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Combined discrete and continuous gravity observations at Mount Etna
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Journal of Volcanology and Geothermal Research 123 (2003) 123^135 www.elsevier.com/locate/jvolgeores
Combined discrete and continuous gravity observations at Mount Etna Daniele Carbone a; , Gennaro Budetta a , Filippo Greco a , Hazel Rymer b a
Istituto Nazionale di Geo¢sica e Vulcanologia, Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy b Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, UK MK7 6AA Received 28 October 2001; received in revised form 1 May 2002; accepted 1 July 2002
Abstract Systematic investigation of discrete gravity measurements has continued at Mount Etna since 1986. The network now covers an area of 400 km2 with about 70 stations 0.5^3 km apart. Mass redistributions occurring at depths ranging between about 8 km below sea level and a few hundred metres below the surface (magma level changes within the shallower parts of the feeding conduits) have been identified from these data. Conventional (discrete) microgravity monitoring on a network of stations furnishes only instantaneous states of the mass distribution at continuously active systems. In order to obtain information on the rate at which the volcanic processes (and thus mass transfers) occur, three stations for continuously recording gravity where installed on Mount Etna in 1998. A 16-month long sequence from one of the continuously running stations (PDN, located 2 km from the active northeast crater at the summit of Etna volcano) is presented. After removing the effects of Earth Tide and tilt, the correlation of the residual gravity sequence with simultaneous recordings of meteorological parameters acquired at the same station was analysed. Once the meteorological effects have also been removed, continuous gravity changes are within 10 WGal of gravity changes measured using conventional microgravity observations at sites very close to the continuous station. This example shows how discrete and continuous gravity observations can be used together at active volcanoes to get a fuller and more accurate picture of the spatial and temporal characteristics of volcanic processes. < 2003 Elsevier Science B.V. All rights reserved. Keywords: Etna volcano; gravity variations; continuous recording; modelling
1. Introduction Micro-gravity studies through discrete measurements have been shown to provide valuable information on the processes occurring within active volcanoes both during eruptive and quiescent periods (Jachens and Eaton, 1980; Sanderson, 1982;
* Corresponding author. Fax: +39-(0)-95-435801. E-mail address: [email protected] (D. Carbone).
Eggers, 1983; Rymer and Brown, 1987; Rymer et al., 1993a,b, 1995; Budetta and Carbone, 1998; Budetta et al., 1999). One of the main drawbacks of repeated discrete network monitoring is the lack of information on the rate at which the volcanic processes occur. The problem is that only changes in the subsurface mass distribution between successive surveys (ranging on Etna between about one month and one year and depending on the snow coverage, availability of transport and personnel, etc.) can be assessed. The rate of
0377-0273 / 03 / $ ^ see front matter < 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0377-0273(03)00032-5
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change between successive measurement times is unknown and therefore there remains ambiguity as to the nature of the causative processes. If one also considers the need to reduce the exposure of personnel in active areas, it is easy to understand the importance of developing monitoring techniques that operate continuously and automatically in remote regions for several months at a time. A promising technique to satisfy these needs is continuous gravity monitoring (Goodkind, 1986). Continuous gravity studies at active volcanoes have been scarce in the past because of the logistic di⁄culties of running them in places where the conditions are far from the clean, ideal laboratory and so it is quite di⁄cult to attain the required precision in the data. Most previous studies have focussed on continuous measurements acquired at sites remote from any active craters in order to either obtain precise tidal gravity factors (Davis, 1981; d’Oreye et al., 1994) or to determine instrument-related correction algorithms for the main external perturbations (El Wahabi et al., 1997). In a recent paper Jousset et al. (2000) have found a correlation between the residual drift of a LaCoste and Romberg D meter which recorded continuously between 1993 and 1995 at a site 4 km away from the summit of Merapi Volcano (Indonesia) and seismic and volcanic activity. In addition they found variations in the ratio between the observations and the theoretical response of the Earth (tides), also correlated with the volcanic activity.
2. Discrete gravity measurements at Mount Etna Discrete gravity measurements have been carried out at Mount Etna since 1986. The gravity network (Fig. 1) is now composed of about 70 stations 0.5^3 km apart, covers an area of about 400 km2 and consists of four integrated subarrays (Budetta et al., 1999): (1) the Main Network (established in 1986), arranged as a ring around the volcano at elevations of between 800 and 2000 m above sea level (a.s.l.) ; (2) the Summit Pro¢le (established in 1992), which runs from Piano Provenzana (PPR; at the
lowest elevation of 1770 m a.s.l.) to the Rifugio Sapienza (RSA; 1890 m a.s.l.), crossing the summit zone; (3) the East^West Pro¢le (established in 1994), between Za¡erana (450 m a.s.l.) and Adrano (600 m a.s.l.), connecting with the Summit Pro¢le and the Main Network at the Rifugio Sapienza; and (4) the Base Reference Network (established in 1994) of four stations, placed at the towns of Adrano, Belpasso, Za¡erana and Linguaglossa, each of which is considered to be stable with respect to volcanic activity; all the other networks are connected to this reference network. The subarrays have di¡erent characteristics regarding the density of the traverse, access to stations (determined by snow coverage), and the time required to take measurements. Each subarray can be occupied independently, optimising the £exibility in taking measurements to accommodate the changeable activity and accessibility of the volcano. Measurements over the whole array are usually conducted every six months. Some parts of the array are reoccupied more frequently (approximately monthly measurements along the E^W and Summit Pro¢les, although snow coverage restricts measurements along the Summit Pro¢le to the summer months). Discrete measurements along the Etna gravity network have allowed mass redistributions occurring over a wide range of depths to be detected. Between June 1990 and June 1991, gravity increases in the summit region of the volcano, not accompanied by signi¢cant ground deformation (all height changes less than a few cm), were measured and interpreted in terms of an in£ux of magma into the summit feeder pipe and into a pre-existing SSE-trending fracture system prior to the 1991^1993 £ank eruption (Rymer et al., 1993a,b; 1994). Magma drainage from the upper levels of Etna’s central feeding system marking the closing stages of the 1991^1993 event could be detected through analysis of gravity variations (Budetta and Carbone, 1998). Mass redistributions at about 1 km a.s.l. have been detected during the September 1994^July 1996 period through measurements along the Etna Summit Pro¢le and have been associated
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Fig. 1. Sketch map of Mt Etna showing stations on the Main Network (¢lled circles), the Summit Pro¢le (triangles), the East^ West Pro¢le (small open circles), and the Basal Reference Network (large open circles). The three continuously running stations (¢lled squares) and the traces of the E^W and Summit pro¢les (lines) are also shown.
with the 1995^1996 explosive activity (Budetta et al., 1999). Mass redistributions 3^6 km below sea level producing negligible elevation changes at the surface and thought to be the e¡ect of magma injections within a NNW^SSE oriented elongate zone were detected at Etna several times during the last ten years (1987^1989, June^September 1992, and 1994^1996 periods; Budetta and Carbone, 1998, Budetta et al., 1999).
3. Continuously running gravity stations at Mount Etna
(Fig. 1). The continuously running stations are located ca. 10 km south of the active craters at the Serra la Nave Astrophysical Observatory (SLN; 1740 m a.s.l.), 2 km NE of the summit Northeast Crater at the Pizzi Deneri Volcanological Observatory (PDN ; 2920 m a.s.l.) and just 1 km S of the summit Southeast Crater at the Torre del Filosofo refuge (TDF; 2919 m a.s.l.). These three sites were chosen mainly because of the buildings present in each of them (in the case of PDN and TDF, the closest buildings to Etna’s active craters) which provide protection from the elements. 3.1. Setup of continuously running Etna stations
In 1998 three continuous gravity stations equipped with LaCoste and Romberg (L and R) spring gravimeters were installed on Mount Etna
The conditions at a continuous gravity station close to an active crater are far from the clean,
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Fig. 2. Three-component setup for acquisition and transmission of gravity data at Etna’s remote stations.
ideal laboratory and so it is quite di⁄cult to attain the required precision in the data. Because of this, continuous gravity observations at active volcanoes have not developed as quickly as other geophysical techniques. However, technological improvements in recent years have allowed most of these di⁄culties to be overcome. At Etna we have developed a three-component setup (Fig. 2) for typical ¢eld stations in the active volcanic environment which is robust, easy to remove and re-establish, and cheap (the gravity meter being by far the most expensive item). The power system employs solar panels connected to trickle-charged batteries. To provide a constant power supply (within a few hundredth of a Volt) to the feedback of the recording L and R meters a dc^dc converter coupled with a lowdropout tension stabiliser is used.
