Research Of Forward And Inversion In3-D CSAMT Combining With Actual Data

Since the invention of controlled source audio-frequency magnetotelluric(CSAMT) at seventies of last century, it has became an important means ofelectromagnetic exploration, it has been widely used, whether on the onshoreexploration, marine electromagnetic exploration or airborne electromagneticexploration, it occupies an important position. With the progress of the study on thetheory of CSAMT method and prospecting instrument, the data processing andinterpretation methods has put forward higher requirements and urgent demand of3Ddata acquisition and accurate and effective data processing method, in view of thissituation, this paper we carry on the theoretical data three-dimensional numericalsimulation and limited memory quasi Newton method inversion, and study combiningwith the controlled source electromagnetic exploration actual data.In this paper, we carry on the CSAMT theory data3D forward by the staggeredgrid finite difference method. For the layered background model with low resistivityand high resistivity anomalies, the characteristics of electric field distribution in thepresence is analyzed and summarized.For the inversion aspects, this paper used the limited memory BFGS method(L-BFGS) to inverse. The method is one of quasi-Newton method, the advantage ofwhich is not need to store a large Hessian matrix or its inverse matrix, instead bystoring m steppes iteration information before, thereby reducing the space occupiedby the computer when the inversion. For calculation results of the theoretical model,this paper use the limited memory BFGS method, by giving different inversion initialbackground resistivity to data of different frequencies to inversion, and the resultswere analyzed. The result shows that this method for theoretical data can accuratelyreflect the information of anomaly including the resistivity, position and shape. It isproved that the limited memory quasi Newton method to the actual data of3Dinversion is feasible.At last, the paper carries out the inversion simulation of the actual CSAMT data.The inversion is carried out after editing the data into the structure of theoretical data,and we select single frequency for calculation. From the real data inversion results wefind that as the actual data is more complex, and numerical inversion of the resistivityis not precise enough, we can only determine the distribution of undergroundstructures according to the resistivity. Therefore, it is necessary for a more comprehensive and in-depth research on the actual data inversion.In this paper, the following conclusions are drawn from the analysis of thetheoretical and practical data:(1) Using the finite difference staggered grid method to model forward，analysis the calculation results we can summarize the law of electric field response,the laws also can provide ideas and reference for the inversion;(2) The application of L-BFGS3D inversion in theoretical model shows that ifwe choose the suitable inversion parameters, the method can reflect the location andresistivity of the anomalous body, and the rms drops fast, the model data convergesobviously;(3) We carry on L-BFGS inversion using the measured data. The resistivity ofthe actual data results can be clearly distinguished, the underground geologicalstructure can be judged by the distribution of resistivity.