Projects of the Royal Observatory of Belgium (ROB) at the Lanzarote Geodynamical Laboratory (LGL)

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1 m 47 #. m 1 -l} il'jjtil,**. (2001) :í't Journal of the Geodetic Society of Japan Vol. -!7, :-lo. 1, (2001), pp Projects of the Royal Observatory of Belgium (ROB) at the Lanzarote Geodynamical Laboratory (LGL) Michel van Ruymbeke!", Andre Somerhausen!', Bernard Ducarme!', Ricard Vieira", José Amoso'" and Emilio Velez 2 ) 1) Observatoire Roya! de Belgique, 3 Ave Circulaire, 1180Bruxelles, Belgium. 2) Inst.of Astronomy and Geodesy (CSIM-UCM), Univ.Complutense de Madrid, Facultad de Matematicas Madrid, Spain (Received October 16,2000; Revised January 3, 2001; Accepted January 3, 2001) Abstract A series of experiments prepared at the Royal Observatory of Belgium, are running in the three Lanzarote laboratories under the EDAS concept (van Ruymbeke et al.,1993,1997a). The main purposes of our observations which started ten years ago (Fernandez et al., 1990), cover the domains of the tidal effects including oceanic tides interactions, ground deformation and environmental parameters monitoring. We prepare instruments adapted to the special conditions of volcanic medium in the mind of the EDAS concept which is adapted to that project. In the Timanfaya area, we are observing the interactions between a shallow magma chambers and the tidal signals. A correlation exists between the drift rate of a LaCoste & Rornberg gravimeter and the local seismic activity which seems to induce heat flow changes. In the cave "Jameo del Agua", it is possible to record the oceanic tide in excellent conditions due to the filtering of the sea-waves by a few kilometers long siphon connecting the site to the open sea. Original probes are designed to check the long term stability of pressure sensors. A vertical pendulum register the tiit of the ground induced by the oceanic loading. High precision thermometers (van Ruymbeke et al., 1999) assume the monitoring of the vertical thermal gradient which could be influenced by the modulation of the salinity by the water flow from aquifers. The HiCum method is used to detect the admittance between the different parameters. 1. Introduction The aims of I:anzarote Geodynamical Laboratories (LGL) project include a multi-parameter approach to develop methods dedicated to hazard precursors researches. Analysis of various signals after major events seems to confrrm by post-diction, the possibility a relationship between the event itself and some signals prior to this event. The involved phenomenon are extracted from the noise induced by many kind of sources. It is essential to be sure about the meaning of the conclusions obtained from the analysis of signals. A global approach including cross coupling effects between the various parameters reduces the risk of misunderstanding of processes. * michelvanruymbeke@oma.be

2 464 Michel van Ruymbeke et al. The improvements of instrumentation and data treatment allow to use tidal signals for the monitoring of the geophysical medium evolution.tides are a permanent and well defined modulation acting everywhere on each part of the system. So they are highly suitable to characterize transfer functions of a medium, including processes conceming mechanical energy, hydraulic energy and thermal energies.among the transfers of energies between the geophysical medium M and the outside one should distinguish two types of interacting energies. On one hand, one must consider large energies which are responsible for catastrophic events (such as earthquakes, volcanic eruptions,...). On the other hand, one must take into account smaller energies which will induce small scale modifications of physical parameters (earth-tides, variations of stress, inclination, temperature, humidity,...) without destruction of the Medium. The principle of our approach to detect eventual precursors consist to determine the transfer function that relates the physical parameters to the small energies. A modification of the transfer function would be the evidence of a change in the medium M. By monitoring the changes of this transfer function it could be possible to determine a threshold value of the physical parameters that induce a catastrophic event. It is thus necessary to follow the evolution of the different physical parameters in respect with time and to relate them to the variations of energies in order to determine the evolution of the transfer function in time. A significant change would then be interpreted as a precursor of a catastrophic event. Therefore such method applied in LGL, requires a multi-parameter and multi-location approach with continuous recording of data in order to obtain the information. To illustrate this approach, we shall describe two experiments set-up in the "Timanfaya" park and in the lava tube "Jameo del Agua" 2. Monitoring of High Temperature Variations in the Geothermal Area of Timanfaya In the southem part of Lanzarote (Canary Archipelago),lies a National park named Timanfaya. It covers 51 sq. km and represents the most recent volcanic activity in this islands. The last historical eruption formed a geothermal alteration at the «Islote de Hilario», consisting in a residual magma chamber located at a depth of approximately 2-4 km. Temperatures of 610 C have been recorded at a depth of 13 m and between 100 C and 120 D C near the surface. The seismic activity of this zone is quite interesting as small magma-phreatic quakes occur almost every day. We have observed a correlation between the drift rate of a LaCoste&Romberg gravimeter installed in an underground laboratory located just above the magma chamber, and the shallow seismic activity (Figure 1). The amplitude of gravity changes are to large to result of an up-lift which has to reach more than sixty cm. Considering the complexity of water interactions which could explain this behaviours of the gravimetric signature, it is justified to sense environmental parameters in this area. Completing a series of probes dedicated to the ground deformation monitoring, we set up an array of EDAS- HTEMP (van Ruymbeke et al.,1997a,b) which are underground temperature sensors specially designed, to detect an eventual correlation between the occurrence of the quakes and the variations of the underground temperature.