The acquisition system comprises the gravity meter itself, which outputs analogue signals representing the feedback force and the long and cross levels and sensors for the atmospheric temperature and pressure. The meters are equipped with remote controlled stepper motors to reset them. The whole acquisition system is placed inside a thermally insulating polystyrene container (Fig. 3). Data are acquired every second. The average over 60 measurements is then calculated and stored in the solid-state memory of the data logger (at 1 datum/min). After being temporally stored in the solid-state memory of the data logger, data are dumped to the INGV Sezione di Catania (Catania, Italy) automatically every 24 h by the transmission system which employs a cellular phone. Using suitable software on a computer in Catania it is also pos-
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sible: (1) to remotely activate the stepper motor which turns the meter dial allowing the meter to be reset (Fig. 3), and (2) to monitor in real time all the parameters recorded.
4. Continuous data from PDN station 4.1. Data presentation Many shortcomings (mainly electronic) a¡ected data from L and R PET-1081 at TDF during the ¢rst eight months of its recording period (the instrument was newly acquired when installed at TDF in September 1998). Data were corrupted to such an extent that it became impossible to separate the geophysical from other-sources-gen-
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erated signal. The appearance of £uctuations in the calibration factor by as much as 20% with periods of a few days leaded us to suspect an electronic defect a¡ecting the feedback system (MVR concept; VanRuymbeke, 1989; LaCoste and Romber, 1997) ¢tted to this new instrument. L and R ET-13 working at SLN was a¡ected by pressure-driven instrumental e¡ect yielding strong apparent gravity changes (Ando' and Carbone, 2001) and indicating that the instrument was not suitably compensated for pressure variations (the pressure seals were no longer airtight). PDN, on the other hand, is the Etna station which best ¢ts the required standards for a continuous remote gravity station in the volcanic environment. Thanks to the ruggedness of the station setup and reliability of the sensor (L and R
Fig. 3. The Pizzi Deneri Volcanological Observatory (a) hosts one of Etna’s continuously running gravity stations. To reduce the e¡ect of atmospheric temperature the gravimeter has been placed inside a two level thermally insulating polystyrene container (b). A step motor, mounted on the black lid of the gravimeter (c), allows the instrument to be remotely reset via cellular phone.
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Fig. 4. Raw gravity data recorded at PDN station during the October 1998^February 2000 time interval.
D-157 works at PDN), a high-quality data sequence free from long gaps has been acquired at this station. Since the present work is mainly intended to show the possibilities of combined continuous and discrete observations through spring gravimeters at active volcanoes, in the following sections just data from the PDN continuous station will be presented, analysed and compared with data acquired through discrete measurements at a site very close to the continuous station. Fig. 4 shows the raw gravity recording at PDN for the time period between October 1998 and February 2000. The ¢rst degree instrumental drift rate is about 285 WGal/month. During the above period eight interruptions of the continuous record with length ranging between 5 and 90 h occurred. Interruptions to the data set were due to: (1) instrumental drift bringing the proof mass out of the feed-back range in periods when it was impossible to reset the meter remotely and to get to the station (snow coverage) ; (2) temporary failures of the powering system of the stations; and (3) seismic shocks sending the proof mass of the instruments to the edge of the feed-back range (the feed-back force takes a variable time to restore the equilibrium). The mean background noise is less than 1 WGal. 4.2. Analysis of the data 4.2.1. Tide and inclination e¡ects To model the tidal gravity e¡ect on Etna, we used the Eterna 3.30 data processing package (Wenzel, 1996). The Eterna package allows the parameters of the main tidal components to be calculated on grounds of the available recorded
data. The data were ¢rst pre-processed, using suitable computing kernels of the Eterna package, to remove spikes, interpolate gaps and correct for jumps. After the pre-processing step, the parameters of the main tidal components were evaluated by a least squares harmonic analysis method using the Analyze computing kernel. These parameters allow tidal gravity on Etna to be modelled to better than a few tenths of WGal. Tilt changes are measured at each station along two perpendicular axes X and Y using the electronic levels ¢tted to L and R instruments. These devices are simple air-damped pendulums the position of which is sensed by a capacitance bridge (LaCoste and Romber, 1997). Their resolution (about 2.5 Wrad) is surely suitable for our purposes. Tilt variation during the period covered by the present study were typically within a few Wrad (up to about 50 Wrad). The gravity e¡ect of the tilt changes was calculated using the formula (Scintrex Limited, 1992): Ng ¼ gð13cosX cosY Þ
ð1Þ
where g is an average gravity value (980.6 Gal). The maximum amplitude of the tilt changes e¡ect is about 10 WGal. 4.2.2. The e¡ect of meteorological parameters Each continuously running gravity station of the Etna array has been equipped with atmospheric temperature and pressure sensors. Fig. 5 shows residual gravity (a), temperature (b) and pressure (c) observed at PDN during the period October 1998 to February 2000. The instrumental drift (modelled as a ¢rst degree curve), the Earth Tide and tilt e¡ects have been removed from the
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Reduced Gravity (µGal)
1000
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(a)
800 600 400 200 0 -200 -400 -600 -800 -1000
24
(b)
22 20 18 16 14 12
730
(c)
725 720 715 710 705 700 695 690 Oct 98
Dec 98
Feb 99
Apr 99
Jun 99
Aug 99
Oct 99
Dec 99
Feb 00
Fig. 5. Reduced gravity (a), after removal of the best linear ¢t, the theoretical Earth Tide and tilt e¡ects, temperature (b) and pressure (c) observed at PDN station during the October 1998^February 2000 period.
gravity signal. The most evident feature is the strong correlation between longer-wavelength gravity and temperature variations. Meteorological parameters are expected to affect continuously recording spring gravimeters (Torge, 1989; El Wahabi et al., 1997; Bonvalot et al., 1998; El Wahabi et al., 2000). Apparent gravity changes depend on the temporal development and magnitude of the meteorological change that caused them as well as on the insulation and compensation of the gravimeter. Thus, the correction formulas are instrument-speci¢c and often frequency-dependent. To investigate the complex relationship between the gravity signal and meteorological parameters, spectral correlation analyses have been performed using Welch’s averaged periodogram method. In
particular the coherence function Cxy (indicating how well the meter output corresponds to each meteorological parameter at each frequency) is given by: Cxy ¼ ðMPxyM2 Þ=ðPxx PyyÞ
ð2Þ
where Pxy represents the cross spectral density, while Pxx and Pyy represent the power spectral density (Papoulis, 1991). As for temperature, a signi¢cant correlation (Fig. 6a) appears over the lowest frequency band (T greater than about 1 month; coherence values greater than 0.6). Moving towards the higher frequencies, after a sharp drop, the coherence values remain within 0.2, indicating the absence of correlation. Over the lowest frequency band the maximum admittance value is close to
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Fig. 6. Transfer functions in the frequency domain between gravity and temperature (a,b) and between gravity and pressure (c,d) signals acquired at PDN station.
about 200 WGal/‡C (Fig. 6b); that means that apparent changes of the order of hundreds of WGal can be observed as the result of signi¢cant temperature changes. Using data sequences longer than ours, other authors have found that temperature changes can cause an instrumental e¡ect on spring gravimeters which can reach over a yearly period an amplitude of about one mGal (El Wahabi et al., 1997, 2000). However, air humidity, which is closely dependent on temperature, could be directly responsible for the instrumental gravity changes correlated with temperature (Bastien and Goodacre, 1990; El Wahabi et al., 2000). The coherence diagram for pressure shows that the correlation between gravity and pressure (Fig. 6c) is signi¢cant (coherence values around 0.6) just over the spectral band ranging between 0.1 and 1 c.p.d. (T ranging between about 1 and 10 days). Over this spectral band, the mean admittance value is around 0.3^0.4 WGal/mbar (Fig. 6d), in good agreement with the value of the theoretical local admittance given by the combined
e¡ect of the gravitational attraction of the air column and the distortion of the Earth’s surface resulting from barometric changes (Spratt, 1982; Mu«ller and Zu«rn, 1983; Niebauer, 1988; Merriam, 1992; Sun et al., 1995; Berrino et al., 1997). Signi¢cant coherence values together with admittance values close to theoretical ones over this band indicate the absence of important instrumental (apparent) changes linked to atmospheric pressure changes on one hand and the absence of other important e¡ects able to lower correlation values and corrupt admittance values on the other one. Over the other parts of the spectrum (f 6 0.1 c.p.d. and f s 1 c.p.d.), e¡ects not related to pressure changes become predominant (low coherence values, high background noise in the admittance diagram). To correct the gravity signal for the temperature e¡ect, the procedure outlined by Ando' and Carbone (2001) has been followed. In particular, a sixth degree polynomial ¢lter has been used to remove the seasonal temperature e¡ect while the
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e¡ect of non-seasonal temperature changes has been removed through a non-linear (Neuro^ Fuzzy) form of a model structure which takes into account the time delay between temperature and gravity (Ando' and Carbone, 2001). In Fig. 7a the residual gravity signal, after the temperature e¡ect has been removed, is presented. As observed above in the frequency analysis, the gravimeter at PDN (L and R D-157) is not affected by atmospheric pressure changes. Thus, the signal from this meter is only to be corrected for the normal local in£uence of the atmospheric pressure on the gravity ¢eld (standard admittance value of 30.365 WGal/mbar). The maximum peakto-peak amplitude of the e¡ect of the atmospheric pressure is about 10 WGal over the period considered. It can be seen that to correct the gravity signal for this e¡ect improves its quality signi¢cantly over changes within a period of several days, a¡ected by pressure changes related to (anti-) cyclones (Fig. 7b).
