problem, recorders or signal processors are available that incorporate the 

 automatic or time variable gain control features described above. 



96. Recorders contain a selection of stylus sweep speeds that control 

 vertical scale, and chart transport speeds that control horizontal scale. 

 Newer recorders have reversible sweep direction, a very useful feature that 

 allows parallel lines, run on opposite headings, to be directly compared as if 

 both were run on the same heading. Since adjacent seismic profiles are usu- 

 ally run in opposite directions, out on one line and in on the next, reversi- 

 ble sweep features are of great value in later comparative analysis and 

 interpretation. 



Ancillary equipment 



97. In addition to the basic components of seismic reflection systems, 

 a number of ancillary components have been developed to increase the capabili- 

 ties and improve the quality of the system as a whole. These components are 

 valuable additions that are well worth the relatively small cost of their 

 rental compared to overall equipment and personnel costs. In many cases, com- 

 ponents such as motion compensators and automatic record annotators can actu- 

 ally decrease costs by limiting downtime due to weather and reducing personnel 

 requirements . 



98. Tape recorders . Tape recorders, having two or more channels, can 

 be used to advantage with seismic reflection systems to record the key pulse 

 and the unprocessed return signals. This provides an additional record from 

 which the graphic record can be reproduced in the event of loss or damage of 

 the original. In addition, the raw acoustic data can be repeatedly processed 

 after the survey using different combinations of band pass filters processing 

 other variables such as scale and chart speed settings, which can improve 

 overall record quality and enhance selected aspects of the data. Such records 

 are particularly useful where the acoustic source creates pulses with a broad 

 band of frequencies produced from boomers and sparkers. Multiple-channel 

 recorders can be used to simultaneously record other survey data such as side 

 scan and navigation information. 



99. Motion compensators . Seismic reflection records are adversely 

 affected by wave conditions because of the cyclic vertical motions imparted to 

 the acoustic source and receiver, which give a jagged saw-tooth character to 

 the lines marking reflectors. This irregularity complicates interpretation 

 and introduces a significant element of error in measuring the precise depth 



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