Page 85 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

64 Maritime Archaeology: A Technical Handbook, Second Edition
tion, the magnetic effects of the individual items. The effect is very local- ized, but it can be utilized in a predisturbance survey to determine the extent of the buried wreck site (Green 2002; Green et al., 1967). This is discussed in Chapter 5.
2. Search Considerations
Obviously with a towed magnetometer search some preliminary consid- erations are necessary. First, it is essential to have an estimate of the size of the object being searched for, so that the most effective deployment of the sensor head can be achieved. Knowing the size of the object, it is possible to calculate the detection range from the Hall equation. The detector head must then be streamed behind the search vessel at a depth that will give the best lateral coverage. Maintaining a height equal to the maximum detec- tion range of the object being searched for is useless, as the lateral cover- age or lane width being searched will be negligible. Experiments have shown (Green, 1969) that the optimum height above the seabed is half the detection range, which gives a width of coverage equal to 1.7 times the detection range. In other words, the increased difficulty and danger of towing the detector head just above the bottom adds little value to the detection range after half the detection range.
Because the velocity is the most critical variable in determining the depth of the sensor head, any variation in the velocity will make a considerable difference to the depth of the head. Therefore, it is essential that the survey is either carried out with the detector heavily weighted to the required depth (the weights being nonferrous and kept several meters away from the sensor), or that a method of depth determination be used. Most high- quality (and thus expensive) magnetometers have some form of depth sensor. However, this does not give a direct reading of the height above the seabed, which is, of course the critical issue for both safety and knowing the distance for the Hall equation. Ideally, an echo sounder transducer located near the head would give the head–seabed distance. A less complex and easier system to install is a fine (1–2 mm in diameter) plastic tube taped to the cable and open-ended at the bottom end. The top end is attached to an accurate pressure gauge which is calibrated in the depth of the seawater. A constant air flow valve is connected to a high-pressure air source to give a regular and minimum flow of air down the tube no matter what the depth. The depth is then given by measuring the pressure required to maintain the output of the constant flow valve. Alternatively, the strain gauge depth sensor is a more sophisticated solution which is available in some commercial versions. These systems give the depth of the head, not the head–seabed distance, so an echo sounder seabed depth measurement is