Electromagnetic field response characteristics excited by magnetic core coil for borehole TEM with scanning detection

Ultra-small coil is generally used by transient electromagnetic logging as the transmitting device. Due to the low intensity of excited transient electromagnetic field, only the rock layer near the borehole wall can be detected, but the remote radial detection of the outer borehole and the accurate location of geological anomalies cannot be realized. Based on the theory of transient electromagnetic field in whole space, the scanning detection method for borehole transient electromagnetic method(BTEM)is proposed. The magnetic core coil is used to excite the electromagnetic field, enhance the transmitting magnetic moment, and improve the radial detection distance of BTEM. At the same time, by flexibly adjusting the direction of the coils, the borehole wall can be scanned at 360° to realize all-round detection in the borehole radial direction, the spatial resolution and positioning accuracy of BTEM for the water-bearing geological body can be improved. Three-dimensional geological-geophysical model of borehole whole space is established, optimized mesh generation strategy is proposed, and special grid structure is used to encrypt and standardize the magnetic core coil in the borehole to improve the accuracy of numerical simulation. Using the finite element method, the electromagnetic field response of BTEM excited by magnetic core coil is calculated under the condition of uniform medium, and then the transient electromagnetic field response of different devices for the model of low resistivity geological anomaly is also calculated. The response characteristics of transient electromagnetic field with coplanar dipole device and coaxial dipole device and with different geometric parameters are analyzed. The whole space all-time apparent resistivity calculation and time to depth conversion algorithm are used to image the resistivity in whole space. The research shows that the electromagnetic field strength can be significantly enhanced by adding a magnetic core to the ultra-small coil of BTEM,and the amplitude of the enhancement is proportional to the diameter and length of the magnetic core. In comparison, the enhancement of the electromagnetic field response is more significant by increasing the length of the magnetic core. The detection result of BTEM with magnetic core coil in whole space can better reflect the transient electromagnetic field anomaly caused by low resistivity geological body in the surrounding rock layer of the borehole radial direction. Both the coplanar dipole device and the coaxial dipole device can obtain ideal detection effect, and the resolution of resistivity imaging in whole space is high, In comparison, the anomaly amplitude obtained by the coplanar dipole device is larger, and the positioning of the center of anomaly body is more accurate, while the description of the overall shape and boundary of the anomaly body by the coaxial dipole device is more accurate.