[S-TT41] Seismic monitoring and processing system

[STT41-05] Development of a laser interferometric broadband seismometer for seismic observation in extreme environments

*Akito Araya¹, Keisuke Onodera², Taichi Kawamura³, Takeshi Tsuji⁴, Satoshi Tanaka⁵ (1.Earthquake Research Institute, The University of Tokyo, 2.Institute for Planetary Materials, Okayama University, 3.Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, 4.Department of Systems Innovation, The University of Tokyo, 5.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency)

Keywords:laser interferometer, broadband seismometer, extreme environment, lunar seismic network

Various types of seismometers are used for seismic observations, and they are selected according to their performance in terms of detection accuracy and bandwidth, as well as conditions such as the temperature environment, the installation method, the size, and the power consumption. Seismic observation in extreme environments, such as deep underground and lunar/planetary exploration, requires high environmental tolerance, such as continuous operation in high temperature with small size and low power consumption. Compared to measurement methods based on semiconductor devices, optical measurement can solve some of these issues, and a laser interferometer in particular offers high detection performance and environmental tolerance.

We are developing a broadband seismometer using a laser interferometer for seismic observation in deep underground and in lunar and planetary exploration. In order to measure long-period seismic waves as a broadband seismometer, it is necessary to use an almost astatic long-period pendulum as a seismic sensor. Therefore, when low-frequency detection performance and vibration tolerances for transportation and installation are highly required, it becomes a challenge to design a seismometer in the conflicting conditions. In this study, we developed a Laser Interferometer Unit (LIU) that can be combined with various types of pendulums suitable for required performance and resources to form a broadband seismometer. A laser beam is introduced into the LIU via an optical fiber, and if a mirror is attached to the mass of the pendulum, the displacement of the mass is detected with high precision as an interference signal. The interference signal is taken out from the LIU via optical fiber and converted into an electrical signal by external detectors. This signal is fed back to the mass of the pendulum to work as a broadband seismometer.

By attaching the LIU to a pendulum with a natural frequency of approximately 4 Hz based on a commercially available seismometer and applying broadband seismometer control, we successfully constructed a compact broadband seismometer with a flat frequency response in velocity from 0.1 Hz to 50 Hz. The LIU was also attached to a penetrator seismometer, which was developed for the lunar survey program (LUNAR-A) and has excellent vibration resistance; its broadband frequency response and low-noise performance were characterized. In addition, we have attempted to calibrate the seismometer output with respect to the laser wavelength, and have shown that the seismometer can be calibrated on its own with high accuracy. It is useful to ensure the quality of data in long-term observations in extreme environments that may not be easily accessible. We are now developing a package of broadband seismometers incorporating LUNAR-A type seismometers as a space instrument, aiming to deploy a seismic observation network on the Moon.