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Characterization of the response of spring-based relative gravimeters during paroxysmal eruptions at Etna volcano
© Greco et al.; licensee Springer. 2014
Received: 4 December 2013
Accepted: 15 May 2014
Published: 29 May 2014
Gravity time sequences collected at Etna volcano by continuously recording spring-based relative gravimeters showed significant variations in temporal correspondence with paroxysmal eruptions. Since the observed gravity variations can only be partially related to subsurface mass redistribution phenomena, we investigated the instrumental effects due to ground vibrations such as those that accompany explosive activity. We simulated the performances of relative gravimeters with laboratory experiments to estimate their response to vertical and horizontal excitations. Laboratory tests were carried out using a vibrating platform capable of accelerating the instruments with intensities and frequencies, in both the vertical and horizontal directions, observed in the ground vibrations associated with paroxysmal events. The seismic signals recorded at Etna volcano during the 10 April 2011 lava fountain were analyzed to retrieve the parameters used to drive the vibration platform. We tested two gravimeters used for Etna volcano monitoring: the LaCoste & Romberg D#185 (Lafayette, CO, USA) and the Scintrex CG-3 M#9310234 (Concord, ON, Canada). The experiment results highlight that the vibrations resembling the seismic waves propagated during paroxysmal events cause an amplitude response in the gravity readings on the order of several hundred microgals (μGal). Generally, the relationship between the vibrations and the gravimeter response is nonlinear, with a fairly complex dependence on the frequencies and amplitudes of the signals acting on the gravimeters.
Gravity measurements are a powerful method to investigate mass and density changes, which are useful in many branches of earth science. In particular, repeated gravity surveys conducted at active volcanoes have proved to be effective for monitoring purposes, since they are able to detect subsurface magma movements before volcanic unrest (Williams-Jones and Rymer 2002; Battaglia et al. 2003; Furuya et al. 2003; Carbone and Greco 2007; Bonaccorso et al. 2011a; Greco et al. 2010, 2012; Del Negro et al. 2013). Gravity monitoring of volcanic areas is traditionally performed by means of time-lapse surveys, allowing the detection of temporal gravity changes over relatively long periods (from months to years; e.g., Berrino 2000; Carbone et al. 2003a; Bonaccorso et al. 2011a; Greco et al. 2010, 2012; Pistorio et al. 2011; Del Negro et al. 2013). Continuous gravity measurements at volcanic sites are rarely performed (Berrino et al. 1997, 2006; Bonvalot et al. 1998; Branca et al. 2003; Carbone et al. 2003b, 2006; Kazama and Okubo 2009) because of the harsh environmental conditions encountered close to the active craters, where it is difficult to obtain reliable data. Nevertheless, continuous measurements could potentially track rapid changes (from minutes to days) in subsurface mass redistribution, which are impossible to monitor using only discrete measurements.
The Istituto Nazionale di Geofisica e Vulcanologia (INGV) in Catania has been operating an array of continuously recording gravity stations in the summit area of Etna volcano since 1998 (Carbone et al. 2003b). The stations are equipped with spring-based relative gravimeters, which represent the best compromise in terms of quality and cost. These types of instruments can be deployed at sites very close to active craters, where significant gravity changes associated with volcanic activity are expected (Greco et al. 2008; Bonaccorso et al. 2011b, [c]).
Over the 2011 to 2013 period, the eruptive activity at the Etna volcano has been dominated by frequent, intermittent episodes of spectacular lava fountains associated with fast-moving lava flows and tephra columns occurring from the New SE Crater in the summit area (Ganci et al. 2012). The average duration of these paroxysmal eruptions is approximately 1 to 2 h (Ganci et al. 2012), with lava fountains up to 800 to 1,000 m high (Calvari et al. 2011) erupting approximately 1 × 106 m3 of pyroclastics and about twice as much lava (Calvari et al. 2011; Ganci et al. 2012).
Continuous observations with spring gravimeters conducted at Etna volcano during these short-lived but intense paroxysmal events have shown significant variations in the amplitude of the gravity signals, which correspond temporally with the increase in amplitude of seismic activity (Bonaccorso et al. 2011a, [b]). Since all relative gravimeters are more or less subject to offsets (tares) due to shock when subjected to severe vibrations for long periods of time (Seigel 1995), we investigated if the observed large gravity variations (hundreds of microgals (μGal); 1 μGal = 1 × 10−8 m/s2) could be ascribed not only to the subsurface mass redistribution phenomena but also to the mechanical response of the gravimeter to ground acceleration caused by seismic wave propagation, both in the vertical and in the horizontal components. In such a case, the observed gravity signal should be the combination of (i) a component due to the subsurface mass or density variations accompanying the ascent of magma to the Earth's surface and (ii) a spurious component due to the effects of transient ground acceleration caused by seismic wave propagation.
