Ocean Bottom Seismometers are deployed remotely and without intervention - therefore, their actual horizontal orientation on the seafloor is unknown. OBSIP instruments do not carry orientation devices (magnetic compasses, gyros, etc.) because accurate instruments are cost and power prohibitive, and current low cost instruments are of insufficient accuracy. Therefore, horizontal orientation of the OBS must be determined from measured data. The ambient noise inherent in OBS data can make this process difficult.
There are several viable methodologies for determining OBS orientation. OBSIP has used the process described by (Stachnik et al., 2012) and variations of that process to orient instruments deployed in the Cascadia project. More details of the process used for Cascadia and OBSIP success with that methodology can be found in the report "Cascadia Amphibious Array Ocean Bottom Seismograph Horizontal Component Orientations"
The MATLAB code used to process the Cascadia is available for community use and may be downloaded here:
MATLAB OBS Horizontal Orientation Code (Updated: 11/14/14)
or a perl version here:
perl OBS Horizontal Orientation Code (Outside Link)
Cascadia Horizontal Orientations can be found on the Cascadia Project Page:
Stachnik, J.C., A.F. Sheehan, D.W. Zietlow, Z. Yang, J. Collins, and A. Ferris (2012), Determination of New Zealand Ocean Bottom Seismometer Orientation via Rayleigh-Wave Polarization: Seismol. Res. Lett., 83, 704–712, doi:10.1785/0220110128.