Page 183 - Carbonate Sedimentology and Sequence Stratigraphy

174 WOLFGANG SCHLAGER

lithology RC synthetic seismograms
with different frequencies
A)
1 1/2 1/4
GWC

B)

A B C D E F
Fig. C.9.— Fresnel zone and lateral seismic resolution. A) Cross
Fig. C.7.— The effect of pulse breadth (seismic frequency) on sections of layers with width expressed as fraction of Fresnel ra-
seismic resolution illustrated by synthetic seismograms. Frequency dius. B) Seismic images of layers in A. Layers smaller than 1 Fres-
in (F) is about four times higher than in (C). After Anstey (1982), nel radius no longer appear flat. Rather, they are imaged as reflect-
modified. ing point sources that create a diffraction hyperbola. After Neidell
and Pogglialiolmi (1977). (Reprinted by permission of the AAPG
whose permission is required for further use).
acoustic impedance
boundary
where V is the average sonic velocity of the overburden, t
is two-way travel time, and f c is the frequency of the seismic
waves. Thus, the Fresnel zone increases (and resolution de-
creases) with increasing velocity, increasing travel time and
maximum zone of decreasing frequency.
Fresnel uniform sense of
zone particle motion
source SEISMIC ATTRIBUTES

magnitude of From the basic measurements of travel time, amplitude
particle motion and frequency of seismic traces, attributes can be calculated
to better examine structural and stratigraphic patterns . Par-
spherical ticularly in 3D data, attributes may be powerful tools to vi-
wavefronts
sualize subsurface geology (e.g. Brown, 1996). It must be
kept in mind, though, that despite their vastly different ap-
pearance, attributes of the same data set all are derived from
the same basic measurements. Therefore, they are not inde-
pendent of each other. Seismic attributes derived from time
Fig. C.8.— Spherical wave fronts impinge on a flat reflector. Only are particularly useful for illustrating structure. Attributes
the cross-hatched part of the reflector contributes to the reflected related to amplitude and frequency are particularly useful
wave. The Fresnel zone is even narrower because the contribution in stratigraphic and sedimentologic work. A general word
of the distal parts of the cross-hatched zone is insignificant. Af-
ter Neidell (1979). (Reprinted by permission of the AAPG whose of caution is in order: algorithms for calculating seismic at-
permission is required for further use). tributes seem to appear on the market much faster than they
can be tested and calibrated on geologic objects. Thus, the
geologic meaning of many attributes is not well constrained.
spherical wave fronts hitting a flat reflector. The Fresnel
zone is the region of the reflector where particle motion is
in the same direction and therefore contributes positively to
the reflected wave. The radius of the Fresnel zone, r ,is
f
given by
r =(V/4)(t/ f c ) 1/2
f