Study On Effects Of Magnetic Measurement Environment In A Borehole And Test Methods Of Magnetometer Performance

Borehole magnetic observation is one of the important means of earthquake prediction.Many measurements show that the geomagnetic field changes significantly before and after an earthquake.However,borehole magnetic measurement environment is totally different from that on the ground.The performance and magnetic measurement results of magnetometer are greatly affected by the ferromagnetic casing pipe and the high-temperature,high-pressure environment in borehole.The casing pipe has magnetic shielding effect on the magnetic measuring device placed in it,which can directly weaken the magnetic abnormal signal.The high temperature and high pressure in borehole can also decrease the magnetic measurement performance.In this paper,the magnetic shielding effect of the ferromagnetic casing pipe in borehole is simulated with the multi-physical field simulation software COMSOL firstly,and the magnetic shielding effect of the casing and the simulation method are proved to be correct by experiments.The effect of magnetic shielding on magnetic anomaly detection is proved by simulation and experiment with a small magnet as anomaly source.It also shows that the magnetic gradient tensor is less affected by the magnetic shield of the casing than the magnetic field.Through the parametric simulation analysis,the magnetite concentration corresponding to the minimum visible magnetic anomaly caused by magnetite with 5 different grain size are obtained.And the smaller the grain size of magnetite,the greater the possibility of exploring magnetic anomalies.Secondly,the influence of high-temperature and high-pressure environment on magnetic measurement is studied.The influence of high-pressure environment on magnetic measurement is simulated through its effect on magnetic characteristics of the casing pipe.It is found that the greater the stress on the casing,the less obvious the magnetic shielding.The axial magnetic field component Bz at the center of the casing increases linearly,and the stress increases Bz 5 n T for every 50 MPa,while the total magnetic field B and the radial magnetic field components Bx and By increase exponentially with the increase of stress.Then,the influence of high temperature on magnetic measurement is modeled by the functional relationship between hysteresis parameters and temperature for the magnetic core of fluxgate magnetometer.With the increase of temperature,the saturation magnetic induction intensity,residual magnetic induction intensity and coercivity of magnetic core gradually decrease,the shape of hysteresis loop keeps getting narrower,and the maximum permeability increases.That means that as the temperature increases,the hysteresis loss and remanence of the fluxgate decrease,while the sensitivity increases.Finally,the test methods of important performance indexes of fluxgate magnetometer and the influence of ambient temperature on these performance indexes are studied.Through simulation and experiment,it is verified that a direct coupling effect is produced between the magnetic shield cylinder and the coil placed in it.Although the scale factor of the coil is changed,it still has great uniformity in its uniform region.The performance indexes of uniaxial fluxgate are tested in different temperature environments to study the variation trend of each performance index of fluxgate with temperature.It is found that the variation trend of fluxgate of the same type was consistent,which proves the advantage of magnetic gradient tensor measurement in the high temperature environment of boreholes.The study of this paper is of great significance to improve reliability of borehole magnetic measurement,fluxgate performance and its long-term stability.