The basic seismic reflection survey process has remained unchanged in concept since the recording of the first seismic reflection over 50 years ago. A quantum of energy from a controlled source is transmitted into the ground at or near the surface. The energy travels through the sedimentary section gathering information wfrom each reflecting horizon, and returns to the surface. It is captured by a geophone as a short, continuous signal and shunted through amplifiers to a system for permanent recording.
The reflection process consumes the input energy at a rapid rate. The initial seismic return may have a million times greater amplitude than that existing at the end of its short life. Wide dynamic range is a dominant characteristic of the seismic signal, and it is perhaps the major engineering consideration in the design of seismic data acquisition equipment. For the purpose of this paper, dynamic range may be defined as the ratio between the mimimum and maximum signal strength over a range for which signal distortion through a given system can be held below acceptable levels. A system which can record and recover a signal of one millivott with acceptable distortion, and continue to perform satisfactorily up to a level of one volt has a dynamic range of 1,000:1.
A logarithmic scaling unit, the decibel, is commonly used to measure dynamic range. Expression of the ratio in decibels is obtained by multiplying the log10 of the numerical ratio by 20; thus a ratio of 1,000:1 becomes 20 × 3