5. Discussion During the October 1998^February 2000 peri-
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od, discrete surveys were accomplished along the Summit Pro¢le (section 2; Fig. 1) in June 1999, July 1999, September 1999 and October 1999. In Fig. 7 gravity data acquired through discrete measurements at station CO (at a distance of about 500 m from PDN; Fig. 1) are reported (black dots). Unfortunately, no elevation data from the summit Etna zone are available for the period covered by the discrete gravity measurements. However, since the height variations on Etna are usually too small to signi¢cantly a¡ect surface gravity measurements (Sanderson, 1982; Rymer et al., 1993a,b, 1995; Budetta and Carbone, 1998; Budetta et al., 1999), the observed gravity changes have been assumed to be only the result of subsurface variations in mass. The reduced continuous gravity changes from PDN station di¡er from gravity changes assessed simultaneously by discrete relative observations at CO by no more than 10 WGal (Fig. 7b). This value is within the error a¡ecting discrete relative gravity measurements along the summit Etna stations (Budetta and Carbone, 1998). This indicates that the corrected signal from the PDN continuous station re£ects the actual changes of the local gravity ¢eld.
Fig. 7. PDN station: reduced gravity (after removal of the best linear ¢t, the theoretical Earth Tide and tilt e¡ects) corrected for the temperature e¡ect (a), and for the temperature and pressure e¡ects (b). Filled circles are gravity changes assessed by discrete relative observations at CO (measurements errors on discrete gravity changes along the Summit Pro¢le are typically of the order of 10 WGal), a site very close to the continuous station (see Fig. 1).
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Between October 1998 and February 2000 a nearly continuous activity was observed at the four Etna summit craters. During the ¢rst part of the above period the Southeast Crater (SEC) was the most active one: it produced 21 paroxysmal fountaining episodes between September 1998 and February 1999. During the last episode (February 4) a fracture formed on the £ank of the crater and lava started pouring from it. The other summit craters remained quiet during this period. In the summer of 1999 activity from the Voragine (VOR) and Bocca Nuova (BN) craters resumed with a gradual increase. On September 4 a strong lava fountain episode (the eruption column rose V2 km high before being blown downwind) was produced by the VOR. A revival of Strombolian activity occurred a few hours after this event at the SEC where a new fracture also formed at the base of the cone, renewing lava emission. Activity at BN became more energetic after the event on September 4 and climaxed on September 20 with vigorous lava fountaining. After this event, the activity shifted to the Northeast Crater (NEC), which had Strombolian activity in its central pit, and then to the BN again in early October. During the second half of October, the BN crater produced spectacular Strombolian activity with episodes of high lava fountaining. As a consequence of this activity the crater was ¢lled with pyroclastic material and lava over£owed on October 17 onto the W £ank of the volcano producing a lava ¢eld extending towards Monte Nunziata (1750 m a.s.l.). After November 3, the activity declined to low levels. The volume of lava erupted from the BN between October 17 and November 3 is probably in the range of 15^20U106 m3 (Smithsonian Institution, 1999). This places the October^November activity from the BN among the largest summit eruptions recorded at Etna during the past 200 years (Smithsonian Institution, 1999). During the December 1999^January 2000 period BN produced intermittent mild Strombolian activity. In late January eruptive activity resumed at SEC. In February lava emission mostly from fractures on the N and S £anks of the SEC cone occurred. Eruptive episodes began to diminish in frequency and intensity after February 18. Activity at BN during the eruptive epi-
sodes at SEC (end of January^February 2000) continued at relatively low levels while the other two summit craters remained essentially quiet. A 3-D inversion has been performed to assess the characteristics of the source which caused the gravity decrease measured through discrete measurements along the Summit Pro¢le between June and September 1999 (Fig. 8c). The calculation (of the trial-and-error type) has been carried out through a 3-D programme called GRAVERSE (Carbone, 2002) which simulates the gravity e¡ect of the source (buried homogeneous mass) by rep-
Fig. 8. Gravity changes along the Summit Pro¢le during the June^July 1999 (b) and June^September 1999 (c) periods. They are attributed to a mass decrease in a source zone, modelled as a prism-shaped body, the top of which is at about 1.8^2 km above sea level (the projection onto the surface of this body is shown in (a)). Errors a¡ecting gravity changes along the Summit Pro¢le are typically within 15 WGal.
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resenting it as a lattice of point masses. Assuming a prismatic geometry and an orientation following the trend of Etna’s Northeast rift (Gardun‹o et al., 1997), a good ¢t with observation is achieved for a source 1.5 km long, 0.5 km in vertical extent and with a top about 1.8^2 km a.s.l. (the projection onto the surface of this source is shown in Fig. 8a). The mass decrease is about 2U1010 kg. The continuous sequence acquired at PDN evidences at least two complete cycles of gravity (mass) increase/decrease (Fig. 7b). Given their similarity as for period (about six months) and amplitude (about 100 WGal peak-to-peak), they could reasonably be the e¡ect of the same source. The hypothesis of water-table £uctuations as a source for the observed gravity changes can be ruled out because of two reasons: (1) water-table £uctuations usually have a quasi-seasonal period; and (2) at Etna the maximum gravity e¡ect of water-table £uctuations is about 20 WGal peakto-peak (Budetta et al., 1999; Carbone, 2002). It is more likely that the gravity changes observed along the Summit Pro¢le in 1999 re£ect cyclic mass variations a¡ecting the magma within the inferred dike. Accordingly, the calculated mass change of 2U1010 kg implies about 107 m3 of magma being redistributed between June and September 1999. The spectral amplitude of the volcanic tremor observed at both PDN and ESP stations (placed respectively within a few metres from the homonymous gravity station and in the Southeast sector of the volcano, very close to station CV ; Fig. 1) during the 1998^1999 period shows a marked negative correlation with the gravity sequence acquired at PDN continuous station over the 6-month-period component of the signal (Privitera, pers. commun.). Given that the volcanic tremor is generated by non-stationary £uid £ows due to gases escaping through open magma-¢lled conduits (Seidl et al., 1981; Schick, 1988) and its amplitude is related to the intensity of turbulent motions (Leonardi et al., 1999), it is likely that the increasing of the degassing processes in the inferred source (and possibly also in other parts of the upper Etna’s plumbing system in physical continuity with the inferred source) leads to both a gravity (mass) decrease and an increase in the spectral amplitude of the volcanic
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tremor and thus to the observed negative correlation. It is also noteworthy that most paroxysmal explosive episodes observed during the studied time interval took place during the decrease parts of the above gravity cycles, especially during the August^October 1999 period. This observation is also in agreement with the above hypothesis of changes in the gas content of the source magma body as the mechanism triggering the observed cycles of gravity increase/decrease (Rymer and Brown, 1987).
