Geoscience Reference
In-Depth Information
correct value for t 0 , t NMO = 0.062 s when α 1 = 5.5 km s 1 and t NMO = 0.044 s
when α 1 = 6.5 km s 1 . Clearly, a signal with predominant frequency of 20 Hz or
less (i.e., time for one cycle > 0.050 s) cannot give an accurate value for α 1 .
Higher-frequency signals give more accurate velocity analyses, as discussed in
Section 4.4.4. The best way to obtain reliable interval-velocity measurements in
such cases is to supplement the reflection profiles with wide-angle-reflection
profiles. Such profiles, perhaps 50-80 km in length, allow deep wide-angle
reflections to be recorded. Wide-angle reflections have larger amplitudes than those
of normal-incidence reflections (Fig. 4.39), and their travel times (Eqs. (4.65) and
(4.67)) give more reliable values for the interval velocities.
Example: amplitude of reflections
For normal-incidence reflections the P-wave reflection coefficient is given by
Eq. (4.62). An increase in impedance will therefore give a positive reflection
coefficient and a decrease in impedance will give a negative reflection coefficient.
As a simple example, imagine a layer of sediment with an impedance of 4 × 10 6
kg
m 2
s 1
sandwiched in sediment with an impedance of 3 × 10 6
kg m 2
s 1 . The
reflection coefficient at the top of the layer is
4
1
7 = 0 . 143
while the reflection coefficient at the base of the layer is
3 4
3 + 4 =
3
4 + 3 =
1
7
=− 0 . 143
To see a real example of the use of reflection coefficients, consider the
bottom-simulating reflector (BSR) which is a common feature of marine
seismic-reflection lines. The BSR arrives about 200-300 ms after the seafloor
reflection, has the opposite polarity, follows the seabed reflection (hence its name)
and so frequently cuts across reflections from any sedimentary stratigraphy. The
BSR is a consequence of the presence of gas hydrates within the sediments. A gas
hydrate is a rigid water-molecule cage that encloses and is stabilized by methane or
other hydrocarbons. Hydrates are stable at temperatures over 0 Catthe elevated
pressures reached in water over 300 m deep - effectively an 'ice' that is stable above
0 C. These hydrates have a narrow stability field, the base (controlled by
temperature) being only a few hundred metres below the seafloor. The base of the
stability field marks the boundary between high-velocity hydrated sediments above
and normal or gas-filled sediments below. Since there is a change in physical
properties of the sediment at this boundary, it will yield a seismic reflection.
Figure 4.43 shows a seismic-reflection line across the Cascadia margin where the
Juan de Fuca plate is subducting beneath North America. The BSR is clearly
visible, as is the deformation of the sediments filling this oceanic trench (see also
Section 9.2.2). Figure 4.44 shows the detail of the seafloor and BSR reflections - the
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