Solving the Down-hole Installation Problem: A Self-Orienting Seismic System

Orientation of a seismic sensor is a key item of metadata required for analysis, however this is often inaccurate or missing from stations where the operator cannot directly access the sensor to see its orientation, for example, in boreholes or on the ocean bottom.  Nanometrics has developed a unique, patented system incorporating a miniature MEMS gyrocompass in the seismometer to determine its orientation with an accuracy of +/-0.5 degrees, which is generally not achievable by other methods.  Software in the data recorder automatically queries this orientation and incorporates it in the channel metadata.  Seismometers incorporating this gyrocompass also have self-leveling functionality, so the tilt of the data is known as well as the orientation.  This North-finding capability was initially developed for ocean bottom use and is now being integrated into down-hole systems using the Trillium 120 Borehole or Posthole seismometer with the Centaur data recorder.

This self-orienting seismic system requires only a single software command to find its orientation and automatically incorporate it in the metadata.  It removes any concern about the accuracy of the orientation, since the gyrocompass measures the direction of the Earth’s axis of rotation, which is the definition of geographic North.  This is an improvement over a magnetic compass, which measures magnetic, not geographic, North and may not accurately measure magnetic North due to disturbances from nearby magnetic material, such as the borehole casing.  Furthermore, this North-finding capability eliminates the complex operations currently used to control the orientation of borehole sensors, such as installing a “bishop’s hat” fixture with expensive equipment or cross-correlating seismic data between the borehole sensor and a surface sensor.  The orientation determined by the gyrocompass will automatically accompany the seismic data to the data center, so it will always be available to the user.  Optionally, a rotation transform can be applied in the Centaur data recorder, to supply data that is already aligned to North.