磁力探測對搜尋水下目標物之應用研究

海洋磁力探測技術常被利用在偵測海床上磁性物質，測量之海洋磁力值經過處理後繪製成磁力異常分布圖，研究目標物磁力異常特徵之後就可幫助判定鐵磁性目標物位置，故海洋磁力測量可廣泛應用在偵測海床上電光纜位置判定、搜尋人造磁性目標物、沈船定位、探討磁性物質汙染以及考古作業等方面。本研究使用Geometrics 公司所設計之G-880銫原子光學激發型磁力計來作磁力測量，使用拖船長度大約為10公尺，磁力計後拖距離控制在20公尺到30公尺左右，盡量避免拖船上人為磁力干擾的影響。磁力參考選擇與測區最近的中央氣象局磁力觀測站作為磁力基站。鐵磁性物體因物體形狀差異、所在地球磁場方向有所不同，其產生之磁力異常特徵亦會不同。最典型的磁力異常特徵就是偶極磁力異常模式（dipole）與管型磁力異常模式（pipeline）；本研究將提出兩個海洋磁力測量實例分別研究及討論這兩個磁力異常特徵模式。琉球與台灣本島間海床上電纜所產生之磁力異常符合管型磁力異常模式，垂直經過電纜的磁力剖面都有明顯之磁力異常，經過資料處理，將磁力值網格化後繪製成的磁力異常圖顯示出電纜受地球磁場磁化影響，如同一個海底的巨型磁體，其磁力方向與電纜擺放方向相同。電纜體積大，磁化程度強，磁力異常最大值幾乎達到60 nT，透過數條磁力剖面定位出的電纜位置相當精準。龍門海域的磁力調查結果則顯示了偶極磁力異常模式。經過處理且網格後的磁力異常圖顯示了在測區東北方與西南方各有一個磁力異常區。在同個背景地球磁場下，這兩個磁力異常區大小不同，且指向也不同。異常區大小與磁性目標物深度有關，距離磁力計越遠，所造成的磁力特徵波長越長，反之亦然；指向與目標物磁化的方向有關，在目標物上方的磁場方向就是目標物磁場與地磁向量合成的結果，故觀察磁場方向可以判定目標物的指向。磁力測量所判定的定位、目標物指向、目標物深度結果與測掃聲納影像結果相符，證明海洋磁力測量在搜尋磁性目標物上的成功。為了使海洋磁力測量更精確及實用，研究中將討論磁力計與航向的關係並且實地實驗。光學激發型磁力儀因為其運作原理，而有低敏銳區。當地球磁場通過這個區域，儀器將會無法感測到磁力強度，這就是磁力計的「死區」。台灣地區海域在近五年（2013年左右）磁偏角為37.1度，磁傾角為-4.09度，經過實驗發現，將磁力計設定在轉向（rotation）0度、傾斜度（tilt）0度下會有最好的收訊結果。海床測量通常是集合多種手段一起進行，磁力測量如何與其他搜尋方式結合是極為重要的。當側掃聲納影像上無法判別目標物是否為磁性物質時，加上磁力測量資料可以幫助判定，且透過研究磁力異常特徵也可預測目標物指向、深度以及大略重量。根據此論文中多個案例研究，海洋磁力探測在海事應用上可廣泛應用，且成果也頗理想，惟須注意探測作業中人為磁力干擾的控制與評估，磁力計是否正確設定，以及定位系統的精確，才能確保磁力資料判釋上的正確。

關鍵字

海洋磁力探測；銫原子光學激發型磁力計；磁力異常特徵

並列摘要

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.

並列關鍵字

marine magnetic survey ； cesium optimal pumping magnetometer ； magnetic anomaly

參考文獻

Boyce, J. and E. G. Reinhardt (2004). "Marine Magnetic Survey of a submerged Roman Harbour, Caesarea Maritima, Israel." International Journal of Nautical Archaeology 33.1: 15.

Boyuce, J., M. Pozza and B. Morris (2001). "High-resolution magnetic mapping of contaminated sediments in urbanized environments." The Leading Edge: 886-890.

International Association of Geomagnetism and Aeronomy, W. G. V.-M. (2010). "International Geomagnetic Reference Field: the eleventh generation." Geophysical Journal International 183(3): 1216-1230.

Nabighian1, M. N., V. J. S. Grauch2, R. O. Hansen3, T. R. LaFehr4, Y. Li1, J. W. Peirce5, J. D. Phillips2 and M. E. Ruder6 (2005). "The historical development of the magnetic method in exploration." Geophysics 70: 29.

Nabighian, M. N. (1972). "The analytic signal of two-dimentional magnetic bodies with polygonal cross section: its properties and use for automated anomaly interpretation." Geophysics 37: 507-517.

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磁力探測對搜尋水下目標物之應用研究

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