Recalibration of OBSIP Instrument Orientations

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

Ocean bottom seismometers (OBSs) sit on the seafloor and record earthquakes. They are deployed from a research vessel. The instrument is released on the ocean surface, the OBS sinks and lands on the seafloor. How it lands and how the instruments are aligned with the geographic coordinate system is unknown. A gimbal system surrounding the seismometer makes sure that it is upright, but no check is made on which direction the instrument is facing. Seismometers on land can be oriented using a compass, but this is not currently possible on the seafloor. The analysis of seismic data, however, is dependent on knowing the orientation of the instrument. To address this problem, a standardized, automated tool will be developed to provide the best possible estimates for OBS instrument orientations. The tool is based on arrival angle measurements on seismic surface waves from shallow earthquakes around the globe. After development of the new automated tool it will be validated by comparing results to older high-precision, but hands-on and time consuming measurements that have been made. The resulting code will be released publically and become a community tool. Many seismic data analyses, such as receiver functions, shear-wave splitting and Love wave dispersion, rely on the processing of a complete 3-component seismogram. For these analyses, knowledge of the alignment of the horizontal components with respect to the geographic coordinate system is a prerequisite. On land, this information is easy and routinely obtained through cross-check with a compass. In the oceans, free-fall ocean bottom seismometers (OBSs) sink with unknown orientation. Acoustic surveys of the instrument are cost-prohibitive, and the determination of the orientation of seismometer components is usually left to the individual data end user. Reorientation is done concurrently with the intended data analysis, often without detailed assessment of biases from uneven event coverage and wave propagation in complex media. This project will develop a standardized automated tool to provide optimal estimates for OBS instrument orientations. Long-period Rayleigh waves likely remain the best tool to determine instrument orientations given the restrictions and noise levels of an ocean environment. The new toolbox will be carefully bench marked against an older, well-established interactive-screen tool. The latter has proven impractical to be used by others because it contains old, non-compliant legacy code. The new tool will consist of a combination of Python and Fortran codes and will provide automated access to data at the IRIS Data Management Center.