The new IMGC-02 transportable absolute gravimeter: measurement apparatus and applications in geophysics and volcanology

V DErrico V DErrico,G Corrado G Corrado,S Desogus S Desogus,A Germak A Germak,G DAgostino G DAgostino,G Berrino G Berrino,D Quagliotti D Quagliotti,C Origlia C Origlia,G Ricciardi G Ricciardi

doi:10.4401/ag-3038

Abstract

The research carried out at the Istituto Nazionale di Ricerca Metrologica (formerly Istituto di Metrologia «G. Colonnetti») aiming to develop a transportable ballistic absolute gravimeter ended with a new version of the instrument, called the IMGC-02. It uses laser interferometry to measure the symmetrical free rising and falling motion of a test mass in the gravity field. Providing the same accuracy achieved with previous versions, the instrumental improvements mainly concern size, weight, data processing algorithms and operational simplicity. An uncertainty of 9 ?Gal (1 ?Gal=1×108 m·s?2) can be achieved within a single observation session, lasting about 12 h, while the time series of several observation sessions show a reproducibility of 4 ?Gal. At this level, gravity measurements provide useful information in Geophysics and Volcanology. A wide set of dynamic phenomena, i.e. seismicity and volcanic activity, can produce temporal gravity changes, often quite small, with an amplitude ranging from a few to hundreds of microgals. Therefore the IMGC absolute gravimeter has been employed since 1986 in surveying the Italian active volcanoes. A brief history of the gravimeter and the description of the new apparatus, together with the main results of ongoing applications in Geophysics and Volcanology are presented.

Highlights

The best accuracy achievable in measuring the acceleration due to gravity g is estimated to be few microgals (1 μGal=1×10–8 m⋅s−2) and concerns modern absolute ballistic gravimeters
Since 1968, starting with the technical assistance of the Bureau International des Poids et Mesures – BIPM, the construction and maintenance of transportable absolute gravimeters has been continued at the Istituto di Metrologia «G. Colonnetti» ( Istituto Nazionale di Ricerca Metrologica – INRIM) (Cerutti, 1967, 1971; Cerutti et al, 1974)
Only two rise and fall systems participated in the last International Comparison of Absolute Gravimeters – ICAG, organized by BIPM and held in 2005: the IMGC-02 and the TBG absolute gravimeter, Ukraine

Summary

Introduction

The best accuracy achievable in measuring the acceleration due to gravity g is estimated to be few microgals (1 μGal=1×10–8 m⋅s−2) and concerns modern absolute ballistic gravimeters. 3, confirm that the IMGC-02 has reached a point in its development where the measurement results agree within the expanded uncertainty, estimated to be about 9 μGal Such an uncertainty level is achieved with one observation session that lasts about 12 h, even if the apparatus showed a measurement reproducibility of about 4 μGal at the INRIM gravity laboratory. Geodynamics, seismicity, and volcanic activity can produce temporal variation in the gravity field, often quite small, within the amplitude range from a few to hundreds of microgals For this reason, since 1986, the IMGC absolute gravimeter has been employed in surveying the Italian active volcanoes in co-operation with the Osservatorio Vesuviano (presently Istituto Nazionale di Geofisica e Vulcanologia INGV) and the University «Federico II», both in Napoli (Italy) (Berrino et al, 1988; 1999; Berrino, 1995). New features and technical solutions of the IMGC-02 are described later on together with an overview of the ongoing applications in Geophysics and Volcanology

Description of the apparatus

Conclusions