Page 26 - Marine Magnetometer Processing

Processing using time-series plots alone has its limitations as it does not give a good idea of where anomalies are in relation to one another. The same survey data can be shown on a chart as a series of track lines showing the path of the magnetometer in a two-dimensional plan view. Colouring the track lines according to the measured magnetic field allows us to visualise the same data in a different way. In figure 21 the red areas show regions of increased magnetic field strength, the green areas show regions of reduced field strength and the yellow areas show the undistorted or background field. Using a chart we can see more clearly how the anomalies relate to one another and also see if any anomalies have been detected on multiple lines. Figure 21: Chart plot with coloured track lines Figure 22: Chart plot showing target point size proportional to target estimated mass in tonnes Using a combination of time-series plot and chart you should be able to identify the significant targets within the survey area. This is often an iterative process where you start with the time-series plot, mark those targets on a chart, which then suggests other areas in the data where you could look for more targets. This can be helpful when deciding whether a small anomaly is really an anomaly or just noise as its position may correlate with other anomalies or a debris trail. Marine Magnetometer Processing \[25\] © 3H Consulting Ltd Often the required result of a survey is simply a list of targets that need to be investigated. A chart showing the locations of the targets gives a better idea of how the targets are grouped and whether there is any pattern to the distribution, such as caused by a debris trail from a shipwreck. The size of the anomalies can also provide useful information. We can show targets as points with the size of the point representing the size of the anomaly in nT, so the larger anomalies will show up more easily than the smaller ones, but groups of targets can still be seen (Fig. 22).