The solution for a transient electric dipole in em geosci uses `erf`:  (http://em.geosci.xyz/content/maxwell1_fundamentals/dipole_sources_in_homogeneous_media/electric_dipole_time/analytic_solution.html) This is inconsistent with Ward and Hohmann (`erfc`. Note that `erfc(x) = 1-erf(x)`)  cc. @sgkang, @dccowan