We performed extensive laboratory tests to evaluate the degree of dependence between the ground accelerations, similar to those accompanying explosive activity, and the gravimetric response of two instruments operating at Etna for volcano monitoring purposes: the LaCoste & Romberg D#185 (Lafayette, CO, USA), used for continuous gravity recording, and the Scintrex CG-3 M#9310234 (Concord, ON, Canada), typically used for time-lapse surveys and occasionally deployed for continuous measurements. Here, we show the results of the characterization of these two gravimeters using a vibrating platform to induce appropriate accelerations, in both the vertical and horizontal directions, through calibrated oscillating systems. We simulated the conditions occurring during the paroxysmal events, driving the vibration platform with signals resembling the seismic sequences recorded at the Etna volcano during the recent lava fountains. Experimental results highlight that when vibration is applied to the gravimeters with amplitudes and frequencies comparable to those observed during explosive activity, significant changes in gravity readings can be recorded.
Gravity and seismic signals recorded during the 10 April 2011 lava fountain
Maximum peak-to-peak acceleration values
a x, p-p
Ground vibrations and gravimeter response
Several studies have been carried out that take into account gravity changes detected during different eruptive events at the Etna volcano (Branca et al. 2003; Carbone et al. 2006; Bonaccorso et al. 2011a, [b]). Essentially, the observed gravity changes were interpreted as being due to local mass redistributions triggered by the magma/gas dynamics in the shallow portion of the volcano's plumbing system. Nevertheless, the ascent of magma to the Earth's surface and its eruption are commonly accompanied by earthquakes and seismic tremor, resulting in ground vibrations (Alparone et al. 2003; Cannata et al. 2008) which may affect the measurement system of a spring gravimeter. It is well known that relative spring gravimeters, when subjected to shocks, can react with a sudden change in the reading level (tares). The possibility that ground vibration might contribute to the noise level of gravimeter measurements has already been suggested by Greco et al. (2008). However, although instrumental effects can occur during volcanic processes, the effects have never been quantified until now.
The vibrating platform is an ideal tool to characterize the dynamic behavior of the mechanical components of gravity instruments (Tobyáš et al. 1999; Aliod et al. 2003). We calibrated the spring gravimeters by analyzing their response to controlled harmonic accelerations exerted by the vibrating platform. The amplitude-frequency analysis allows investigation of the dynamic response of the gravimeters at specific frequencies and determination of the critical resonance frequencies.
where a z is the output signal of the gravimeter (in milligal (mg)), and b(i) is the amplitude of the i th excitation signal (in mg); i stands for any horizontal (h) and vertical (z) component. High coupling factor values indicate the frequencies at which there is a maximum transfer of spurious signals in the gravimeter response (resonance frequencies). To determine the factors K(h)z, the movable aluminum platform was excited simultaneously along the x and y directions by the horizontal actuators, and laser #1 was used to measure the induced displacement (Figure 6a). In this case, laser #2 was used only to monitor possible undesired movements in the vertical component of the platform. To determine the coupling factor K(z)z, the experimental setup, appropriately modified by moving the piezoelectric actuators below the aluminum platform, allows forcing of the vibration platform in the z direction and measuring the induced displacements using laser #2.
Results and discussion
We simulated the performance of relative gravimeters in the laboratory by exposure to forced vibrations at intensities and frequencies that can be encountered in the near field during paroxysmal events. The vibrating platform was driven on the basis of the peak-to-peak values estimated in the x, y, and z acceleration components at the SLN and BVD stations derived from seismic signals (Table 1). According to the analyses results, and considering the limit of the equipment, the amplitude values of the inertial acceleration (ranging between 0.18 and 3.5 mg) were produced by moving the platform by a few tens of micrometers. Furthermore, to excite any possible resonances in the sensors, the tests were performed in the frequency range between 1 and 25 Hz, which are the dominant frequencies of the seismic signals during paroxysmal events.
In order to evaluate the gravimeter response as a function of the horizontal and vertical excitations (x, y, and z), the laboratory test can be divided into three different steps: during the first step, the excitation was switched off; then, in the second step, the platform was forced to move with a horizontal or vertical displacement; finally, in the third step, the excitation was switched off again. Below, we describe the details of the laboratory tests that were performed by applying controlled vibrations in the horizontal and vertical directions to the LaCoste & Romberg D#185 and Scintrex CG-3 M#9310234 gravimeters.
LaCoste & Romberg D#185 gravimeter
We were particularly interested in investigating the dependence of the LaCoste & Romberg D#185 on the vibrations because this gravimeter was in operation at the Etna BVD station (Figure 1a) during the 10 April 2011 lava fountain. During the tests, the gravity data were acquired every second. The average over 60 measurements was then calculated and stored in the solid-state memory of a CR10X Campbell Scientific data logger (at 1 datum/min; Logan, UT, USA). The tidal effect correction was performed a posteriori with additional software. Together with the gravimetric data, the instrument also provided the measurement of the tilt angles of its base. Figure 6c shows the experimental setup with the arrangement of the gravimeter and the data logger.
Amplitude values ranging between 2 and 3.5 mg, produced by imposing platform movements around a few tens of micrometers in the frequency range between 2 and 19 Hz, were taken into account, reproducing the amplitudes and frequencies typical of those recorded at the BVD gravity station during the Etna paroxysmal events.
