Geological interpretation - 3-D Seismic Interpretation - page 109

cyclic change in sea-level is to cause a change in accommodation space that will cause
deposition at any particular point to change from more landward to more seaward, and
back again. If there is a systematic impedance difference between landward deposits
and seaward deposits (caused perhaps by their being sand-prone and shale-prone, re-
spectively), then there will be cyclic changes in impedance. During one half-cycle the
impedance will generally be increasing, so reflection coefficients at individual thin beds
will all be positive, and over the other half-cycle they will all be negative. Superposed
on these cycles there may be quite large impedance jumps (and resulting reflection co-
efficients) at depositional hiatuses. However, the cyclic effects will dominate if there is
resonance with the periods found in the seismic pulse, which is why seismic reflectors
tend to be chronostratigraphic markers linked to cyclic sea-level change. Incidentally,
this is an additional reason why well synthetics may not match actual seismic data; the
resonant reinforcement of the signal may be quite sensitive to details of the wavelet
used to create the synthetic. If it does not have a smooth broad-band spectrum, then
the resonant effects may be suppressed at frequencies where the wavelet spectrum is
deficient.

4.3 Interpretation tools

Geological interpretation of seismic data may be simple if there is adequate well control,
but in many cases the interpreter has to make inferences from the appearance of observed
bodies. This may include both their external form and, if resolution is good enough for it
to be visible, the geometry of internal reflections. Some ways of looking for distinctive
features are as follows.
(1) Vertical sections. Standard displays as discussed in chapter 3 may be adequate to
show the geometry of individual bodies, particularly if they are thick enough to
show distinctive internal reflections, such as the dipping foreset beds of a delta
front. The top and base of the unit containing the foresets can be picked by the
same methods as used for structural interpretation, and in some cases it may be
possible to map a number of vertically stacked or laterally equivalent units. A tool
that may be useful is the instantaneous phase display, which is derived from the
seismic trace as follows (Barnes, 1998; Taner et al. , 1979) . Suppose we define the
envelope of the seismic trace at any particular TWT as the maximum value that
the trace can have when modified by applying a single phase rotation to the entire
trace. In principle, this could be found by observing how the trace changes when the
phase is rotated through the range 0 to 360 ◦ ; at any TWT, the maximum value that
the trace assumes during the rotation is the envelope or instantaneous amplitude,
and the phase rotation that gives rise to this maximum amplitude is the instantaneous
phase (reversed in sign). Finding these values for all times on the trace gives us
the envelope and instantaneous phase traces. The actual calculation is in practice