1. A method of identifying microseismic events generated by hydrocarbon extraction activities in seismic data, the seismic data comprising at least first seismic data traces acquired at a first seismic receiver and second seismic data traces acquired at a second receiver spatially separated from the first receiver, the method comprising: positioning the first and the second seismic receivers in a borehole, wherein the first and the second seismic receivers are vertically separated in the borehole; identifying a potential doublet in the first seismic trace by determining a first measure of similarity for a pair of microseismic events in the first seismic trace; using characteristics of the potential doublet to identify a corresponding potential doublet in the second seismic trace, wherein the corresponding potential doublet comprises a pair of microseismic events in the second seismic trace; determining a second measure of similarity between the pair of microseismic events in the second seismic trace; determining an overall measure of similarity, wherein the overall measure of similarity is indicative of similarity between the pair of microseismic events acquired at the first seismic receiver and of similarity between the pair of microseismic events acquired at the second seismic receiver.