Frequency values corresponding to the maximum coupling between excitation and LaCoste & Romberg D#185 response
Coupling factor K(h)z
7 × 10−5
2 × 10−4
4 × 10−5
3 × 10−5
Table 2 reports the values of the resonance frequencies, together with the obtained coupling factors. In particular, at the frequency of 4.5 Hz, the coupling factor was extremely high (2 × 10−4).
Figure 7b shows the coupling factor K(z)z calculated for the LaCoste & Romberg D#185 gravimeter with respect to different vertical excitation frequencies. The results show that the coupling factor was significant in the range of 1 to 11 Hz, but rapidly decreased at frequencies higher than 11 Hz (the decoupled zone). The coupling factor is directly related to the excitation frequency, but in this case, there were no peaks of maximum dependence such as those observed for the horizontal excitations.
Scintrex CG-3 M#9310234 gravimeter
Frequency values corresponding to the maximum coupling between excitation and Scintrex CG-3 M response
Coupling factor K(h)z
4 × 10−3
25 × 10−3
1 × 10−3
Exploiting the gravity temporal sequences collected at the Etna volcano during a short-lived but violent lava fountain at two sites very close to the eruptive vent, we conducted laboratory tests to investigate on the instrumental effects due to the ground vibrations that accompanied the explosive activity. The tests were focused on identifying the coupling factors between the gravimeter responses (in the z direction) and the controlled vibrations in the x, y, and z directions, at the frequencies and amplitudes observed in the seismic waves recorded at the Etna volcano during the lava fountains. In this way, we determined the vibration frequencies to which there was a maximum transfer of spurious signals in the gravity output. Amplitude values ranging between 0.18 and 3.5 mg, produced by imposing platform movements around a few tens of micrometers in the frequency range between 1 and 25 Hz, were taken into account. The results revealed different behaviors and a marked sensitivity of the LaCoste & Romberg D#185 and Scintrex CG-3 M#9310234 gravimeters when subjected to vibrations of a few tens of micrometers, both in the horizontal and vertical directions. Both instruments showed a coupling zone in the frequency range between 2.5 and 14 Hz for the horizontal excitations. The maximum coupled zone was in the frequency range between 1 and 7 Hz for the vertical oscillations affecting the Scintrex CG-3 M#9310234 and between 1 and 11 Hz for the vertical oscillations affecting the LaCoste & Romberg D#185.
The laboratory results highlighted that vibrations resembling the seismic waves propagated during paroxysmal events drive the gravimeters to their main resonant frequencies, causing an amplitude response in the gravity readings on the order of several hundred μGals, comparable to those observed during paroxysmal episodes. The laboratory tests were aimed at validating the hypothesis that seismic waves such as those recorded during the explosive activity, in both the horizontal and vertical directions, can cause significant offsets in the gravity measurement as a consequence of the nonlinear effects on the instruments. This dependence, in the first instance, can be considered quadratic (the gravimeter responds quadratically to an increase in the amplitude of the frequency excitations). This effect is mainly due to amplitude modulation of the seismic waves propagating during the event at certain frequencies. The modulating signal, through the nonlinearity, is transferred to the measurement band of the gravimeters, causing a significant apparent variation. Throughout the tests, the vibration platform was driven using an acceleration signal equivalent to those recorded by seismometers during the 10 April 2011 eruptive event at Etna. Clearly, in the real case, the envelope of the seismic signals, a packet of modulated waves, plays the main role instead of a specific and fixed carrier, as used in the tests.
The nonlinearity of the effects restricts the possibility of finding a linear transfer function from the excitation signal in the x, y, and z directions due to the ground motion, to the gravimetric response. Moreover, due to the complicated dependence of the response of the gravimeters on the frequencies and amplitude of the input signals, it is very difficult to filter the spurious signals from the gravity sequences using the seismic signal acquired simultaneously with the gravity signal.
Our experiment results indicate that the use of continuously recording spring-based gravimeters to monitor the volcanic activity requires some precautions. Specifically, to reduce the dependence of the relative spring gravimeters on vibrations such as those recorded during the explosive activity that produces lava fountains, some a priori and a posteriori techniques should be adopted to separate the gravity reading into an instrument-dependent signal (i.e., due to the mechanical perturbation of the sensor caused by seismic waves) and volcano-related components. For example, among the a priori techniques, a hardware damper system could be implemented as a support for the spring gravimeter; in this way, the high-frequency components in the three directions (x, y, and z) that can affect the gravimeters could largely be filtered out. For the a posteriori solutions, an approach based on neural networks could be developed to evaluate the instrumental response to the ground acceleration acting on spring gravimeters at different frequencies and amplitudes.
The final form of this manuscript benefited from the constructive comments of anonymous reviewers. The work was developed within the framework of TecnoLab, the Laboratory for Technological Advance in Volcano Geophysics, organized by INGV-CT and UNICT. We are also indebted to Salvatore Alparone for providing seismic data.
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