Passive Seismic Survey Recording
Passive surveys as developed by Acoustic Reservoir Imaging, LLC, use off-the-shelf seismometers and telemetry cables, plus a data recorder mounted on an all terrain vehicle. No heavy equipment or artificially generated energy are employed. Instead, the surveys rely on the eigenfrequencies of the earth.
The illustration shows the ARI work flow.
Seismometers.
Because ARI passive surveys measure amplitude differentials, the surveys use vertical axis seismometers. While research is assessing the potential value of triaxial sensors for amplitude surveys, single-axis sensors appear to provide all of the data needed for accurate detection of hydrocarbon reservoirs.
Seismometer Coordinates.
In an active survey using artificially induced signals, the data for all sensors are synchronized by time to the physical location of each individual sensor. In a passive survey, the data for each seismometer are independent, but the map coordinates of each sensor are important for determining the size and shape of any hydrocarbon deposits.
Before going to the field, the location of each seismometer to be used in the survey is mapped by longitude and latitude, then programmed into a GPS. The sensors are laid out in an array, considering distance between sensors and topographic conditions.
The field operator sets the sensors at the specified coordinates, then runs data cables to the centrally located data recorder.
Current practice records data for one to six seismometers simultaneously. When additional recording points are needed, the seismometers are moved to other locations and recorded. This is possible because time coordination of the data is not important.
In the future, ARI surveys will use dozens of sensors to enable the mapping of larger areas at one time.
Noise Reduction.
As with any seismic survey, useful signals must be separated from background noise. To help attenuate noise during recording, the seismometers are buried slightly below the surface. This reduces noise generated by wind and changes in ambient temperature, for example.
Recording.
The raw analog data from the seismometers are digitized and recorded. Recording duration varies from 1000 to 3000 seconds. The recording duration depends on the amount of background noise—such as truck traffic or grazing lifestock—with longer durations needed where noise levels are higher.