Generating MODFLOW grids from boundary representation solid models

Complex stratigraphy can be difficult to simulate in MODFLOW models. MODFLOW uses a structured grid that requires that each grid layer be continuous throughout the model domain. This makes it difficult to explicitly represent common features such as pinchouts and embedded seams in a MODFLOW model. In this paper, we describe a method for automatically generating MODFLOW-compatible grids from boundary-representation solid models. Solid models are data structures developed originally for computer-aided design applications that define the geometry of three-dimensional objects. Solid models can be used to represent arbitrarily complex stratigraphy. The elevations defined by the solids are then extracted from the solids in a manner that preserves the continuous-layer requirement imposed by MODFLOW. Two basic approaches are described: The first method adjusts the MODFLOW grid dimensions (layer elevations) to fit the solid model boundaries, and the second method creates a regular MODFLOW grid and adjusts the material properties to match the changes in stratigraphy. One of the main benefits of using solid models to define stratigraphy for MODFLOW models is that it provides a grid-independent definition of the layer elevations that can be used to immediately re-create the MODFLOW grid geometry after any change to the grid resolution.