Dynamic range of the seismic system

Dynamic range usually refers to a range of levels within which a system can handle a signal with a specified fidelity or accuracy. The constraints are set by uncontrolled energy, generated in the system itself, which imposes uncertainties and sets limits on the possible range. This uncontrolled energy is of two types. Firstly, there is always noise in the system which is simply added to any signal which passes through the system. Secondly, there is signal distortion; i.e. unwanted signal-dependent energy, which is also generated in the system. The two types of energy are usually referred to as: 1. system noise; 2. noise behind the signal. Typical figures for a seismic instrument are: system noise -0.3uV rms; noise behind the signal-80 db below signal level for signals under 30 uV rms. The recovery of hidden signals will be hampered when their levels fall below the system noise or below a certain threshold under the total signal level. This recovery is partly an absolute and partly a relative matter. The available technical specificatians of the hardware involved, such as the geophones, the cables and the instrument, enable us to easily assess the dynamic range of this hardware. Things become more complex if environmental disturbances are taken into account. If, for example, the source characteristics vary from shot to shot, if geophones have not been planted properly, or if differential statics are present within an array, then these perturbations will distort the ideal case.