6. Concluding remarks Through the experiments accomplished at Etna during the past few years many steps forward have been made towards the regular acquisition of high-quality continuous gravity sequences. The main issue with continuous gravity measurements at active volcanoes is to achieve data to a suitable standard (as for quality and continuity) against an adverse environment (high altitude, inaccessibility for several months at a time, lack of mains electricity for power, variable temperature, pressure and humidity, seismicity, corrosive gases) and using instruments which are suitable for laboratory conditions. To achieve this goal, some conditions must be satis¢ed : (1) the station setup must be rigid enough and remote-operable; (2) the e¡ect of external (mainly meteorological) perturbations must be measured at each station and suitably removed from the gravity signal ; and (3) reliable discrete micro-gravity measurements performed along an array which comprises benchmarks very close to the continuous stations must be available. The combined use of discrete and continuous gravity measurements is an unique tool both for studying the internal dynamic of a volcano and for surveillance purposes. Discrete measurements, if accomplished along a suitable network referred to stations su⁄ciently far away from the active area, allow both the actual magnitude of the subsurface mass change and the position and spatial characteristics of the source to be de¢ned. Con-
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tinuous measurements, on the other hand, allow the temporal evolution of the source to be followed all year long and with a temporal resolution which would be unthinkable if only discrete measurements were available and thus allow any hypothesis on the source mechanism to be better constrained. As shown before, such a combined system has already allowed important conclusions to be drawn at Mount Etna. Our current target is to consolidate our experience (in particular as for the algorithms for continuous data reduction) using the longer sequences now available and perform correlation analyses between the time sequences of gravity and other geophysical parameters (volcanic tremor, SO2 £ux, ground tilt, etc.) to discover possible common sources to the di¡erent signals and thus both improve the current knowledge of Etna’s internal plumbing and recognise possible precursors to paroxysmal events.
References Ando', B., Carbone, D., 2001. A methodology for reducing the signal from a continuously recording gravity meter for the e¡ect of meteorological parameters. IEEE Trans. Instrum. Meas. 50, 1248^1254. Bastien, R., Goodacre, A.K., 1990. The e¡ect of humidity variations on long-term tidal gravity recordings. Bull. Inf. Mare¤es Terr. 106, 7506^7510. Berrino, G., Corrado, G., Magliulo, R., Riccardi, U., 1997. Continuous record of the gravity changes at Mt Vesuvius. Ann. Geo¢s. 40, 1019^1028. Bonvalot, S., Diament, M., Germinal, G., 1998. Continuous gravity recording with Scintrex CG-3M meters: a promising tool for monitoring active zones. Geophys. J. Int. 135, 470^ 494. Budetta, G., Carbone, D., 1998. Temporal variations in gravity at Mt Etna (Italy) associated with the 1989 and 1991 eruptions. Bull. Volcanol. 59, 311^326. Budetta, G., Carbone, D., Greco, F., 1999. Subsurface mass redistributions at Mount Etna (Italy) during the 1995^96 explosive activity detected by microgravity studies. Geophys. J. Int. 138, 77^88. Carbone, D., 2002. Gravity monitoring of Mount Etna (Italy) through discrete and continuous measurements. Ph.D. Thesis, The Open University, Milton Keynes. d’Oreye, N., Ducarme, B., Hendicks, M., Laurent, R., Somerhausen, A., Van Ruymbeke, M., 1994. Tidal gravity observations at Mount Etna volcano. In: Volcanic deformation
and tidal gravity e¡ects at Mount Etna, Sicily, EEC Science, Final Rep. project ERB40002PL900491, pp. 60^80. Davis, P.M., 1981. Gravity and tilt earth tides measured on an active volcano, Mt Etna, Sicily. J. Volcanol. Geotherm. Res. 11, 213^223. Eggers, A.A., 1983. Temporal gravity and elevation changes at Pacaya volcano, Guatemala. J. Volcanol. Geotherm. Res. 19, 223^237. El Wahabi, A., Ducarme, B., Van Ruymbeke, M., d’Oreye', N., Somerhausen, A., 1997. Continuous gravity observations at Mount Etna (Sicily) and Correlations between temperature and gravimetric records. Cah. Cent. Eur. Ge¤odyn. Se¤ismol. 14, 105^119. El Wahabi, A., Dittfeld, H.J., Simon, Z., 2000. Meteorological in£uence on tidal gravimeter drift. Bull. Inf. Mare¤es Terr. 133, 10403^10414. Gardun‹o, V.H., Neri, M., Pasquare', G., Borgia, A., Tibaldi, A., 1997. Geology of the NE-Rift of Mount Etna (Sicily, Italy). Acta Vulcanol. 9, 91^100. Goodkind, J.M., 1986. Continuous measurement of nontidal variations of gravity. J. Geophys. Res. 91, 9124^9125. Jachens, R.C., Eaton, G.P., 1980. Geophysical observations of Kilauea volcano, Hawaii. 1. Temporal gravity variations related to the 29 November, 1975, M = 7.2 earthquake and associated summit collapse. J. Volcanol. Geotherm. Res. 7, 225^240. Jousset, P., Dwipa, S., Beauducel, F., Duquesnoy, T., Diament, M., 2000. Temporal gravity at Merapi during the 1993^1995 crisis: an insight into the dynamical behaviour of volcanoes. J. Volcanol. Geotherm. Res. 100, 289^320. LaCoste and Romber, 1997. General Catalog. Austin, TX. Leonardi, S., Caltabiano, T., Gresta, S., Mulargia, F., Privitera, E., Romano, R., 1999. Cross-correlation between volcanic tremor and SO2 £ux at Mt. Etna volcano, 1987^1995. Acta Volcanol. 11, 255^258. Merriam, J.B., 1992. Atmospheric pressure and gravity. Geophys. J. Int. 109, 488^500. Mu«ller, T., Zu«rn, W., 1983. Observation of gravity changes during the passage of cold fronts. J. Geophys. 53, 155^162. Niebauer, T.M., 1988. Correcting gravity measurements for the e¡ect of local air pressure. J. Geophys. Res. 93, 7989^ 7991. Papoulis, A., 1991. Probability, Random Variables, and Stochastic Processes, 3rd. ed. McGraw-Hill. Rymer, H., Brown, G., Ferrucci, F., Murray, J.B., 1994. Dyke intrusion mechanisms on Etna 1989^1993 and microgravity precursors to eruption. In: Villari, L. (Ed.), The 1991^1993 Etna eruption. Acta Vulcanol. 4, 109^114. Rymer, H., Brown, G.C., 1987. Causes of Microgravity change at Poa¤s volcano, Costa Rica: an active but non-erupting system. Bull. Volcanol. 49, 389^398. Rymer, H., Cassidy, J., Locke, C.A., Murray, J.B., 1995. Magma movements in Etna volcano associated with the major 1991^1993 lava eruption: evidence from gravity and deformation. Bull. Volcanol. 57, 451^461. Rymer, H., Murray, J.B., Brown, G.C., Ferrucci, F., McGuire,
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D. Carbone et al. / Journal of Volcanology and Geothermal Research 123 (2003) 123^135 J., 1993a. Mechanisms of magma eruption and emplacement at Mt Etna between 1989 and 1992. Nature 361, 439^441. Rymer, H., Murray, J.B., Brown, G.C., Ferrucci, F., McGuire, J., 1993b. Mechanisms of magma eruption and emplacement at Mt Etna between 1989 and 1992. Nature 361, 439^441. Sanderson, T.J.O., 1982. Direct gravimetric detection of magma movements at Mount Etna. Nature 297, 487^490. Schick, R., 1988. Volcanic tremor-source mechanism and correlation with eruptive activity. Nat. Hazard 1, 125^144. Scintrex Limited, 1992. Autograv Operator Manual Version 4.4. Concord, Ont. Seidl, D., Schick, R., Riuscetti, M., 1981. Volcanic tremors at Etna: a model for hydraulic origin. Bull. Volcanol. 44, 43^56. Smithsonian Institution, 1999. Bulletin-November, 24 (11).
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Spratt, R.S., 1982. Modelling the e¡ect of atmospheric pressure variations on gravity. Geophys. J. R. Astron. Soc. 71, 173^186. Sun, H.P., Ducarme, B., Dehant, V., 1995. Theoretical calculation of atmosheric gravity Green’s functions. Cah. Centre Eur. Ge'odyn. Se'ismol. 11, 223^237. Torge, W., 1989. Gravimetry. Walter de Gruyter, Berlin. VanRuymbeke, M., 1989. A new Feedback System for Instruments Equipped with a Capacitive Transducer. Proceedings of 11th International Symposium on Earth Tides, Helsinki, pp. 51^60. Wenzel, H.G., 1996. The nanogal software: Earth tide data processing package ETERNA 3.30. Bull. Inf. Mare¤es Terr. 124, 9425^9439.
