Journal of the Geological Society, London, Vol. 148, 1991, pp. 571-576, 6 figs. Printed in Northern Ireland

Soil gas emanations as precursory indicators of volcanic eruptions

J.-C. BAUBRON', P . ALLARDZy3, J . -C. SABROUX2,4, D. TEDESC02.3, & J . - P . TOUTALN3.5

Bureau de Recherches Gkologiques et Minisres, SGN/GCH, 45060 Orlkans Cedex 02, France 'Centre des Faibles Radioactivitks, CNRS-CEA, 91198 Gif sur Yvette Cedex, France

Ossentatorio Vesuviano, 249 Via A . Manzoni, 80123 Napoli, Ztaly 4Volcanological Survey of Indonesia, Jalan Diponegoro 57, Bandung 40122, Indonesia

50bservatoire Volcanologique du Piton de la Fournaise, ZPG, 97418 Plaine des Cafres, France

Abstract: Field measurements conducted on several active volcanoes in Italy, the Lesser Antilles, and Indonesia demonstrate the common Occurrence of diffuse soil gas emanations from the volcanic piles, at distances from active craters or fumarolic zones. These emanations consist essentially of carbon dioxide and rare gases and their genetic link with crater fumaroles and/or magma degassing at depth can be verified both chemically and isotopically. We emphasize here the potential use of these fluids for continuous volcano monitoring and eruption forecasting.

Geochemical monitoring of volcanic fluids may provide valuable information on magma degassing processes and related changes in the activity of volcanoes (e.g. Allard 1983; Sabroux 1985). Ideally, such monitoring should be, and usually is, performed on the hottest gas vents (eruptive gases, plumes, fumaroles): those most closely connected to the uprise of magmatic volatiles from depth. Because of difficulties in a permanent access to such fluids, due to either hazard or topography, and because of their inherent character (high temperature, corrosive nature), only a discontinuous survey is generally possible, which necessarily limits our understanding of possible variations in terms of eruption forecasting. In trying to overcome these problems, we have recently focused attention on the cold or low temperature gases which percolate diffusively through volcanic piles, at a distance from active craters.

The use of hydrogen in soil gas for volcano monitoring was pioneered by Sat0 & McGee (1980). The importance of carbon dioxide emanations on an active volcano was discovered by Carbonnelle & Zettwoog (1982) and Carbonnelle, Dajlevic et al. (1985) on an erupting volcano. These authors showed that huge amounts of carbon dioxide, enriched in helium and radon, escape diffusively from the flanks of Mt Etna (Sicily), in addition to high-temperature degassing from its summit craters. Further investigations have demonstrated both the magmatic origin of these lateral emanations (Allard et al. 1987 a, b; Allard et al. in press) and their potential use as markers of sub-surface thermal anomalies and/or active faults (Baldi et al. 1984; Baubron 1986-1989 a, b; Baubron & Sabroux 1984; Baubron, et al. 1986, Baubron et al. 1987; Aubert & Baubron 1988). He and CO, mapping in volcanic soils was simultaneously initiated by Italian groups (e.g. Bertrami et al. 1984; Lombardi et al. 1984; Lombardi & Mappi 1986).

In the past five years, we have extended the study of soil gases to several volcanoes in a dormant stage, in Italy (Vesuvius, Vulcano, Solfatara), the Lesser Antilles (Soufrikre of Guadeloupe) and in Indonesia (Lamongan, Dieng). Numerous measurements were made on these sites, which show that the phenomenon of diffuse gas release also affects these volcanoes and can be related to their level of

activity (Baubron & Sabroux 1984; Baubron 1986-1989a, b; Baubron et al. 19866; Baubron et al. 1987; Baubron et al. 1989, 1990; Allard et al. 1988, 1989). Once their genetic link with magma degassing and/or crater fumaroles has been established, soil gas emanations provide reliable conditions for continuous monitoring (easy access, wide distribution at safe distances from active craters, low amount of corrosive components, possible proximity to electrical power, etc.). In this paper, we briefly present the methodology and the potential applications of this new geochemical approach of volcano monitoring, using some of our results as examples.

