Theory Method And Application Of Underwater Mobile Gravity Measurement

Marine gravity data is widely used in geophysics,mineral resources exploration,military application and ballistic missile launching,and is an important part of marine geophysical information.Although the gravity data obtained by satellite altimetry and ship borne gravity survey can solve the problem of gravity characteristics of several kilometers or even larger areas,the geological characteristics of smaller scale(sub kilometer space wavelength)still need to be combined with underwater and near seabed survey to overcome the signal attenuation caused by too far away from the potential field source.Underwater mobile gravity survey can continuously carry out near bottom gravity survey.For example,Autonomous Underwater Unmanned Aerial Vehicle(AUV)can also allow surface mother ship to perform multiple tasks at the same time,thus reducing the high cost and access threshold related to underwater gravity measurement,which is an important direction of the development of marine gravity measurement in the future.Compared with the common airborne and ship borne mobile gravity measurement,the underwater mobile gravity measurement can not use satellite positioning signal,and needs to use a variety of auxiliary positioning equipment such as underwater acoustic positioning,strapdown inertial navigation and depth meter to obtain accurate three-dimensional position information.Meanwhile,the motion attitude of underwater objects is quite different from that of aircraft and ships,and the influence mechanism of gravity measurement is different?In this paper,following the main line of underwater mobile gravity measurement,taking AUV equipped with strap down gravimeter as the main research direction,combined with theoretical research,actual navigation data and experimental analysis,respectively established underwater mobile gravity measurement model,analyzed the influence of AUV platform on gravity measurement,designed an effective gravity measurement platform,and proposed the algorithm and data processing flow suitable for the strapdown gravimeter The rationality and feasibility of the hardware and software platform are verified by the lake experiment.The main work and achievements of this paper are summarized as follows(1)The basic principle and method of underwater mobile gravity measurement are studied.The underwater mobile gravity measurement model and corresponding error model are established.The sensor,attitude,position and velocity error sources of the strapdown gravimeter are analyzed emphatically.The feasibility of 1 Mg Al measurement accuracy and the performance requirements of underwater positioning equipment are discussed.After calculation,when the gravity sensor is consistent with the strapdown Airborne Gravimeter,the positioning accuracy of underwater acoustic positioning system reaches 0.5%of the measured slant distance,the sounding accuracy of water pressure depth gauge reaches 5 cm,and the velocity measurement accuracy of Doppler log reaches 0.1 M/s.(2)In this paper,the influence of AUV's underwater motion on gravity measurement is analyzed,and the induced gravity calculation formula of vector gravity measurement is derived.Based on the actual navigation data of three types of AUV independently developed in China,the corresponding induced gravity is calculated.For vector gravity measurement,the AUV with large displacement should be used,and the pitch angle and heading angle should be reduced as far as possible through the overall optimization design,and the steering and acceleration and deceleration motion should be strictly controlled during the measurement;for scalar gravity measurement,the AUV with multi thruster combination is the best choice for underwater mobile gravity measurement,and the gravimeter layout is the best choice The induced gravity will be greater than 1 Mg Al when it is more than 4 m in the X direction and 2 m in the Y direction from the motion center.(3)A set of underwater mobile gravity measurement platform based on AUV is designed,including the design of platform overall,control system,navigation system,load and gravity measurement system.The control model of eight thrusters is established,and the control system of AUV gravity measurement platform based on underwater navigation fuzzy control method is adopted.This paper analyzes the performance requirements of underwater mobile gravity measurement for navigation equipment,and designs an integrated navigation system composed of INS+DVL,DGPS+SBL,water pressure depth gauge,underwater acoustic altimeter and collision avoidance sonar.The navigation system also provides the underwater position,velocity and depth information required for gravity calculation.(4)The improved Kalman filtering method is used to fuse the multi-source data needed for gravity calculation.According to the attitude,velocity and position error model of sins,the state equation and measurement equation of gravity measurement are derived.The standard,extension and traceless Kalman filtering for inertial navigation and gravity data calculation are analyzed respectively.According to the characteristics of gravity measurement signal and AUV noise,this paper analyzes the application of FIR low-pass filtering method with finite and infinite impulse response in gravity signal extraction,optimizes Kalman filtering and FIR low-pass filtering algorithm,and puts forward the method flow of gravity signal extraction and calculation.Aiming at the problem of accurate pose estimation for underwater mobile gravity measurement,the fusion estimation method is studied.The indirect estimation mode based on error state and the direct estimation mode based on pose state are studied.The state model is established by deriving the state evolution equation;the observation model is constructed according to the external measurement equation,and the number of indirect mode and direct mode is formed respectively According to the fusion state space model.On this basis,an improved algorithm is proposed for the numerical problem of the indirect mode method;for solving the direct mode continuous discrete state space model,the numerical integration method of continuous time update and the virtual sampling iteration method of discrete observation update are proposed,forming the continuous discrete iterative extended Kalman filter(cd-iekf)algorithm of direct mode pose estimation.The effectiveness of the proposed method is verified by the motion data of different carriers generated by industrial GNSS simulator and the simulation measurement data of IMU with different accuracy levels.(5)An underwater mobile gravity measurement verification system composed of AUV gravity measurement platform,gravimeter and survey ship was constructed.In January 2020,four surface and two underwater lines were measured in Mulan Lake water area of Wuhan city.The accuracy before and after adjustment was 0.67 mgal and 0.42 mgal,respectively mgal,the consistency of underwater gravity measurement and water surface measurement is proved by verification experiment,and the feasibility of underwater mobile gravity measurement is verified;furthermore,based on the cd-iekf algorithm proposed in this paper,gravity measurement data processing and gravity extraction are carried out,and the internal coincidence accuracy of gravity anomaly measurement after adjustment is 0.16 mgal,which proves that the proposed algorithm has good robustness to initial conditions and dynamic estimation performance.(6)The geophysical mechanism of gravity gradient is discussed.The gravity gradient and its spatial distribution caused by homogeneous hemisphere,particle and cuboid are established.Taking the U.S.Ohio class ballistic missile nuclear submarine as an example,this paper focuses on the gravity gradient signal and its measurement of inhomogeneous density objects,including the gravity vertical gradient anomaly caused by the mass loss of submarine shell and interior.The detection range of typical submarine under different gravity gradient instrument accuracy conditions is calculated.According to the height difference between submarine and AUV of 500 if the accuracy of the gradiometer reaches 10^-4E(E(?)tv(?)s),the search width can reach830 m.