PART V: GEOPHYSICAL TECHNOLOGY 



65. Geophysical survey techniques are widely used methods for gathering 

 subsurface geological and geotechnical data in terrestrial and subaqueous 

 environments. Three types of geophysical survey systems are typically used 

 for borrow source exploration programs: Fathometers, seismic reflection sub- 

 bottom profilers, and side -scan sonar. These systems are used to obtain 

 information on sea floor geomorphology , small-scale bottom features such as 

 ripple marks and rock outcrops, and the underlying rock and sediment units 

 (Williams 1982) . All three systems are electrically powered acoustic devices 

 that function by propagating sound pulses in the water, and displaying the 

 data graphically on a chart recorder. The description of the Fathometer in a 

 subsequent section will illustrate the basic principles of operation for all 

 three systems. 



Position Control 



66. Accurate navigation and position control is essential for borrow 

 source exploration programs. CERC Coastal Engineering Technical Aid 80-4 

 (Prins 1980) recommends an accuracy of 3 m; however, in most cases, satisfac- 

 tory control can be obtained with a system with an accuracy of 15 m. Posi- 

 tioning and navigation controls presently in general use are low- frequency 

 radio-wave, microwave, and laser-type devices. In addition, a satellite-based 

 system is in the process of development and is expected to be available for 

 general use in horizontal positioning in 1991. 



67. Loran C, a pulsed low- frequency radio wave navigation system, is 

 presently widely used on private and commercial vessels. It has long range, 

 uses an inexpensive receiver, and does not require the portable onshore tran- 

 sponder stations required for microwave systems. However, the accuracy of 

 Loran C is not sufficient for use in borrow source exploration programs except 

 in preliminary field reconnaissance operations. 



68. Microwave navigation and positioning systems have been in general 

 use for many years and have proven to be suitable for high accuracy surveys. 

 Many microwave systems are on the market and they have frequently been 

 employed by Corps of Engineer agencies for borrow exploration and other 

 coastal studies. Microwave systems are relatively expensive. Their typical 



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