The noise level depends on the geological situation and on the proximity of sources, some of which are usually associated with the infrastrucure. A seismograph installed on solid basement rock can be expected to be fairly insensitive to local disturbances while one sitting on a thick layer of soft sediments will be noisy even in the absence of identifiable sources. As a rule, the distance from potential sources such as houses and roads should be large compared to the thickness of the sediment layer.
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 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. Tilt may be caused by traffic, wind, or the barometric pressure. Such noise can be identified by its linear polarization and its correlation with the barometric pressure.