Page 79 - Acquisition and Processing of Marine Seismic Data

70 2. MARINE SEISMIC DATA ACQUISITION

FIG. 2.28 (A) Near- and far-field zones of a marine seismic source. Near-field signature is measured in the zone between
the physical dimensions of the source array (d) and the dominant wavelength of the signal (λ) while the far-field signature is
measured with a hydrophone located approximately 300 m below the source array. (B) Typical near-field signature. P is peak
amplitude of initial pressure, B is amplitude of the first bubble effect, T is bubble period, and T R is the rise time. (C) Typical far-
field signature. P-P is peak-to-peak amplitude of initial pressure.

individual guns of a gun array interfere con- acoustic pressure to maximum bubble pres-
structively and arrive at a distant hydrophone sure). Both parameters are measured from
within one sampling interval (Dragoset, 2000). zero-to-peak value for near-field signatures
The far-field signature is measured with a and peak-to-peak value for far-field signatures.
hydrophone typically located 300 m below the Near-field signatures have an initial high pres-
array (Fig. 2.28a) so that the signal appears as sure pulse, which then falls to negative pressure
if it is emitted from a point source. If we locate values below the hydrostatic pressure followed
the hydrophone to the aft or side of a gun array, by oscillating amplitudes due to the successive
the signatures change from one location to collapse and expansion of the air bubble
another due to the array effect, so the far-field (Fig. 2.28B). This initial pressure creates the
hydrophone must be directly below the array highest amplitude of the signature, which then
so that the pressure pulses from each individual gradually decreases with time. The effect of
gun arrive at the hydrophone at the same time. the source ghost is very small on the near-field
Near- and far-field signatures are commonly signature because of the location of the near-field
used to delineate the source strength (maximum hydrophone. The important parameters that can
acoustic pressure radiated by the source) and be obtained from the near-field signature are the
peak-to-bubble ratio (the ratio of maximum bubble period (T) and peak-to-bubble ratio (PBR),