A small-loop double-coil decoupling transient electromagnetic device for high-sensitivity near-surface detection

Small-loop transient electromagnetic (TEM) devices, several meters in size, are increasingly applied to near-surface geophysical exploration in space-limited environments. But the transmitter (TX) coil significantly affects the early time data acquisition of the receiver (RX) coil during current turn-off, resulting in the loss of shallow signals and creating a significant blind zone problem. We report a double-coil decoupling TEM (DCDTEM) device to solve this problem. The novel DCDTEM device ensures a weak primary magnetic flux region between the two coplanar TX coils with the same current direction and different radii. Simulation results indicate that the transmitting field of the DCDTEM device is more focused than the traditional central loop device, ensuring a stronger coupling with the target. The 3D TEM forward-modeling results show that the DCDTEM device has the highest detection sensitivity, together with a relatively high lateral resolution compared with other small-loop TEM devices. We further carry out several physical experiments to test the response characteristics of the DCDTEM device with reference to the offset and the ratio of turns between the TX and RX coils. The experimental results ensure that the position of the zero magnetic flux point can be accurately calibrated and mitigate interference from TX coils. Our DCDTEM device is also applied in a field test on water pipe detection, demonstrating its practicality in near-surface detection.