Deep underground rotation measurements: GINGERino ring laser gyroscope in Gran Sasso (1702.02789v1)

Abstract: GINGERino is a large frame laser gyroscope investigating the ground motion in the most inner part of the underground international laboratory of the Gran Sasso, in central Italy. It consists of a square ring laser with a $3.6$ m side. Several days of continuous measurements have been collected, with the apparatus running unattended. The power spectral density in the seismic bandwidth is at the level of $10^{-10} \rm{(rad/s)/\sqrt{Hz}}$. A maximum resolution of $30\,\rm{prad/s}$ is obtained with an integration time of few hundred seconds. The ring laser routinely detects seismic rotations induced by both regional earthquakes and teleseisms. A broadband seismic station is installed on the same structure of the gyroscope. First analysis of the correlation between the rotational and the translational signal are presented.

• The paper demonstrates that GINGERino achieves a resolution of 30 prad/s by applying advanced noise correction techniques.
• It utilizes a 3.6 m square optical cavity with GPS-synchronized data acquisition and an Extended Kalman Filter for backscattering noise reduction.
• The findings correlate rotational signals with seismic data, underscoring its potential in precise geophysical and geodetic research.

Deep Underground Rotation Measurements: GINGERino Ring Laser Gyroscope

The present paper discusses the implementation and early results from GINGERino, a large frame laser gyroscope installed deep underground at the Gran Sasso Laboratory in Italy. The paper explores the sensitivity and accuracy of this apparatus in detecting ground motion, primarily through rotational signals, enabling both geophysical and geodetic research applications.

Overview and Experimental Setup

The GINGERino project is centered around a single-axis He-Ne ring laser gyroscope designed to measure the Sagnac effect induced frequency shifts with high precision. This setup incorporates a 3.6 m square optical cavity with a laser operating at a 633 nm wavelength. The design leverages a robust infrastructure comprising four spherical mirrors with a 4 m radius of curvature enclosed in a steel vacuum chamber. This is affixed to a massive granite structure to reduce susceptibility to environmental distortions.

The paper describes how the device provides continuous measurements of the Earth's rotation rate, maintaining a high degree of sensitivity. Specifically, the gyroscope obtained a resolution of 30 prad/s after applying a sophisticated backscattering noise subtraction technique alongside traditional noise correction strategies.

Data Acquisition and Processing

Data acquisition for GINGERino employs a remote-controlled system, minimizing human-induced disturbances. The setup ensures frequencies are monitored through GPS-synchronized signals permitting accurate temporal alignment with other measurement instruments. This facilitates the crucial integration of translational and rotational data from installed seismometers. The paper documents the rigorous approach to noise management, emphasizing the implementation of an Extended Kalman Filter for effective backscattering noise reduction.

Observations and Analysis

GINGERino effectively detected rotational signals attributed to seismic waves including both regional earthquakes and teleseisms. Results include a comprehensive analysis of the correlation between the rotational data collected by the gyroscope and translational data from a co-located seismic station. Noteworthy are the results comparing seismic wave phase velocities, estimated through the correlation coefficient between rotation rate data from the gyroscope and transverse ground acceleration data from seismic sensors.

Moreover, probabilistic power spectral densities render a comprehensive profile of the local seismic noise environment, demonstrating proximity to the New Low-Noise Model (NLNM) — an assessment noteworthy for site stability analysis.

Practical and Theoretical Implications

The sensitivity of GINGERino at the level of $10^{-10}$ rad/s within the seismic bandwidth suggests substantial utility for monitoring geophysical processes with precision. This capability enhances the potential for augmenting traditional seismological methods, transitioning into applications capable of contributing valuable insights into geodynamic phenomena, including rotational microseismic noise and geodetic effects of Earth's rotational vector.

Future Directions

Advancements in noise reduction and gyroscopic sensitivity could lead to novel applications in Earth science and fundamental physics, possibly extending to precision tests of general relativity effects such as the Lense-Thirring effect, which remains an objective of broader projects like GINGER. Continuous developments in active stabilization techniques alongside improved environmental isolation and data processing algorithms are anticipated to markedly enhance measurement accuracy, potentially contributing to significant insights into both short and long-term geodynamic processes.

In conclusion, the installation and operation of GINGERino represent a strategic step forward in the measurement of rotational ground motion, suggesting compelling directions for future experimental and theoretical exploration.