| Airborne time-domain electromagnetic method is a geophysical survey means, which is based on the law of electromagnetic induction. The electromagnetic detection devices are placed on the aircraft to detect the earth. By inversing the measured data, we can obtain the distribution of the underground geologic conductivity. But this process takes too long because of the large amount of the measured data. While conductivity depth imaging has been proved useful and efficient to show the underground conductivity distribution, and has a more widespread application in the actual survey. In addition, the CDI results of different earth models have the certain guiding sense to the detection instrument design.The article is relied on the National Natural Science Foundation of the holistic inversion of fixed-wing airborne electromagnetic data. We calculate the magnetic fields B and magnetic induction d B/dt and obtain the transformation of standard step-response. We study the conductivity depth imaging algorithm based on step-response data. Furthermore, the suggestion of the detection instrument design has been given via the comparison of CDI’s results. The major findings and content are as follows:Based on the basic principles of airborne electromagnetic method, we calculate the B field and the d B/dt response when the system waveform is step current, and the B field and the d B/dt response of arbitrary waveform can be abtained by convolution. In practical AEM, the d B/dt response can be fleetly obtained from the measured induction electromotive force, thus the B field response can be calculated from d B/dt response by numerical integration.By studying the system waveform’s influence on electromagnetic response and CDI, response from any arbitrary waveform can be converted to the standard step response by singular-value decomposition in Tau domain. As the decompositions of the data from step current and any arbitrary waveform are analogous, we convolve the pure exponential decays of discrete time constant gained from step response with the transmitter waveform as sampled at the receiver coil, thus we obtain a set of basic functions particular to the AEM system with arbitrary transmitter waveform. By decomposing arbitrary waveform data with the above-mentioned basis functions, we get the amplitude coefficients, corresponding to each time constant base. The standard step-response is then calculated easily by combining the amplitude coefficients with the pure exponential decays of step response.Based on the relation between the B field response launched by step current and image depth, we can obtain the image depth by using the theory of linear regression to fit the B field response-depth, thus the image depth and CDI depth corresponding to each time channel are calculated. Based on the basic principles of airborne electromagnetic method, the time and conductivity integral equation is deduced. With the conductivity of the depth corresponding to each time channel as the note, we use a set of cubic polynomials to represent the conductivity which is separately defined on each interval, and obtain the equations with conductivity to be determined. Defining a first-derivative smoothness and a second-derivative smoothness as two constrained quantities, and using the least-square sense, the matrix equation respect to conductivity has been gained. Combine the conductivity by solving the matrix equation and the CDI’ depth, thus we complete CDI algorithm. When applied into the one-dimensional and quasi-two-dimensional forward simulation data, the CDI based on step-response gets more ideal fitting effects.Compared the CDI results of CDI based on step-response and lookup method when applied into the quasi-two-dimensional forward simulation data, the CDI based on step-response gets more ideal fitting effects and has better detection capability. And the detection capability of the anomalous body for the airborne electromagnetic survey has been studied as the theoretical direction. |
PDF Full Text Request