Page 31 - Marine Magnetometer Processing

measurements collected in the other (Fig. 28). This may be caused by the towfish being too close to the towing vessel when working in shallow water. Again an additional cross line of magnetometer data can be used to correct the data, this is a line run at 90° to the main survey lines. In processing, the values of data points on each survey line are shifted up or down so that the signal values are the same where the cross line and survey line meet. A better answer can be obtained if more than one cross line is recorded and used to compute the shift for each survey line. Wave Noise Wave noise or swell noise is created by vertical motion of the magnetometer in the water column and like other sources of noise it is better to avoid recording it rather than trying to remove it from the recorded data. Wave noise shows up as a regular, periodic change in the amplitude of the magnetic signal (Fig. 29). With a long, slow swell the wavelength (or more correctly the wavenumber) of the noise is correspondingly low and in some cases it can be reduced by high pass filtering. Wave noise from a choppy sea may unfortunately have a similar wavenumber to small magnetic anomalies so filtering out the wave noise will also remove the legitimate anomalies being searched for. As it often cannot be removed the wave noise effectively increases the noise floor value, masking small anomalies and making processing much more difficult to do. Figure 28: Lines 40nT different in absolute value, alternate lines run in different directions Figure 29: Wave noise causing 'tiger stripes' on the chart plot As it can be difficult to remove the effects of wave motion it is far better to collect magnetic survey data on calm days and to whatever is possible to reduce the effect of wave motion when towing the magnetometer. Marine Magnetometer Processing \[30\] © 3H Consulting Ltd