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Field Technique Design for Seismic Reflection Exploration
Seismic field techniques (for conventional sources) can be described by specifying twelve factors: far-trace offset, near-trace offset, group interval, charge size, charge depth, multiplicity, sample rate, low-cut filter, geophone frequency, record length, geophone array, and spread type. Traditionally, these parameters have been constrained by hardware (e.g., far-trace offset or group interval by available cables), or have been established by picking the “best record” in a comparison (e.g., charge size). Sometimes geophone and source patterns were designed by computing an array which, in theory, cancelled a measured type of coherent noise. Often field techniques are “the same as the last survey” (which may have had a different objective, or may have been restricted by a hardware limitation which no longer applies).
This is no longer good enough. Techniques are practically unlimited by hardware (although we must now often ask what we can afford to do). Because we are now interested in attributes of reflections rather than just reflection times, the effect of field parameters on the reflections must be carefully weighed; this leads logically to design of field techniques around the known or predicted characteristics of the target reflections. Where comparison shooting is necessry, the comparison must be objective rather than subjective. Designing arrays on the basis of unwanted noise is increasingly undesirable because as resolution has increased the wave number range of high frequency, high-velocity reflection components has overlapped the wave numbers of low-frequency, low-velocity ground roll. Such coherent noise may now be better handled in processing.
With the widespread use of migration even in areas of low dip, additional parameters (such as line length) which are not normally considered as part of the field techniques have become critical.
In view of the increasingly wide range of field parameters which are practical, and the extension of interpretation beyond mere structural mapping, it is essential that the purpose of the survey be clearly and completely defined before the acquisition parameters are set, and that the parameters be designed logically from this purpose.
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