| With the development of the economy, the demand for energy is increasing. Theefficient resource exploration has been imperative. Fixed-wing airborne time-domainelectromagnetic method(FTEM) is one geophysical survey means, which is based onthe law of electromagnetic induction. The electromagnetic detection devices areplaced in the fixed-wing aircraft to detect the earth, which is high efficiency, low cost,can be widely used increasingly in the mineral exploration, geological survey, gasprograms, hydrographic survey and so on. Thus, conductivity depth imaging (CDI) ofthe huge number of data has been proved useful and efficient to obtain the distributionof the geologic conductivity.The article is relied on the National Natural Science Foundation of the holisticinversion of fixed-wing airborne electromagnetic data and the sub-topic of the specialresearch on the major national scientific research equipment,”airborne transientelectromagnetic system data processing and inversion imaging”. In order to meet theneeds of fixed-wing airborne electromagnetic detection equipment’s design, we studythe forward algorithm of the three component and the conductivity depth imagingalgorithm of simple-component or total field response. Based on the multi-componentand the bird attitude measurement, we have developed a joint conductivity-depthimaging approach based on multi-component data. The major findings and content areas follows:Based on the basic principles of airborne electromagnetic method, the methodsof the multi-component off-time response which launched by step pulse has beenstudied. According to the convolution nature, we obtain the multi-component off-timeresponse corresponding to arbitrary emission waveform. Based on the biot-savart law,the primary field of arbitrary emission waveform has been obtained and then the fullfield response can be calculated. We quantitative analysis how the trapezoidal and thehalf-sine current waveform variation influences the transient response. The resultsshow that uder the same trapezoidal transmit pulse of4ms, when the width of therising time of the trapezoidal pulse has the error of0.2ms, the width of the falling time of the trapezoidal pulse has the error of0.1ms, the relative error of theresponse will be less than4%for adopting the.trapezoidal transmit waveform. If weadopted the half-sine as the transmit waveform, uder the same half-sine transmit pulseof4ms, when the width of the transmit pulse has the error of0.4ms, the relativeerror of the response will be less than4%. Meanwhile, the more the transmit pulseclose to the rectangular pulse, the larger the response amplitude can be obtained.While, the total field data is commonly used for conductivity-depth imaging withregard to the fixed-wing airborne system, because it is difficult to obtain the correctmulti-component responses in the inertial coordinate system due to the limitation ofbird attitude measurement for its receiving device. Based on the forward algorithm,we copmare the B field and the dB dt response. The results show that the Bfiled has significantly greater energy at low frequencies and is advantageous fordetecting conductive targets. Copmpared the result which adopted the B field andthe dB dt resopnse as the conductivity-depth imaging data, B field gets moreideal fitting effects. Meanwhile, we study the impact of the altitude on theelectromagnetic response and conductivity depth imaging.The result shows that thealtitude can not only affect the amplitude of the electromagnetic response, but alsocause the larger error of the imaing precision. Finally, the influence of the transmitcurrent to the conductivity depth imaging has been studied. We find that the smalleroscillation of the transmit current also can causes the lager error of the imaingprecision.Nowadays, the bird attitude can be recorded by IUN or three GPS (Krazter,2007). Therefore, the development of the above makes it possible to obtain the correctmulti-component data in the inertial coordinate. Traditional fixed-wingelectromagnetic survey which using total field as the data for conductivity depthimaging, lost multi-component measurement information. Therefore a jointconductivity depth imaging approach based on the lookup method of two-componentB field response is proposed. Based on the fixed-wing electromagnetic forward, thispaper establishes a table ofBx-Bz-conductivity-altitude, uses joint lookup method oftwo-component B field and interpolation algorithm to determine the conductivityand then takes advantage of the skin depth formula to obtain the depth, finally obtainsthe results of the joint conductivity depth imaging. Compared the CDI results oftwo-component B field, total field and simple component lookup method whenapplied into the one-dimensional and quasi-two-dimensional forward simulation data, the CDI of two-component B field gets more ideal fitting effects and the imagingprecision is improved by7%.The detection capability of the anomalous body for the fixed-wing airborneelectromagnetic survey has been studied as the theoretical direction. When theresolution of the instrument is0.01nT, We obtained the limit of the resolution fordifferent model. Compared the CDI results of two-component B field, total fieldand simple-component lookup method when applied into the quasi-two-dimensionalforward simulation data, the CDI of two-component B field gets more ideal fittingeffects and has better detection capability. |
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