1. A method of modifying a directionality function of a virtual source used in seismic interferometry, said method comprising: recording seismic wave data incident on each individual seismic receiver of an array of seismic receivers, wherein each receiver of the array of seismic receivers has an associated location within the array; performing a domain transform on the recorded seismic wave data over the array locations of the receivers to separate the recorded seismic wave data into different transform components, wherein each transform component corresponds to a type of wave that has some directionality; measuring a signal strength measurement for each transform component; and determining multiplication factors to convert the measured signal strength for each transform component into a desired strength for each transform component.