The transient electromagnetic method (TEM) is an efficient geophysical prospecting method widely used in mineral exploration, groundwater detection, etc.
When working by TEM, firstly, the transmitter injects the current into the transmitter loop, which will excite the primary magnetic field in the ground. When the transmitter current is abruptly turned off, then the receiver measures the changing rate of the secondary magnetic field using the receiver loop. Ideally, the primary field is therefore absent while measuring. The transmitter loop of TEM system has large inductance and small resistance. Its power supply utilizes rechargeable batteries and the H-bridge inverter is used to generate bipolar pulse current. In most TEM systems, the typical waveform of transmitter current is referred to as a bipolar square pulse. However, in practice, the real current waveform is often distorted and different from an ideal square pulse because of the dynamic characteristics resulting from the inductive load (the transmitter loop), the energy-storage components and the parasitic parameters of circuit elements. The waveform quality of the transmitter current affects the measuring accuracy.
Therefore, there has been a lot of research to further shorten the fall time of current pulse and improve the linearity of falling-edge.
Jang Chol Jin, a researcher at the Nano Physics Engineering Institute, has proposed a TEM transmitter topology using the passive voltage clamp circuit and the divided transmitter loop. The proposed scheme can greatly shorten the fall time of current pulse and improve the linearity of falling-edge, which is of great significance in mine TEM surveys or shallow TEM detections. Besides, it has simple circuit structure.
The details are found in his paper “Fast Turn-off of Pulse Current using the Clamping Circuit and Divided Loop in TEM Transmitter” in “Informatica” (SCI).
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