Number of seismic sources

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Problem 12.4a

Whereas seismic ships sometimes tow several streamers, only rarely do they use more than two source arrays. Why?

Solution

The ship must move at a steady rate to keep the towed equipment under control. Also it must travel an appreciable distance between air-gun releases to avoid overlap of the recordings from successive shots. A speed of 6 knots means about 3 m/s. The data received by the streamer ordinarily lasts for about 15 s following a source release, so that the source firing interval must be at least this large if overlap of data is to be avoided. During 15 s, the ship travels about 45 m, about the maximum without excessive smearing. With two source arrays firing alternately, minimum source spacing is thus about 90 m. The use of more than two sources would result in unacceptable source spacing and gaps in the continuous coverage.

Problem 12.4b

Why do some surveys use more detector locations than source locations whereas others do the opposite?

Solution

The cost of a seismic survey depends primarily on the time required. Depending on local conditions, laying out detectors may be either fairly rapid or slow, and similar considerations apply to movement of the source. Hence the optimum combination of numbers and disposition of detectors and sources in order to achieve a given density of coverage varies from situation to situation.

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