Development Of Broadband Low-frequency Continuous-wave Electromagnetic Method Instruments

With the development of our society, economic and urbanization are more andmore common. Housing demolition and reconstruction、subway construction areoverwhelming. We should pay more attention to the geological structure. Shallowstructure of the original underground—Waterway、electric circuit and Natural gaspipeline had been broken by barbaric construction. Thus causes economic losses andcasualties. Shallow surface exploration is particularly important. Shallow surfacegeological structure and line pipe, etc. are reflected the electrical anomaly andmagnetic anomaly. Non-destructive detection of underground structures in thepre-construction by electromagnetic method instrument is necessary. Calculate thedistribution through space, software maps and describe the underground pipeline willbring help to the construction. Multi-frequency electromagnetic launch multiplefrequency electromagnetic wave at the same time. Penetration of each frequencyelectromagnetic waves is not the same. So multi-frequency electromagnetic methodcan achieve unusual probe in different depths of the underground.Here describes a broadband low-frequency continuous-wave electromagneticmethod instrument. By drawing on the experience of the design plan of relatedproducts and advanced equipment design experience home and abroad, proposed a setof design plan. System based on FPGA and DSP technology, realized multi-frequencyhigh-power signal transmission、multi-frequency signal matching receiver、signalacquisition and transmission at a high speed.、DSP data processing、FPGA globalcontrolling、store data in U disk、data processing and mapping by the host compute. Atthe same time in the hardware design，design and product the transmitting coil andreceiving coil，in which design is a high-power op amp at the transmitter part. Enlargethe waveform emitted from DA in order to achieve the power transmissionrequirements. Transmit coil design into the multi-turn parallel mode，number of turnsare equal to normal wound coil turns.，reduce the resistance and inductance. In thehigh frequency, peak large and signal to noise ratio higher waveform can also beissued. Design a reference coils is a reference coil in front of the receiver coil， Reference coils are composed by two groups of coils which number of turns andcross-sectional area are the same. The number of turns multiplied by thecross-sectional area is far less than the receiving coil number of turns multiplied bythe cross-sectional area. By calculating，induction of the coil output voltage isproportional to the number of turns of the coil and the cross-sectional area， inverselyto the cubic of distance. Put the reference coil in the between receiving coil andtransmitter coil. Take the overall system to higher ground. In the case of only thetransmit coil excitation，mobilize the position of the reference coils，make the inducedelectromotive of the two reference coil are equal to the receiving coil’s. One of thetwo reference coils is connected reversely to the receiving coil in order to offset theprimary field. Another reference coil is used to receive the primary field. We canknow the information of the abnormal body through these two data.The acquisition part are: front-end match enlarge、high frequency filtering、powerfrequency notch、signal driving and then collected by AD, in which we shouldselect dual-channel AD, primary field and secondary field can be collected at the sametime. Two channels of the dual-channel AD have the same indicators、technology.analog/digital isolation of the receive part using magnetic dipole isolation. Eachpiece of board is a separate power supply because of the power limited of DC-DC.Each board is connect by a magnetic dipole isolation chip, will not work disorder athigh and low reference question. During a simple digital filtering, extract signalamplitude of each frequency by a LIA. The results are stored in the U disk by a MCU.The software part of the design is FPGA、 DSP and MCU and the hostcomputer Matlab, LabVIEW, specific design as follows: FPGA control DA, makeDA issue the desired waveform. Then loaded into transmitter coil through the poweramplifier. Generate switching sequence at the same time to control the programmablefilter, AD and DSP. Make data high-speed acquisition by the control of FPGA.After collecting extracted each required frequency from the other frequency and noise.Transmitted the information to the MCU by UART. Make the stored data converted todecimal format at the host computer by LABVIEW, make a simple filtering，nonlinearregression using matlab, and then mapping by Surfer.