The Seismic Reflection Method
The goal of a high resolution seismic survey is to provide an
image of the subsurface that is as detailed as possible, within
the limits imposed by the nature of acoustic wave
propagation in the earth. The 2D seismic method entails
propagation of the acoustic waves through the earth from a
surface pattern of source and receiver points.
Energy is introduced by using one of a number of possible
seismic sources. Common sources include seismic shotgun,
small dynamite charges, weight drop or rammer sources. In
some environments, such as gassy deltaic sediments, a shear
wave source and geophones are used. The returning
wavefield is recorded in the seismograph from a spread of
geophone receiver groups. The number of geophones used
may be as few as 24 in favourable environments to many
times more where high subsurface sampling redundancy is
required.
The survey procedure entails collection of a seismogram,
then advancing the energy source a fixed distance down the
survey line and repeating the process to receive another
seismogram. This method known as common mid-point
(CMP) provides a very high degree of redundancy of
sampling of the energy received from a given reflector at
depth. The redundancy is used in the data processing
procedure to develop a high fidelity image of the subsurface.
If all receiver locations are used as shot points, the
multiplicity of data on one subsurface point (called CMP or
CDP fold) is equal to one half of the number of recording
channels.
A number of seismic events are present in each seismogram
in addition to the reflections of interest. Data processing
steps designed to attenuate this unwanted energy, and
enhance the reflections of interest are carried out. The
ultimate product of the reflection processing is a corrected
cross section with reflection events ready for interpretation.
The SEGY formatted seismic data is imported into the
Seismic Micro Technologies (SMT) 2D/3D seismic
interpretation package, together with survey line position
information. This software is a comprehensive 2D/3D
seismic interpretation program that provides interpretive and
horizon picking tools, integrated into a map and section
database, entity management, and display system.
The initial step in the seismic data interpretation process is
the correlation of the major geological structures to seismic
reflectors. The process of tracing reflectors within a given
seismic line, and from line to line at tie points, requires
careful phase correlation of the events. During this process
sedimentational character, and stratigraphic features and
faulting are considered. A velocity model is developed
during processing, and by correlation with borehole geologic
and velocity logging information. The times associated with
each of the interpreted reflectors are then converted from
time sections into depth sections using this model.
Final products include colour contour maps and sections and
isopach maps of geologic horizons. As well, volumetric
computations for regions and horizons may be developed.
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