| Transient electromagnetic method(TEM)is widely used in advanced geological exploration,city construction engineering and mineral mining exploration.The transient electromagnetic method sends a pulsed magnetic field into the geological body through an ungrounded transmitter loop,receives the eddy current of secondary field which is generated by electromagnetic induction in the geological body,and detects the abnormal geological body.Since the self-inductance and mutual inductance between the transmitting and receiving coils produce a primary field which interferes with the secondary field signal,brings a certain blind zone in shallow detection application.To solve the problem of shallow detection by transient electromagnetic method,some scholars have proposed a TEM of equivalent inverse magnetic flux.The electromagnetic model and test shows that which can effectively eliminate the primary magnetic field signal,and has a broad application prospect.The equivalent reverse magnetic flux transient electromagnetic method uses the transmitting current to pass through two same transmitting coils with opposite winding directions,and the receiving coil is arranged on the zero magnetic flux position to detect the secondary field signal.Between two identical transmitting coils,the receiving coil is put on the position of zero magnetic flux with mechanical placement adjustment.This adjustment requires:(1)two transmitting parameters are the same;(2)Mechanically adjusting the receiving coil is difficult to operate and easy to change its position.In order to solve the above two problems,a self-balanced opposing coils transient electromagnetic instrument is designed.By independently driving the transmitting coils currents,place the receiving coil on the zero magnetic flux position instead of mechanical adjustment mode.This article focuses on the development in three parts:(1)Design a transient electromagnetic instrument using the principle of opposing coils transient electromagnetic;(2)Use microcontrollers to set a specified current signal,independently drive the upper and lower transmitting coils,and return according to the primary field of the received signal.The change of the work value adjusts the current of one of the coils until the minimum value of the work value is reached to achieve the purpose of accurately determining the zero magnetic flux surface;(3)The use of multichannel transmitting and collecting coil devices to improve detection efficiency.This article first uses ANASYS Maxwell software to model,verify that there is a zero magnetic flux surface in the equivalent reverse flux transient electromagnetic method,and secondly verify that changing the current size can indeed make the receiving coil in the position of the zero magnetic flux surface.Then the hardware system and software system of the instrument are introduced in detail.Finally,the effectiveness of the instrument is verified through performance testing and field detection experiments in Yunnan mines.The working process of the instrument is to use the master computer control software to operate the instrument,transmit electromagnetic eddy current signals to the detection area through the transmitting coil,and use the receiving coil to receive the secondary magnetic field induced by the geological body.The received signal is processed by the microprocessor and directly uploaded to the tablet computer.After analysis,the results of the geological advanced report and the two-dimensional visualization result map were obtained on the spot,which were used to analyze the geological structure of the coal mining area and improved the efficiency of detection and construction.The structure design is presentedd including aspects of receiving coil preprocessing circuit,transmitting module,receiving module,power module and WIFI module communication circuit.The instrument uses the STM32F429 microprocessor as the main controller,and two sets of coil devices are designed in the four-transmit and two-receive mode.First,design of receiving coil signal pre-amplification circuit.The instrument controls and drives the isolation device SI8233 to output weak current to control a strong current,and transmits a bipolar pulse current signal to the transmitter coil through an H-bridge constant current circuit.The magnetic field is quickly offset by the upper and lower transmitting coils,and the secondary field signal generated by the geological body induced by the downward conduction eddy current is captured by the receiving coil,after signal conditioning,it is converted into a digital signal by the ADS1278 sampling chip and transmitted to the microprocessor for processing.data processing.Finally,the main control chip and the master computer control software adopt Socket wireless communication based on the TCP/IP protocol to realize Socket wireless data collection.A supporting embedded signal acquisition software was developed.It is equipped with RT-Thread real-time operating system,and uses task scheduling to realize functions such as receiving commands,transmitting current,signal acquisition,data transmission and adjusting zero magnetic flux position.The master computer software is developed based on the Qt platform,and includes functional modules such as project file setting,transmitting parameter setting,waveform display,file management and so on.After the collection is completed,sent data to the upper computer software.The master computer software draws single-shot waveforms and multi-channel waveforms according to the received data,and saves them as engineering data files.A series of tests are performed on the instrument's emission current,emission frequency,receiving voltage and software functions.The instrument's emission current measurement accuracy reaches 0.005 m A,the current pulse width is adjustable at 10,20,and 40 ms,and the off time is 0.5 to 300 ms.All important indicators of the instrument have reached the proposed design requirements.And a Yunnan mine is selected,and the exploration experiment was carried out in the mine.The experimental results are presented in a two-dimensional visualized result graph.According to the changes in apparent resistivity,the distribution positions of the low-resistance anomaly area and the high-resistance anomaly area are analyzed,which is basically consistent with the actual geological drilling situation.All the experimental results verify the effectiveness of the instrument and accuracy. |
PDF Full Text Request