Email: helm@gps.caltech.edu

B.S., University of Minnesota, 1961; M.S., University of California (San Diego), 1965; Ph.D., 1967. Assistant Professor, Caltech, 1970-74; Associate Professor, 1974-79; Professor, 1979-2000, 2011-; Smits Family Professor, 2000-11. Director, Seismological Laboratory, 1998-2003.

Seismic wave propagation and fine structures of the upper earth, as determined by P and S wave inversion; particularly interested in mapping ultra-low-velocity zones of the core-mantle boundary.