Page 143 - Introduction to Petroleum Engineering

130 RESERVOIR GEOPHYSICS
coefficient. The seismic trace also includes noise, which is always present in realistic
data acquisition. In practice, traces from a single source are recorded at multiple
receivers. A family of seismic traces is called a gather.
Time‐lapse seismology, also known as 4‐D seismic, is the comparison of two 3‐D
seismic surveys taken in the same geographic location at different times. The fourth
dimension in 4‐D seismic is time. The difference between the two 3‐D seismic sur-
veys should show changes in the same rock volume due to changes resulting from
operations. The structure should be the same while the pressure and saturation
distributions change. Thus, 4‐D seismic can be used to identify the movement of
fluids between wells, improve the quality of reservoir characterization, and highlight
bypassed reserves in reservoirs where a signal can be detected.

7.4.2 Data Processing
Data processing is used to prepare seismic data for interpretation. One of the most
important data processing tasks is to transform travel time to depth. The time‐to‐
depth conversion makes it possible to view seismic traces as functions of depth and
compare them to geological and engineering measurements which are typically
expressed as functions of depth. The conversion of travel time measurements to
depths depends on the velocity of propagation of the acoustic signal through the
earth. Seismic velocity varies with depth and depends on the properties of the media
along the travel path. The collection of velocities used in the time‐to‐depth conversion
is called the velocity model, and the process is known as depth migration. If evidence
becomes available that suggests the velocity model needs to be changed, it may be
necessary to process seismic data again with the revised velocity model.
Vertical seismic profiles (VSP) or checkshots in wellbores can improve the quality
of velocity models. A checkshot is conducted by discharging a vibration source at the
surface and recording the seismic response in a borehole receiver. The checkshot is a
VSP with zero offset if the vibration source is vertically above the receiver. The
checkshot is a VSP with offset if the vibration source is offset relative to the receiver.
A reverse VSP is obtained by placing the source in the wellbore and the receiver
on the surface. The velocity model can be validated or refined by comparing well log
and core sample data to VSP and checkshot data.
Data processing is also used to maximize the signal to noise ratio of the seismic
data. There is always noise in seismic data. Noise arises from such sources as ground
roll, wind, vibrations from other operations, and interference with other seismic
waves that intersect the path of the signal. A variety of mathematical techniques have
been developed to maximize the signal and minimize the noise.

7.4.3 Data Interpretation
Seismic data that have been subjected to data processing are ready for interpretation.
Processed seismic data are reviewed in conjunction with data from other disciplines
to provide a better understanding of the fluid content, composition, extent, and geom-
etry of subsurface rock. Some applications of the interpretation process include