Page 86 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 3: Search and Survey 65
required for the head–seabed distance to be calculated. In some respects this is a good system, because the echo sounder can warn the operators if there is a sudden change in depth of the seabed, which may cause the detec- tor head to snag the bottom.
More recently, successful trials have been made using a side scan sonar and a magnetometer combination. When the magnetometer is located rea- sonably close to the side scan tow “fish,” it gives information about the seabed and the distance of the detector head from the bottom. All these data then can be combined within the side scan computer data logger. At the Department of Maritime Archaeology at the Western Australian Maritime Museum we have integrated a side scan sonar with a magne- tometer. The side scan system we use is a Marine Sonic Sea Scan PC system which, apart from logging the side scan data, allows data from the magne- tometer to be recorded in a data file together with the GPS information. In this way it is possible to log the information on the magnetic field strength and, as the information is georeferenced, create magnetic contour plans. If the magnetometer tow fish is deployed a short distance behind the side scan, the side scan trace can be used to determine the height of the unit above the seabed. So that when a magnetic anomaly is encountered its mass can be estimated from the approximate Hall equation.
In the situation where a position fixing system is not available, it is pos- sible to locate an anomaly by throwing a buoy overboard when the anomaly is first observed. The course is then reversed and another buoy thrown over- board when the anomaly is again observed, thus bracketing the target. This process can be repeated on a course at right angles to the first anomaly to help to define its position more accurately.
The only effective way to operate a magnetometer survey is to carry out the survey first and then investigate the anomalies. The temptation to examine each anomaly as it occurs should be avoided, as this is both time- consuming and very inefficient. Most magnetometers have a digital readout and, in some cases, have a paper trace or a data output through an RS232 interface. Whatever system is used, it is necessary to relate the magnetic intensity figures with the position being plotted. Where there is only a digital readout, the values should be recorded against time, for example, every 10 seconds; however most systems have a paper trace or chart recorder that gives a continuous intensity reading (Figure 3.25). Thus, it is only necessary to mark the point on the trace at which each fix is taken. In the situation where a GPS is used, if the position data and time can be logged on a data logger, then it is only necessary to record the time on the trace. Obviously, the ideal situation is where the magnetic intensity is recorded together with the position automatically. There are a number of integrated systems that allow that to happen.