Development Of High Precision And High Dynamic Range Transient Electromagnetic Receiver

Transient electromagnetic method（TEM）is a promising geophysical exploration method with wide applications.However,nearsurface TEM exploration is faced with problems such as large mutual inductance between the receiving coil and transmitting coil,and fast attenuation of the TEM signal.In view of the above problems,this paper carried out the research on the integrated TEM transceiver coil and dualchannel parallel acquisition of the TEM signal.First,the integrated-transceiver coil for TEM and the analog buffer used to isolate the induction coil from the front-end acquisition unit for TEM instrument are developed.Using the integrated transceiver coil can reduce the mutual inductance and reduce the structural complexity of the TEM instrument.The designed coil size is 0.8×0.8 m2,the number of turns is 39,the effective physical area is 24.96 m2,the internal resistance is 0.498 Ω,and the resonant frequency of the designed coil is 305 kHz respectively.A buffer is integrated near the coil,which not only provides damping resistance for the coil,but also provides a low-impedance input signal for the TEM processing circuit of the receiver,which can effectively reduce the crosstalk on the transmission line.Secondly,an acquisition board for receiving TEM signals is developed,which uses two parallel acquisition channels to process the early and late TEM signals respectively,in order to cope with the huge difference in the intensity of the early and late signals.The early signal processing circuit participates in the whole process of signal reception to ensure the integrity of the data,the amplification factor is 1 times,and the output signal is transmitted to the ADC（Analog-to-digital converter）through a 10-fold attenuation circuit;the late signal processing circuit controls the gating through an analog switch,and the amplification factor is controlled by a relay,which is 25,125,and 625 times optional,the input is connected to ground through a 50 Q resistor before ungated to avoid the amplifier being affected by early TEM signals.Thirdly,use FPGA（Field Programmable Gate Array）to control the working state of the circuit and process the TEM signal.Control functions include switching of analog switches and relays,ADC mode selection and signal transmission.Processing functions include 50 Hz interference suppression in TEM signals,superposition of TEM signals,etc.The above functions can not only enhance the signal-to-noise ratio of TEM signals,but also greatly reduce the data processing burden of embedded systems.Finally,the functions and parameters of the receiver are analyzed and tested.The simulation analysis results show that the noise floor of the integrated transceiver coil and its buffer is 2.59 nV/Hz^1/2;the equivalent input noise of the early-time signal processing circuit is 46 nV/Hz^1/2,and the equivalent input noise of the late-time signal processing circuit is 1.22 nV/Hz^1/2.The measured results show that the noise power spectral density of the early-time signal processing circuit is-128 dB,and the noise power spectral density of the late-time signal processing circuit is-164 dB.The above analysis and test results show that the function of the receiver is normal,and the parameters basically meet the design requirements.Finally,according to the test results,an improved method is given to reduce the noise floor of the TEM receiver and increase its dynamic range.