Subsurface Visualization And Monitor Technique Based On Electrical Resistivity Tomography For Complicated Topography

Subsurface visualization technique is a new subject, which has become prevalent since 1990’s. The aim of this technique is describing the geophysical results with the visualization method. Nowadays, with the development of computer science, this technique has been widely used in many fields such as geophysical exploring, hydrology and environment. The big advantage of this technique is the ability to handle with huge number of data and more visually oriented. Besides, geophysical data used by this technique are mainly consisting of electrical resistivity data, seismic reflection data and ground penetrating radar data, which are all suit for tomography construction. In particular, the electrical resistivity tomography (ERT) is applied widely for its non-invade explore, powerful image and 3-D dynamic monitor. However, with the increasing need for improved characterization of the near surface, this method also meets many challenges such as complicated topography, topography effect removing and tomography resolution issue. Based on that, this research is aim to solve all these problems and the main contents and conclusions are showed as follow:1. The modeling of ERT for complicated topography:the best solution for topography involved ERT modeling is the finite element method with tetrahedral mesh. It was proved to be most flexible mesh which can represent topography with high accuracy. However, exist mesh generation software always have some limitations to get the high quality mesh. In that case, a tetrahedral mesh generation method based on constrained Centroidal Voronoi-Delaunay triangulation (CCVDT) was proposed. And the method gives the right size tetrahedral so that the modeling program can work with high accuracy. Additionally, we compiled a program for getting point sets by using the DEM of research area which will ensure that the generated tetrahedral mesh is obtaining for real terrain. Therefore, combined with electrical field theory and boundary condition, the modeling method for complicated topography can be built. And the program was proved to be reasonable and effective.2. The methods for removing the effect of complicated topography: Based on the modeling method above, the relation between apparent resistivity and complicated topographic feature has been clarified. The results of apparent resistivity feature under the different topographies show that topographic resistivity anomalies are serious in areas where slope angle are more than 15 degree. Different arrays such as pole-pole and wenner array give different resistivity reflections about the same topography. Besides, with the increase of slope angle, the topography effects become more significant. Meanwhile, much attention has been paid to the direction where the electrodes lines were laid, the topographies along the direction perpendicular to electrodes line could also affect the results heavily. Based on that, the ways to remove topography influence for 2-D and 3-D survey are respectively proposed. For a 2-D survey, topography effect can be easily removed by the method in which the apparent resistivity values for a homogeneous earth model with the observed topography are calculated to correct the measured data. But, the method is not available for 3-D survey. The 3-D resistivity inversion algorithm was proposed to solve complex topographies problems under 3-D survey. And it was proved to be stable and reliable which have the relative image error of less than 30% and the correlation coefficient of more than 90%.3. A new strategy for the reconstruction of resistivity by using inversed seismic data as prior model is presented. It’s an improvement of existing techniques and combines the ERT data with the seismic data. ERT inversion algorithm has been proved could accelerate the calculation greatly by comparing to that of RES2DINV. The strategy was applied to synthetic data as well as to field data. The converted prior model provided less runtime and high accurate results comparing to average prior mode. For one block model, the stabled RMS level difference is not obvious between two initial models. But, the better position and size information of anomaly was obtained from the converted prior model reconstruction scheme. In the case of two blocks model, the RMS level drop heavily when the converted prior model was used after first iteration which manifest the advantage of the strategy in reconstruction complicated subsurface structure. In particular, the strategy works well for the complicated Fault model where the velocity model does not agree with resistivity model completely. From the results of synthetic models, we can conclude that the new strategy is reliable and reasonable for better understanding and characterization of subsurface structures. The application of the multi-geophysical approach proved to be very useful in the interpretation of the field data from the Beishan region. The average prior model shows differences in the thickness of the geological layers, while the converted model gives a more reasonable geological scenario according to RMS level. However, more independent data is still needed to verify that our method presents more accurately subsurface structure.4. The application of subsurface visualization technique in hydrology: the traditional way for hydrology progress monitor and access the hydrogeological parameter have some kind of problems lead to great need in solution from geophysical field. The research executes the ERT modeling in the pump test. And the results show that the method has the ability to monitor the water-level fluctuations with high accuracy. Besides, we try to get hydrogeological parameter with ERT data. Finally, the method has been applied in Beishang china, and the results are positive.