3 Projects of the Royal Observatory of Belgium at the Lanzarote Geodynamical Laboratory 465 Number of seisms ,..,..,..,..,.., -e a,.. -a, m m m m m q> m m m m m m., o re re o =>..., ::> -a, s. " 6. ts :lo Ó.ó >. C: -'- " 6. :> ó ::> c: re o- ::> o ;:;: '" '" O O V? z u....., c( 7 :::< ch V? '",, N N N o m en ch r-!. <b.;, ID N N N N N Fig. 1. The upper curve is the record of the gravimeter signal which show a lmgal annual termo In parallel we put an histogram of the seismic activity. A correlation exists between the two signals with about one month of delay. These sensors are based on the principie of thennal expansion of solids. A metallic cylinder is placed into a metallic tube having a slightly larger diameter in such a way that both elements are co-axial with less than one mm of distance between them. We measure the value of the capacitance to monitor temperature changes by differential dilatation between inner cylinder and outer tube (Figure 2). Fig. 2. The variation of TO induces thermal expansion which modifies the electric capacitance between the cylinder and the external tube. The principie of the measurement is based on the fact that the central cylinder due to its mass and to the gap that separates it from the intennediary tube will barely be affected by the changes in temperature of the outside geophysical medium. On the contrary, the intennediary tube which has a much smaller volume and is also much closer to the heat source will fol!ow the changes in temperature of the outside medium. Let' s recal! here that al! the thermal energy that reaches the central element has to cross al! the others parts. This allows us to use a simple electric analogy (Figure 3) to analyse the behaviour of the model of the HTEMP sensor. This analogy as sumes that the cel! R3-C3 is fully influenced by the cel! R2-C2.

4 466 Michel van Ruymbeke et al. RI R2 R3 el 1 Fig.3. Electric analogy of the 'HTEMP' sensor. The capacitances stand for the layers of metal that are being crossed by the heat flow. Heat energy is stored in those metallic pieces and their capacitance is proportional to their mass and their specific heat. el and e2 respectively represent the capacity of the outer tube and the intermediary tube. e3 stands for the central cylinder. Since the mass of the outer tube and intermediary tube is much smaller than the mass of the central solid cylinder, it can already be stated that the capacitance of the two tubes will be much less than the one of the central cylinder. Rl represents the contact resistance between the outside medium and the thermometer. As the sensor is completely surrounded by this medium and the thermal contact with it is good, it can be assumed that Rl is very low. In the further simplification of the model, R1 will be considered equal to zero. The resistances R2 and R3 model the air gaps between the elements. When flowing from one element to the next one, heat flow is hindered by the poor conducting properties of air. This is, the greater the resistance, the larger the gap. A first prototype of HTEMP sensor had been installed in It was placed in the upper part of a 12 meter deep bore-hole. The temperature of the air that comes out of that hole is of about 200 e. The hot air flow is simply due to the free convection on the walls of the tube. In order to carry out experiments on the HTEMP sensor, three more prototypes were built and installed. Conclusive recordings confirrned that this high temperature thermometer based on the concept of differential dilatation of metals is applicable. The recordings of the on-site testing of the 'HTEMP' sensor in Timanfaya (Lanzarote, Canarian Islands) are presented in (vanruymbeke et al., 1997c). It can be noticed that thermal agitation signals occur simultaneously with seismic agitation. 3. Examples of Jameo del Agua Systems Monitoring Oceanic Tides EtTects The site of observation named "Jameo del Agua" is located in a lava tunnel which is plunging in the sea for more than one and half kilometer.. The filtering of the short period waves is due to the few openings existing between the under-sea part of the tunnel and the open-sea'tidal variations of water levels could reach few meters with a very smooth motion in a site where we have access to the sea-water (Figure 4). This is a suitable conditions for the monitoring of the mean sea-level variations on very long periods. However local ground deformation should be studied with techniques like permanent GPS to allow the correction of the tidal probes anchored in front of an absolute geodetic reference.

