and Hydrogeology 1 INTRODUCTION TO HYDROGEOPHYSICS 12 SUSAN S. HUBBARD and YORAM RUBIN 1 Lawrence Berkeley National Laboratory, Earth Sciences Division, Berkeley, CA 94720 USA. sshubbard@lbl. gov 2 Department of Civil and Environmental Engineering, UC Berkeley, Berkeley, CA 94720, USA In this chapter, we discuss the need for improved hydrogeological characterization and monitoring approaches, and how that need has provided an impetus for the development of an area of research called hydrogeophysics. We briefly describe how this research area has evolved in recent years in response to the need...