The effect of inertial accelerations on the higher frequency components of the signal from spring gravimeters

Experimental and theoretical studies have shown that, due to the magma/gas dynamics in the upper part of a volcano’s plumbing system, gravity changes can develop over periods between a few tens of seconds and several hours. The mass transport, implied by certain fast-evolving volcanic processes, also constitute the source mechanism of seismic waves with frequencies over the lower limit of the seismic band. These seismic waves could affect the measuring system of spring gravimeters, that are increasingly used as continuously running devices to monitor and study active volcanoes. As a consequence, under some circumstances, the signal from a continuously running spring gravimeter will be the combination of the gravity field component and the inertial acceleration component, the latter due to the ground motion. In such cases, the inertial acceleration must be separated from the gravity signal to assess the amount of mass redistributed during the studied process. To achieve this separation, the frequency response curve of the spring gravimeter to inertial accelerations must be calculated, since it is not supplied by manufacturers. In this paper, we present a method to retrieve the above curve, using simultaneous recordings during the transit of teleseismic waves, of a LaCoste & Romberg D gravimeter and a Nanometrics Trillium 40 broadband seismometer, whose frequency response curve to ground acceleration is known a-priori. The use of teleseismic waves is particularly useful for our purpose since teleseisms are not associated with a local mass redistribution; the gravimeter will thus be affected only by the ground motion, making the above calculation possible. Our results show that, because of the instrumental damping, the effect of the inertial acceleration is reduced in the output signal from the gravimeter to 0.5 and 0.1 of its original value, at frequencies between 0.02 and 0.07 Hz, respectively. The robustness of the calculated frequency response curve is proven using independent simultaneous signals from gravimeter and broadband seismometer.

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