Development Of Borehole 3rd-order Magnetic Gradient Tensor Measurement System

Borehole magnetic survey is one of the important means to explore the earth's resources.Based on the magnetic characteristics of mineral resources,it can explore the blind ore and the spatial location of magnetic mineral materials in a certain scale around the well.It can avoid the interference of surface human,terrain and environment and provide effective technical support for the census of magnetic minerals and the exploration of deep non-ferrous metal minerals.One of the inevitable trends of the development of borehole magnetic survey is the increasing order of the observed physical quantities,from measuring the scalar of the total geomagnetic field intensity to measuring the magnetic field vector,and then to measuring the magnetic gradient tensor.The improvement of the order of the observed physical quantities can effectively overcome the interference of the geomagnetic field itself with time,which is more sensitive to the abnormal field caused by magnetic materials,and can provide more spatial geometric information about the structural characteristics of the magnetic source,which is more suitable for searching and locating deep minerals in the well.The rotating modulation frequency point method is one of the measurement methods of the third-order magnetic gradient tensor.Firstly,the rotating magnetic sensor converts the signal in the spatial domain into the signal in the time domain,and then transforms the signal in the time domain into the signal in the frequency domain through the Fourier transform.Finally,the magnetic gradient tensor components with different orders are extracted in the frequency domain.The advantage of this method is that it can accurately measure the third-order magnetic gradient tensor information by using a magnetic sensor.Based on this principle,this paper completes the hardware development and software design of the third-order magnetic gradient tensor measurement system in the well,mainly including the design of signal acquisition system,which completes the high-precision conversion of magnetic field signal from analog to digital;the design of motion control system,which accurately controls the rotation of the electric machine;the design of alignment system,which realizes the positioning of the measured data in the geodetic coordinate system The design of umbrella shaped external support structure can improve the stability of test points in the field well experiment,and the design of waterproof structure shell is suitable for the harsh test environment in the well.Finally,the magnetic field signal measured by the instrument is preprocessed and analyzed to get the third-order magnetic gradient tensor data.In the process of measuring the third-order magnetic gradient tensor in the well,it is inevitable that there are various factors such as measurement component error,installation accuracy error and downhole test environment,so the data quality can not be guaranteed.In view of the above situation,this paper proposes a unified calibration framework suitable for all order magnetic gradient tensors.Its biggest advantage is that the required correction coefficient is constant,which effectively solves the problem that the number of correction coefficients increases sharply when the order increases.In this framework,the magnetic gradient tensor is regarded as a whole,and the constant modulus of each order of magnetic gradient tensor before and after coordinate rotation is taken as the constraint condition.The least square algorithm is used to solve the correction coefficient,which can improve the detection accuracy of the instrument.The accuracy and practicability of the instrument are verified through the test and application experiment of the third-order magnetic gradient tensor measurement system in the well.Design the dynamic rotation experiment of magnetic dipole,complete the frequency domain calibration of magnetic dipole,calculate the magnetic moment information;magnetic dipole line experiment,compare the actual measured value with the simulation theoretical value,verify the accuracy of the third-order magnetic gradient tensor measurement system in the well;resolution test experiment,design two circular coil junctions with the ratio of the distance from the coil to the origin and the coil radius of 1.188 The resolution of the third-order magnetic gradient tensor measurement system is at least 1090 n T/m~2.The calibration method experiment improves the measurement accuracy of the instrument,and the improvement ratio reaches 1.433.Finally,the borehole detection experiment proves that the third-order magnetic gradient tensor measurement system is practical.