A Joint Inversion Based On The Data Of Multi-source And Multi-crosshole And Its Application In The Electromagnetic Tomography

It is based on both the status and the developing direction of the electromagnetic tomography technique, and the geological engineering investigations for the engineering exploration, the environmental protection, the cultural relics survey, the disaster prevention to carry out the study on the high-resolution cross-hole electromagnetic tomography based on the multi-source, multi-crosshole data. The studied contents mainly include five parts: the first one, to develop the theory and methods for the electromagnetic propagation in the complex mediums such as the Karst regions, the goave, the crushed zone, the water layers, and the coal layers, etc. The numerical characteristics of the electromagnetic field based on the different geological models are investigated by using a large number of numerical calculations, where the models are designed in accordance with the actual geological conditions. Secondly, the tomographic inversion has been developed in this study to reconstruct the images for either absorption coefficient or attenuation factor of the electromagnetic waves. The electric field value, the result of the numeric modeling, was added 5% random perturbations to be regarded as the observations, which were then input into the tomographic system to check the method and resolution. The third one, a joint inversion was developed based on multi-source, multi-crosshole data, in an attempt to better constraint the properties of matter near the drilling holes. The fourth, the multi-source multi-crosshole joint inversion has been applied to the practical engineering geological survey. In this study, three examples are illustrated to display the excellent behavior of the tomographic inversion. The three applications are the geological investigation for the railway subgrade in Yuhuai railway (K419+710～K419+850), the Karst investigation for a bridge in national main line between Erlianhaote and Hekou, the exploration for the Tianshengxia bridge site basis on the Guiyang-Kunming railway, and the soil cave exploration for Zhengzhou-Xi'an passenger dedicated line. Finally, data from the forward modeling and inversion, the calculated programs for nodeling and inversion, and the actual data processing are collected by a software system for the electromagnetic tomography which is developed by using VC++ language in this study.Forward model is the basis of electromagnetic tomography. In the previous forward model of the electric field attenuation, an approximate is used to neglect the medium effect of the source region on the electromagnetic wave propagation. Thus the effects from the near field have to be added into the attenuation imaging. As a result, the accuracy of imaging is greatly degraded. Considering the effect of the source (near-field region) medium on the electrical wave propagation, this study investigate the attenuation characteristics of the electric field, and try to construct a forward model for the application of high-resolution electromagnetic tomography. First of all, an electric field attenuation factor is introduced as the reciprocal of the electric absorption coefficient. A successive linear tomography method is established for the attenuation factor. Then, an excitation with point-source nature is introduced to replace the near-field processing, which is substituted into the imaging equation. Both the excitations and the attenuation factor are obtained by solving the equation. The joint inversion would effectively eliminate the near-field effects from the electrical conductivity of rocks in source regions, so as to overcome the lack of a simple source approximate calculation in the traditional methods of electromagnetic imaging. In the joint inversion for both attenuation factor and source function, the sparse linear equations are solved by using the least square QR decomposition method (LSQR), which let the equation quickly convergence. It is based on both the conductivity and permeability of the actual geological structures, such as the different rocks, soil layers, water layers, hollow caves, etc, to compute the absorption coefficient distribution of the electromagnetic wave, and to further construct the forward models for the numeric test. In the forward calculations, the source excitations are assumed by considering the input current intensity and source regional medium. Through model testing, the characteristics of the electric field strength for the different types of geological bodies are obtained. A large number of the actual observational data was used to check the reliability of the electric field based on the attenuation tomography presented in this study.A database has been built by integrating the numeric test data, the engineering examples, the forward method and inversion. Using VC++ programming language, I establish a visually tomography software which is based on the multi-source, multi-crosshole electromagnetic data.