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Journal of Volcanology and Geothermal Research 123 (2003) 123^135 www.elsevier.com/locate/jvolgeores
Combined discrete and continuous gravity observations at Mount Etna Daniele Carbone a; , Gennaro Budetta a , Filippo Greco a , Hazel Rymer b a
Istituto Nazionale di Geo¢sica e Vulcanologia, Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy b Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, UK MK7 6AA Received 28 October 2001; received in revised form 1 May 2002; accepted 1 July 2002
Abstract Systematic investigation of discrete gravity measurements has continued at Mount Etna since 1986. The network now covers an area of 400 km2 with about 70 stations 0.5^3 km apart. Mass redistributions occurring at depths ranging between about 8 km below sea level and a few hundred metres below the surface (magma level changes within the shallower parts of the feeding conduits) have been identified from these data. Conventional (discrete) microgravity monitoring on a network of stations furnishes only instantaneous states of the mass distribution at continuously active systems. In order to obtain information on the rate at which the volcanic processes (and thus mass transfers) occur, three stations for continuously recording gravity where installed on Mount Etna in 1998. A 16-month long sequence from one of the continuously running stations (PDN, located 2 km from the active northeast crater at the summit of Etna volcano) is presented. After removing the effects of Earth Tide and tilt, the correlation of the residual gravity sequence with simultaneous recordings of meteorological parameters acquired at the same station was analysed. Once the meteorological effects have also been removed, continuous gravity changes are within 10 WGal of gravity changes measured using conventional microgravity observations at sites very close to the continuous station. This example shows how discrete and continuous gravity observations can be used together at active volcanoes to get a fuller and more accurate picture of the spatial and temporal characteristics of volcanic processes. < 2003 Elsevier Science B.V. All rights reserved. Keywords: Etna volcano; gravity variations; continuous recording; modelling
1. Introduction Micro-gravity studies through discrete measurements have been shown to provide valuable information on the processes occurring within active volcanoes both during eruptive and quiescent periods (Jachens and Eaton, 1980; Sanderson, 1982;
* Corresponding author. Fax: +39-(0)-95-435801. E-mail address: [email protected] (D. Carbone).
Eggers, 1983; Rymer and Brown, 1987; Rymer et al., 1993a,b, 1995; Budetta and Carbone, 1998; Budetta et al., 1999). One of the main drawbacks of repeated discrete network monitoring is the lack of information on the rate at which the volcanic processes occur. The problem is that only changes in the subsurface mass distribution between successive surveys (ranging on Etna between about one month and one year and depending on the snow coverage, availability of transport and personnel, etc.) can be assessed. The rate of
0377-0273 / 03 / $ ^ see front matter < 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0377-0273(03)00032-5
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change between successive measurement times is unknown and therefore there remains ambiguity as to the nature of the causative processes. If one also considers the need to reduce the exposure of personnel in active areas, it is easy to understand the importance of developing monitoring techniques that operate continuously and automatically in remote regions for several months at a time. A promising technique to satisfy these needs is continuous gravity monitoring (Goodkind, 1986). Continuous gravity studies at active volcanoes have been scarce in the past because of the logistic di⁄culties of running them in places where the conditions are far from the clean, ideal laboratory and so it is quite di⁄cult to attain the required precision in the data. Most previous studies have focussed on continuous measurements acquired at sites remote from any active craters in order to either obtain precise tidal gravity factors (Davis, 1981; d’Oreye et al., 1994) or to determine instrument-related correction algorithms for the main external perturbations (El Wahabi et al., 1997). In a recent paper Jousset et al. (2000) have found a correlation between the residual drift of a LaCoste and Romberg D meter which recorded continuously between 1993 and 1995 at a site 4 km away from the summit of Merapi Volcano (Indonesia) and seismic and volcanic activity. In addition they found variations in the ratio between the observations and the theoretical response of the Earth (tides), also correlated with the volcanic activity.
2. Discrete gravity measurements at Mount Etna Discrete gravity measurements have been carried out at Mount Etna since 1986. The gravity network (Fig. 1) is now composed of about 70 stations 0.5^3 km apart, covers an area of about 400 km2 and consists of four integrated subarrays (Budetta et al., 1999): (1) the Main Network (established in 1986), arranged as a ring around the volcano at elevations of between 800 and 2000 m above sea level (a.s.l.) ; (2) the Summit Pro¢le (established in 1992), which runs from Piano Provenzana (PPR; at the
lowest elevation of 1770 m a.s.l.) to the Rifugio Sapienza (RSA; 1890 m a.s.l.), crossing the summit zone; (3) the East^West Pro¢le (established in 1994), between Za¡erana (450 m a.s.l.) and Adrano (600 m a.s.l.), connecting with the Summit Pro¢le and the Main Network at the Rifugio Sapienza; and (4) the Base Reference Network (established in 1994) of four stations, placed at the towns of Adrano, Belpasso, Za¡erana and Linguaglossa, each of which is considered to be stable with respect to volcanic activity; all the other networks are connected to this reference network. The subarrays have di¡erent characteristics regarding the density of the traverse, access to stations (determined by snow coverage), and the time required to take measurements. Each subarray can be occupied independently, optimising the £exibility in taking measurements to accommodate the changeable activity and accessibility of the volcano. Measurements over the whole array are usually conducted every six months. Some parts of the array are reoccupied more frequently (approximately monthly measurements along the E^W and Summit Pro¢les, although snow coverage restricts measurements along the Summit Pro¢le to the summer months). Discrete measurements along the Etna gravity network have allowed mass redistributions occurring over a wide range of depths to be detected. Between June 1990 and June 1991, gravity increases in the summit region of the volcano, not accompanied by signi¢cant ground deformation (all height changes less than a few cm), were measured and interpreted in terms of an in£ux of magma into the summit feeder pipe and into a pre-existing SSE-trending fracture system prior to the 1991^1993 £ank eruption (Rymer et al., 1993a,b; 1994). Magma drainage from the upper levels of Etna’s central feeding system marking the closing stages of the 1991^1993 event could be detected through analysis of gravity variations (Budetta and Carbone, 1998). Mass redistributions at about 1 km a.s.l. have been detected during the September 1994^July 1996 period through measurements along the Etna Summit Pro¢le and have been associated
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Fig. 1. Sketch map of Mt Etna showing stations on the Main Network (¢lled circles), the Summit Pro¢le (triangles), the East^ West Pro¢le (small open circles), and the Basal Reference Network (large open circles). The three continuously running stations (¢lled squares) and the traces of the E^W and Summit pro¢les (lines) are also shown.
with the 1995^1996 explosive activity (Budetta et al., 1999). Mass redistributions 3^6 km below sea level producing negligible elevation changes at the surface and thought to be the e¡ect of magma injections within a NNW^SSE oriented elongate zone were detected at Etna several times during the last ten years (1987^1989, June^September 1992, and 1994^1996 periods; Budetta and Carbone, 1998, Budetta et al., 1999).
3. Continuously running gravity stations at Mount Etna
(Fig. 1). The continuously running stations are located ca. 10 km south of the active craters at the Serra la Nave Astrophysical Observatory (SLN; 1740 m a.s.l.), 2 km NE of the summit Northeast Crater at the Pizzi Deneri Volcanological Observatory (PDN ; 2920 m a.s.l.) and just 1 km S of the summit Southeast Crater at the Torre del Filosofo refuge (TDF; 2919 m a.s.l.). These three sites were chosen mainly because of the buildings present in each of them (in the case of PDN and TDF, the closest buildings to Etna’s active craters) which provide protection from the elements. 3.1. Setup of continuously running Etna stations
In 1998 three continuous gravity stations equipped with LaCoste and Romberg (L and R) spring gravimeters were installed on Mount Etna
The conditions at a continuous gravity station close to an active crater are far from the clean,
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Fig. 2. Three-component setup for acquisition and transmission of gravity data at Etna’s remote stations.
ideal laboratory and so it is quite di⁄cult to attain the required precision in the data. Because of this, continuous gravity observations at active volcanoes have not developed as quickly as other geophysical techniques. However, technological improvements in recent years have allowed most of these di⁄culties to be overcome. At Etna we have developed a three-component setup (Fig. 2) for typical ¢eld stations in the active volcanic environment which is robust, easy to remove and re-establish, and cheap (the gravity meter being by far the most expensive item). The power system employs solar panels connected to trickle-charged batteries. To provide a constant power supply (within a few hundredth of a Volt) to the feedback of the recording L and R meters a dc^dc converter coupled with a lowdropout tension stabiliser is used.
