A method of characterizing changes in a hydrocarbon reservoir comprising: receiving a base seismic trace (102, 110) of a base seismic survey and a monitor seismic trace of a monitor seismic survey, the base seismic survey and the monitor seismic survey being associated with the hydrocarbon reservoir and taken at different respective times by applying a seismic waveform at approximately the same position on a ground leading to respective propagation paths for a seismic waveform through the ground at the different respective times; generating a predicted monitor seismic trace based on the base seismic trace (102, 110) and the monitor seismic trace, wherein generating the predicted monitor seismic trace from the base seismic trace (102, 110) comprises: estimating initial velocity perturbations occurring between the base seismic trace (102, 110) and the monitor seismic trace, the estimated velocity perturbations relating to changes in velocity for the seismic waveform through the hydrocarbon reservoir; applying a time shift to the base seismic trace (102, 110) based on the initial velocity perturbations; and compensating for amplitude changes (112, 122) between the base seismic trace (102, 110) and the monitor seismic trace, the time shift being applied to the amplitude changes (112, 122) to generate the predicted monitor seismic trace, wherein the base seismic trace (102, 110), monitor seismic trace, and predicted monitor seismic trace are different seismic traces; minimizing, until a predetermined convergence is reached, a difference between the predicted monitor seismic trace and the monitor seismic trace by iteratively estimating the estimated velocity perturbations to obtain final estimated velocity perturbations; and producing a velocity perturbation map using at least the final estimated velocity perturbations, the velocity perturbation map indicating changes in geological characteristics of the hydrocarbon reservoir between the base seismic survey and the monitor seismic survey. receiving a base seismic trace (102, 110) of a base seismic survey and a monitor seismic trace of a monitor seismic survey, the base seismic survey and the monitor seismic survey being associated with the hydrocarbon reservoir and taken at different respective times by applying a seismic waveform at approximately the same position on a ground leading to respective propagation paths for a seismic waveform through the ground at the different respective times; generating a predicted monitor seismic trace based on the base seismic trace (102, 110) and the monitor seismic trace, wherein generating the predicted monitor seismic trace from the base seismic trace (102, 110) comprises: estimating initial velocity perturbations occurring between the base seismic trace (102, 110) and the monitor seismic trace, the estimated velocity perturbations relating to changes in velocity for the seismic waveform through the hydrocarbon reservoir; applying a time shift to the base seismic trace (102, 110) based on the initial velocity perturbations; and compensating for amplitude changes (112, 122) between the base seismic trace (102, 110) and the monitor seismic trace, the time shift being applied to the amplitude changes (112, 122) to generate the predicted monitor seismic trace, wherein the base seismic trace (102, 110), monitor seismic trace, and predicted monitor seismic trace are different seismic traces; estimating initial velocity perturbations occurring between the base seismic trace (102, 110) and the monitor seismic trace, the estimated velocity perturbations relating to changes in velocity for the seismic waveform through the hydrocarbon reservoir; applying a time shift to the base seismic trace (102, 110) based on the initial velocity perturbations; and compensating for amplitude changes (112, 122) between the base seismic trace (102, 110) and the monitor seismic trace, the time shift being applied to the amplitude changes (112, 122) to generate the predicted monitor seismic trace, wherein the base seismic trace (102, 110), monitor seismic trace, and predicted monitor seismic trace are different seismic traces; minimizing, until a predetermined convergence is reached, a difference between the predicted monitor seismic trace and the monitor seismic trace by iteratively estimating the estimated velocity perturbations to obtain final estimated velocity perturbations; and producing a velocity perturbation map using at least the final estimated velocity perturbations, the velocity perturbation map indicating changes in geological characteristics of the hydrocarbon reservoir between the base seismic survey and the monitor seismic survey.