Orientation to True North for Seismic Stations: Review of Field Practices and New Instruments to Improve Accuracy and Operational Efficiency

Accurate horizontal orientation of three-component seismic sensors remains a persistent weakness in field deployments, despite its direct impact on modern seismological observables and advanced processing methods. While orientation is routinely performed using magnetic compasses for practicality, the effective accuracy achieved in the field is often insufficient and leads to systematic errors that propagate into receiver functions, amplitude and polarization analyses, propagation direction estimates, waveform separation, and moment tensor inversion. Large-scale surveys of permanent and temporary networks report frequent misorientations at the multi-degree level, with a worldwide dispersion typically on the order of several degrees, and a significant fraction of stations exhibiting errors well above 4–10°.

This contribution reviews current practices used to transfer the North direction to seismic stations, with emphasis on the real causes of misorientation in operational conditions: magnetic declination handling (including temporal drift of the magnetic pole), local magnetic disturbances (geology, infrastructure, reinforced concrete, metallic objects, and even the station itself), and the intrinsic limitations of visual alignment and reference transfer. Post-installation orientation estimation techniques, although valuable, often remain an additional and fragile processing step, not consistently preserved as station metadata and not always correctly exploited by end users.

A specific focus is placed on the orientation accuracy required to enable reliable array-derived rotation (ADR), which explicitly links field alignment to scientific performance. Building on published uncertainty analyses, we highlight that station misalignment is among the dominant contributors to ADR uncertainty, and that replacing magnetic compasses by gyrocompass-based procedures can reduce uncertainty by about one order of magnitude while extending the usable wavelength band toward longer periods—thereby unlocking higher-value array analyses.

To address this gap between scientific needs and field reality, we introduce the NJORD product line, designed specifically for seismic station orientation, from cost-effective solutions to higher-performance instruments compatible with static deployments and field. Among these, the Årian true-north finder is a compact, handheld static optical gyrocompass enabling rapid operation (alignment time < 4 minutes), one-day field autonomy, and an export-free approach requiring no aiding sensors (no GNSS, no magnetics), with a stated heading performance of 0.9° seclat RMS.

Årian is intented to shift field-deployment practice, enabling high-quality station orientation to become routine rather than exceptional, improving both data quality and operational efficiency at network scale.