Page 19 - Marine Magnetometer Processing

Figure 14: Caesium magnetometer data showing 10-20nT of wave noise (blue trace raw data, red trace filtered) This type of noise is caused by vertical motion or rolling of the towfish, itself caused by wave action on the towfish or the heaving motion of the boat pulling on the magnetometer tow cable. It is particularly common on surveys where the towfish has been towed under floats on the surface and on shallow surveys where the towfish is affected by vertical movement of waves close to the surface. Practical Data Collection We have seen how the update rate of a magnetometer affects the sample interval and the ability to detect small objects. The noise measured by an instrument will also limit the size of anomalies that can be detected if the noise level is larger than the anomaly signal to be measured. In practice a compromise is needed. Towing the magnetometer faster means that more ground is covered each hour making the survey more efficient, but towing slower allows smaller anomalies to be detected as the measurement samples are closer together. Increasing the update rate will increase the sample interval allowing smaller objects to be detected, but this will also increase the instrument noise for all types of magnetometer (proton, Overhauser, caesium). Increasing the sample interval will increase the noise floor so although the measurements are closer together they are also more noisy so you still can’t detect the small anomalies. Typical surveys producing good quality magnetometer data are run at 4 knots or 2ms-1 with a sample interval of 4Hz (4 measurements per second) giving measurements 0.5m apart. Caesium magnetometers can be run at 10Hz to detect smaller objects but this may produce an increase in instrument noise which may be significant depending on the targets being searched for. Making measurements faster than 10Hz is not recommended for marine magnetic surveys because of the additional increase in noise. The solution to the problem of wave noise is to ensure that the magnetometer towfish is not moving up and down or rolling as it is being towed along. For deeper surveys the unwanted movement can be limited by correct choice of tow method. For shallow surveys the tow method can be optimised but to get the best results the surveys should be done when the sea is calm and there are no waves. Marine Magnetometer Processing \[18\] © 3H Consulting Ltd