The acquisition system comprises the gravity meter itself, which outputs analogue signals representing the feedback force and the long and cross levels and sensors for the atmospheric temperature and pressure. The meters are equipped with remote controlled stepper motors to reset them. The whole acquisition system is placed inside a thermally insulating polystyrene container (Fig. 3). Data are acquired every second. The average over 60 measurements is then calculated and stored in the solid-state memory of the data logger (at 1 datum/min). After being temporally stored in the solid-state memory of the data logger, data are dumped to the INGV Sezione di Catania (Catania, Italy) automatically every 24 h by the transmission system which employs a cellular phone. Using suitable software on a computer in Catania it is also pos-
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sible: (1) to remotely activate the stepper motor which turns the meter dial allowing the meter to be reset (Fig. 3), and (2) to monitor in real time all the parameters recorded.
4. Continuous data from PDN station 4.1. Data presentation Many shortcomings (mainly electronic) a¡ected data from L and R PET-1081 at TDF during the ¢rst eight months of its recording period (the instrument was newly acquired when installed at TDF in September 1998). Data were corrupted to such an extent that it became impossible to separate the geophysical from other-sources-gen-
127
erated signal. The appearance of £uctuations in the calibration factor by as much as 20% with periods of a few days leaded us to suspect an electronic defect a¡ecting the feedback system (MVR concept; VanRuymbeke, 1989; LaCoste and Romber, 1997) ¢tted to this new instrument. L and R ET-13 working at SLN was a¡ected by pressure-driven instrumental e¡ect yielding strong apparent gravity changes (Ando' and Carbone, 2001) and indicating that the instrument was not suitably compensated for pressure variations (the pressure seals were no longer airtight). PDN, on the other hand, is the Etna station which best ¢ts the required standards for a continuous remote gravity station in the volcanic environment. Thanks to the ruggedness of the station setup and reliability of the sensor (L and R
Fig. 3. The Pizzi Deneri Volcanological Observatory (a) hosts one of Etna’s continuously running gravity stations. To reduce the e¡ect of atmospheric temperature the gravimeter has been placed inside a two level thermally insulating polystyrene container (b). A step motor, mounted on the black lid of the gravimeter (c), allows the instrument to be remotely reset via cellular phone.
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Fig. 4. Raw gravity data recorded at PDN station during the October 1998^February 2000 time interval.
D-157 works at PDN), a high-quality data sequence free from long gaps has been acquired at this station. Since the present work is mainly intended to show the possibilities of combined continuous and discrete observations through spring gravimeters at active volcanoes, in the following sections just data from the PDN continuous station will be presented, analysed and compared with data acquired through discrete measurements at a site very close to the continuous station. Fig. 4 shows the raw gravity recording at PDN for the time period between October 1998 and February 2000. The ¢rst degree instrumental drift rate is about 285 WGal/month. During the above period eight interruptions of the continuous record with length ranging between 5 and 90 h occurred. Interruptions to the data set were due to: (1) instrumental drift bringing the proof mass out of the feed-back range in periods when it was impossible to reset the meter remotely and to get to the station (snow coverage) ; (2) temporary failures of the powering system of the stations; and (3) seismic shocks sending the proof mass of the instruments to the edge of the feed-back range (the feed-back force takes a variable time to restore the equilibrium). The mean background noise is less than 1 WGal. 4.2. Analysis of the data 4.2.1. Tide and inclination e¡ects To model the tidal gravity e¡ect on Etna, we used the Eterna 3.30 data processing package (Wenzel, 1996). The Eterna package allows the parameters of the main tidal components to be calculated on grounds of the available recorded
data. The data were ¢rst pre-processed, using suitable computing kernels of the Eterna package, to remove spikes, interpolate gaps and correct for jumps. After the pre-processing step, the parameters of the main tidal components were evaluated by a least squares harmonic analysis method using the Analyze computing kernel. These parameters allow tidal gravity on Etna to be modelled to better than a few tenths of WGal. Tilt changes are measured at each station along two perpendicular axes X and Y using the electronic levels ¢tted to L and R instruments. These devices are simple air-damped pendulums the position of which is sensed by a capacitance bridge (LaCoste and Romber, 1997). Their resolution (about 2.5 Wrad) is surely suitable for our purposes. Tilt variation during the period covered by the present study were typically within a few Wrad (up to about 50 Wrad). The gravity e¡ect of the tilt changes was calculated using the formula (Scintrex Limited, 1992): Ng ¼ gð13cosX cosY Þ
ð1Þ
where g is an average gravity value (980.6 Gal). The maximum amplitude of the tilt changes e¡ect is about 10 WGal. 4.2.2. The e¡ect of meteorological parameters Each continuously running gravity station of the Etna array has been equipped with atmospheric temperature and pressure sensors. Fig. 5 shows residual gravity (a), temperature (b) and pressure (c) observed at PDN during the period October 1998 to February 2000. The instrumental drift (modelled as a ¢rst degree curve), the Earth Tide and tilt e¡ects have been removed from the
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Reduced Gravity (µGal)
1000
129
(a)
800 600 400 200 0 -200 -400 -600 -800 -1000
24
(b)
22 20 18 16 14 12
730
(c)
725 720 715 710 705 700 695 690 Oct 98
Dec 98
Feb 99
Apr 99
Jun 99
Aug 99
Oct 99
Dec 99
Feb 00
Fig. 5. Reduced gravity (a), after removal of the best linear ¢t, the theoretical Earth Tide and tilt e¡ects, temperature (b) and pressure (c) observed at PDN station during the October 1998^February 2000 period.
gravity signal. The most evident feature is the strong correlation between longer-wavelength gravity and temperature variations. Meteorological parameters are expected to affect continuously recording spring gravimeters (Torge, 1989; El Wahabi et al., 1997; Bonvalot et al., 1998; El Wahabi et al., 2000). Apparent gravity changes depend on the temporal development and magnitude of the meteorological change that caused them as well as on the insulation and compensation of the gravimeter. Thus, the correction formulas are instrument-speci¢c and often frequency-dependent. To investigate the complex relationship between the gravity signal and meteorological parameters, spectral correlation analyses have been performed using Welch’s averaged periodogram method. In
particular the coherence function Cxy (indicating how well the meter output corresponds to each meteorological parameter at each frequency) is given by: Cxy ¼ ðMPxyM2 Þ=ðPxx PyyÞ
ð2Þ
where Pxy represents the cross spectral density, while Pxx and Pyy represent the power spectral density (Papoulis, 1991). As for temperature, a signi¢cant correlation (Fig. 6a) appears over the lowest frequency band (T greater than about 1 month; coherence values greater than 0.6). Moving towards the higher frequencies, after a sharp drop, the coherence values remain within 0.2, indicating the absence of correlation. Over the lowest frequency band the maximum admittance value is close to
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Fig. 6. Transfer functions in the frequency domain between gravity and temperature (a,b) and between gravity and pressure (c,d) signals acquired at PDN station.
about 200 WGal/‡C (Fig. 6b); that means that apparent changes of the order of hundreds of WGal can be observed as the result of signi¢cant temperature changes. Using data sequences longer than ours, other authors have found that temperature changes can cause an instrumental e¡ect on spring gravimeters which can reach over a yearly period an amplitude of about one mGal (El Wahabi et al., 1997, 2000). However, air humidity, which is closely dependent on temperature, could be directly responsible for the instrumental gravity changes correlated with temperature (Bastien and Goodacre, 1990; El Wahabi et al., 2000). The coherence diagram for pressure shows that the correlation between gravity and pressure (Fig. 6c) is signi¢cant (coherence values around 0.6) just over the spectral band ranging between 0.1 and 1 c.p.d. (T ranging between about 1 and 10 days). Over this spectral band, the mean admittance value is around 0.3^0.4 WGal/mbar (Fig. 6d), in good agreement with the value of the theoretical local admittance given by the combined
e¡ect of the gravitational attraction of the air column and the distortion of the Earth’s surface resulting from barometric changes (Spratt, 1982; Mu«ller and Zu«rn, 1983; Niebauer, 1988; Merriam, 1992; Sun et al., 1995; Berrino et al., 1997). Signi¢cant coherence values together with admittance values close to theoretical ones over this band indicate the absence of important instrumental (apparent) changes linked to atmospheric pressure changes on one hand and the absence of other important e¡ects able to lower correlation values and corrupt admittance values on the other one. Over the other parts of the spectrum (f 6 0.1 c.p.d. and f s 1 c.p.d.), e¡ects not related to pressure changes become predominant (low coherence values, high background noise in the admittance diagram). To correct the gravity signal for the temperature e¡ect, the procedure outlined by Ando' and Carbone (2001) has been followed. In particular, a sixth degree polynomial ¢lter has been used to remove the seasonal temperature e¡ect while the
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e¡ect of non-seasonal temperature changes has been removed through a non-linear (Neuro^ Fuzzy) form of a model structure which takes into account the time delay between temperature and gravity (Ando' and Carbone, 2001). In Fig. 7a the residual gravity signal, after the temperature e¡ect has been removed, is presented. As observed above in the frequency analysis, the gravimeter at PDN (L and R D-157) is not affected by atmospheric pressure changes. Thus, the signal from this meter is only to be corrected for the normal local in£uence of the atmospheric pressure on the gravity ¢eld (standard admittance value of 30.365 WGal/mbar). The maximum peakto-peak amplitude of the e¡ect of the atmospheric pressure is about 10 WGal over the period considered. It can be seen that to correct the gravity signal for this e¡ect improves its quality signi¢cantly over changes within a period of several days, a¡ected by pressure changes related to (anti-) cyclones (Fig. 7b).
