Geoscience Reference
In-Depth Information
5
Interpreting seismic amplitudes
In areas with favourable rock properties it is possible to detect hydrocarbons directly
by using standard 3-D seismic data. Amplitude interpretation is then very effective in
reducing risk when selecting exploration and production drilling locations. Not all areas
have such favourable rock physics, but it is always useful to understand what seismic
amplitudes may be telling us about hydrocarbon presence or reservoir quality. As well
as amplitudes on migrated stacked data, it is often useful to look at pre-stack data and
the way that amplitude varies with source-receiver offset (AVO). The first step is to
use well log data to predict how seismic response will change with different reservoir
fluid fill (gas or oil or brine), with changing reservoir porosity, and with changing
reservoir thickness. Then we can use this understanding to interpret observed changes
in seismic amplitude or other measures of the size and shape of individual seismic
loops. In principle this process can also be used to interpret amplitudes on 2-D seismic
data, but as we saw in
chapter 1
the power of 3-D seismic lies in the ability to make
maps based on very densely sampled data, allowing us to see systematic amplitude
changes that are only just above the noise level.
5.1
Basic rock properties
We shall consider in detail only
isotropic
rocks, where the seismic velocities are inde-
pendent of the direction of propagation through the rock. In practice, many rocks are
anisotropic
. The velocities of horizontal and vertical paths may be different, perhaps
owing to fine-scale internal layering. Horizontal velocities may vary with azimuth, per-
haps owing to cracks aligned in a particular direction. These effects are complicated
and it is difficult to obtain the rock parameters needed to model them. The isotropic
case is much simpler: two rock properties control the response to sound waves. These
are the acoustic impedance and the ratio of compressional to shear wave velocity
(
V
p
/
V
s
). The acoustic impedance is simply the product of compressional velocity and
density:
AI
=
V
p
ρ.
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