Absolute and Relative Gravity Measurements at Volcanoes: Current State and New Developments Under the NEWTON-g Project

F Greco,G Siligato,F Cannavò,A A Messina,D Carbone

doi:10.1007/1345_2020_126

Abstract

AbstractGravity changes associated with volcanic processes occur over a wide range of time scales, from minutes to years and with magnitudes between a few and a few hundred microGal. High-precision instruments are needed to detect such small signals and both time-lapse surveys along networks of stations, and continuous measurements at single points, are accomplished. Continuous volcano gravimetry is mostly carried out through relative gravimeters, either superconducting instruments, providing higher quality data, or the more widely used spring meters. On the other hand, time-lapse surveys can be carried out with relative (spring) gravimeters, that measure gravity differences between pairs of stations, or by absolute gravimeters, capable of measuring the absolute value of the gravitational acceleration at the observation point. Here we present the state-of-the-art of terrestrial gravity measurements to monitor and study active volcanoes and the possibilities of new gravimeters that are under development. In particular, we present data from a mini array of three iGrav superconducting gravimeters (SGs) at Mount Etna (the first network of SGs ever installed on an active volcano). A comparison between continuous gravity measurements recorded through the iGrav#016 superconducting gravimeter at Serra La Nave station (1730 m a.s.l.) and absolute gravity data collected with the Microg LaCoste FG5#238 gravimeter in the framework of repeated campaigns is also presented. Furthermore, we introduce the Horizon 2020 NEWTON-g project (New Tools for Terrain Gravimetry), funded under the FET-OPEN Research and Innovation Actions call, Work Programme 2016–2017 (Grant Agreement No 801221). In the framework of this project, we aim to develop a field-compatible gravity imager, including an array of low-costs Micro-Electro-Mechanical Systems (MEMS)-based relative gravimeters, anchored on an absolute quantum gravimeter. After the design and production phases, the gravity imager will be field-tested at Mt. Etna (Italy) during the last 2 years of the project.

Highlights

High-precision gravity measurements provide a powerful tool for volcano monitoring, since they highlight processes that induce bulk mass/density changes
In spite of its potential, volcano gravimetry is not widely adopted by research institutions and agencies in charge of monitoring active volcanoes. This depends on several factors, including: (1) the cost of instrumentation, (2) problems that are inherent with the use and deployment of instruments intended for laboratory conditions in the harsh environments that characterize the summit zones of most active volcanoes, and (3) difficulty in interpreting gravity changes that may result from volcanic processes, as well as hydrological effects, and instrumental artefacts
The network has been developed and has evolved over the years and currently it consists of: (a) 71 benchmarks, covering an area of about 400 km2, for relative gravity campaigns (LaCoste & Romberg model D and Scintrex CG-3M/CG-5 gravimeters were used over time); (b) three continuously running gravity stations equipped with iGravs superconductive gravimeters by GWR Instruments, Inc.; (c) 14 stations for absolute gravity (AG) measurements using the Microg LaCoste FG5#238

Summary

Introduction

High-precision gravity measurements provide a powerful tool for volcano monitoring, since they highlight processes that induce bulk mass/density changes. In spite of its potential, volcano gravimetry is not widely adopted by research institutions and agencies in charge of monitoring active volcanoes This depends on several factors, including: (1) the cost of instrumentation, (2) problems that are inherent with the use and deployment of instruments intended for laboratory conditions in the harsh environments that characterize the summit zones of most active volcanoes, and (3) difficulty in interpreting gravity changes that may result from volcanic processes, as well as hydrological effects (changes in the undergroung water mass), and instrumental artefacts. We introduce the field-compatible gravity imager, that will be developed under the NEWTON-g project, including an array of lowcosts MEMS-based relative gravimeters, anchored on an absolute quantum gravimeter This measuring system will provide continuous imaging of gravity changes associated with variations in subsurface fluid properties, with unparalleled spatio-temporal resolution and, with fundamental implications for risk management.

New Tools for Terrain Gravimetry

Conclusions