Experimental Verification and Research for the Distortion in the Integrated Frequency Responses of the Sealed Pressure Case and Magnetic Field Sensor

Although the magnetotelluric sounding method applied to land is mature, many technical problems arise when they are transplanted into the seafloor environment, one of which is how to put magnetic field sensors down to the seafloor to complete measurements. To protect magnetic field sensors from intense erosion and high pressure, suitable sealed pressure cases must be designed. Considering the magnetic measurement and seafloor operation, the sealed pressure case should be nonmagnetic and transportable. Among all optional materials, LC4 super-hard aluminum alloy has the highest performance to price ratio. However, it doesn′t mean that the sealed pressure case made by LC4 will be perfect in performance. In fact, because of its weak magnetism, the pressure case made by LC4 has distorting effect on frequency responses of the magnetic field sensors sealed in it. This distorting effect doesn′t affect the use of the magnetic field sensor, but if we want to eliminate its impact, we should study it through experimental measurements. In our experimental tests, we used frequency swept magnetic field as excitation signal, and then measured responses of the magnetic field sensor before and after being loaded into the sealed pressure case. Ultimately, we obtained normalized abnormal curves for the frequency responses, through which we could reveal how the sealed pressure case impacts the responses of the magnetic field sensor. Experiment results suggest that the response distortion induced by the sealed pressure case appears in the mid and high frequency area. Using experiment results as standardized data, the frequency responses collected by seafloor magnetotelluric measurements can be corrected to produce authentic information about the seafloor field source.