Following the GRAPES-3 discovery of a transient weakening of Earth's magnetic shield through observation of a cosmic ray burst on 22 June 2015, we have been involved in an effort to search for more such events in 20 years of archived data. An important step in analyzing the data is to simulate the cosmic ray trajectories in the geomagnetic field influenced by the interplanetary magnetic field during a geomagnetic storm. The simulation of 22 June 2015 burst required over two months of running time on the 1280 core GRAPES-3 computer cluster in Ooty. We have developed a fast, efficient technique which has allowed us to simulate the 22 June 2015 event in a fraction of a second with the same resolution as the earlier computational-intensive method. This has laid the foundation for quick analysis of a large number of such events in archival data, and of future events virtually in real-time. Details of this method will be presented during the conference.