A small-loop double-coil decoupling TEM 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 environment. 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 transmitter (TX) coils with the same current direction and different radii. Simulation results indicate that the transmitting field of the DCDTEM device is more focused compared to the traditional central loop device, ensuring a stronger coupling with the target. 3-D TEM forward modeling results show that the DCDTEM device has the highest detection sensitivity, together with a relatively high lateral resolution compared to other small-loop TEM devices. We further carry out several physical experiments to test the response characteristics of the DCDTEM device with reference to both 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. The proposed DCDTEM device is also applied in a field test on water pipe detection, demonstrating its practicality in near-surface detection.