| As the rapid development of national economy in China, a large amount of shipping is demanded more and more strongly. Therefore, particularly in harbor, pier and navigation route, the coastal engineering projects have become one of the key areas for our national infrastructures. With the forward advance of the Maritime Silk Route and the Yangtze River’s golden waterway, the marine construction projects in China will, in the near future, not only be continued in deep and upgraded in domestic, but also take an even bigger slice of international market. However, when exploiting the valuable coastline resources, the coastal projects such as the port, wharf, fairway, marine reclamation land and bridge conflict very possibly with the pre-existing artificial structures and the derelicts those submarine pipelines, artificial reefs, shipwrecks and explosives etc. These unknown objects may results into engineering accidents and the corresponding financial loss. Therefore, at the early stage of the engineering constructions, the underwater obstacles must be surveyed to avoid the hidden dangers. Nevertheless, when detecting the underwater objects, the higher requirements of the veracity are demanded by the new management systems of the construction and the safety. For example, when probing the underwater obstacles, not only the position but also the size and the number of the targets need to be provided, so as to apply sufficient evidences for the dodge, removing dangers and salvages. In brief, detecting the underwater objects has become the important issue in the marine engineering geophysical exploration.Recently, integrated geophysical exploration is utilized to probing the general underwater objects and the artificial verification is supplemented. The main methods include the multi-beam sounding, the Sub-bottom Profiler Probe, the side-scan sonar and the marine magnetic survey etc. The characteristics of each method are described as following:the multi-beam sounding with the features of large coverage and highly sampling density, has the ability of discovering the target and locating the horizontal position precisely so that this method is usually employed to probe in case of large region; The Sub-bottom Profiler Probe is suited to exploring the depth, shape and size of the obstacle; The side-scan sonar is mainly adopted to probing the location and shape of the uncovered obstacle; The marine magnetic survey is valid for the highly magnetic obstacles such as the ferromagnetic bodies and the cement. Actually, the obstacles often are made by the ferromagnetic materials and the cement and very possibly covered by the seabed mud and sand. Therefore, the marine magnetic survey becomes one of the most used technical means.However, there exist some important problems in the marine magnetic survey for detecting the underwater objects. These problems are, for example, as following:the variation of the physical properties inside the object leads to the complex magnetic anomalies and thus erroneous judgments; it is difficult to distinguish the target anomaly from the background field and the contaminations caused by the passing ships; the underwater objects are sensitivity to environmental changes, thus the verification of the inferred parameters such as the depth of the pipes is hard to be carried out or the validation results are not good; the quantitative indicators of the inversed parameters of the target objects from the magnetic anomaly are still necessary to be improved in details. Therefore, to improve the accuracy and the reliability for probing the underwater magnetic objects, there is the necessity that developing and studying the data processing and interpreting techniques according to the characteristics of the unknown objects. Consequently, these studies have important operation and academically significance to drive the development of the underwater obstacle detecting technique and thus the applying promotion of the geophysical exploration in the coastal engineering.In this paper, the basic theory of the magnetic anomaly calculation and the impacts of the inducing magnetization and the remanent magnetization on the magnetic probing above the water are investigated first. Then the contents are determined, respectively, which are stated bellow:(1) Noise reduction by filtering on the magnetic measurements. The empirical mode decomposition (EMD) and the Hum filtering method are investigated by the theoretical analysis and the synthetic tests. The results indicate that there are some residual noises by the Hum filtering method while the noise is reduced more thorough and thus the filtering results are smoother by the EMD method.(2) Separation and extraction of the target magnetic anomaly. The Bidimensional Empirical Mode Decomposition (BEMD) and the wavelet transform are theoretically analyzed and then applied to multi-scale decomposition of the superposed magnetic anomalies field. According to the features of the multi-scale fields, the local anomalies and the regional field are re-constructed. At last, the corresponding results by the BEMD and the wavelet transform are compared with those by the matched filtering method. The studies suggest that the BEMD and the wavelet transform have different advantage respectively, and thus in the practical application multiple methods of the multi-scale decomposition should be utilized to obtain a better results of the separation and the extraction of the target magnetic anomaly.