A cross-hole electromagnetic CT forward and inversion method in low resistivity formation

The rapid development of urban underground space development and rail transit has an increasing demand for inter hole electromagnetic wave CT detection in low resistivity Karst development areas and fault zones. Due to the high absorption of electromagnetic waves by low resistivity formations, frequency reduction is necessary to ensure the detection distance. The existing CT forward and inversion modeling methods for inter hole electromagnetic waves are based on high-frequency ray theory to calculate amplitude measurement information. The error in using CT forward and inversion modeling for low-frequency electromagnetic waves in low resistivity formations will increase, affecting the inversion resolution. A joint method based on amplitude and phase is used to realize forward and inverse calculation for low-frequency electromagnetic wave CT under low resistivity formation conditions. The tensor Green’s function in one-dimensional Sommerfeld integral form in two-dimensional layered media is derived, and the influence of surface wave effects is calculated. The envelope features of surface wave effects are extracted. The iterative inversion method based on Laplace operator constraints is used to improve the resolution of low-frequency signal inversion. The technology was successfully applied to the 70 m distance inversion in the Sanjianghui area of Hangzhou and the 27 m distance inversion in a Karst development area of a certain subway line in Longquan Mountain, Wuhan. The inversion results in Hangzhou were consistent with the formation stratification, and a Karst cave with a height of 1.1 meters was successfully detected in Longquan Mountain. This method has been fully validated.