5. Discussion During the October 1998^February 2000 peri-
131
od, discrete surveys were accomplished along the Summit Pro¢le (section 2; Fig. 1) in June 1999, July 1999, September 1999 and October 1999. In Fig. 7 gravity data acquired through discrete measurements at station CO (at a distance of about 500 m from PDN; Fig. 1) are reported (black dots). Unfortunately, no elevation data from the summit Etna zone are available for the period covered by the discrete gravity measurements. However, since the height variations on Etna are usually too small to signi¢cantly a¡ect surface gravity measurements (Sanderson, 1982; Rymer et al., 1993a,b, 1995; Budetta and Carbone, 1998; Budetta et al., 1999), the observed gravity changes have been assumed to be only the result of subsurface variations in mass. The reduced continuous gravity changes from PDN station di¡er from gravity changes assessed simultaneously by discrete relative observations at CO by no more than 10 WGal (Fig. 7b). This value is within the error a¡ecting discrete relative gravity measurements along the summit Etna stations (Budetta and Carbone, 1998). This indicates that the corrected signal from the PDN continuous station re£ects the actual changes of the local gravity ¢eld.
Fig. 7. PDN station: reduced gravity (after removal of the best linear ¢t, the theoretical Earth Tide and tilt e¡ects) corrected for the temperature e¡ect (a), and for the temperature and pressure e¡ects (b). Filled circles are gravity changes assessed by discrete relative observations at CO (measurements errors on discrete gravity changes along the Summit Pro¢le are typically of the order of 10 WGal), a site very close to the continuous station (see Fig. 1).
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Between October 1998 and February 2000 a nearly continuous activity was observed at the four Etna summit craters. During the ¢rst part of the above period the Southeast Crater (SEC) was the most active one: it produced 21 paroxysmal fountaining episodes between September 1998 and February 1999. During the last episode (February 4) a fracture formed on the £ank of the crater and lava started pouring from it. The other summit craters remained quiet during this period. In the summer of 1999 activity from the Voragine (VOR) and Bocca Nuova (BN) craters resumed with a gradual increase. On September 4 a strong lava fountain episode (the eruption column rose V2 km high before being blown downwind) was produced by the VOR. A revival of Strombolian activity occurred a few hours after this event at the SEC where a new fracture also formed at the base of the cone, renewing lava emission. Activity at BN became more energetic after the event on September 4 and climaxed on September 20 with vigorous lava fountaining. After this event, the activity shifted to the Northeast Crater (NEC), which had Strombolian activity in its central pit, and then to the BN again in early October. During the second half of October, the BN crater produced spectacular Strombolian activity with episodes of high lava fountaining. As a consequence of this activity the crater was ¢lled with pyroclastic material and lava over£owed on October 17 onto the W £ank of the volcano producing a lava ¢eld extending towards Monte Nunziata (1750 m a.s.l.). After November 3, the activity declined to low levels. The volume of lava erupted from the BN between October 17 and November 3 is probably in the range of 15^20U106 m3 (Smithsonian Institution, 1999). This places the October^November activity from the BN among the largest summit eruptions recorded at Etna during the past 200 years (Smithsonian Institution, 1999). During the December 1999^January 2000 period BN produced intermittent mild Strombolian activity. In late January eruptive activity resumed at SEC. In February lava emission mostly from fractures on the N and S £anks of the SEC cone occurred. Eruptive episodes began to diminish in frequency and intensity after February 18. Activity at BN during the eruptive epi-
sodes at SEC (end of January^February 2000) continued at relatively low levels while the other two summit craters remained essentially quiet. A 3-D inversion has been performed to assess the characteristics of the source which caused the gravity decrease measured through discrete measurements along the Summit Pro¢le between June and September 1999 (Fig. 8c). The calculation (of the trial-and-error type) has been carried out through a 3-D programme called GRAVERSE (Carbone, 2002) which simulates the gravity e¡ect of the source (buried homogeneous mass) by rep-
Fig. 8. Gravity changes along the Summit Pro¢le during the June^July 1999 (b) and June^September 1999 (c) periods. They are attributed to a mass decrease in a source zone, modelled as a prism-shaped body, the top of which is at about 1.8^2 km above sea level (the projection onto the surface of this body is shown in (a)). Errors a¡ecting gravity changes along the Summit Pro¢le are typically within 15 WGal.
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resenting it as a lattice of point masses. Assuming a prismatic geometry and an orientation following the trend of Etna’s Northeast rift (Gardun‹o et al., 1997), a good ¢t with observation is achieved for a source 1.5 km long, 0.5 km in vertical extent and with a top about 1.8^2 km a.s.l. (the projection onto the surface of this source is shown in Fig. 8a). The mass decrease is about 2U1010 kg. The continuous sequence acquired at PDN evidences at least two complete cycles of gravity (mass) increase/decrease (Fig. 7b). Given their similarity as for period (about six months) and amplitude (about 100 WGal peak-to-peak), they could reasonably be the e¡ect of the same source. The hypothesis of water-table £uctuations as a source for the observed gravity changes can be ruled out because of two reasons: (1) water-table £uctuations usually have a quasi-seasonal period; and (2) at Etna the maximum gravity e¡ect of water-table £uctuations is about 20 WGal peakto-peak (Budetta et al., 1999; Carbone, 2002). It is more likely that the gravity changes observed along the Summit Pro¢le in 1999 re£ect cyclic mass variations a¡ecting the magma within the inferred dike. Accordingly, the calculated mass change of 2U1010 kg implies about 107 m3 of magma being redistributed between June and September 1999. The spectral amplitude of the volcanic tremor observed at both PDN and ESP stations (placed respectively within a few metres from the homonymous gravity station and in the Southeast sector of the volcano, very close to station CV ; Fig. 1) during the 1998^1999 period shows a marked negative correlation with the gravity sequence acquired at PDN continuous station over the 6-month-period component of the signal (Privitera, pers. commun.). Given that the volcanic tremor is generated by non-stationary £uid £ows due to gases escaping through open magma-¢lled conduits (Seidl et al., 1981; Schick, 1988) and its amplitude is related to the intensity of turbulent motions (Leonardi et al., 1999), it is likely that the increasing of the degassing processes in the inferred source (and possibly also in other parts of the upper Etna’s plumbing system in physical continuity with the inferred source) leads to both a gravity (mass) decrease and an increase in the spectral amplitude of the volcanic
133
tremor and thus to the observed negative correlation. It is also noteworthy that most paroxysmal explosive episodes observed during the studied time interval took place during the decrease parts of the above gravity cycles, especially during the August^October 1999 period. This observation is also in agreement with the above hypothesis of changes in the gas content of the source magma body as the mechanism triggering the observed cycles of gravity increase/decrease (Rymer and Brown, 1987).
