Abstract
Continuous gravity observations performed in the last few years, both at Mt. Etna and Stromboli, have prompted the need to improve the tidal analysis in order to acquire the best corrected data for the detection of volcano related signals. On Mt. Etna, the sites are very close to each other and the expected tidal factor differences are negligible. It is thus useful to unify the tidal analysis results of the different data sets in a unique tidal model. This tidal model, which can be independently confirmed by a modeling of the tidal parameters based on the elastic response of the Earth to tidal forces and the computation of the ocean tides effects on gravity, is very useful for the precise tidal gravity prediction required by absolute or relative discrete gravity measurements. The change in time of the gravimeters sensitivity is also an important issue to be checked since it affects not only the results of tidal analysis but also the accuracy of the observed gravity changes. Conversely, if a good tidal model is available, the sensitivity variations can be accurately reconstructed so as to retune observed tidal records with the synthetic tide, since the tidal parameters are assumed to be constant at a given location.
Highlights
Continuous gravity recordings in volcanic areas, coupled with continuous GPS observations, are considered a good detection tool of the mass changes linked to magma transfer processes and, to recognize forerunners of paroxysmal vol-Spring gravity meters are still the most widely used instruments for microgravity studies in volcanic areas due to their relatively low cost and small size, which make them easy to transport and install
A first interest of tidal gravity observations on volcanoes is to build up an experimental model that can be compared with a modeling of the tidal parameters based on the elastic response of the Earth to tidal forces and the computation of the ocean tides effects on gravity (Section 4)
Refined tidal analysis requires a long series of observations
Summary
Continuous gravity recordings in volcanic areas, coupled with continuous GPS observations, are considered a good detection tool of the mass changes linked to magma transfer processes and, to recognize forerunners of paroxysmal vol-. The main focus of continuous gravity observations on an active volcano is the detection of. The maker provides a calibration constant but, when recording the very tiny (less than 0.3 mgal) tidal changes, one should generally apply a scale factor or normalization constant. This quantity can be determined either by recording side by side with well calibrated instruments (Wenzel et al, 1991; Ducarme and Somerhausen, 1997) or by checking the scale factor on specially designed short baselines e.g., the Hannover vertical base line (Kangieser and Torge, 1981; Timmen and Wenzel, 1994). The sequences analyzed are not continuous and were recorded using both LaCoste and Romberg (LCR) and Scintrex spring gravimeters, located The STR station (φ= 38.80°N, λ=15.2270°E, h=200 m) is the only one located at Stromboli and the gravity meter for the recordings is the LCR D-157. Data were recorded at 1datum/min sample rate (each datum is the average calculated over 60 measurements) through a CR10X Campbell Scientific data-logger (A/D bits: 13) (for more details see the website http://www.campbellsci.com/cr10x/)
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