disclosed is a method for characterising the evolution of a reservoir by determining a seismic wavelet which links observed seismic data to a sequence of reflectivities . the method comprises obtaining seismic data representing seismic changes which have occurred between a first time and a second time , said seismic data comprising a plurality of seismic traces ; and performing an optimisation operation simultaneously on the seismic traces so as to optimise for said seismic wavelet . the optimisation operation may be performed without using known reflectivity data as an input .