The Separation Method Of Inductive-induced Polarization Effect In Time Domain Electromagnetic Method And The Measurement System Design Using Low-temperature SQUID

The time domain electromagnetic method is a geophysical exploration method based on electromagnetic induction theory.It has been widely used in the mineral resources exploration,geological disaster investigation,environmental monitoring,et al.,especially in the exploration of metal mineral resources at over 500 meters below ground.Lead,zinc,silver,et al.are critical industrial materials,mainly exist in the form of sulfides,which often exhibit polarization effects under the excitation of electromagnetic fields.The polarization effect signal in time domain electromagnetic method is a rapid decay curve crossing zero and then reversal sign,the signal at a late time is tough to observe.In practical,since the traditional measurement system mostly uses the induction coil as the sensor,the bandwidth and sensitivity of the coil are mutually limited,which causes the measurement system hardly to measure the induction-polarization effect.To overcome the difficulties in identifying,separating and measuring induced polarization effect in time domain electromagnetic method.Under the support of the key scientific and technological research project of Jilin Science and Technology Support Program,“SQUID-based magnetic polarization-induction dual-field joint detection key technology research,” the sub-project "Development of Low-Temperature Superconducting Transient Electromagnetic System," the paper starts the research on the identification,separation and the measurement of induced polarization effect.The main innovative work is as follows:(1)To separate polarization effect in time domain electromagnetic detection,using lowfrequency approximation theory of Debye model,and the numerical simulation is realized by three-dimensional frequency domain finite difference method,and typical anomalous bodies are analyzed.The induced-polarization response characteristics,the fake polarization phenomenon caused by factors such as the parameters of the measurement system are discussed,which provides a theoretical basis for accurately understanding and identifying the polarization effect generated by underground media.(2)Based on the analysis of the induction and polarization response with a magnetic source,the induction and polarization responses in a resistive medium are approximated independently,and a negative value fitting compensation method is proposed.With the help of the method,the time constant characterizing the polarization information can be found and the decay time of the inductive signal is effectively prolonged.(3)In fact,the induction-polarization signal is complex,with extremely small amplitude,difficult to accurately measure,et al.,it is necessary to develop the acquisition insrtrument with high precision such as low temperature SQUID.The SQUID system achieves the measurement accuracy of 7.5fT/?Hz@10kHz,and realizes the stable operation under the strong human-induced electromagnetic interference environment.(4)Using the self-developed low-temperature SQUID system,field experiments were carried out in Hengsha Island,Shanghai,and Inner Mongolia Taohemu.In the strong interference environment,the anti-interference test of the measurement system is completed,and the magnetic field data of the long-time window of 300 ms is effectively measured in the low-resistance area.In the polymetallic ore polarization area,induced polarization signal at the fT-level is successfully measured for the first time in China.The magnetic field signal of the polarization effect and the anomaly loop experiment were carried out to verify the stability and reliability of the measurement system.The research effectively separate the induction-polarization effect in time domain electromagnetic detection,extract the induction and polarization information of the subsurface medium,and provide a theoretical basis for the induction-polarization multiparameter inversion.The induction-polarization measurement system based on the lowtemperature superconducting sensor is developed to provide new measurement equipment for studying the mechanism of induction-polarization effect,and on the other hand to realize metal ore detection in the second deep space of underground.