(3) Magnitude transforms and enhancements of the object’s magnetic anomaly. Some conclusions can be drawn based on the analysis of magnetic anomaly modulus data:the magnetic anomaly modulus Ta and its byproducts (the magnetic anomaly modulus R, E, Q and L) are not or slightly affected by the magnetization direction in both 2-D and 3-D situations. In which case, they can be used to interpret the magnetic anomalies without worrying the interferences caused by the remnant magnetization and demagnetization. At the meantime, the ASA method is thoroughly studied in this thesis, which leads to the understanding that the ASA can locate the horizontal boundaries of underwater structures and decrease the affection of submerged objects" remnant magnetization and demagnetization. The conclusions are demonstrated using two synthetic examples which contain a cubic model and a combined cuboids model. The results indicate that using the ASA and magnetic anomaly modulus data can effectively depict the submerged objects’characters, despite the distortions, based on the transformed anomalies gained from the downward continuation process. The improved regularization method has also shown a better performance dealing with the low frequent part of the magnetic anomaly comparing to the general Tikhonov method, which enables a better access for the low frequent anomaly and shows more distinct and focused images in theoretical experiments.(4) Imaging inversion of magnetic susceptibility for submerged objects. In this thesis, the minimum structure inversion method has been employed to locate and characterize the underwater target objects. The differences between two outcomes, one is calculated based on the L1 norm and the other is based on the L2 norm, are analyzed and compared. For a cuboid model, the former has more regular shaped and distinct boundaries, while the later shows smoother ones. Moreover, for a combined cuboids model, the locations and horizontal boundaries of the cuboids can be obtained using both methods, yet the result based on the L1 norm is still better regarding to the distribution of magnetic susceptibility.(5) Field examples. In this section, the field examples are used to test the practical validity of the methods and techniques in this paper, such as the sunken ship survey in Madang, the seabed drainage pipeline survey in Daya Bay, the seabed gas pipeline survey in YaI3-1 and the magnetic survey for detecting the immersed tunnel before constructing the Hong Kong-Zhuhai-Macau Bridge. The field examples suggest that the methods and techniques in this paper are helpful into improving the data processing level and the precision of determining the parameters of the underwater objects. Therefore, the magnetic survey has good applicative effects and wide application prospects.The main innovation is presented as follows:(1) Based on the characteristics of realistic anomaly body underwater, some related experimental investigation aimed at theoretical simulation and new method and technology are carried out, and the achievement is directive in practice.(2) The target underwater can be located in the horizontal direction, since the improved stable downward continuation is applied and the transformation parameter which is less sensitive to the residual magnetization is chosen. Theory proof and practical application testify the better effect(3) On the basis of inversion theory, magnetic susceptibility image using the smallest model can verify and depict the target underwater. L1 norm and L2 norm are taken in the process of inversion, and the results are compared and analyzed then. For the model of a single rectangular body, the inverse result with the L1 norm can depict the boundary clearer, while the one with L2 norm is smoother.(4) The survey case of sunken ship in Madang shows that the methods can get the accurate quantitative indicators of the sunken ships and the application effect is preferable. The methods include the denoising based on the EMD, the elimination of the residual magnetization through the adoption of the modulus of magnetic anomaly (Ta, R, E, Q, L) and the ASA, the stable regularized downward continuation and the magnetic susceptibility image based the smallest model. The survey case of the drainage pipeline in Daya Bay shows that the modulus of magnetic anomaly can locate and depict the shape of the cement pipe buried under the sea floor fast and accurately, with a broad space for development in future. The survey case of nature gas line under the sea floor in Yal3-1 and the magnetic experiment in the immersed tunnel of the Hong Kong-Zhuhai-Macao Bridge shows that magnetic susceptibility image can recover the iron tube and reinforced concrete immersed tube with high accuracy. The survey cases above have been verified by other geophysical methods mutually and diving fishing directly. |
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