| Borehole magnetic exploration is an important means of geophysical exploration.Compared with ground magnetic measurement and aeromagnetic survey,downhole magnetic measurement is closer to magnetic anomaly and develops a three-dimensional information survey,which can provide richer and more accurate underground magnetic anomaly distribution information than traditional magnetic field and its three components measurement.Borehole magnetic survey can be realized by total magnetic field and three components,but both of them are easily affected by the background geomagnetic field and magnetic casing,and sensitive to the direction error.The magnetic gradient tensor is used to measure the gradients of three orthogonal magnetic field components.It has the advantages of high spatial resolution,rich information and insensitivity to the direction relatively.It also suffers less influence by the time and spatial changes of the geomagnetic field and the casing pipe in a downhole,which is more suitable for downhole exploration.In this paper,the magnetic gradient tensor is solved by using the principle of rotating modulation method.The rotatory modulation method uses the FFT to transform the time-domain signal to the frequency domain by rotating the different derivatives of the magnetic potential,and then processes the signal in the frequency domain to obtain the information of specific harmonics and calculate the second-order magnetic gradient tensor.This method does not limit the type of magnetic sensor,and can solve the full magnetic tensor value.It can improve the dc and low frequency performance of the instrument,as well as the gradient unbalance.In this paper,the principle of measurement is rotating modulation method and the subject of study is the world's first borehole magnetic gradient tensor all of full tensor instrument(based on ratating modulation method)developed in Australia,and the borehole magnetic gradient tensor developed by our project team.To further improve the measurement accuracy of the tensor gradiometer,the system errors have been analyzed and classified according to the effect of errors.Major errors include scale factor error of fluxgate itself,installation errors under three drum coordinates,and transformation errors between drum coordinates and the instrument coordinate.Minor errors cover errors of finite-length measurement parameters,error of ignoring high-order tensors,the deviation of the rotation axis,and temperature drift.We have developed the error models for major errors and calibrated the full-tensor gradiometer by using the two independent tensor rotation invariants.1,000 simulations with random errors show the root mean squares of relative error are less than 8% after calibration.Through simulation experiment,the experimental conditions of calibration algorithm verification experiment were formulated,and field experiment was carried out in the magnetic field uniform region.The improvement ratio of tensor magnitude value was 3.74.The difficulty of magnetic measurement in borehole is the alignment of underground magnetic field information in geodetic coordinate system.By using the MEMS gyroscope and accelerometer of attitude angle measurement device and the fusion algorithm of kalman filter,the underground three-component reorientation assisted by geomagnetism is realized.Positioning precision of the pitch angle and roll angle is 0.1 °,azimuth accuracy is 0.5 °.The validity of the algorithm is verified by the simulation experiment of a non-magnetic turntable.The feasibility of calibration algorithm and attitude Angle calculation method is verified by data processing of downhole test.The paper analyzed and corrected the error of the magnetic gradient tensor used in the magnetic survey in borehole based on the rotating modulation method,which further improves the measuring precision of the instrument and provides certain reference and technical support for the actual measurement of the magnetic gradient tensor in borehole. |
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