HIGH PRECISION THERMAL MEASUREMENTS IN THE VOLCANIC TUBE OF 'LA CORONA' (LANZAROTD

HIGH PRECISION THERMAL MEASUREMENTS IN THE VOLCANIC TUBE OF 'LA CORONA' (LANZAROTD J. Fernández; H. Van Ruymbeke and R. Vieira Instituto de Astronomía y Geodesia Facultad de Ciencias Hatemátlcas Ciudad Universitaria 28040-HADRID Observatolre Royale de Belglque Avenue Clrculalre, 3 1180 Bruxelles ABSTRACT Slnce 1988, under the exlstlng collabor~tlon between the Instituto de Astronomía y Geodesia, the Observatolre Royale de Belglque and the European Center for Geodynarnlcs and Selsmoloqy,lhermal measurements ha ve been made In the geodynamlc statlon Cueva de los Verdes (Lanzarote, Canary Islands), that have be en used to check the quallty of the sensors used as well as the thermal stablllty of the laboratory. Severa! experlmenls have be en performed:measllrements of vertical ~nd horizontal thermal proflles, contlnuous record s of rack temperature, In dlfferent polnls, and ambiental temperature In the tunnel, where the laboratory Is located. The mea sures of the temperature profiles have heen made In three observatton campalngs whose results we are offer~~g here In.We have achleved preclslons In the range of 100C. We have found a vertical gradlent 0.030C/m In the vertical thermal proflle. The long and short perlod varlatlons of rock and ambiental temperature ha ve been studled. The results of the harmonlc analysls of those temperatures are presented here. 1 INTRODUCTION Within the investigation project that the Instituto de Astronomía y Geodesia is developlng in the Canary Islands, collaboratlng wi th the Observatolre Royale de Belglque and the European Center for Geodynamics and Seismology there are thermal studies made since 1988 in the geodynamic statlon Cueva de los 17

Verdes located in the volcanic tunnel of the La Corona volcano in Lanzarote, Canary Islands (Vieira et al.,1988(a), 1988(b), 1988 (el, 1989 (a), 1989 (b), 1989 (cl, 1990). Ihe observation of thermal profiles in the rock which allowed us to stablish the precision of the sensors used to take punctual measurements was made in a first phase. Afterwards the sensors were left installed in a permanent way to study the variations of long and short periods of temperature in rock. During this time it was as well made a continuous register of ambiental temperature and relative humidity in the tunnel, in the area where the station is placed. Measurements of thermal profiles have been repeated in two more occasions. 2. THERMAL PROFILES Ihere are several profiles observed, Figure 1, (Fernández et al., 1988). Ihree of them located in the tunnel area known as Los Coropiales, even and easy to reach: one of them horizontal of 12 points in around 14 meters whose objetive was to study the best observation method; another one tridimensional of 8 points enclosing both tunnel walls; and a third profile, made to study the vertical gradient, with 21 points. Ihere were as well made observations on an horizontal profile that links the Auditorium, located in an area previous to Los Coropiales, with the geodynamic station (around 250 meters). Io observe the temperature there were made perforations in the rock of about 10 cm depth and a diameter of around 1.5 cm. In these last ones there were introduced tubes of plastic material to avo í d that falling ground would prevent the direct contact of the sensor with the rock. Ihe sensors used are based on the use of thermally sensitive resistences. Ihere were used two type of thermometers, one with direct output in ohms and the other one with out.put in mv through electronics (Van Ruymbeke et al., 1989; Van Ruymbeke, 1990). Some of the observations have been made leaving the sensor in the perforation for differents times before the reading, searching for the interval wi th the best resul t (Fernández et al., 1988). 2.1 Results Ihe observation of the thermal profiles has been made in three campaigns (Fernández et al., 1988, Van Ruymbeke et al. 1989) in 1988, 1989 and 1990. Ihe most representative results can be seen on Iables 1 and 2. Standard deviations of measurements in the first campaign are affected because the perforations were very near in the time. Minor standard deviations were collected as results of the second campaign, once this interference effect 18

