Seismic data acquisition is the first of the three distinct stages of seismic exploration, the other two being seismic data processing and seismic interpretation. Seismic acquisition requires the use of a seismic source at specified locations for a seismic survey, and the energy that travels within the subsurface as seismic waves generated by the source gets recorded at specified locations on the surface by what is known as receivers. Before seismic data can be acquired, a seismic survey needs to be planned, a process which is commonly referred to as the survey design. This process involves the planning regarding the various survey parameters used, e.g. source type, receiver type, source spacing, receiver spacing, number of source shots, number of receivers in a receiver array, number of receiver channels in a receiver spread, sampling rate, record length etc. With the designed survey, seismic data can be recorded in the form of seismic traces, also known as seismograms, which directly represent the "response of the elastic wavefield to velocity and density contrasts across interfaces of layers of rock or sediments as energy travels from a source through the subsurface to a receiver or receiver array."
Survey parameters
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 comprise 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, or it can be a multi-component geophone designed to record p-waves and s-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.
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. According to the Nyquist criterion, the frequency with which the seismic signal needs to be sampled should 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 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. 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.
