Page 89 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

68 Maritime Archaeology: A Technical Handbook, Second Edition
Figure 3.27 Example of a magnetic survey conducted from a vessel over a geomagnetic anomaly, thought at first to be a World War II wreck site.
in the case of the GPS, from where the antenna is located, Figure 3.28). Thus, if one steams up one lane and down the next lane past the same anomaly, and layback is not taken into account, then the targets will appear separated by twice the cable length. Access to a computer and a chart plotter will speed the processing of large surveys and, in many cases, the layback can be fed into the data logger during the survey.
3. Magnetometer Sensitivity
The sensitivity of a magnetometer depends on a number of variables including the background noise experienced when operating a magne- tometer in seawater. In general, the explanation of this noise is complex, the usual noise level for a proton magnetometer is about 5 nT. The source of noise in the seawater environment can be divided between small, random variations in the magnetic field intensity. These variations are caused by physical effects which are largely microphony in the cable and general instrumental noise. Other physical effects include the physical movement