Note: In the final version of the New Manual, parts of this section will be incorporated into Chapter 4. They are reproduced here in order to make Chapter 5 more complete as a standalone text.
Electronic seismographs can be designed for any desired magnification of the ground motion. A practical limit is however imposed by the presence of undesired signals which must not be magnified so strongly that they obscure the record. Such signals are usually referred to as noise and may be of seismic, environmental, or instrumental origin; the instrumental self-noise may come from mechanical or electronic sources. We will discuss seismic and environmental noise in the present section and instrumental self-noise in the next.
Seismic noise has many different causes. Short-period noise is at most sites predominantly manmade and somewhat larger in the horizontal components than in the vertical. At intermediate periods (2 to 20 s), marine microseisms dominate. They have similar amplitudes in the horizontal and vertical directions. At long periods, horizontal noise may be larger than vertical noise by a factor up to 300, the factor increasing with period. This is mainly due to tilt which couples gravity into the horizontal components but not into the vertical (see 3.3). Tilt may be caused by traffic, wind, or local fluctuations of the barometric pressure. Large tilt noise is sometimes observed on concrete floors when an unventilated cavity exists underneath; the floor then acts like a membrane. Such noise can be identified by its linear polarization and its correlation with the barometric pressure. Even on an apparently solid foundation, the long-period noise often correlates with the barometric pressure [Beauduin et al. 1996]. If the situation cannot be remedied otherwise, the barometric pressure should be recorded with the seismic signal and used for a correction. For very-broad-band seismographic stations, barometric recording is generally recommended.