Optimized inversion of TEM and CSAMT detection in goaf water accumulation area of shallow coal seam

Comprehensive geophysical prospecting is an effective method to delineate the water accumulation area in the gobin shallow coal seams.The conventional TEM method has calculation error when inverting non- central point data.The controllable source audio magnetotelluric method (CSAMT) has a wide detection frequency range and poor resolution to shallow geological bodies.In this paper,one-dimensional forward modeling of large fixed-loop TEM is used to calculate the transient electromagnetic response of different measuring points at different positions offset from the center of the transmission loop.Compared with the center point,the larger the deviation distance,the more serious the early signal distortion.Based on the one-dimensional forward modeling theory of large fixed-loop TEM,the non-center point data is calculated inversely,and the calculation results accurately restore the electrical characteristics of the model.The CSAMT frequency is optimized and combined with the buried depth of the target layer,the frequency points within the corresponding buried depth range are encrypted to improve CSAMT′s ability to distinguish shallow buried geological bodies.Through forward simulation and measured data processing,the results show that large fixed-loop TEM one-dimensional inversion can eliminate the early signal deviation caused by offset distance and restore the true resistivity response of the model.CSAMT with optimized frequency points can collect abundant information to realize the detection of shallow buried geological bodies.The results of this two optimal methods are verified by each other,which eliminates the multiple solutions of a single method and makes the detection results more reliable.It is an effective method for the accurate exploration of the gob water area in the shallow buried coal seam,and the results are accurate.