Ambient Earth noise and instrumental noise

Introduction The seismic frequency band can be defined from 0.3 mhz to tens or hundreds of hz, in which the lower limit is defined by the period of the gravest observed free mode of the Earth (0s2). Digital seismic recordings in this frequency band always contain noise, which essentially can be attributed to ambient Earth noise and instrumental noise, or self-noise, of the recording system. Instrumental noise of today’s seismic sensors and high resolution digitizers is usually not considered during the interpretation of seismic data. however, this type of noise may dominate at lower frequencies (e.g. below 0.02 hz) or at higher frequencies at sites with very low ambient Earth noise levels. In cooperation with Utrecht University (Faculty of Geosciences) KNMI developed a new method to measure instrumental noise in seismic recording systems using ambient Earth noise recordings1). This new method extracts self-noise and relative transfer functions of the recording systems from the measurements only, and does not require a priori information about the transfer function of each channel. As a consequence the method reveals under which conditions the interpretation of data may be biased by the recording system.