Yuji Otake, Akito Araya and Kazuo Hidano

ERI, Earthquake Research InstituteUniversity of Tokyo

Development of a Highly-Development of a Highly-sensitive/sensitive/Broad-band Servo-type Broad-band Servo-type Seismometers using Magnetic Seismometers using Magnetic LevitationLevitation

Earth

To observe cheaply easily the vertical/Horizontal components of normal free oscillations of the Earth at many points.

To observe Telseismic Events Earthquake

Use for Earth Tomography

10-6 m/sLess than 0.1 Hz

Seismometer

Purpose of DevelopmentPurpose of Development

Less than 10-10 m/sLess than 0.01 Hz

Atmospheric PressureNormal Free Oscillations

　 By using the NS alternative magnetization, we can obtain a uniform potential energy just above the magnet surface. A perpendicular direction referred to the movement direction of a pendulum must be mechanically constrained with a hinge, because of Earnshaw’s low.

NSNSNSNS

NSNSNSNS

Levitation

　　 A horizontal component seismometer with A horizontal component seismometer with magnetic levitation using plane permanent magnetic levitation using plane permanent magnets with NS alternatively magnetized magnets with NS alternatively magnetized strips.strips.

X-Y-Z Stage

Hi ngeVi brati on

Magenti c Levi tati on Force

Constrai nt

A principle of an astatic rotational pendulum usinA principle of an astatic rotational pendulum using a plunger permanent magnetg a plunger permanent magnet in a uniform magnetic field environment. in a uniform magnetic field environment.

The positive rotational moment of a plunger permanent magnet generated by a magnetic field B cancels a negative rotational moment acted at a weight by gravity.

Uni form Magneti c Fi el d B

Permanent Magnet

N

S

Rotati onal Axi s wi thout Fri ct i on

Wei ght

Gravi ty

Force generated by B

Relative Amplitude

Angle [rad]

By gravity

By magnetic force

1

1 1.5 0.5

0.5

-0.5

-1

-1 -1.5 -0.5

The vertical component seismometer using The vertical component seismometer using an astatic rotational penduluman astatic rotational pendulum

Capacitance Position Sensor

Feedback Coil

Magnetic Spring

Pendulum Weight

Window-frame Magnet

Plunger Magnet

Free oscillationNatural Period : about 5s

10- 3 10- 2 10- 1 100 101 102

10- 10

10- 9

10- 8

10- 7

10- 6

10- 5

10- 4

10- 3

frequency (Hz)

Vel

ocity

(m

/s)/

Sqrt

(Hz)

TheThe 　　 wave form and the frequency spectrum owave form and the frequency spectrum of the earth’s background vibration noise at Matuf the earth’s background vibration noise at Matushiro, March 04”shiro, March 04”

buoyancy

- 0.4

- 0.35

- 0.3

- 0.25

- 0.2

0 50 100

Time (S)

Am

plitu

de (

V),

0.1

V =

10**

-5 m

/s

Marine Microseism10-6 m/s

March 12, 04” AM 2h 2min

1010-- ８８ (m/s)/√Hz(m/s)/√Hz

Rotational PendulumRotational Pendulum(m/s)/√Hz

10**-5

10**-4

10**-6

10**-7

10**-8

10**-9

10**-10

10**-3

10**-210**-3 10**-1 10**-0

10**1 10**2Frequency [Hz]

The details will be presented in the poster.

summarysummary

1. We proofed the principle of the vertical astatic rotational pendulum.

2.2. The noise level of the rotational pendulum seismo The noise level of the rotational pendulum seismometer in a frequency range over 1 Hz was less thmeter in a frequency range over 1 Hz was less than 10an 10-9-9 (m/s)/√Hz. (m/s)/√Hz.