use include the fact that the pulses generated are predictable, controllable and hence repeatable. Additionally, it uses air to generate the source which is readily available and free of cost. Lastly, it also has a relatively smaller environmental impact for marine life compared to other marine seismic sources; an aspect that deters the use of vibratory sources for marine acquisition. Air-guns are typically used in groups or arrays (i.e. multiple air-guns of different volumes) to maximise the signal-to-noise ratio and to minimise the appearance of bubble pulses or oscillations on the traces. Receiver type Hydrophone A hydrophone is a seismic receiver that is typically used in marine seismic acquisition, and it is sensitive to changes in pressure caused by acoustic pulses in its surrounding environment. Typical hydrophones utilise piezoelectric transducers that, when subjected to changes in pressure, produce an electric potential which is directly indicative of pressure changes. As is the case with air-guns, hydrophones are often also employed in groups or arrays which consist of multiple hydrophones wired collectively to ensure maximum signal-to-noise ratio. Geophone A geophone is a seismic receiver that is often chosen in land acquisition to monitor the particle velocity in a certain orientation. A geophone can either be a single-component geophone which is designed to record p-waves (compressional waves), or it can be a multi-component geophone designed to record p-waves and s-waves (shear waves). Geophones require sufficiently strong coupling with the ground to record the true ground motion initiated by the seismic signal. This is of considerable importance for higher frequency components of the seismic signals, which can be altered substantially with respect to their phase and amplitude due to poor coupling. In the figure on the right, a geophone is shown; the conical spike on the geophone is dug into the ground for coupling. As is the case with hydrophones, geophones are often arranged in arrays as well to maximise the signal-to-noise ratio as well as to minimise the influence of surface waves on recorded data. Sampling interval and Nyquist criterion The seismic signal that needs to be recorded by the receivers is inherently continuous and hence needs to be discretised. The rate at which this continuous signal is discretised is referred to as the sampling interval or sampling rate (see Sampling (signal processing) for more details). According to the Nyquist criterion, the frequency with which the seismic signal needs to be sampled should