To understand what information can be obtained
from seismic data requires a basic knowledge of the seismic method.
Seismic data is acquired by setting off a source of seismic energy,
e.g. an airgun or vibrator array, and recording the energy reflected
from horizons or reflectors in the sub-surface (figure 1). A seismic
survey is acquired by repeating this procedure for a series of
shot and receiver locations. The data recorded are reorganised
so that the mid-point between shot and receiver is common for
each source-receiver pair. These groups of records are called
CMP gathers (Common Mid-Point
gathers) and they are the basic building block of the seismic
method. CMP gathers are reduced from a group of traces, all with
the same mid-point, to a single record of the stacked seismic
section.
Figure 1: From Acquisition to CMP Record
Reflections on a CMP gather follow a generally hyperbolic path and can be characterised by three parameters:-
[A summary between these properties of the seismic data and the layer properties shown in Figure 2.]
The interval velocity of the material in the sub-surface is obtained from an analysis of the shapes of the hyperbolae. The vertical resolution depends on the separation in time of successively deeper reflectors which can be individually recognised. Velocity analysis can only give a coarse description of the sub-surface, in terms of layers greater than 100m or 300ft thick.
Figure 2 : The relationship between layer properties and seismic data.
The Poisson's ratio variation in the sub-surface is estimated from the change in reflection amplitude as the source-receiver offset increases. The rate of change is related to the Poisson's ratio contrast across the layer boundaries generating the reflections. Consequently, the Poisson's ratio of a layer cannot be estimated from seismic data alone. However, the seismic data do provide a good measure of the relative changes of Poisson's ratio between neighbouring layers.
The acoustic impedance (AAI) of the sub-surface is estimated from the amplitude of the reflection at zero source-receiver offset. Migrated seismic sections provide this amplitude information. The amplitude of a reflection is related to the acoustic impedance contrast across a layer boundary. As with the Poisson's ratio of the layers, the seismic data can only provide information on the relative changes of acoustic impedance between neighbouring layers.
Seismic data help estimate three geophysical parameters of the sub-surface:-
The interval velocity. The Poisson's ratio contrast across layer boundaries. The acoustic impedance contrast across layer boundaries.
[A description of the techniques used to estimate these parameters from seismic data is given in the Overview section of the Tools Report.]
Poisson's ratio and AAI contrasts relate to the difference in properties across a layer boundary, and are not layer properties. Since only layer properties can be related to the lithology and fluid content of a reservoir, the Poisson's ratio and acoustic impedance contrasts need to be transformed to layer properties. To transform from interface to layer parameters requires both well data and a geological interpretation of the sub-surface. This means that any estimate of layer Poisson's ratio or acoustic impedance is not unique.
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