Geoscience Reference
In-Depth Information
Wavelet Phase
Zero Phase wavelet and
selected frequency
components
0 3 6 12 24 40 72Hz
Phase describes the relative timing of the
various frequency components that make
up the seismic wavelet
1
180
Phase
angle
Amplitude
0
-180
0
20
60
20
60
Frequency Hz
Frequency Hz
Fig. 3.2 The phase spectrum related to the timing relationship of the various frequency
components in the wavelet, which in this example is zero-phase.
Of all the wavelets that have this property and have any particular amplitude spectrum,
the minimum-phase wavelet is the one that has the most concentration of energy close
to the start time. This does not necessarily mean that the leading loop is the largest;
some minimum-phase wavelets have their greatest amplitude in the second loop. These
wavelets are important because the actual source signature from explosives or air guns
is close to minimum-phase. However, as noted in chapter 2 , the wavelet is often con-
verted to zero-phase during processing. This produces a wavelet like that shown in
fig. 3.2 , symmetrical about the zero-time and so with energy at negative times and
not causal. This wavelet is preferred for interpretation because the strong central peak
at time zero is easy to relate to the reflector concerned. In practice, the processor's at-
tempt to convert the wavelet to zero-phase is rarely perfect, and mixed-phase wavelets
are common.
The choice of wavelet can make a considerable difference to the appearance of
synthetic traces (Neidell & Poggiagliolmi, 1977) . There are several possible approaches.
A simple method is to make synthetics with theoretical wavelets of different frequency
content, both zero- and minimum-phase, and look for a good visual match to the real
seismic. This may be adequate to identify the loops corresponding to key stratigraphic
markers, so is a useful start to making structural maps. A totally different approach is to
start from a measured source signature, and to calculate the way that it is modified by
 
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