Dynamic Range and the Seismic System – What Has Been Accomplished and Where Must We Go

Abstract

The combination of earth absorption and limited dynamic range results in limited recoverable bandwidth from our seismic data. Limited bandwidth means limited clarity of our images of the subsurface. Therefore, much emphasis has been placed on improving dynamic range in past years. Through to the end of the 1980’s, one of the recognized weakest links in our seismic system was the analogue- to-digital converter in the recording instruments. The instantaneous dynamic range of this system was, in practice, only about 70 dB.

The development of the Delta-Sigma modulator (the so called “24-bit” recording system) was brought to large production land seismic systems in 1991. This A-D converter offered between 100 and 110 dB of instantaneous dynamic range when implemented in a field system.

However, the A-D converter is just one component in a complex system of producing seismic sections. We must regard the system as a chain of processes including the geophone, analogue cable segments, preamplifier, converter, tape format, tape media, digital processing and filtering, data display and interpretation. Our end product will be limited by the weakest link in the chain.

It is important to understand the theory of the Delta Sigma converter in order to appreciate the importance of sample rate and modulator order in achieving dynamic range. This is a different tool that is poorly understood. Like any good tool, when mis-used, its effectiveness is limited.

It is important to recognize the limits imposed by harmonic distortion within the pre-amplifier. We are further limited by distortion introduced by the geophones and cables in front of the recorder. Distributed telemetry systems and low distortion (close tolerance) geophones have begun to address these problems, but we have much work ahead of us to strengthen these parts of our seismic system. This presentation will outline some recent developments in this area.

As instrument manufacturers strive to improve dynamic range in field acquisition, moduwhat have we been doing with respect to other links in the chain of the seismic system? Are we employing the best processing algorithms to preserve and utilize the dynamic range of field data? I will present evidence that frequency domain processing results in a loss of dynamic range. Are work station displays preserving the available information? Are processors and interpreters continuing to re-set standards for bandwidth preservation?

It is my sincere hope to use this paper to outline the bandwidth limitations due to limited instantaneous dynamic range due to each element of the seismic system. I will review some of the progress that has been made in the past twenty years. and I hope to stimulate thought on some progress we can readily achieve in the coming years.