We have developed a robust 2D joint inversion scheme incorporating the new concept of cross-gradients of electrical resistivity and seismic velocity as constraints so as to investigate more precisely the resistivity-velocity relationships in complex near-surface environments. The results of joint inversion of dc resistivity and seismic traveltime data from collocated experiments suggest that one can distinguish between different types or facies of unconsolidated and consolidated materials, refining a previously proposed resistivity-velocity interrelationship derived from separate inversions of the respective data sets. A consistent interpretive structural model can be obtained from the joint inversion models.