Noise Reduction in Magnetically Shielded Room Using by Active Microtremor Isolation System

Abstract

In biomagnetic measurements such as magnetoencephalography and magnetocardiography, very weak magnetic fields have been measured. Magnetic sensitivity can be limited by microtremor of magnetically shielded room (MSR) used for the measurements as well as environmental magnetic noise at low frequencies caused by streetcars (trains), motorcars traffics and elevators. There are two possible mechanisms for a contribution of vibration of the MSR to magnetic noise: movements of the MSR shielding panels change magnetic field inside the MSR and the vibrating SQUID magnetic sensors receive static magnetic field as time-varying magnetic field.
Thus, it is important to investigate the relationship between magnetic noise and vibration of the MSR, and method for eliminating the vibration is desirable. In this paper, we measured the magnetic noise in the MSR using a 148-channel SQUID magnetometers (BTI) when artificial vibrations were applied to the MSR by means of heal impacts and evaluated correlation between the vibration and magnetic noise. Further an active microtremor isolation system (TACMI) was installed as a base of MSR to reduce the microtremor of the MSR. We also measured reduction of vibration and magnetic noise in an MSR actually mounted on TACMI.