be at least equal to or greater than twice the maximum frequency component of the signal i.e. fsample ≥ 2fmax,signal. The challenge that remains is that the highest frequency component is usually not known during acquisition to be able to calculatedly determine the sampling rate. Therefore, estimates need to be made of the highest possible frequencies contained within the signal; usually, sampling rates higher than these estimates are preferred to ensure that temporal aliasing does not occur. Record length Despite the term length, the record length refers to the time duration (typically listed in milliseconds) over which the receivers are active, recording and storing the seismic response of the subsurface. This recording time should usually start slightly before the source is initiated to ensure that the direct waves are received as the first arrivals on the near-offset receivers. Additionally, the record length should be long enough to ensure that the latest expected arrivals are recorded. Typically, for deeper exploration surveys, the record length is adjusted to the order of multiple seconds (6 seconds is common). 15 to 20 seconds is common for deep crustal exploration. Since the recorded traces can always be clipped for later arrivals during data processing, the record length is normally preferred longer than necessary rather than shorter. References Geophysical survey Seismology measurement