Development Of Borehole Magnetic Gradient Tensor Measurement System Based On Rotating Modulation Method

With the gradual depletion of surface and shallow mineral resources,the search for deep and concealed minerals has become an urgent task to solve the resource crisis.Borehole magnetic measurement is very suitable for searching for deep and concealed minerals because the probe goes deep into the underground which is far away from the interference of magnetic material on the surface and closer to the deep underground minerals.Borehole magnetic measurement develops gradually from magnetic total field and three-component to magnetic gradient tensor.Magnetic gradient tensor has many advantages,such as rich information and little influence by geomagnetic field,and it is more advantageous in searching and locating deep and concealed minerals.Rotating modulation method is a novel tensor measurement method,which is different from the traditional differential magnetic gradient tensor system.In the process of rotating magnetic tensor measurement,the magnetic sensor rotates around the rotating axis at a certain speed,and modulates useful information to a specified frequency point,and uses Taylor expansion formula and Fourier analysis method to analyze the signal in frequency domain and extract the specified frequency.The magnetic gradient tensor is obtained from the signal characteristics.Rotating magnetic gradient tensor measurement system can use two or even a single-axis magnetic sensor to obtain full tensor information.Reduction in the number of sensors can simplify the complexity of the system which is of great significance for the realization of tensor measurement system,system optimization and error calibration.In addition,the rotating magnetic gradient tensor system can measure multiple-order tensors by increasing the number of coordinate rotations to obtain more abundant magnetic field information.It is of great positive significance to broaden the research direction of tensor measurement and to carry out multiple-order tensor measurement.This paper starts from the theoretical derivation of the measuring principle ofrotation modulation method,and analyzes and verifies the feasibility and advancement of the rotation modulation method by simulation.Then the rotating magnetic gradient tensor measurement system is designed and constructed according to the measuring principle of rotation modulation method,which includes the structural design of rotating motion machinery,the design of control and acquisition circuit,and the alignment design of instrument coordinate system with geodetic coordinate system and the design of interactive software of the system,then a magnetic gradient tensor measurement system based on rotation modulation method is constructed.Finally,relevant experiments are designed and implemented,and the experimental results are analyzed.The magnetic dipole experiment is designed to verify the correctness of the rotating magnetic gradient tensor measurement system by analyzing the experimental data and comparing it with the simulation data.The simulation experiment of the underground magnetosphere is designed to simulate the situation of the tensor system passing through the underground magnetosphere during logging,which lays a foundation for the magnetic tensor measurement and data processing in the field test of the wells.And the comparative experiment of the rotation modulation method and the differential flux-gate tensor system is designed.The practicability of the rotating magnetic tensor measurement system is clarified.The tensor invariant correction experiment is designed to correct the error of the tensor system and improve the accuracy of the system,and the improved ratio of the tensor modulus after correction reaches 3.74.The resolution test of the magnetic gradient tensor system is carried out,and the resolution of the rotating magnetic tensor system can reach 8.76 n T/m.Finally borehole detection experiments are carried out to prove the practicability of the instrument.