Figure 26. Side-scan sonar in operation 



system is sensitive to vessel motion and is most suitable for use during calm 

 conditions. 



Commonly available side-scan sonar equipment, at a frequency of 

 100 kHz, is capable of surveying the seafloor to over 500 m to either side of 

 the vessel track line; thus, a total swath of 1 km or more can be covered at 

 each pass. To provide higher resolution output at close range, some systems 

 are capable of dual operation using both 500-kHz and 100-kHz frequency 

 signals. The data are simultaneously recorded on separate channels of a four- 

 channel recorder. Digital side-scan sonar systems are available that perform 

 signal processing to correct for both slant range to seafloor targets and survey 

 vessel speed. The resulting records show true x-y location of seafloor objects, 

 analagous to maps or aerial photographs. The digital data can be recorded on 

 magnetic media, allowing additional signal processing or reproduction at a 

 later date. 



Ground-penetrating radar (GPR) is a relatively new technique for subsur- 

 face exploration. In contrast to the acoustic systems described above, GPR is 

 used subaerially. The radio portion of the electromagnetic spectrum is emitted 

 from the source and reflected back to the sensors. The transparency of geo- 

 logic materials varies. Sands and limestones are typically reasonably 



Chapter 3 Field Data Collection and Observation 



61 



