Research On Magnetotelluric Forward Simulation of Typical Karst Geothermal Reservoir

Electromagnetic method have been proved to be effective in geothermal exploration. Faced with the problems of fine description of buried depth, fine depiction of fault and fine prediction of water-bearing area by electromagnetic method in geothermal reservoir exploration. Based on the geological features of typical work area, physical property-geological model from three aspects is established, including the karst reservoir buried depth, fracture characteristics and water-bearing of reservoir. Finite element method is used to simulate and analyze the model to understand the identification rule and limitation of electromagnetic response of karst reservoir under different geological conditions. It is of guiding significance to the acquisition, processing and interpretation of electromagnetic method of karst reservoir. The results show as follow: (1) The inversion depth of the stratum within 1000 m is basically consistent with the actual model. However, with the increase of the buried depth of the thermal reservoir, the low frequency signal of the shallow thick layer is limited, and the error of the inversion depth of the stratum increases, which needs to be corrected by the relevant geological data. (2) The different fault distance, dip angle and fracture zone width of the fault will cause the gradient change of the apparent resistivity contour. The larger the dip angle of the formation, the steeper the change of the apparent resistivity contour. The larger the fault distance, the greater the difference between the apparent resistivity contours on both sides of the fault, while the change of the fracture zone width on the apparent resistivity contour can be ignored. (3) In the case of water-bearing of reservoir, if the fault is filled with water, the apparent resistivity isoline pull phenomenon is obvious, and the water-bearing on the upper and lower wall is opposite to the actual profile interpretation. If the karst is filled with water, it is related to the buried depth and length of the karst body and the electrical property of the overlying strata, and the apparent resistivity isoline is symmetrical concave.