Recorder 



94. The main functions of the recorder are to trigger the acoustic 

 source, and display the data being generated on a strip chart record showing 

 bottom and sub-bottom reflection horizons. The recording element consists of 

 a stylus on a continuous belt that crosses the chart paper at a selected rate 

 of speed called the sweep speed. When the stylus is at the upper margin of 

 the chart, the sound source is triggered and emits an acoustic signal, and as 

 the stylus continues across the chart, electric potentials created by return 

 signals are routed to the stylus, which makes a mark on the wet or dry type 

 electrosensitive chart paper. In this way, the time lapse between pulse 

 initiation and return signals for each reflector is graphically displayed, and 

 as the chart paper advances with each sweep, a coherent profile line is 

 created. The recorded time differences between the zero line and reflector 

 traces can be converted to distances by use of appropriate conversion scales. 

 The scales are based on the following expression: 



V 



D= ^ x t (3) 



where 



D = depth to reflector 



V = velocity of sound in the media traversed 



t = two-way travel time 

 This is the same formula used for conversion of Fathometer data. However, 

 instead of one medium, water, the signal also travels through one or more sub- 

 bottom layers having different sound velocity characteristics. For the rela- 

 tively shallow water and limited subbottom penetration of the systems used for 

 borrow source exploration, a velocity of approximately 1,500 m/sec in the 

 water column and 1,600 m/sec in unconsolidated sub-bottom material is reason- 

 ably accurate in most cases. 



95. The intensity of marks made on the graphic recorder showing the 

 reflectors can be varied by means of a gain control to bring out the weaker 

 reflectors; however, a too high gain will cause excess burning of the traces 

 of strong reflectors such as the bottom-water interface. To alleviate this 



38 



