Conductivity Depth Imaging Of Airborne Electromagnetic Data Based On Double-pulse Transmitting Current

Airborne electromagnetic exploration is a geophysical exploration method based on the electromagnetic induction theory as the basis,with aircraft as a platform equipped with electromagnetic exploration device.It has many advantages such as low cost,high efficiency,fast speed,good passage,wide coverage and so on.Airborne electromagnetic systems mainly transmit bipolar single pulse to complete the electromagnetic exploration task.With the development of airborne electromagnetic exploration technology,CGG company developed the latest MULTIPULSE system can transmit double-pulse,which transmits a half-sine pulse of high power at the front,and then transmits a trapezoidal pulse with low power.It not only keeps the deep target detection ability,but also improves the shallow resolution.In this paper,we have developed double-pulse CDI approach based on the model fusion and the look-up table.The major findings and content are as follows:Based on the basic principle of airborne electromagnetic exploration,the forward algorithm of electromagnetic response with double pulse emission current is deduced by linear convolution,then we calculate the full-time electromagnetic response of the double pulse emission current.According to the forward calculation,the electromagnetic response of single and double waves under the condition of multiple target conductivity is analyzed and compared.Finally,we analysis how the parameters of double wave emission current influences the electromagnetic response.The results show that if the half sine pulse width is 4ms,the trapezoidal pulse width is 1ms,the double wave length of 10 ms,when the half sine pulse width range is 10%,the trapezoidal wave rising edge and falling edge width range is 4%,and the double wave distance variation range is 0.2%,the relative error of the response will be less than 4%.Based on the traditional single wave look-up table CDI,the double-pulse conductivity depth imaging with model fusion is proposed.When the electrical conductivity is 0.1S/m,the corresponding CDI results are obtained by using the single wave look-up table for the electromagnetic response of the half sine wave and the trapezoidal wave.Ascending order two sets of depth to get the fusion depth,and the conductivity of the two groups is obtained by the interpolation algorithm,the fusion conductivity is obtained by weighting the variable parameters.Based on the results of one-dimensional and quasi-two-dimensional simulation,we cannot detect the depth conductive layer from the CDI results of trapezoidal pulse,and the curve for the CDI results of half-sine pulse is unable of indicating that the existence of shallow conductive anomaly.However,the CDI result with model fusion not only clearly reveals the shallow depth of the high resistivity,and can better reflect the deep conductive anomaly.According to the characteristics of double-pulse response,we propose a new method of double wave CDI with look-up table.Based on the forward calculation of electromagnetic double wave response,this paper establishes a table of tB-sB-conductivity-altitude,uses joint lookup method of two-response(tB,sB)and interpolation algorithm to determine the conductivity,and then takes advantages of skin depth formula to obtain the depth,finally obtains the results of the joint Conductivity depth imaging.Based on the results of one-dimensional and quasi-two-dimensional simulation,we can obtain that the CDI results from trapezoidal pulse fails to indicate the resistive basement under conductivity layer,and the CDI results from half-sine pulse can not reflect the shallow geological conducting information.However,the double-pulse CDI results from look-up table can clearly identify the underground target from shallow to deep.Based on the electromagnetic response forward,the electromagnetic response of the low resistivity model and the high resistance model is calculated when the depth of the thin layer and the surrounding rock conductivity change.We calculate the relative error of apparent resistivity after it can be imaged by look-up table and model fusion.When the resolution limit is 10%,we analyzed the resolving power of airborne electromagnetic system to thin layer.Compare the CDI results of lookup table with single pulse,the CDI results of double pulse with model fusion and with look-up table,the double wave conductivity depth imaging has better resolving power to the abnormal body.