As we have already described, we obtain records of ground motion detected at each geophone over some time interval. The relevant piece of information that we would like to extract from these records is the time of arrival for the first arriving seismic energy.

One such record is shown to the right. A discussion on how first arrivals can be chosen has already been given. Suffice it to say that on this record it is fairly easy to see that the first arriving seismic energy comes in at the time corresponding to the blue line. The record shown, however, is noise free. With the inclusion of noise, the choice of time of the first arrival becomes much more complicated and, in truth, should be considered part of the interprational process.

With noisy data, it is often easier to choose first arrivals by comparing ground motion recorded at a variety of offsets. In the example shown below, for instance, it is much easier to distinguish the small refracted arrivals on the far offset traces when a group of these traces are plotted together in a record section.