Page 90 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 3: Search and Survey 69
GPS antenna Survey vessel
Layback
Approximate cable length
Fish
Height of fish above bottom
Depth of fish
Figure 3.28 A schematic showing the principle of layback. Cable equations are complex, but an estimation of the horizontal distance from the detector head behind the boat is important when conducting accurate surveys.
of the detector head caused by turbulence and movement of towing cable. Such effects are likely to cause the detector to experience variations in mag- netic field strength, thus causing noise. In addition, the inductive effect of the polarizing field of the proton magnetometer system is likely to be affected by being surrounded by a conductor (seawater). Other sources of noise encountered in general operation is electromagnetic noise originat- ing from electrical discharge such as engines, lightning, electrical sparks, radio transmission, and sunspot activity. These usually result in large random spikes of noise.
4. Aerial Magnetometer
If the detector head can be removed from the seawater medium, both the noise-related effects of towing and the noise effects of the medium can be eliminated. Thus in shallow water it is often a preferable option to run the system out of the water with the advantage of the improved signal–to- noise ratio. The other option, in air, is to use a cesium vapor magnetome- ter which usually has a resolution of about 0.001 nT, although in reality this is probably about 0.05 nT due to a variety of other noise-induced problems.
Thus, in certain circumstances, there can be a real benefit to operating in air. Because the cesium magnetometer allows a much higher sampling rate, the survey can proceed at a much faster rate and can even be deployed on an aircraft. This immediately has implications relating to cost-effectiveness of a survey. Although the cesium magnetometer is inherently expensive, the