reservoir in Alberta, Canada (Hansteen et al. 2010).
When steam is injected, the stiffness of the rock is
reduced, leading to a velocity decrease. This velocity
decrease leads to a corresponding increase in time
shift, as observed in Fig. 19.33.
Hilbich (2010) uses time-lapse refraction seismic
tomography to study seasonal changes in the ground
ice in the Alps. He concludes that the method is
capable of detecting temporal changes in alpine
permafrost and can identify ground ice degradation.
A major strength of the method is according to Hilbich
the high vertical resolution potential and the ability to
discriminate phase changes between frozen and
unfrozen material over time. A significant limitation
is the low penetration depth due to strong velocity
contrasts between the active layer and the permafrost
table.
Presently, time lapse refraction is not well
established as a 4D technique. The few examples
presented so far illustrate that the technique has a
potential. One weakness is that it is hard to map the
time lapse anomalies precisely in depth. Fortunately,
full wave form inversion techniques have developed
significantly over the last decade (Virieux and Operto
2009, Sirgue et al. 2009). A few examples of this
technique have also been presented for 4D
applications (Routh et al. 2009). Therefore, there is
hope that time lapse refraction methods might develop
into a more precise and accurate technique. Despite
this lack of more advanced analysis methods, it is
important to underline that if 4D refraction is used as
a tool for detecting shallow gas leakage, the important
issue is to detect and locate approximately where the
leakage occurs. Detailed and accurate mapping of the
4D anomaly is not crucial. Should more accurate
mapping be required, this may be done across the
located area by conventional reflection survey.
Fig. 19.33 Time lapse refraction example from Peace River heavy oil field in Alberta. Significant velocity reduction caused by
steam injection into row 8. (Figure from Hansteen et al. 2010)
19 4D Seismic
511
When steam is injected, the stiffness of the rock is
reduced, leading to a velocity decrease. This velocity
decrease leads to a corresponding increase in time
shift, as observed in Fig. 19.33.
Hilbich (2010) uses time-lapse refraction seismic
tomography to study seasonal changes in the ground
ice in the Alps. He concludes that the method is
capable of detecting temporal changes in alpine
permafrost and can identify ground ice degradation.
A major strength of the method is according to Hilbich
the high vertical resolution potential and the ability to
discriminate phase changes between frozen and
unfrozen material over time. A significant limitation
is the low penetration depth due to strong velocity
contrasts between the active layer and the permafrost
table.
Presently, time lapse refraction is not well
established as a 4D technique. The few examples
presented so far illustrate that the technique has a
potential. One weakness is that it is hard to map the
time lapse anomalies precisely in depth. Fortunately,
full wave form inversion techniques have developed
significantly over the last decade (Virieux and Operto
2009, Sirgue et al. 2009). A few examples of this
technique have also been presented for 4D
applications (Routh et al. 2009). Therefore, there is
hope that time lapse refraction methods might develop
into a more precise and accurate technique. Despite
this lack of more advanced analysis methods, it is
important to underline that if 4D refraction is used as
a tool for detecting shallow gas leakage, the important
issue is to detect and locate approximately where the
leakage occurs. Detailed and accurate mapping of the
4D anomaly is not crucial. Should more accurate
mapping be required, this may be done across the
located area by conventional reflection survey.
Fig. 19.33 Time lapse refraction example from Peace River heavy oil field in Alberta. Significant velocity reduction caused by
steam injection into row 8. (Figure from Hansteen et al. 2010)
19 4D Seismic
511