Page 95 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

74 Maritime Archaeology: A Technical Handbook, Second Edition B. OTHER TOWED DETECTOR SYSTEMS
There are a number of interesting methods that could possibly be used to locate wreck sites. In many cases these systems have been suggested, but never tried in practice. For example, it is possible that a form of towed metal detector could be used for wreck site location. However, these systems depend on the 10-6 law rather than the 10-3 law of the magnetometer. In simple terms, if one doubles the distance from an object then the signal is reduced by 1/8 using a magnetometer, whereas it is reduced by a 1/64 in a metal detector. As a result of this short range, such systems have limited use. Others have suggested using a sensitive, differential conductivity meter to locate sites. To date neither of these systems is available either commer- cially or in a development stage. Many systems, while sounding theoreti- cally attractive, either do not work, or do not perform as well as advertised or claimed. Therefore any new or unproven system should be treated with a great deal of caution until the claimed performance to detect sites can be clearly demonstrated.
VI. ACOUSTIC SYSTEMS A. ECHO SOUNDER
The paper trace or the color screen echo sounder is one of the simplest acoustic wreck location systems. With this system a wreck, standing above the seabed, can usually clearly be seen on an echo sounder trace (Figure 3.31). Such a system of wreck location is limited, because the narrow beam width of most modern shallow water echo sounders results in a very narrow search path. The echo sounder trace can be used quite effectively to pre- cisely pinpoint a wreck site once its general location has been determined. This is particularly important when plotting a site with a GPS. Two varia- tions of the simple echo sounder are described next.
B. SCANNING SONAR
The scanning sonar is a system where the sonar transducer rotates, like a radar antenna, and the reflected signal is used to produce a sonar plan of the seabed. The beam is directed approximately horizontally and a time variable gain is used in the electronic system to amplify the signals reflected from a distance. The farther away the target, the weaker the signal and