Experimental Study On Effectiveness Of Wide Field Electromagnetic Theory

Wide-field electromagnetic method (WFEM) is a new frequency domain controlled source electromagnetic prospecting method proposed by He Jishan academician.In order to study the validity of this new method, the Dayangshu basin (an oil and gas prospect area) was selected to do fitst field experiment, and use it as the topic of this paper.Firstly, the history of development of electromagnetic prospecting methods are reviewd, the advantages and disadvantages of MT and CSAMT are discussed, the technical background of WFEM are introduced in this paper. Under the guidance of electromagnetic field theory, theoretical expressions of horizontal electrical dipole and vertical magnetic dipole in the uniform and layered conductive half space are derived as the theoretic base. The basic principle of WFEM, method of collecting apparent resistivity and the main advantages of WFEM are discussed. The difference between bipolar source and standard dipole source in actual use is studied,then errors using bipolar instead of dipole source is analyzed and calculated, and points out the applying range of use bipolar source to replace dipole source.The geographic position, regional geological construct, characters of stratum and lithology and oil and gas prospecting perspective of Dayangshu basin are recounted, electrical parameters of rock measured at Dayangshu area before this experimant are emphatically narrated, feasibility of electromagnetic method used to Dayangshu area and types of geoelectrical section which could be meeted are analyzed.It is recounted such as selected survey regions, laid surveying lines and points, working parameters in field, finished quantity and quality, etc.Selecting100、110、120、130、140and149measuring point of LYC1line (6points together), geological and geophysical interpretations of Dayangshu area are made. Designed geoelectrical model for interpretaion, theoretically calculating forward apparent resistivity curve and to compare it with the measured apparent resistivity value, parameters of the model to be repeatedly corrected until to the theoretical forward apparent resistivity curve fited with measured apparent resistivity curve in a satisfying precision.According to the results of interpretation, the physical interpretating geoelectrical section of LYC1line wouls be drawes. Then this section is compared with geologicalsection exposed by drilling hole and seismic exploring informations, The geological section of LYC1line interpreted by WFEM are obtained. The interface depth between second electrical layer and third electrical layer obtained by WFEM (which is1350-1500m) and the interface depth between ganhe group and jiufengshan group of Cretaceous (which is1600m) are nearly equal. The oil and gas show of jiufengshan group are exposed by Yangcan1well. The reflect to Dayangshu basin oil and gas formation of WFEM and CSAMT is compared and studied, the effectivity of WFEM to prospect Dayangshu basin oil and gas formation would be affirmedThe results of WFEM effectivity experiment at dayangshu basin WFEM are stimulative. So it is suggested that prospecting oil and gas resource used WFEM should be further made at Dayangshu or similar area.