3-D Monte Carlo Simulation of Bedrock Geology and Discrete Fracture Networks in Crystalline Bedrock Beneath the Mirror Lake Watershed, Hubbard Brook Experimental Forest, Woodstock, NH

Abstract:

Fracture properties are strongly correlated to the bedrock types underlying the watershed. The principal rock types are schist intruded by igneous rocks composed of granite and minor pegmatites. The rock types were logged in 40 bedrock wells (Johnston and Dunstan, 1998) and on adjacent roadcut exposures (Barton, Camerlo, and Bailey, 1997). These were used to constrain Monte Carlo simulations of the 3-D geology under the Mirror Lake Watershed, using FracMan® software developed by Golder Associates, Inc. The simulation shows sub-horizontal planar intrusions of the igneous rocks decreasing in volume from east to west under the watershed.

Once the 3-D geology was simulated, the fracture networks were simulated, separately for the igneous rocks and the schist. The fracture properties of - spatial position, orientation, trace length, aperture, and interconnectedness are constrained within a Monte Carlo simulation of the fracture network using the FracMan® software.

The Monte Carlo simulation of the fracture network reveals multiple 3-D high conductivity pathways extending across the bedrock underlying the Mirror Lake watershed. The remainder of the fracture network consists of dead ends that contribute to the storativity of the network but little to the overall conductivity. This behavior is similar to that observed in percolation theory at or above the percolation threshold.