Geochemical tracers Volcanic soil gas emanations consist primarily of carbon dioxide, associated with hydrogen and rare gases (He, Ar, Rn) and are variably diluted by air (NZ, OJ. Sulphur compounds are usually missing, while methane sometimes occur. Because of their characteristics, summarized below, carbon dioxide, helium and radon were selected as preferential tracers.

Carbon dioxide Beside water vapour, it is the major gas species in both volcanic fluids and magmas. It is a good tracer of sub-surface magma degassing, since its low solubility in silicate melts at low to moderate pressure favours its early exsolution (e.g. Stolper & Holloway 1988). Among other possible carbon dioxide sources, the atmospheric contribu- tion is usually negligible. Carbonate sediments and organic matter in the volcanic basements may be significant, but their contrasted 13C/12C ratios (If0 f 1 and -25 f 5%, respectively), compared to that of mantle-magmatic sources (#-6 f 2), can be used to distinguish their contribution to the gas (e.g. Allard 1983). Chemical ratios and isotopic data on associated gases may also help in clarifying the distinction. COz is measured in the field by infra-red spectrometry with a 0.5% accuracy.

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572 J.-C. BAUBRON E T A L .

Helium This light rare gas escapes early from magmas at depth and migrates easily through fractured rock layers. Its chemical inertia, high mobility, and low solubility in water thus make it a privileged tracer of deep events. Furthermore, the 3He/4He isotopic ratio (R) allows a clear-cut distinction between atmospheric dilution ( R , = 1.4 X 10-6), contribu- tions of radiogenic He from the crust (R/R,#O.l to 0.01), and mantle-magmatic degassing (R/R, from 5 to 35, depending on sites; see the review by Lupton 1983). Air dilution of that gas can be evaluated according to the He/Ne ratio, neon being almost exclusively atmospheric. Soil helium is measured by mass spectrometry, either in the field or soon after sampling (in teflon bag and/or evacuated glass bottles) with a 2% accuracy.

222Radon This heavy radioactive rare gas, generated from radium in the rocks, has both a short decay period (3.8 days) and a low intrinsic mobility. Radon in soils is usually transported by waters and/or gases (mainly CO,). So, its emission at the surface is usually related to either shallow thermal anomalies or high gas flows (e.g. Baubron & Sabroux 1984). In situ measurement of the =’Rn activity in soil gases is done by alpha counting, either directly with a ionization chamber or after gas sampling in evacuated ZnS scintillating bottles, with a 10% accuracy.

Reliable detection, mapping and survey of magmatic gas leaks from the surface of volcanoes may thus be attempted using the concentration excess (relative to air), the chemical ratios, the isotopic characteristics, as well as the flux of these three gaseous components. In the following figures, the concentrations of CO, and He are expressed in volume units, while the activity of Rn is reported in pCi per litre of gas.

Procedures

Sampling Volcanic soil gases are analysed or sampled at depths of 0.7-1.0m in the ground, in order to minimize the effect of meteorological variations. For that purpose, we use 1 cm diameter stainless steel probes, filled with a teflon capillary, which are introduced into the ground with a sliding hammer. These are further connected to analysers or sampling containers. Analysers are either portable (CO, infra-red spectrometer, Rn scintillation counter) or transported in a four-wheel-drive van equipped with a quadrupole mass spectrometer, a specific He mass spectrometer and a computer for data collection and processing elaboration.

Prospection of soil gas emanations Prior to their monitoring, a detailed reconnaissance of volcanic soil gas leaks is necessary. This first stage implies numerous field measurements along traverses, generally with a 10m step, across the main structural features of the volcanic pile, as inferred from geological maps and aerial pictures. About 30 points a day can be currently analysed. The location, distribution, and chemical characteristics of the major gas leaks can thus be identified.

60t I t

saw* Fig. 1. Soil gas traverse (A on Fig. 2) across a faulted zone. 10 m sampling interval. Grotta di Palizzi, base of Fossa cone, Vulcano, Italy, 1989. Black areas correspond to ‘anomalies’.

As illustrated in Fig. 1, clear gas anomalies often relate to either visible or hidden active faults. The origin of these anomalies can be evaluated first from their spatial distribution and their chemistry, compared to those of other fluids on the volcano (hot fumaroles, water well or drill hole atmospheres, gas bubbles in natural springs or pools). The results can be displayed in the form of geochemical maps (Fig. 2), which indicate the area1 extension of the gas leaks, their possible relationship to different hydrothermal systems, and possible sites of future eruption (magmatic intrusion). Further insight into the origin of the gases and their link with hot crater fumaroles and/or magma degassing is provided by isotopic analyses in the lab (c, He, D, 3H, etc.). On the basis of such data, a few best sites can be selected for monitoring.

