Geology Reference
In-Depth Information
HIGH FREQUENCY
Loops close together
1500
T = 21ms
Resolution = T/2.31 = 9.1ms
Vp = 2500m/s
Vertical resolution = 11.3m
LOW FREQUENCY
Loops further apart
T = 27ms
T/2.31 = 11.7ms
Vp = 3000m/s
Vertical resolution = 17.6m
Figure 3.17 Estimating the dominant period in a seismic section.
but which have different slopes on the amplitude
spectra, together with synthetics generated from a
reflection series with different types of thin beds.
The wavelet with steep slopes has a narrow main lobe
but significant side lobe energy. This wavelet seems to
show some detectability of thin beds with same polar-
ity reflections but is poor at identifying thin beds with
opposite polarity reflections, owing to the presence of
side lobe energy. By contrast, the opposite appears to
be the case with a wavelet with shallower slopes, a
broad main lobe and minimal side lobe energy. In
practice, seismic processors make judgements on the
trade-off between the desire for a narrow main lobe
and the effects of reverberations due to high filter
slopes.
2000
Figure 3.18 Frequency, velocity and vertical resolution.
un-migrated seismic image the Fresnel zone diam-
eter is given by (Sheriff 1977 ) :
s
z+ 4
2
¼
2
z 2 ,
ð
3
:
6
Þ
F d
where z
wavelength.
Un-migrated seismic objects that are smaller than
the Fresnel zone are, therefore, not uniquely identi-
fied. In the days when prospect maps were generated
with un-migrated seismic it was important to calcu-
late the Fresnel zone. It was used, for example, to
determine an adequate stand-off distance from faults
and to ensure that the well was drilled on the
up-thrown side. Migration, however, plays a signifi-
cant role in enhancing lateral seismic resolution.
Figure 3.21 shows how the Fresnel zone is compressed
in the inline direction for a 2D migration. Note that
3D migration collapses the Fresnel zone to a small
circle (ideally around one half of the wavelength in
¼
depth and
λ ¼
3.7.5 Lateral resolution
For most practical purposes lateral seismic
resolution is not as important for amplitude inter-
pretation as vertical resolution, but it is still worth
considering briefly as it may be a factor in evaluating
the significance of lateral shifts in quantitative well
ties ( Chapter 4 ). The starting point for considering
lateral resolution is the fact that reflections are the
result of constructive interference over an area of
the wavefront called the Fresnel zone (Sheriff, 1977 )
( Fig. 3.20 ). The reader is referred to a useful discus-
sion of the Fresnel zone by Lindsey ( 1989 ) . For an
35
Search WWH ::




Custom Search