6. Concluding remarks Through the experiments accomplished at Etna during the past few years many steps forward have been made towards the regular acquisition of high-quality continuous gravity sequences. The main issue with continuous gravity measurements at active volcanoes is to achieve data to a suitable standard (as for quality and continuity) against an adverse environment (high altitude, inaccessibility for several months at a time, lack of mains electricity for power, variable temperature, pressure and humidity, seismicity, corrosive gases) and using instruments which are suitable for laboratory conditions. To achieve this goal, some conditions must be satis¢ed : (1) the station setup must be rigid enough and remote-operable; (2) the e¡ect of external (mainly meteorological) perturbations must be measured at each station and suitably removed from the gravity signal ; and (3) reliable discrete micro-gravity measurements performed along an array which comprises benchmarks very close to the continuous stations must be available. The combined use of discrete and continuous gravity measurements is an unique tool both for studying the internal dynamic of a volcano and for surveillance purposes. Discrete measurements, if accomplished along a suitable network referred to stations su⁄ciently far away from the active area, allow both the actual magnitude of the subsurface mass change and the position and spatial characteristics of the source to be de¢ned. Con-
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tinuous measurements, on the other hand, allow the temporal evolution of the source to be followed all year long and with a temporal resolution which would be unthinkable if only discrete measurements were available and thus allow any hypothesis on the source mechanism to be better constrained. As shown before, such a combined system has already allowed important conclusions to be drawn at Mount Etna. Our current target is to consolidate our experience (in particular as for the algorithms for continuous data reduction) using the longer sequences now available and perform correlation analyses between the time sequences of gravity and other geophysical parameters (volcanic tremor, SO2 £ux, ground tilt, etc.) to discover possible common sources to the di¡erent signals and thus both improve the current knowledge of Etna’s internal plumbing and recognise possible precursors to paroxysmal events.
References Ando', B., Carbone, D., 2001. A methodology for reducing the signal from a continuously recording gravity meter for the e¡ect of meteorological parameters. IEEE Trans. Instrum. Meas. 50, 1248^1254. Bastien, R., Goodacre, A.K., 1990. The e¡ect of humidity variations on long-term tidal gravity recordings. Bull. Inf. Mare¤es Terr. 106, 7506^7510. Berrino, G., Corrado, G., Magliulo, R., Riccardi, U., 1997. Continuous record of the gravity changes at Mt Vesuvius. Ann. Geo¢s. 40, 1019^1028. Bonvalot, S., Diament, M., Germinal, G., 1998. Continuous gravity recording with Scintrex CG-3M meters: a promising tool for monitoring active zones. Geophys. J. Int. 135, 470^ 494. Budetta, G., Carbone, D., 1998. Temporal variations in gravity at Mt Etna (Italy) associated with the 1989 and 1991 eruptions. Bull. Volcanol. 59, 311^326. Budetta, G., Carbone, D., Greco, F., 1999. Subsurface mass redistributions at Mount Etna (Italy) during the 1995^96 explosive activity detected by microgravity studies. Geophys. J. Int. 138, 77^88. Carbone, D., 2002. Gravity monitoring of Mount Etna (Italy) through discrete and continuous measurements. Ph.D. Thesis, The Open University, Milton Keynes. d’Oreye, N., Ducarme, B., Hendicks, M., Laurent, R., Somerhausen, A., Van Ruymbeke, M., 1994. Tidal gravity observations at Mount Etna volcano. In: Volcanic deformation
and tidal gravity e¡ects at Mount Etna, Sicily, EEC Science, Final Rep. project ERB40002PL900491, pp. 60^80. Davis, P.M., 1981. Gravity and tilt earth tides measured on an active volcano, Mt Etna, Sicily. J. Volcanol. Geotherm. Res. 11, 213^223. Eggers, A.A., 1983. Temporal gravity and elevation changes at Pacaya volcano, Guatemala. J. Volcanol. Geotherm. Res. 19, 223^237. El Wahabi, A., Ducarme, B., Van Ruymbeke, M., d’Oreye', N., Somerhausen, A., 1997. Continuous gravity observations at Mount Etna (Sicily) and Correlations between temperature and gravimetric records. Cah. Cent. Eur. Ge¤odyn. Se¤ismol. 14, 105^119. El Wahabi, A., Dittfeld, H.J., Simon, Z., 2000. Meteorological in£uence on tidal gravimeter drift. Bull. Inf. Mare¤es Terr. 133, 10403^10414. Gardun‹o, V.H., Neri, M., Pasquare', G., Borgia, A., Tibaldi, A., 1997. Geology of the NE-Rift of Mount Etna (Sicily, Italy). Acta Vulcanol. 9, 91^100. Goodkind, J.M., 1986. Continuous measurement of nontidal variations of gravity. J. Geophys. Res. 91, 9124^9125. Jachens, R.C., Eaton, G.P., 1980. Geophysical observations of Kilauea volcano, Hawaii. 1. Temporal gravity variations related to the 29 November, 1975, M = 7.2 earthquake and associated summit collapse. J. Volcanol. Geotherm. Res. 7, 225^240. Jousset, P., Dwipa, S., Beauducel, F., Duquesnoy, T., Diament, M., 2000. Temporal gravity at Merapi during the 1993^1995 crisis: an insight into the dynamical behaviour of volcanoes. J. Volcanol. Geotherm. Res. 100, 289^320. LaCoste and Romber, 1997. General Catalog. Austin, TX. Leonardi, S., Caltabiano, T., Gresta, S., Mulargia, F., Privitera, E., Romano, R., 1999. Cross-correlation between volcanic tremor and SO2 £ux at Mt. Etna volcano, 1987^1995. Acta Volcanol. 11, 255^258. Merriam, J.B., 1992. Atmospheric pressure and gravity. Geophys. J. Int. 109, 488^500. Mu«ller, T., Zu«rn, W., 1983. Observation of gravity changes during the passage of cold fronts. J. Geophys. 53, 155^162. Niebauer, T.M., 1988. Correcting gravity measurements for the e¡ect of local air pressure. J. Geophys. Res. 93, 7989^ 7991. Papoulis, A., 1991. Probability, Random Variables, and Stochastic Processes, 3rd. ed. McGraw-Hill. Rymer, H., Brown, G., Ferrucci, F., Murray, J.B., 1994. Dyke intrusion mechanisms on Etna 1989^1993 and microgravity precursors to eruption. In: Villari, L. (Ed.), The 1991^1993 Etna eruption. Acta Vulcanol. 4, 109^114. Rymer, H., Brown, G.C., 1987. Causes of Microgravity change at Poa¤s volcano, Costa Rica: an active but non-erupting system. Bull. Volcanol. 49, 389^398. Rymer, H., Cassidy, J., Locke, C.A., Murray, J.B., 1995. Magma movements in Etna volcano associated with the major 1991^1993 lava eruption: evidence from gravity and deformation. Bull. Volcanol. 57, 451^461. Rymer, H., Murray, J.B., Brown, G.C., Ferrucci, F., McGuire,
VOLGEO 2581 10-4-03
D. Carbone et al. / Journal of Volcanology and Geothermal Research 123 (2003) 123^135 J., 1993a. Mechanisms of magma eruption and emplacement at Mt Etna between 1989 and 1992. Nature 361, 439^441. Rymer, H., Murray, J.B., Brown, G.C., Ferrucci, F., McGuire, J., 1993b. Mechanisms of magma eruption and emplacement at Mt Etna between 1989 and 1992. Nature 361, 439^441. Sanderson, T.J.O., 1982. Direct gravimetric detection of magma movements at Mount Etna. Nature 297, 487^490. Schick, R., 1988. Volcanic tremor-source mechanism and correlation with eruptive activity. Nat. Hazard 1, 125^144. Scintrex Limited, 1992. Autograv Operator Manual Version 4.4. Concord, Ont. Seidl, D., Schick, R., Riuscetti, M., 1981. Volcanic tremors at Etna: a model for hydraulic origin. Bull. Volcanol. 44, 43^56. Smithsonian Institution, 1999. Bulletin-November, 24 (11).
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Spratt, R.S., 1982. Modelling the e¡ect of atmospheric pressure variations on gravity. Geophys. J. R. Astron. Soc. 71, 173^186. Sun, H.P., Ducarme, B., Dehant, V., 1995. Theoretical calculation of atmosheric gravity Green’s functions. Cah. Centre Eur. Ge'odyn. Se'ismol. 11, 223^237. Torge, W., 1989. Gravimetry. Walter de Gruyter, Berlin. VanRuymbeke, M., 1989. A new Feedback System for Instruments Equipped with a Capacitive Transducer. Proceedings of 11th International Symposium on Earth Tides, Helsinki, pp. 51^60. Wenzel, H.G., 1996. The nanogal software: Earth tide data processing package ETERNA 3.30. Bull. Inf. Mare¤es Terr. 124, 9425^9439.
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