Geology Reference
In-Depth Information
12
SEISMIC REFLECTION
The seismic reflection method absorbs more than 90% of the money spent
worldwide on applied geophysics. Most surveys are aimed at defining oil-
bearing structures at depths of thousands of metres using hundreds or even
thousands of detectors, and are beyond the scope of this topic. However,
some reflection work is done by small field crews probing to depths of, at
most, a few hundred metres. The instruments used in these surveys were
originally very simple but may now have as much inbuilt processing power
as the massive processing laboratories of 30 years ago, and field operators
need to have some understanding of the reasons why they are being presented
with so many options.
12.1 Reflection Theory
Ray-path diagrams, introduced in Chapter 11, provide insights into the
timing of reflection events but give no indication of amplitudes.
12.1.1 Reflection coefficients and acoustic impedances
The
acoustic impedance
of a rock, usually denoted by
I
, is equal to its
density multiplied by the seismic P-wave velocity. If a seismic wavefront is
normally incident
(incident at right angles) on a planar interface between
two rock layers with impedances
I
1
and
I
2
, then the
reflection coefficient
(RC), i.e. the ratio of the amplitude of the reflected wave to the amplitude
of the incident wave, is given by:
RC
=
(
I
2
−
I
1
)
/
(
I
2
+
I
1
)
If
I
1
is greater than
I
2
, the coefficient is negative and the wave is reflected
with phase reversed, i.e. a negative pulse will be returned if a positive pulse
was transmitted, and vice versa.
The amount of energy reflected first decreases and then increases as the
angle of incidence increases. If the velocity is greater in the second medium
than in the first, there is ultimately total reflection and no transmitted wave
(see Section 11.1.5). However, most small-scale surveys use waves reflected
at nearly normal incidence.
