3. EM/GPR Imaging Technologies -Invited-
4-D Inversion of Geophysical Monitorng Data Acquired over Dynamically Changing 4-D Earth Model
Jung-Ho Kim, Sam-Gyu Park, Myeong-Jong Yi and Jae Gon Kim
Korea Institute of Geoscience & Mineral Resources (KIGAM), Korea.
Abstract
In the geophysical monitoring to understand the change of subsurface material properties with time, the time-invariant static subsurface model is commonly adopted to reconstruct a time-lapse image. This assumption of static model, however, can be invalid particularly when fluid migrates very quickly in highly permeable medium in the brine injection experiment. In order to alleviate this problem, we develop a new least-squares inversion algorithm assuming that the subsurface model will change continuously in time. Instead of sampling a time-space model into numerous space models with a regular time interval, a few reference models in space domain at different times pre-selected are used to describe the subsurface structure continuously changing in time; the material property at a certain space coordinate are assumed to change linearly in time. Consequently, finding a space-time model can be simplified into obtaining several reference space models. The regularization along time axis is introduced in order to stabilize iterative inversion process as well as to calculate meaningful subsurface images varying with time. After verifying the performance of the developed algorithm through some numerical experiments of crosshole resistivity tomography, the proposed algorithm was applied to the monitoring data of crosshole resistivity tomography jointly performed with the dye tracer experiment. All the time-lapse data were simultaneously inverted by the developed algorithm and the resultant time-lapse and difference images were compared with the stain images formed by the migration of dye. The difference images and the dye stain images were matched very well with little inversion artifacts. The analyses of difference images, moreover, enabled us to identify the different types of flow pattern of solute in the undisturbed residual soil, which implies the effectiveness of the proposed algorithm.
Last modified: Tue Jun 27 20:01:59 2006