ABAP Frequently Used reports- How to search??

Dear Members,

We have a system with more than 3000-4000 'z' reports, and that system we are going to upgrade. For that we need to know which report is used with what frequency. that includes background reports,too.

Now what we need to know is that, is there any short-cut method so that we can find out the frequency of the usage of those reports?

If yes,then what is the method?

If no,then how to go about it?

Sugata