The characterization of three-dimensional inhomogeneities from reflection measurements is typically done in a three-dimensional measurement geometry where sources and receivers are independently deployed over some measurement surface. While such a geometry is theoretically sound for characterizing three-dimensional inhomogeneities, it is resource-intensive since it either requires a two-dimensional array of receivers and associated data acquisition and storage instrumentation or, when limited data acquisition resources are available, it is extremely time consuming. An alternative to full three-dimensional data acquisition is to employ the so called 212-dimensional reflection geometry where sources and receivers are deployed over a line (a two-dimensional measurement geometry) and this line is sequentially move in a direction perpendicular to the measurement line. Here it is shown that the 212-dimensional reflection measurement geometry is, with appropriate processing, rigorously valid for characterizing three-dimensional weak inhomogeneities. ¿ 1999 American Geophysical Union