| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 102 === Marine magnetic survey is broadly applied to detect ferrous material on seafloor. After processing, the measured value can be transformed to magnetic anomaly map, researcher can locate ferrous target by means of analyzing magnetic features that generated by the ferrous target. Locating electric cables on seafloor, detecting underwater artificial target, searching ship wreckage, studying magnetic contamination and archaeological research are some examples of applications about marine magnetic survey.
In this research, we had G880 cesium optical pumping magnetometer measuring total magnetic field, the length of survey ship is about 10 meters. To avoid influence of magnetic disturb, the distance between the ship and magnetometer remained 20 to 30 meters. The nearby Center Weather Bureau magnetic observatory stations were chosen for magnetic ground reference stations.
Different directions of geomagnetism and various shape of ferrous targets change magnetic anomaly features. The most classical magnetic features are dipole mode and pipeline mode. In this dissertation, two marine magnetic survey case are pointed out to study and discuss two distinct magnetic features respectively.
The result of electric cable between Liuqiu and Taiwan survey show us dipole magnetic features, and the magnetic anomalies are quite obvious. After data processing, the gridded magnetic map present that the magnetism cables generate is similar to which a linear magnet does, in addition, direction of magnetism is along the cables. Due to the giant volume, the cables are highly magnetized, the maximum of magnetic anomalies even reached 60 nT. The predicted location of cables is close to the actual one.
The result of lomgmen magnetic survey present dipole anomaly feature. The gridded magnetic map points out that there are two magnetic anomalies in the survey area; one is in the north-eastern area, the other is in the west–southern area. In the same geomagnetism field, these two values and directions of anomalies are different. The research conclude that the fact is highly relevant to the depth and heading of the target. The wavelength of anomalies is a function of the distances between targets and magnetometer, and the heading of magnetic source could change the direction of its anomalies. Therefore, the researcher could interpret depth and heading of the source by studying the magnetic field above it. The side scan image in the same survey area prove the accuracy of the interpretation of magnetic survey.
In order to improve the accuracy, we discussed how headings of ship could change the quality of measured data. According to the basic theory of magnetometer, the sensor can not collect data, when the angle between geomagnetism and magnetometer fall into “dead zone”. After the experiment, the magnetometer would have the best performance in measuring when it was set to both 0 degree in rotation and tilt angle. ( The inclination and declination of geomagnetism in Taiwan area is -4.09° and 37.1° respectively. )
Marine survey usually comprises of multiple measuring methods, so the integration between different kinds of information is important. For instance, combined with side scan image, the magnetic survey can pick out ferrous targets of interest, and present the heading, depth, and roughly-measuring weight of ferrous material by studying the magnetic anomalies.Based on the several cases and study in this dissertation, we could conclude that marine magnetic survey is able to apply to maritime research widely and present convinced result in detecting and locating ferrous target. To ensure the accuracy, controlling magnetic noise, adjusting magnetometer correctly, and enhancing location system is necessary in marine magnetic survey.
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