To survey a subterranean structure, measured seismic data is processed using an algorithm for deriving a single-mode absorption parameter. A first absorption parameter for a mode-converted seismic wave is computed based on an output of the algorithm. The first absorption parameter represents an absorption effect of the mode-converted seismic wave by the subterranean structure, where the mode-converted seismic wave is reflected from the subterranean structure in response to a source seismic wave of a different mode than the mode-converted seismic wave.