Page 103 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

82 Maritime Archaeology: A Technical Handbook, Second Edition
scan sonar is constantly monitoring position, the location of the source of the sonar image is known for each sonar “ping.” Because the course is known, the azimuth or direction of each ping is known. As the range is also known, it is possible to approximately track the path of the sonar sweep. For each point on the graphic image of the seabed that is output from the side scan sonar, a precise location can be given. With complex software it is then possible to take the graphic image (usually a tiff or jpeg file) and georeference the image. Thus when the image is displayed it is shown in its correct orientation (see Figure 3.36a). One problem with side scan sonar images is that they usually display the water column and then the seabed. This means that precise georeferencing of the sonar image is not possible unless the geometry is taken into consideration. Thus, it is possible to obtain the precise location of an individual target using the Marine Sonic software as the GeoTIFF images are not totally accurate. One way around this is to use a post-processing software package that removes the water column and compensates for the acoustic pathway so that the GeoTIFF is a true rep- resentation of the seabed. There are a number of programs that do this of which Chesapeake Technology’s SonarWeb is a good example (see Figure 3.36b).
Finally, these georeferenced images can be incorporated into a GIS to provide a sonar mosaic that can be then be overlayed on map and aerial photography (see Figure 3.37). The sonar mosaic is invaluable since one can examine wide expanses of seabed in gorrect geographical orientation (rather than one continuous trace without any real idea of relative orien- tation) and to ensure that the seabed is completly surveyed.
F. SUB-BOTTOM PROFILER
Sonar which penetrates below the seabed, in some instances, can reveal buried structures. More often, sub-bottom profiling sonar is used to examine geological and sedimentary formations, but at times the sub-bottom echo has indicated buried cultural objects (Frey, 1972). It is, however, question- able if this system has ever found a buried ship. During the Amsterdam survey (Marsden, 1974), a sub-bottom sonar survey indicated a high prob- ability that there was a buried object at the site, but it is far from clear that this could have been determined if the site was not already known to have been there. Because the device was operated over a known site that had been previously buoyed, this was hardly a scientific test of the system. Although the trace certainly showed some unusual features in the area, these may have been geological. Likewise, with the Mary Rose sub-bottom survey, it is again doubtful that the acoustic return was actually responding