Conversion of Highly Complex Faulted Hydrostratigraphic Architectures into MODFLOW Grid for Groundwater Modeling
Abstract
The USGS MODFLOW is widely used for groundwater modeling. Because of using structured grid, all layers have to be continuous throughout the model domain. This makes it difficult to generate computational grid for complex hydrostratigraphic architectures including thin and discontinuous layers, interconnections of sand units, pinch-outs, and faults. In this study, we present a technique for automatically generating MODFLOW grid for complex aquifer systems of strongly sand-clay binary heterogeneity. To do so, an indicator geostatistical method is adopted to interpolate sand and clay distributions in a gridded two-dimensional plane along the structural dip for every one-foot vertical interval. A three-dimensional gridded binary geological architecture is reconstructed by assembling all two-dimensional planes. Then, the geological architecture is converted to MODFLOW computational grid by the procedures as follows. First, we determine bed boundary elevation of sand and clay units for each vertical column. Then, we determine the total number of bed boundaries for a vertical column by projecting the bed boundaries of its adjacent four vertical columns to the column. This step is of importance to preserve flow pathways, especially for narrow connections between sand units. Finally, we determine the number of MODFLOW layers and assign layer indices to bed boundaries. A MATLAB code was developed to implement the technique. The inputs for the code are bed boundary data from well logs, a structural dip, minimal layer thickness, and the number of layers. The outputs are MODFLOW grid of sand and clay indicators. The technique is able to generate grid that preserves fault features in the geological architecture. Moreover, the code is very efficient for regenerating MODFLOW grid with different grid resolutions. The technique was applied to MODFLOW grid generation for the fluvial aquifer system in Baton Rouge, Louisiana. The study area consists of the '1,200-foot' sand, the '1,500-foot' sand, the '1,700-foot' sand, and the '2,000-foot' sand and includes the east-west trending Baton Rouge fault and Denham Springs-Scotlandville fault. 288 electric well logs in East and West Baton Rouge Parishes were analyzed to construct the aquifer system, which was discretized into 93 rows, 137 columns and 76 layers of MODFLOW grid. Each computation cell has a size of 200 m by 200 m. The cell thickness varies from 3.0 m to 12.8 m. The constructed grid was used to develop a regional groundwater model that includes the '1,200-foot', '1,500-foot', and '1,700-foot' sands and the two faults. A parallel version of CMA-ES was developed in high performance computing to calibrate the complicated groundwater model and estimate model parameters. The result of the case study demonstrates the capability of the technique that exactly converts complex hydrostratigraphic architectures including faults into MODFLOW grid. The code ensures that all narrow sand connections including those through the faults are preserved. Using the complex grid, the error of groundwater model structure can be significantly reduced.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.H51G1276P
- Keywords:
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- 1805 HYDROLOGY Computational hydrology;
- 1846 HYDROLOGY Model calibration;
- 1829 HYDROLOGY Groundwater hydrology;
- 1835 HYDROLOGY Hydrogeophysics