Page 20 - Marine Magnetometer Processing

6. Basic Processing Introduction Having understood how anomalies are formed and the limitations of the instruments used to record them we can now take some magnetometer data and identify any anomalies and the positions of their associated targets. Often the magnetometer survey is undertaken to identify iron objects in the survey area that will then be investigated by divers or ROV. So the most important final product is a list of targets and their positions with some additional information about them. A typical dataset will contain measurements of the magnetic field at known positions. The dataset should also include a measurement of the depth of the towfish at each point and/or the altitude of the towfish above the seabed. Often the survey boat used to collect the data will sail a regular pattern of survey lines a given distance apart. Using this information we can identify the anomalies in each survey line, estimate a position for the associated targets then estimate the mass of iron for each that would cause the measured anomaly. The procedure for identifying targets and anomalies is the same for any data set: • Each survey line is inspected in turn and any significant anomalies marked • A target is associated with each anomaly except where anomalies on adjacent lines are close enough to have been caused by the same target • A list of targets and their properties is then created • Where possible, the target positions can be compared with the results of a side scan sonar or multibeam sonar survey so any sonar targets can be correlated with the magnetometer targets. For each survey line the first step is to identify the anomalies so before that can happen you need to decide what will be considered to be an anomaly. Surveys are usually done for one of two reasons; either to find an object of a known size or to find all iron objects bigger than a given size. For this we need to be able to estimate the mass of iron associated with an anomaly. Calculating Mass There is a direct relationship between the mass of iron in a target, the distance between target and magnetometer and the size of magnetic anomaly it produces. The relationship is defined in the Hall equation. The Hall equation relates the mass, anomaly size and distance: ∆ = 10 × × (3) ∆ = is the anomaly size in nT w = mass in kg d = slant distance in metres a/b = aspect ratio , length / width The distance between target and the magnetometer sensor is a slant distance, the direct distance between target and magnetometer even if the magnetometer does not pass directly over the target. However, for simplicity we usually assume that the target is on the seabed and under the towfish so the slant distance can be calculated from the altitude of the magnetometer towfish above the seabed. The altitude is often calculated from measurements of water depth from an echo sounder and the depth of the towfish measured by the towfish itself. Marine Magnetometer Processing \[19\] © 3H Consulting Ltd