Four-dimensional or time-lapse microgravity monitoring has been used effectively on volcanoes for decades to characterize the changes in subsurface volcanic systems. With measurements typically lasting from a few days to weeks and then repeated a year later, the spatial resolution of theses studies is often at the expense of temporal resolution and vice versa. Continuous gravity studies with one to two instruments operating for a short period of time (weeks to months) have shown enticing evidence of very rapid changes in the volcanic plumbing system (minutes to hours) and in one case precursory signals leading to eruptive activity were detected. The need for true multi-instrument networks is clear if we are to have both the temporal and spatial reso-lution needed for effective volcano monitoring. However, the high cost of these instruments is currently limiting the implementation of continuous microgravity networks. An interim approach to consider is the development of a collaborative network of researchers able to bring multiple instruments together at key volcanoes to investigate multitemporal physical changes in a few type volcanoes. However, to truly move forward, it is imperative that new low-cost instruments are developed to increase the number of instruments available at a single site. Only in this way can both the temporal and spatial integrity of monitoring be maintained. Integration of these instruments into a multiparameter network of continuously recording sensors is essential for effective volcano monitoring and hazard mitigation.
We present a new local Bouguer anomaly map of the Central Volcanic Complex (CVC) of Tenerife, Spain. The high-density core of the CVC and the pronounced gravity low centred in the Las Canadas caldera (LCC) in greater detail than previously available. Mathematical construction of a subsurface model from the local anomaly data, employing a 3-D inversion enables mapping of the shallow structure beneath the complex, giving unprecedented insights into the sub-surface architecture of the complex, and shedding light on its evolution.
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 broad-band 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 broad-band seismometer.
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