(ap, l∂ nat’ ik) The locus for a given traveltime of wave energy reflected or refracted at a surface. Wavefronts are aplanatic surfaces for reflection times observed at the source point; see Figures A-15 and W-3. Aplanatic surfaces can be drawn (for example) to define a salt-sediment interface (Figure A-15c); each combination of source point and geophone position defines one aplanatic surface. If the velocity and other assumptions are correct, the salt-sediment interface is the common tangent to all of the aplanatic surfaces. See Gardner (1949) and Musgrave et al. (1967).