Research On ZTEM Three-dimensional Forward Modeling And Inversion

Artificial source frequency-domain airborne Electromagnetic(AEM) method has the advantage of effective data acquisition procedure. Unfortunately, due to the transmitting frequency is about 200~195000Hz, the effective recognition of the geological body is limited several or hundreds of meters under the ground surface.The magnetotelluric(MT) method collects the electromagnetic information that results from natural sources, and it has the character of great exploration depth and wide applicable range, and has been widely applied in many fields. However, a practical limitation of MT technique is that surveys are costly and time consuming. In an effort to continue use the penetration advantage of natural sources, we consider collected the electromagnetic data that result from natural sources in an aircraft. It is this understanding that prompted the development of Audio Frequency Magnetics(AFMAG) technique. This has resulted in the Z-Axis Tipper Electromagnetic(ZTEM)Technique. In ZTEM, the useable bandwidth is about 25~720Hz, the vertical component of the magnetic field is recorded by airborne electromagnetic data acquisition system above the entire survey area, while the horizontal orthogonal fields are recorded at a ground-based reference station. MT data processing techniques yield frequency domain tipper data. As a result, ZTEM can provide information about 3-D electrical conductivity structure efficiently.In this paper, we develop and implement the 3D ZTEM forward modeling algorithm using the staggered-grid finite difference method that based on the magnetic value, and the algorithm has been verified by calculating a theoretical geo-electrical model. Numerical modeling for classic theoretical models are calculated in order to analyze the anomaly features of ZTEM, included the tipper and its divergence. In addition, based on a data-space OCCAM approach of 3D MT inversion algorithm, we develop a 3D data-space OCCAM inversion algorithm to invert tipper data of ZTEM. The algorithm is applied to synthetic examples for inverting, and shows the ability of recovering the model in lateral boundary constraint is superior to vertical constraint. The synthetic example demonstrates the validity ofthe data-space OCCAM approach for inverting tipper data of ZTEM. This algorithm is suitable for inverting the tipper data acquired from the ZTEM and achieve 3D geological structure imaging of large area so as to obtain geo-electrical structure underground.