Abstract
The combination of earth absorbtion 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 strenghten 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, what 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.
Biography
Norman M. Cooper received his BSc with a major in Geophysics from U.B.C. in 1977. After a short experience with the mining industry he joined Amoco Canada Petroleum Company. With Amoco, he was allowed a free hand in experimenting with unique field and processing methods. This practical experience, combined with Amoco's excellent training program, armed Norm with some basic geophysical tools. Prior to leaving Amoco, he became involved with their training center in Tulsa, Oklahoma, periodically teaching portions of courses as part of their geophysical training program.
In 1981 Norm moved to Voyager Petroleums Ltd. There he became more directly involved with seismic field operations and program design and gained experience quickly in a high-productivity environment. His field of experience was broadened to encompass all of Western Canada as well as minor exposure to the Beaufort, East Coast and Northern States.
By late 1983, Norm felt the urge for independence and founded Mustagh Resources Ltd. Since then he has been very active in geophysical consulting, survey design, and general exploration. He developed "Introduction to Seismic Methods", a one week course which has been presented to many groups as diverse as data brokers, processors, party managers and geologists. By teaching this course periodically for members of the Canadian Association of Geophysical Contractors, he has been exposed to a great variety of perspectives regarding field practices. Norm has also taught introductory courses in geophysics for SAIT. Now his courses have expanded to include special training programs in seismic acquisition, 3-D design, vibroseis theory, and instrumentation.
Most recently, he has been very active in 3D program design and quality assurance of field operations. He has worked in areas as diverse as the Green Mountains of Libya to the Jungles of Borneo, and from the Rocky Mountains of Canada to the basalt plains of Northern Ireland. He has worked on seismic programs in 23 countries spanning six continents.
Norm is a member of the CSEG, SEG, CAGC, APEGGA, CSPG, and CWLS.
