Research On Electromagnetic Sounding Excitation Method Based On Selective Harmonic Elimination Technology

Along with the increase of demand for energy resources and the reduction of easily mined and explored resources,high efficient and high precision detection method for complex terrain has become an important research aspect of geophysical exploration field.Ground-Airborne Frequency Domain Electromagnetic Method(GAFDEM)and Helicopter-borne Time-Domain Electromagnetic Method(HTEM)are by now the most widely applied detection methods.GAFDEM uses high power transmitting of ground electromagnetic method(GEM)and fast non-contact acquisition of airborne electromagnetic method(AEM)measurement method.Meanwhile,the frequency measurement used by GAFDEM is to reaction the information of the subsurface resistivity based on total fields obtained under the condition of uninterruptible transmitting current.With helicopter as the carrier,HTEM uses air-launched and air-received measurement method,and has advantages of fast exploration,low cost,large area coverage and so on.Meanwhile,the time domain measurement used by HTEM is to research secondary field without primary field,which simplifies the research of the anomalous field.The above two methods can achieve high efficiency survey in exploration blind zone of complex terrain,but the detection precision is limited by the transmitting waveform that generate primary field.GAFDEM generally uses 2~n pseudorandom wave as the standard transmitting waveform.According to the frequency sounding method,for high-precision exploration of specific depth targets,GAFDEM obtains the required frequency points by transmitting multiple sets of pseudo-random waves,but multiple transmissions and repeated flight measurements will seriously reduce the detection efficiency and increase the detection cost.HTEM generally uses pulse width modulation(PWM)technology to generate a bipolar trapezoidal wave transmitting current.Since the power storage capacity of the airborne transmitting system is limited,the inverter is expected to be implemented with low switching frequency to reduce the switching and snubber losses.However,low switching frequency directly affects the transmitting waveform quality,thereby affects the detection accuracy.So it is an urgent need to develop an new electromagnetic excitation method for improving the detection accuracy and efficiency of HTEM and GAFDEM.The transmitting system of GAFDEM and HTEM can be achieved by using pulse width modulation technology.Selective Harmonic Eliminated Pulse Width Modulation(SHEPWM)technology is one of the calculating method of the pulse width modulation technology can uses the precise mathematical model to realize the design of GAFDEM and HTEM transmitting system.However,for the conventional SHEPWM method with one quarter periodic symmetry,the output waveform can only be a waveform with one quarter periodic symmetry.It is not applicable for the HTEM and the GAFDEM.Therefore,this paper relaxes restriction condition of the conventional SHEPWM to semi periodic symmetry and periodic asymmetry,and then applied in GAFDEM and HTEM.As a result,the optimized design of the transmitting waveform can be realized.The corresponding research results are listed as below:(1)The restriction condition of the SHEPWM is relaxed to semi periodic symmetry.So the solution of nonlinear equations extend to 0~?,the odd harmonic components of output waveform of SHEPWM inverter can be controlled.And the SHEPWM method with semi periodic symmetry is applicable for the design of output waveform with one quarter periodic symmetry and semi periodic symmetry.The restriction condition of the SHEPWM is relaxed to periodic asymmetry.So the solution of nonlinear equations extend to 0~2?,the odd harmonic and DC components of output waveform of SHEPWM inverter can be controlled.And the SHEPWM method with periodic asymmetry is applicable for the design of output waveform with one quarter periodic symmetry,semi periodic symmetry and periodic asymmetry.(2)Applying the SHEPWM method with periodic asymmetry to GAFDEM.Compared with 2~n pseudorandom method,the SHEPWM method with periodic asymmetry can effectively control the number,amplitude,phase,bandwidth and distribution of the main frequency of the transmitting current,which provides the optimal transmitting waveform for the exploration targets at arbitrary depth and improves exploration efficiency and exploration accuracy.(3)Applying the SHEPWM method with semi periodic symmetry to HTEM.Compared with PWM method,the SHEPWM method with semi periodic symmetry has obvious advantage in the stability control of the transmitting current at the flat top and the suppress of the transmitting current reverse overshoot.Under the premise that the switching frequency is one-third of PWM,the transmission waveform quality and detection accuracy are improved.The results of laboratory experiments and field tests show that,the HTEM electromagnetic excitation method based on SHEPWM and the GAFDEM electromagnetic excitation method based on SHEPWM gain a good test effect,enjoy prospects for commercial application,and have certain guiding significance for the upgrading and updating of the electromagnetic prospecting method.