layout are summarized.According to the test,it is found that the improvement of near-field interference by a single far reference station is limited to a certain frequency band,which is in line with the characteristics of short range and far reference improving high frequency data and remote remote reference improving low frequency data.Moreover,the selection of the remote reference distance with small limit can avoid the superposition of the same phase and azimuth interference source signals between the measuring point and the far reference station.On the premise of not related signal noise,the processing method of multi station remote reference data is improved.The near,medium and long distance reference stations are used to denoise some measuring points which are seriously disturbed by the stray current in cities.Then,the optimal results are selected in the frequency division section,and the denoising effect of multi station remote reference in different combination modes is discussed.The results are optimized and combined to obtain the final magnetotelluric sounding curve,The purpose of suppressing the near-field interference of urban stray current is achieved.In order to verify the practical effect of the multi station far reference magnetotelluric sounding method,it is applied to the deep structure research of Jizhong depression.Firstly,the reliability of the method is verified from two aspects.One is to compare the one-dimensional inversion results without far reference and multi station remote reference processing with the proved strata by taking a known geothermal drilling data in the northern part of Langfang as reference;Secondly,taking the seismic profile of the near line as reference,a survey line which is seriously disturbed by the stray current of the city is selected.The two-dimensional inversion results after the far reference processing and the multi station remote reference processing are compared and analyzed.Then the effectiveness of multi station far reference method is evaluated comprehensively.Finally,the geological interpretation of the multi station far reference inversion section with more reasonable results is carried out.The inversion results show the geological structure of "three concave and one convex" in the northern part of Jizhong depression,clarify the faults controlling the concave convex structure in the depression,and obtain the deep structural information of Jizhong depression,which can provide reference for the exploration of deep resources.