was eliminated. The highest are on the profiles observed In the vertical profile of aproximately 0.03 C/m, profile was observed. dispersions obtained in measurements in 1990. we have observed a vertical gradient in the two campaigns in which this 3 PERMANENT 11IERH0HETERS After the first observation campaign of the thermal profiles two thermometers VRL8756 wi th output in mv, number 901 and 902 (J. Flick et al., 1985; Van Ruymbeke et al., 1989) were installed permanently in the tunnel wall located at the back of the small house of the geodynamic station, Figure 2, in two very close points. In a first period the readings were made twice a week, de t ec tlng a maximum annual variation of 1.66 c. There w.as, at this stage, a similar response in both sensors (Van ruymbeke et al., 1989l. In 1990, once some electrical problems solved, the thermometers located in the rock were connected to the data adquisition system of the station (Fernández et al., 1989; Vieira et al., 1990), where data have been registered in a correct way every ten minutes. The sensor number 902 was moved from his original situation, by number 901, to a perforation made in the opposite wall, Figure 2. A thermometer was placed at the roof of the station to register the temperature of the air in the tunnel and a sensor for t.he relative humidity, Figure 2, both of them connected to the data adquisition system too. 3.1 Long and short perlod varlatlons The maximum variations experimented in the rock temperature and in the ambiental one, in the period between the 13/04/90 and the 30/09/90 can be seen on Table 3. Between the days from the 29/06/90 to the 01/08/90 there is no data from the thermometer in rock number 902. The d í urna I vartations f or these temperatures in the same period, every ten days, together with the relative humidity can be seen on Table 4. 3.2 Harmonic analysls The result of the harmonic analysis made with data from the three sensors, both with hourly data and half-hourly ones, shows two waves of significative amplitude, one from the diurnal period and the other from the semidiurnal one (Table 5). Besides these two waves are of very similar amplitudes for the three temperatures. 19

s The existence of the semidiurnal period wave could be due to an influence of the oceanic tide, mainly for the effect of this last one in the level of underground waters. The existence of possible underground aquifers connected with the ocean would not be either an anomalous fact, taking account the origin of the ground (Vieira et al., 1989 (a». 4 CONCLUSIONS The typical devlations of the measurements made on thermal -3-2 profiles are in a range between S-lO y S-lO, and they seem to depend mainly on the diurnal variation of the temperature, once the effect of the perforation was vanished, as in the third campaign, where this was the highest variation, was where the worst results were obtained. In the observations analysis, the diurnal variation of the temperature is considered as instrumental drift, whlch is equivalent to consider that there Is no varlation along the day (Fernindez et al., 1988) being ajusted by a direct llne. If the temperature variation is small, the error made adjusting with a direct line may be small, speclally if the observation lnterval is no very wide; but it seems more logical to adjust by a function with degree two (as minimum) at least in some occasions. We are working to prepare a new analysis of the three campaigns adjusting the drift in every profile (where the diurnal variation of temperature is included) by a direct line and a parabola, keeping for correction the curve which give us a better adjustement. In this way we expect to diminish the standard deviation of measurements. It has been observed that the most suitable method for observation in profiles with a small separation between points (thermal profiles located in Los Coropiales) í to leave the sensor in the perforation ten minutes before taking the reading; and in profiles wi th a greater distance between points (Auditorium-Geodynamic Station profile) to wait twenty minutes. Related with the continuous resgister, it has been observed that the highest diurnal variations are produced in summer, reaching the range of 0.5 degree some day. In warmless days, it is normal that it does not exceed 0.1 degree. It may have a connection between the variations of the temperature in rock with ambiental temperature and the relatative humidity of the air along the day. We are working to obtain an experimental model of the relationship between these three parameters. It would be interesting to consider the days in which rainfalls have taken place. The signal registered wi th sensor 902 is of a far small amplitude that the one registered with sensor 901, intalled in rock too; this is clearly telling us that the isolation of the sensor in respect with the ambiental temperature must be good, or at least better than the 901 one. The harmonic analysis of the 20

