The “low-frequency” range of seismic recordings has been of interest to global seismologists for many decades, but only recently has there been a growing interest in utilizing low-frequency passive seismic information for hydrocarbon exploration purposes. Examples of applications include passive seismic monitoring, interferometry methods using the ambient background field, and generally utilizing lower frequency energy for imaging and seismic inversion. Sources of low-frequency ambient background noise are often not well-understood, but a good understanding is essential to avoid pitfalls during data analysis and interpretation. In this study, I analyze a passive seismic data set from the North Sea with regard to ambient noise sources in the frequency range 1–10 Hz. As we shall see, we can find a multitude of explainable ambient noise sources, some of which are external, and some of which are related to the sensor and recording packages themselves. External noise sources include boats, seismic operations, and other fixed seabed installations. Environmental noise includes wind-generated ocean waves, water currents, and wildlife. The design of the sensor and recording unit in combination with the soft seabed sediments gives rise to resonances particularly detrimental to the frequency range of interest. These resonances are exacerbated by presumably strong water currents in the survey area. Boats can be observed at distances of more than 160 km.

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