Gas flux measurements Measuring the flow rate of soil gas provides an other important information. First, a high gas flux may be one of the criteria for selecting anomalies to be monitored. Second, this allows the whole output of CO,, He and Rn by volcanic soil degassing to be quantified and to be compared with crater fumarolic emissions. The respective contribution of these two kinds of degassing to the total gas output at a given time can thus be related to the level of activity of the volcano. Finally, the relationship between the flux and the concentration of soil gas allows the distinction between steady or unsteady gas emanation processes in grounds of different permeability. The temporal evolution of this relationship at a given site can then be related to seismic or volcanic events, as exemplified in Fig. 3.

SOIL GAS EMANATION AND VOLCANIC ERUPTION 573

Vulcano gas geochemical map

4 Well atmosphere analyses ( w l t h no CO2).

3 Well atmosphere analyses (High He/hlgh C02)

3 Well atmosphere analyses (Low Helhigh C02)

7 Fumarole analyses (tilgh He/hlgh C021 U m Fumarole analyses (Low He/hlgh C02). U

Peculiar so11 gas analyses

so11 gas traverses

H Area w l t h no gas release

Area wlth Important gas releases (High t ielhlgh C02)

Area with Important gas releases (LOW He/hlgh C O ~ )

Fig. 2. Mapping of soil gas leaks in the active part of Vulcano island, Italy, 1989.

Soil gas fluxes are measured using stainless steel containers (60-200 litres, 0.5-1.3 m2 surface), with the open face placed on the ground. The gas flux can be determined by either dynamic (flow-through) or static (accumulation) methods (Allard et al. 19876; Baubron 1988). We can demonstrate that both methods give comparable results. The first one, requiring continuous and simultaneous analysis of both air and a mixture of soil gas plus air, is rather laborious but fast; the second one is easier but takes more time, involving discontinuous analysis (every hour or so), during 6 to 8 hours of gas accumulation. The contribution of biogenic CO, flux on most volcanoes we investigated is generally negligible, between 0.02 to

-5 0 5 10 15 20 25 30 cm ( X h l l

Fig. 3. Relationship between the flux and the concentration of CO, in soil gases. Base of Lamongan volcano, Java, Indonesia, 1988. The function: FIw (1 m-, h-') = 1.17 E-3 (CO, %)' is determined by open circles representative for areas not affected by seismic activity. The filled circles correspond to areas where cracks opened during a seismic crisis in February 1988 and where gas anomalies developed during the following three months. Arrows indicate evolutions during the survey. The diagram shows that during this event there was an increase in the gas flow rate without noticeable and concomitant increase of CO, concentration in the ground, thus indicating an increase of ground permeability.

0.061m-, h-', results lower than the accepted values for cultivated fields: 0.02 to 0.3 1 ,-'h-' (Henin et al. 1969), and the average value for the continental flow: 0.11 1 m-2 h-' (Gaudry et al. 1990).

Monitoring Soil gas monitoring can be conducted in two modes, discontinuous or continuous, depending on the activity of a given volcano and the rate kinetics of its evolution.

Discontinuous measurements This approach is appropriate for dormant volcanoes with a low activity. Repeated measurements of soil gas concentra- tions along specific profiles at a weekly, monthly or yearly frequency can allow the detection of changes in both concentration and area1 extension of soil gas anomalies, in connection with deep seismo-volcanic events (Fig. 4). Field experiments suggest that the simultaneous appearance of concentration levels of 10% CO2, 10 ppm He and 3000 pCi/l Rn at about 1 m depth in volcanic soils at a place without an initial gas anomaly can be considered as a warning of a possible resuming of volcanic activity.

