| Airborne electromagnetic(AEM)is a geophysical exploration method based on aircraft.The transmitting and receiving systems are all equipped on the flight platform,with the advantages of fast speed,good traffic and high efficiency.For AEM systems,there is an increasing demand for shallow geological structure exploration in high-resolution.Based on the high-frequency pseudo-random current,this thesis researches on the time domain identification method with high-precision,and accurately extracting the electromagnetic response of shallow earth.The main work is as follows:(1)According to the AEM theory,the numerical model for an AEM system using pseudo-random current is established.The model includes the modeling of the transmitting system,the observing system and the earth system,with generation of the pseudo-random signal,the primary field,the secondary field,the background field and the noise signal.The mathematical expressions of the observing current signal and AEM response are obtained,which providing a basis for the following research.(2)In order to solve the problem that the poor-accuracy for the traditional identification method,caused by the correlation side-lobe effect,a time-domain identification method using cyclic correlation is proposed.The method includes the calculation of cyclic correlation function in time domain and the solution of Wiener-Hopf equation based on conjugate gradient iteration method.The identification results of the simulation data show that the time domain identification method proposed in this thesis has higher accuracy and a certain suppression effect on the noise in the early data,which is conducive to the accurate identification of the electromagnetic response of shallow earth.(3)Focusing on the main factors such as the first gate time,the primary field and the observing system,the thesis analyzes the influence on shallow exploration,and studies the key technologies to eliminate the influence and extract the electromagnetic response of shallow earth.The technologies include the method of increasing the sampling rate equivalently to advance the first gate time by using the fractional sampling rate and data reorganization,the method of eliminating the primary field by matching the peak value of the product of the primary field coefficient and auto-correlation function with cross-correlation function and adding together in the process of identification,and the method of removing the influence of observing system through the identification results of electromagnetic data at low altitude,high altitude and electromagnetic shielding room.The forward models with the shallow anomaly layer are established to simulate the electromagnetic data of an AEM system using pseudo-random current.The identification results obtained by the time domain identification method and the inversion results calculated by Aarhus Inv program show that the key technologies effectively improve the consistence between the inversion results and the forward models.(4)In order to improve the data quality of an AEM using the pseudo-random current,this thesis proposes the preliminary data processing methods based on the field data.The methods include the data preprocessing methods,such as clipping abnormal data,correcting pseudo-random current waveform and reducing noise,as well as data identification,drawing and inversion.A basic data processing flow for the AEM data using the pseudo-random current is formed.Aiming at shallow AEM exploration using pseudo-random current,this thesis researches on the time domain identification algorithm with high accuracy.And the method of advancing the first gate time,eliminating the primary field and removing the influence of the observing system.Thus,the electromagnetic response of shallow earth is extracted accurately.These methods are the essential technical support for high resolution exploration of shallow geological structure,which has an important theoretical significance and practical value. |
PDF Full Text Request