temperature reglstered in rock ls similar, as to the representative waves, to the obtained with ambiental temperature, which seems to reflect an effect of this last one over the first ones. Something really important Is the Improvement observed in the harmonic analysis when is made with semihourly data instead of hourly ones, Table S, which together with the a curacy ot the register made wl th the data acquisi tion system has allow us to clearly separa te waves of such a small apmplitude, such as those obtained in the analysis of the temperature registered with sensor 902. For the analysis we can use data obtained every ten minutes, which would be interesting thlnking to improve the results of the harmonic analysis, not only in this particular case but in any other one (Vieira et al., 1990). In a near future we are planning to make a study of the conductivity and the thermal flux in different areas of the volcanic tunnel, mainly in the area where the geodynamic station is located. In these studies the thermometers used would be those based in the same technique that we are using at present, but installed between one and two meters deep, avoiding as much as possible the influence of external temperature. A study about the possible correlations among temperatures, ambiental and rock, and the oceanic tide would be as well made, investigating the existing possibility of underground waters connected to the sea. ACKNOllLEDGHENTS This work has been financed wi th funds from the project "Dinámica Superficial y Profunda de la Caldera del Teide y de la Isla de Lanzarote Investigada a partir de Técnicas y Métodos Geodésicos y Gravimétricos de alta Precisión (PB-0022)" (Deep and Superficial Dynamic of the Teide Caldera and of t.he Island of Lanzarote, Investigated from Geodetic and Gravimetric Tecniques and Methods of a High Precision) of the Instituto de Astronomía y Geodesia (l. A. G. ) ( Consejo Superior de Investigaciones Científicas-Universidad Complutense de Madrid ) and the European Center for Geodynamics and Seismology (E.C.G.S.). Acknowledgments are due for their collaboration to Eng. J. Flick, president of the E.C.G.S. and Professor P. Melchior, director of the Observatoire Royale de Belgique, as well as the great support offered by Cabildo Insular de Lanzarote and a11 the staff of the turistic center of Jameos del Agua and the turistic and cultural center Casa de los Volcanes, and very specially to J. Naverán, J. Soto, O. Hernández and M. Espino. We acknowledge as well the help given by J. P. Davidts, A. Van Ruymbeke and J. Arnoso in the observation of the thermal profiles. 21

ssue t. REFERENCES Fernández, J.; and Van Ruymbeke, M.: "Estudio térmico de una zona de la Cueva de los Verdes (Lanzarote, Islas canarias)." Publicaciones de Instituto de Astronomía y Geodesia n0164, pp. 7-13, (1988). Fernández, J.; Vieira, R.; Lambas, F.; and Toro, C.: "Data acquisition systems in the "Valle de los Caidos" and "Cueva de los verdes" stations." Proc. of the XI Int. Symp. on Earth Tides. Helsinki, 12 pp. (In press) (1989). Flick, J.; Van Ruymbeke, M.; and Melchior, P.: "Instrumental Deve lopmen ts at the underground laboratory for geodynamics (Walferdange Grand Ducht of Luxemburg)." Proc. X lnt. Symp. on Earth Tides; pp. 83-94. Ed í R. Vieira; C.S T.C.. Madrid (1985). Van Ruymbeke, M.; Fernández, J.; and Westerhaus, H.: "Temperature measurements". Proc. of the XI Int. Symp. on Earth Tides. Helsinki, 10 pp. (In press) (1989). Van Ruymbeke: "Temperature measurements." (This í ) (1990). Vieira, R.; Fer nández, J.; and Toro, C.: La estación de la Cueva de los Verdes (Lanzarote): primeros resultados de las experiencias realizadas." Revista de la Real academia de Ciencias Exactas, Físicas y Naturales de Madrid. Tomo LXXXII, cuaderno 2. pp 309-312 (1988(a)). Vieira, R.; Fe rnández, J.; Toro, C.; and Camacho A.G.: "Estación Geodinámica del complejo Jameos de Agua-Cueva de los Verdes (Lanzarote). Objetivos científicos y estado actual de las instalaciones." Publicaciones del Instituto de Astronomía y Geodesia, n0164, pp. 1-6, (1988(b»). Vieira, R. ; Fernández, J. ; and Toro, C. :"Las variaciones periódicas de la gravedad observadas en el tunel volcánico de La Cueva de los Verdes (Lanzarote)." Publicaciones del Instituto de Astronomía y Geodesia, n 164, pp. 15-19, (1988 (c )). Vieira, R.; Fernández, J.; Toro, C.; and Camacho, A.G.: "Stuctural and oceanic effects in the gravimetric tides observations in Lanzarote." Proc. of the XI Int. Symp. on Earth Tides. Helsinki, 10 pp. (In press) (1989(a)). 22

