Data Charted As You Read This
Passive Seismic
Given the plethora of expendable boreholes on land
for receiver placement versus the marine environment, it comes as
no surprise that most passive seismic applications thus far have
been land-based.
Still, there have been a few noteworthy applications
in water.
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Given the plethora of expendable boreholes on land
for receiver placement versus the marine environment, it comes as
no surprise that most passive seismic applications thus far have
been land-based.
Still, there have been a few noteworthy applications
in water.
Perhaps the most highly documented offshore project
took place in 1997 at Ekofisk in the North Sea, where micro-seismic
recording identified a fault pattern under a gas cloud, which hinders
normal seismic views. Locations of micro-seismic events recorded
over 18 days indicated the seismicity clustered along lineations
parallel to major structures in the field, according to S.C. Maxwell
at ESG Canada.
The events were attributed to induced movement along
pre-existing fractures.
One of the advantages of micro-seismic is that faults
with small throws can be directly detected. Vertical faults ordinarily
are mapped indirectly with reflection seismology by offset horizons.
Passive seismic monitoring at Ekofisk is taking place
today, with the sensor hooked up to the power and telemetry distribution
system in the cable to deliver data in real time back to the client's
office.
"You can see things going on on the seafloor," said
Input/Output's Dave Ridyard, "and also listen for what's happening
in the subsurface."