1. Department of Energy and Mineral Engineering, The Pennsylvania State University, 231 Hosler Building, University Park, PA 16802 e-mail:
2. Department of Energy and Mineral Engineering, The Pennsylvania State University, 204 Hosler Building, University Park, PA 16802 e-mail:
3. Department of Energy and Mineral Engineering, The Pennsylvania State University, 410 Boucke Building, University Park, PA 16802 e-mail: