  Signals obtained in land seismic surveys are usually contaminated with
coherent noise, among which the ground roll (Rayleigh surface waves) is of
major concern for it can severely degrade the quality of the information
obtained from the seismic record. Properly suppressing the ground roll from
seismic data is not only of great practical importance but also remains a
scientific challenge. Here we propose an optimized filter based on the
Karhunen--Lo\'eve transform for processing seismic data contaminated with
ground roll. In our method, the contaminated region of the seismic record, to
be processed by the filter, is selected in such way so as to correspond to the
maximum of a properly defined coherence index. The main advantages of the
method are that the ground roll is suppressed with negligible distortion of the
remanent reflection signals and that the filtering can be performed on the
computer in a largely unsupervised manner. The method has been devised to
filter seismic data, however it could also be relevant for other applications
where localized coherent structures, embedded in a complex spatiotemporal
dynamics, need to be identified in a more refined way.