Repeated flux measurements also give useful informa- tion. Gas fluxes can vary in accordance with concentrations (Fig. 4), but may also increase faster than the latter as a consequence of increasing ground permeability due to earthquake swarms and micro-fracturation, as shown in Fig. 3 for example. Thus, a shift of the flux/concentration function at a given site toward increasing gas flow can be used as an indicator of higher activity, requiring more intensive investigations. This approach can be applied for the detection of volcano-seismic activity and can be compared to that used for the prediction of tectonic earthquakes (Reimer 1980, 1985; Sugisaki et al. 1980; Sugisaki & Sigiura 1986).

574 J . -C . B A U B R O N E T A L .

87 -9

Fig. 4. Temporal frequency distribution of CO, concentration in soils along a traverse crossing a hidden fracture, before (87/9) and after (88/3 to 5) a seismic crisis (88/2). Base of Lamongan volcano, Java, Indonesia, 1987-1988.

Continuous monitoring On volcanoes in more rapid evolution, permanent monitoring is obviously desirable. Such a survey can be done on a selected soil gas anomaly ,or, alternatively, on related well gases, depending on possibilities. The proximity of electrical power, needed by permanent gas analysers, is one of the constraints. The dimensions of the volcanic edifice is another one, as an extensive coverage of all areas is unrealistic.

We have recently undertaken such continuous monitoring at Vulcano (Italy), a small volcanic cone, during a period of increasing activity. CO,, He and Rn are

100

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20

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8

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6

5

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0 24 4 8 7 2 9 6 120 i

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Fig. 5. Example of continuous monitoring of the concentrations of CO,, He and Rn in the atmosphere of a water well (site 23, Fig. 2). Base of Fossa cone, Vulcano, Italy, 1989. Short oscillations correspond in part to dilution by air due to atmospheric pressure variations. The three species vary altogether, which demonstrates their common derivation.

surveyed in gases emanating from a 14 m deep Water Well ( T = 65"C, site 23 in Fig. 2) which, according to both chemical and isotopic data (Baubron et al. 1990), have the same origin as the crater fumarolic gases. On such a site, diurnal variations are observed with some very fast oscillations due to atmospheric pressure changes at the interface between air and water (Fig. 5). Comparable observations were previously made by Sugisaki (1981) and Sugisaki & Sigiura (1986) during the monitoring of water wells in a seismic area of Japan. Slower but important variations also occur, with or without changes in the gas ratios, which may be due to volcanic effects. The steady Rn/CO, ratio during the very fast oscillations supports a rapid transfer of a hot volcanic gas through the groundwater. Otherwise, the simultaneous temporal varia- tion of He/C02 in both the well gas and the hot crater fumarole (F5, site 1 in Fig. 2) confirms the common and deep feeding of both fluids.

The decrease in both concentration and flux of soil CO, at Vulcano in 1989 (half the 1988 values, Fig. 6) and the corresponding decrease of He/CO, ratio may indicate that

SOIL GAS EMANATION AND VOLCANIC ERUPTION 575

0 1988 m 1989

? '

Fig. 6. Evolutions of the flux and the concentration of soil CO, at the base of Fossa cone, Vulcano, in the period 1988-1989. A clear decrease in both parameters was observed, the maximum CO, concentration in soils dropping from 90% to 55%.

the volcano is temporarily returning to a lower level of activity. This will have to be verified from further monitoring.

Conclusion Investigations of soil gases emanating from active volcanoes, at distances from craters or fumarolic zones, can provide useful information upon the evolution of magma degassing at depth, possible shallow intrusion or forthcoming seismic activity. Using the chemical and isotopic proportions of selected indicators such as CO,, He and Rn, some warning thresholds may be proposed for each volcano once the igneous origin of the gases has been recognized.

Hopefully, continuous monitoring of soil gases or related well gases could assist forecasting renewed activity at dormant volcanoes.

This collaborative study was supported by the Volcanic Hazards program of the Bureau de Recherches Gtologiques et Minitres (Paris, France), the Centre National de la Recherche Scientifique (Paris, France), the Volcanological Survey of Indonesia (Bandung, Indonesia), the Osservatorio Vesuviano (Napoli, Italy), the French Dtltgation Aux Risques Majeurs (Paris, France) and the French Ministry of Foreign Affairs. Part of this work was done during the stays of P. A. and J.-P. T. at the Osservatorio Vesuviano. J.-C. B. and J.-C. S . at the Volcanological Survey of Indonesia.

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Received 6 December 1990; revised typescript accepted 13 June 1990