5 Projects of the Royal Observatory of Belgium at the Lanzarote Geodynamical Laboratory 467 Fig. 4. "Jameo del Agua" laboratory in the site where the free surface of the sea-water is moving under the effects of the oceanic tides In parallel with a permanent GPS receiver, various probes record oceanic-tides, ground flexion, air and water temperature modulations,... The Figures 5 and 6 show examples of records obtained in the Jameo del Agua. The figure 7 shows HICUM output for the M2 lunar main wave (van Ruymbeke et al., 2001). Jameo del Agua <Lanzarote) EDAS probes for uater level a TO l. I'<Jo.:1.". 1\.tI'J' }w.)-nvl W:'IIJt1 VWlj. <Jlt!t,MP"';V':/\lV\.h 'J,'AI. fmni' {twtlvvv'j\\\ rvw/v.'rr,,watertocfitítj p-d or. i WaterTO down i}yr(w{wynfij!iiwil'nlfnill\!.r.il{1wttw 1" 'i,:;..- "fl f;.::o,- "...D,- Fig.5. The four curves are the forty days record s of probes installed in the Jameo del Agua:: - I.A maregraph registering the water level variations which have an amplitude of more than two rneters, - 2.A thermometer registering the variations of water temperature near the surface ( typically one tenth of C ), - 3.A thermometer registering the difference of temperature between the surface and the bottom of the sea, - 4.A thermometer registering the water temperature at the bottom (amplitude of few hundreds of degree). Correlations exists between these four signals

6 468 Michel van Ruymbeke et al. Jameo del Agua (Lanzarote) RPH tiltmeter 8 Maregraph _.._....'" L._.. L......i., *. : ;._. t...._ _ Fig. 6. The lower curve is the records of a vertical pendulum EDAS-RPH registering the tilt of the rocks under the loading effect of the oceanic tides. A correlation exist between the thermorneter recording the gradient of TO (channel 3 of figure 4) and the pendulum record which is for the same time interval.. Maregraph... _\'{at!.iy.el.i9.!1 WaterTO-up Ja.ea: HICUI1 M wotenevellow" Jeeec: HrCUM,,2 XY graphs HICUM-M2-Water T"Up =-----._--:::-_._-_._-_. --._---_... _----_. -,- Fig.7. The left graph is the plot of the HICUM-M2 outputs for the four channels of the figure 4. The anomaly existing on the upper thermometer compared to the bottom one is clearly confrrmed by the HICUM-M2 of the differential thermometer. The right graph is a X-y output with in X-axis, the HICUM-M2 of the bottom thermometer and in the Y-axis, the upper thermometer and the differential thermometer. Such graphs illustrate the non-linearity existing between the oceanic tides and the thermal reaction of the water. 4. Conclusions The Lanzarote project becomes a very wide geodynamical tool dedicated both to the methodology and the observations including various parameters modulated by earth & oceanic tides. A better understanding of such environment could help to improve the interpretation of the registration produced by the very sensitive modern instrumentation.

7 Projects of the Royal Observatory of Belgium at the Lanzarote Geodynamical Laboratory 469 Acknowledgement The authors whish to thank Professor P. Páquet, Director of the Royal Observatory of Belgium for putting all the necessary means at their disposal, Professors R. Vieira & R.Ortiz as well as the ICONA and the Cabildo Insular de Lanzarote for their active support in organizing the on-site testings. This research is made belonging the Commission of the European Cornmunities, DG XII, Environment Prograrnme, Climatology and Natural Hazards Units, in the framework of the contract EV5V-CT and is partly supported by the ECGS and Casa de los Volcanes. References Femandez J., M. Van Ruymbeke, R Vieira (1990): Instrumentacion y Proceso de Datos en Ciencias de la Tierra, Medidas Termicas de Alta Precison en El Laboratorio Geodinamico de Lanzarote. Serie Casa de Los Volcanes n 1. IlI. van Ruymbeke, M., N. d'oreye, A. Somerhausen and N. Grarnmatica (1993): Technological Approach from Walferdange to Lanzarote, the EDAS concept. Proc. of the Workshop New Challenges for Geodesy in Volcanoes Monitoring, Walferdange June 14th - 16th, 1993.Cahiers du Centre Européen de Géodynamique et de Séismologie, 8, 99, van Ruymbeke, M., Fr. Beauducel and A. Somerhausen (1997a): The Environmental Data Acquisition System (EDAS) Developped at the Royal Observatory of Belgium, Proc. of the Workshop "Short thermal and hydrological signatures related to tectonic activity", Cahier du Centre Européen de Géodynamique et de Séismologie, 14, presented during the Seminar 192 der ( Wli-Heraeus- Stiftung) "Microtemperature Signals of the Earth's Crust ", Bad Honnef, Germany, March, 1998,23-35, van Ruymbeke, M. and M. Carbonez (1997b): A Probe for High Temperature Variations Measurements in the Field of Geothermalism, Proc. of the Workshop "Short thermal and hydrological signatures related to tectonic activity ". Cahier du Centre Européen de Géodynamique et de Séismologie, 14, van Ruymbeke, M., R Vieira, R Ortiz and M. Carbonez (1997c): Geothermal Signals Recorded in the Timanfaya Area (Lanzarote, Canarias, Spain), Proc. of the Workshop " Short thermal and hydrological signatures related to tectonic activity ". Cahier du Centre Européen de Géodynamique et de Séismologie, 14, van Ruymbeke, M. and A. Somerhausen (1999): Experiments in Laboratory and Field with Precise Thermometry, presented during the Seminar 192 der ( WE-Heraeus-Stiftung) "Microtemperature Signals of the Earth's Crust", Bad Honnef,Germany, March, 1998,