Study On Technology For High Temperature Superconducting Full Tensor Magnetic Gradient Measurement

As the latest aeromagnetic survey technology,the aeronautical full tensor magnetic gradient measurement has many unique advantages in geophysical exploration,military exploration,underwater and underground unexploded ordnance detection,so is known as the next breakthrough in magnetic prospecting.The Direct Current Superconducting Quantum Interference Device(DC-SQUID)planar gradiometer inherits the features of SQUID,such as high sensitivity,low noise and wide frequency band.Besides,it measures the gradient of the magnetic field directly and can strongly suppress the common-mode magnetic field which can result in the interference to the target gradient signal,so is very suitable for constructing magnetic gradient tensor measurement probe.As a result,Germany,America,Australia and other countries have successively carried out a fruitful study of aeronautical full tensor magnetic gradient measurement system based on DC-SQUID planar gradiometer.Among them,Germany has successfully developed a practical product and conducted a number of geological survey tasks in South Africa.This paper conducted a prospective pre-research aiming at the application of HTS DC-SQUID planar gradiometer to Aeromagnetic full tensor magnetic gradient measurement,and was supported by "Study on Aeromagnetic Three Component Vector Survey System and Full Tensor Measurement Technology" which is a topic of the "863 Program".The main research contents and achievements are as follows:(1)Studied on the High temperature superconducting(HTS)full-tensor magnetic gradient measurement model and then designed the measurement system.At first,the theory of the full-tensor magnetic gradient measurement and the principle of the planar gradiometer and magnetometer based on DC-SQUID were studied.And then,aiming at the situation that the attitude of probe coordinate system isn't fixed during the aeromagnetic measurement,the motion measurement model based on the geographic coordinate system was presented after deeply analyzing the partial differential measurement model.On the basis of the motion measurement model,the first set of magnetic gradient measurement system based on HTS DC-SQUID planar gradiometers was developed in China.(2)Studied on geometry structure optimization of high temperature superconducting full-tensor magnetic gradient measurement probe.Since the DC-SQUID planar gradiometer is vector sensor,the probe structure parameters could affect the measurement resolution for 5 independent components of the magnetic gradient tensor.The paper deduced the relationship between the resolutions for 5 independent components and the structure parameters,and then the maximum of the relative resolutions for 5 components was selected as the objective function to optimize the probe.Finally,the optimal probe structure were obtained with the inclination being 58.7° and the angle between the projection of 1st gradiometer's baseline on xoy plane of probe coordinate system and x-axis being ±15°.The method is also applicable to optimizing other figurate tensor probe based on DC-SQUID planar gradiometer.(3)Studied on imbalance correction technology for HTS DC-SQUID planar gradiometer.The imbalance of DC-SQUID planar gradiometer is inevitable and would result in the ability of gradiometer to suppress the common-mode magnetic field being limited.In the wild,the common-mode noise caused by imbalance would seriously interfere with the target gradient signal for the existence of common-mode background magnetic field.Thus the paper studied the technology based on RLS adaptive fitting algorithm for correcting the imbalance of gradiometer,and reduced the common-mode effect by two orders of magnitude in the field ground experiment.In consideration of the correction accuracy being limited by the uniformity of calibration field,theoretically,a higher precision corrected result would be obtained in the air where the magnetic field uniformity is much better than which on ground.(4)Studied on automatic control technology for DC-SQUID.During aeromagnetic measurement,the measurement system was arranged in a cabin which is dragged by tens of meters long rope,thus can't be touched by the operator sitting in the flight platform.Concerning this specific arrangement mode,the paper developed digital DC-SQUID control unit for remotely adjusting the working parameters of DC-SQUID sensors.With the help of the control unit,operator could timely adjust the parameters of DC-SQUID according to its working state during flight.In addition,based on the control unit,the optimal working point automatic search algorithm was introduced and realized the automatic setting for DC-SQUID working point,which not only greatly reduced the workload of operator but also provide the technical support for magnetic tensor measurement based on Unmanned Aerial Vehicle(UAV)more importantly.(5)Studied on the static performance test and feasibility verification for the HTS magnetic gradient tensor measurement prototype.This paper studied the test method and built the indoor test platform,finally obtained the prototypical parameters such as noise level,sensitivity,dynamic range,bandwidth,rate etc by indoor experiments.On the basis of above test,a magnetic dipole localization experiment was carried out for verifying the feasibility of prototype.In the experience,the prototype stayed in static state and measured the magnetic gradient tensor resulting from a moving clamp.According to positioning algorithm,the motion trace of clamp could be calculated with measurement data.Finally,the inversion result was consistent with the actual moving path,which proved that the prototype is feasible.Above work lays the foundation for the development of the dynamic peRFormance test,field experiment and even practical prototype.