1. A method for producing a seismic attribute classification volume corresponding to a seismic data volume obtained from and corresponding to a subterranean region, comprising the steps of: (a) using the seismic data to calculate values of at least one selected seismic attribute at points throughout said region; (b) selecting at least one cross-section from each attribute data volume; (c) constructing a plurality of polygons on the selected cross sections, and making an initial classification of the attribute within each polygon, said polygons being chosen to be collectively representative of the range of attribute values in the respective data volumes; (d) constructing a probabilistic neural network using the attribute classifications within the polygons to train the network; (e) using the neural network to produce an attribute classification volume for a portion of the subterranean region; (f) repeating steps (c) through (e) until the classifications for the portion of the region are considered satisfactory; and (g) using the constructed probabilistic neural network to produce an attribute classification volume for the entire subterranean region.