Complex sentence: In order to successfully detect the degree of anisotropy in a region there must be more several arrivals that are well distributed in time, too few events cannot detect the change even if they are from similar waveforms.
Split into simpler sentences: In order to successfully detect the degree of anisotropy in a region there must be more several arrivals that are well distributed in time. Too few events cannot detect the change even if they are from similar waveforms.