Page 98 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 3: Search and Survey 77
Figure 3.33 Multibeam image of a shipwreck in the Persian Gulf. Data collected by the RESON SeaBat Multibeam System. (Courtesy of RESON.)
these were less flexible in their use and the rolling of the vessel in anything except flat calm conditions usually made the system inoperable. The side scan transducer emits a fan-shaped pulse of sound which has a narrow beam width in the fore and aft direction and a wide beam width laterally. The fan- shaped beam lies in a plane at right angles to the track of the fish, and the center of the beam is directed slightly downward from the horizontal toward the maximum anticipated range. Nonlinear, time-variable amplifi- cation enhances the signals coming from distant objects and compensates for strong near-field signals. The unit records the intensity of the return of the time-variable signal. In the case of a towed fish, the trace displays three elements: the seabed; the water surface; and a shadow picture of the seabed (see Figure 3.34). Interpretation of the record is often quite complex, par- ticularly because the output often shows the water surface and the seabed as single lines and the adjustment of gain to obtain the best bottom signal can be difficult.
In normal operation the side scan is best operated over a smooth sandy seabed. Usually the system is interfaced with a GPS, so that the image trace is created to scale. Thus in a search for the Sapporo Maru in Truk Lagoon,