Vieira, R.; Fernández, J.; Toro, C.; and Van Ruymbeke, M.: "Estación geodinámica de Lanzarote" ESF meeting on Canarian Volcanism. Edit. E.S.F.-C.S. J.C, pp. 276-277. Lanzarote (1989(b». Vieira, R.; Fernández, J.; Toro, C.; Camacho, A. G. :and Van Ruymbeke, M.: "Dos afios de observaciones en la estación geodinámica de Lanzarote" ESF meeting on Canarian Volcanism. Edit. E.S.F.-C.S. J.C, pp. 278-280. Lanzarote (1989(c». Vieira, R.; Van Ruymbeke, M.; Fernández, J.; Arnoso, J.; and Toro, C.: "The Lanzarote underground laboratory." (This issue) (1990). 23

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AMBIENTAL TEHPERATURE INSTRUHENT NUHBER 001 Period: 13/04/90 to 30/09/90 Hax. : 20.49 e Date: 26/08/90 Hin. : 17.69 e Date: 15/04/90 /j. = (Hax. -Hin. ) = 2.8 e (a) ROCK TEHPERA TURE INSTRUHENT NUHBER 901 Period: 13/04/90 Hax. : Hin. : 19.012 17.714 /j. = (Hax.-Hin.) = 30/09/90 Date: 26/08/90 Date: 28/04/90 (b ) ROCK TEHPERA TURE INSTRUHENT NUHBER 902 Period: 13/04/90 20/06/90 Hax. : 18.023 Date: 20/06/90 Hin. : 17.643 Date: 28/04/90 /j. = (Hax.-Hin.) = 0.380 C Period: 01/08/90 Hax. : 18.738 30/09/90 Date: 27/08/90 Hin. : 18.438 /j. = (Hax.-Hin.) = 0.3 c Date: 01/08/90 (e) TabIe 3 27

TEMPERATURE SENSORS M.R.H. 001 901 902 ( Yo 13-04-90 0.050 0.010 0.004 67 23-04-90 0.090 0.040 0.026 63 04-05-90 0.100 0.010 0.005 71 14-05-90 0.080 0.003 0.002 72 23-05-90 0.080 0.022 0.011 75 03-06-90 0.180 0.013 0.007 78 13-06-90 0.120 0.016 0.009 77 23-06-90 0.080 0.007 0.004 80 28-06-90 0.250 0.083 82 12-07-90 0.230 0.018 85 22-07-90 0.210 0.015 84 03-08-90 0.510 0.069 0.032 89 13-08-90 0.7.20 0.046 0.030 85 23-08-90 0.230 0.019 0.005 87 02-09-90 0.180 0.020 0.012 85 11-09-90 0.250 0.024 0.007 84 22-09-90 0.220 0.047 0.021 85 30-09-90 O. 150 0.020 0.007 84 Table 4. Daily variations in e given, normally every ten days, and Mean Relative Humldity (M.R.H.) In %, correspondlng to the same day. 28

DIURNAL PERIOD INSTRUMENT AKPLITIJDE ( e) E.Q.H. HOURLY DATA HALF H. DATA HOURLY DATA HALF H. DATA 001 0.104 0.083 0.033 0.022 901 0.0107 0.0147 0.0048 0.0033 902 0.0038 0.0046 0.0021 0.0015 (a) SEHIDIURNAL PERIOD 1NSTRUHENT AKPLI11JDE (oc) E.Q.H. HOURLY DATA IIALF H. DATA HOURLY DATA HALF H. DATA 001 0.156 0.154 0.015 0.010 901 0.0103 0.0110 0.0014 0.0008 902 0.0032 0.0032 0.0005 0.0004 (b) rabie 5 29