Research On Error Calibration And Interference Compensation Method Of Oceanic Underwater Towed Three-component Magnetic Survey System

The importance of the ocean in the 21 st century is widely recognized by all major countries in the world.As one of the important tools for human to understand and develop ocean geoscience,high precision ocean magnetic survey technology is extremely promising in many fields.It can provide important technical support for engineering applications such as submarine seismic monitoring,mineralization prediction,oil and gas pipeline detection,and autonomous underwater navigation.Currently,magnetic survey has shifted from scalar to multiparameter vectors,and expanded from ground and airborne to oceanic underwater.As the newest development direction in the field of ocean magnetometry,three-component magnetometry can simultaneously characterize the intensity and direction information of magnetic vectors.It has unique advantages in the interpretation of magnetic anomaly inversion.The fullspace dynamic magnetic field observation method for China’s ocean underwater operating environment is still in the initial stage.There are many influencing factors such as ocean motion induced magnetic field,error of magnetic survey system and carrier interference,which seriously limit its application in related fields.This work is supported by the National Key Research and Development Program of China "Key Technology of Deepwater Oil and Gas Near-Seafloor Gravity and Magnetic HighPrecision Detection".Based on China’s self-developed deep-sea towed exploration system "Di Ting”,an in-depth study was conducted on the system error calibration and interference compensation.The feasibility of the method is verified by simulation and sea trial experiments in the South China Sea.The main research content and results of this paper can be summarized as follows:(1)Completed the design of the underwater towed three-component magnetic survey scheme,which includes the design of the sensors and their performance indexes of the underwater towed three-component magnetic survey system,the design of the underwater towing scheme,the design of the structure of the underwater towing system and the design of the probe rod type pressure chamber.Based on the above design scheme,relevant coordinate systems and the parametric rotation method between them is introduced and analyzed.Finally,the error calibration and interference compensation scheme of the oceanic underwater towed three-component magnetic survey system is proposed.(2)Completed a study on the characteristics of magnetic fields induced by common seawater motions.A mathematical model of the induced magnetic field is constructed based on the simplified motion model of the wave sensing.The regional distribution of the wave induced magnetic field is simulated according to the JONSWAP spectrum.A computational model of the ocean current induced magnetic field is constructed based on the laminar flow model.The superposition theory of the induced magnetic of sea currents and the geomagnetic field is refined,and its effects on the error calibration and interference compensation of the three-component magnetic survey system under different conditions are analyzed.The model for calculating underwater wake induced magnetic field is derived using the Rankine ovoid equivalent.The magnitude of the underwater wake induced magnetic field is clarified by simulation calculations.The above works provides the foundation for the planning of experimental program of oceanic underwater towed three-component magnetic survey.(3)Completed the research on the error calibration method of oceanic threecomponent magnetic survey system based on the principle of plane constraint.The ocean three-component magnetic survey system consists of a magnetic sensor and an inertial navigation system.The sensitive axes of them cannot be aligned precisely by mechanical means,resulting in various errors.The planar constraint method completes the error calibration of the three-component magnetic survey system by spatial full degree of freedom sampling.This method gets rid of the constraints of the planar limit device on the calibration accuracy,while effectively reducing the difficulty of on-site calibration of three-component magnetic survey system.It realizes the accurate acquisition of magnetic survey data in ocean underwater sea trial experiments.In ground simulation experiments the proposed method improves the accuracy of the three-component magnetic survey system to 6n T.(4)Completed the research on the interference compensation method of oceanic underwater three-component magnetic survey system based on the homogenization of solution equation.The magnetization field of the ferromagnetic material in the carrier will superimpose on the three-component magnetic field data resulting in degraded data quality.A comprehensive compensation model was established based on an in-depth study of the interference magnetic field.Based on the spatial projection characteristics of the magnetic vector field,the objective function of the compensation coefficient solution is homogenized.The proposed method can effectively solve the multicollinearity problem in the model;at the same time,the errors introduced by the heading angle are eliminated;the limitations of the vector constraint method are also eliminated,the accurate solution of the compensation parameters can be completed by using the scalar magnetic field value.Both simulation and ground experimental results show that the compensation accuracy of the proposed method are better than that of the vector constrained compensation method.In ground simulation compensation experiment the proposed method improves the accuracy of three-component magnetic data to 10 n T.(5)Completed the first oceanic 2000 m depth towed three-component magnetic survey experiment in China.In this paper,the equipment for the underwater towed three-component magnetic survey experiment was developed under the project of "Key Technology of Deepwater Oil and Gas Near-Seafloor Gravity and Magnetic HighPrecision Detection".The project was jointly completed by the Guangzhou Marine Geological Survey(China)and several research institutes.Among them,Jilin University undertakes the work of magnetic survey data error calibration and interference compensation.The system error calibration and interference compensation method proposed in this paper has improved the accuracy of oceanic underwater three-component magnetic survey data to 20 n T,which provides technical support for the accumulation of highprecision oceanic underwater three-component magnetic field data in China’s Ocean.