154 Seismology and Earth Structure
vt
x
vt r
vt a
Real
Apparent
α
β
Fig. 3.3-35 A dipping layer can be modeled as a line of point diffractors.
On a time section, interference between the diffraction hyperbolas
produces an apparent reflector, as shown schematically (top) and with true
amplitudes (bottom). Because the scaled travel time to the true interface
(vt r ) equals the scaled arrival time on the trace (vt a ), the apparent dip α is
shallower than the true dip β. (After Claerbout, 1985.)
Distance
(a)
(b)
Depth
A
B
W
X
Y
Z
A
B
Distance
t AW
t BX
t BZ
t BY
Time
Fig. 3.3-36 Top: Illustration of several ray paths for reflections at zero
offset from a reflector with a synclinal structure. Bottom: Because these
arrivals, each from a different point on the reflector, have different travel
times, the time section shows an apparent anticlinal, or “bowtie,”
structure. (Kearey and Brooks, 1984.)
Depth
Time
their apexes, and thereby reconstruct the reflectors as a set
of point diffractors. Thus diffraction artifacts like those in Figs
3.3-36 and 37 should be removed, and apparent interfaces with
shallow dips should be converted to interfaces with the steeper
true dips. Figure 3.3-39 shows the improvement to a seismic
section from Kirchoff migration. The resulting migrated time
section can be converted to a depth section using a velocity–
depth function.
The appearance of a migrated section depends on the
assumed velocity. Using a too-slow velocity reduces the length
Fig. 3.3-37 The ends of truncated interfaces
(left) act as diffractors, so a time section
(right) shows spurious down-dip extensions
of the interfaces. (Claerbout, 1976.)
(http://sepwww.stanford.edu/sep/prof/)
vt
x
vt r
vt a
Real
Apparent
α
β
Fig. 3.3-35 A dipping layer can be modeled as a line of point diffractors.
On a time section, interference between the diffraction hyperbolas
produces an apparent reflector, as shown schematically (top) and with true
amplitudes (bottom). Because the scaled travel time to the true interface
(vt r ) equals the scaled arrival time on the trace (vt a ), the apparent dip α is
shallower than the true dip β. (After Claerbout, 1985.)
Distance
(a)
(b)
Depth
A
B
W
X
Y
Z
A
B
Distance
t AW
t BX
t BZ
t BY
Time
Fig. 3.3-36 Top: Illustration of several ray paths for reflections at zero
offset from a reflector with a synclinal structure. Bottom: Because these
arrivals, each from a different point on the reflector, have different travel
times, the time section shows an apparent anticlinal, or “bowtie,”
structure. (Kearey and Brooks, 1984.)
Depth
Time
their apexes, and thereby reconstruct the reflectors as a set
of point diffractors. Thus diffraction artifacts like those in Figs
3.3-36 and 37 should be removed, and apparent interfaces with
shallow dips should be converted to interfaces with the steeper
true dips. Figure 3.3-39 shows the improvement to a seismic
section from Kirchoff migration. The resulting migrated time
section can be converted to a depth section using a velocity–
depth function.
The appearance of a migrated section depends on the
assumed velocity. Using a too-slow velocity reduces the length
Fig. 3.3-37 The ends of truncated interfaces
(left) act as diffractors, so a time section
(right) shows spurious down-dip extensions
of the interfaces. (Claerbout, 1976.)
(http://sepwww.stanford.edu/sep/prof/)